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Published by Priv.-Doz. Dipl.-Ing. Dr. techn. Nicole Dörr Univ.-Prof. Dr.-Ing. Carsten Gachot Dr.-Ing. Max Marian Dr.-Ing. Katharina Völkel 24th International Colloquium Tribology Industrial and Automotive Lubrication Conference Proceedings 2024 24 th International Colloquium Tribology Industrial and Automotive Lubrication 23 rd to 25 th January 2024 Technische Akademie Esslingen Published by Priv.-Doz. Dipl.-Ing. Dr. techn. Nicole Dörr Univ.-Prof. Dr.-Ing. Carsten Gachot Dr.-Ing. Max Marian Dr.-Ing. Katharina Völkel 24 th International Colloquium Tribology - Industrial and Automotive Lubrication Conference documents 2024 Bibliografische Information der Deutschen Nationalbibliothek Die Deutsche Nationalbibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet über http: / / dnb.dnb.de abrufbar. The German National Library lists this publication in the German National Bibliography. Detailed bibliographic data are available in the Internet at http: / / dnb.dnb.de The work including all its parts is protected by copyright. Any use outside the narrow limits of copyright law without the consent of the publisher is inadmissible and punishable. This applies in particular to reproductions, translations, microfilming and storage and processing in electronic systems. The present work was created with great care. However, errors cannot be completely ruled out. Neither the publisher nor the authors or publishers therefore accept any liability for the correctness, topicality and completeness of the work and its electronic components. Das Werk einschließlich aller seiner Teile ist urheberrechtlich geschützt. Jede Verwertung außerhalb der engen Grenzen des Urheberrechtsgesetzes ist ohne Zustimmung des Verlages unzulässig und strafbar. Das gilt insbesondere für Vervielfältigungen, Übersetzungen, Mikroverfilmungen und die Einspeicherung und Verarbeitung in elektronischen Systemen. Das vorliegende Werk wurde mit großer Sorgfalt erstellt. Fehler können dennoch nicht völlig ausgeschlossen werden. Weder Verlag noch Autoren oder Herausgeber übernehmen deshalb eine Haftung für die Fehlerfreiheit, Aktualität und Vollständigkeit des Werkes und seiner elektronischen Bestandteile. © 2024. Alle Rechte vorbehalten. expert verlag Ein Unternehmen der Narr Francke Attempto Verlag GmbH + Co. KG Dischingerweg 5 · D-72070 Tübingen E-Mail: info@verlag.expert Internet: www.expertverlag.de Printed in Germany ISBN 978-3-381-11831-1 (Print) ISBN 978-3-381-11832-8 (ePDF) Technische Akademie Esslingen e. V. An der Akademie 5 · D-73760 Ostfildern E-Mail: maschinenbau@tae.de Internet: www.tae.de 24th International Colloquium Tribology - January 2024 5 Preface Challenging times demand swift and effective solutions to problems. Since the last 5 years, we have been confronted with several serious global challenges. On the one hand, there are geopolitical „flashpoints“ where science unfortunately offers limited guidance, as the ball is mainly in the political arena. On the other hand, the pressing issue of climate change repeatedly reminds us of our vulnerability. In addition, questions regarding sustainable energy supply, resource conservation and the urgent need to take completely new pathways in transport and production to combat global warming are paramount. Buzzwords like e-mobility, hydrogen, circular economy, and digital transformation, while initially rather having been empty words, need to be filled with life, realized by courageous political decisions enabling significant implementation of novel technologies. The multifaceted nature of these issues demands interdisciplinary solutions. Tribology, as the science of friction, wear, and lubrication, is uniquely positioned to address these challenges due to its interdisciplinary nature. As already once noted by the gifted Galileo Galilei, “in matters of science, the authority of thousands is not worth the humble argumentation of a single person”. This underscores the significance of conferences that bring together scientists with diverse perspectives from all over the world. It‘s through these interactions that innovative ideas can develop and mature, potentially providing solutions to critical contemporary questions. The 24 th International Tribology Colloquium of the TAE in Ostfildern offers an ideal communication platform for representatives from industry and science to come together and discuss approaches to solutions for current tribological issues. The conference covers a wide range of tribological topics to advance solutions to challenges as outlined above. Accordingly, 5 main topics were defined that illustrate the main fields of current research in tribology: • New trends in lubricants and additives • Coatings, surface interactions and underlying mechanisms • Machine elements and their application in tribology • Computational methods and digital transformation in tribology • Test and measurement methodologies The conference will be rounded off by excellent plenary and keynote talks on topics of mobility, data handling and efficiency enhancement of machine elements. Last but not least, we invite our “Young Tribologists” to the curtain with 2 dedicated sessions to report on ongoing research work in tribology in their early careers. We are looking forward to a pro-active exchange and stimulating discussions and hope that curiosity will drive us all to advance the field of tribology in its entirety. Together, we can address the multifaceted challenges presented by global crises and create a brighter future. Yours sincerely, Nicole Dörr, Katharina Völkel, Max Marian & Carsten Gachot Steering Committee 24th International Colloquium Tribology - January 2024 7 Table of contents P Plenary Lectures P.1 Sustainability in Winter Sports - The Tribological Perspective 23 Matthias Scherge P.2 Minimizing CO 2 Emissions and Maximize ROI: Implementing Known Tribology and Design for Zero Principles for a Carbon Neutral Industry 25 Roland Larsson, Victoria Van Camp P.3 Dynamic Properties of Lubricants for Electric Vehicles 27 Yan Chen, Hong Liang P.4 E-Fuels and Tribology * Lars Hummel P.5 Supporting Mobility Transition - Alternative Energy Carriers in Tribology 29 Marcella Frauscher, Adam Agocs, Charlotte Besser, Michael Adler, Hannes Hick P.6 Towards Superefficiency 31 Thomas Lohner, Constantin Paschold, Karsten Stahl P.7 The Data Science Frontier in Tribology 33 Nick Garabedian, Ilia Bagov, Malte Flachmann, Nuoyao Ye, Miłosz Meller, Floriane Bresser, Christian Greiner 1 New Trends in Lubricants and Additives 1.1 EV-Lubricants and Additives 1.1.1 Lubricants Technology for Improving the Protection Performance of Reduction Gears in Transaxles for Electric Vehicles 37 D. Takekawa, H. Tatsumi, K. Matsubara, K. Narita 1.1.2 Next-Generation Anti-Wear for EV Lubricants 39 Christelle Chretien 1.1.3 Impact of Lubricating Oils on the Performance for Liquid-Cooled Motor and Battery Thermal Control System Applied to Electric Transaxles 41 K. Narita, Y. Nakahara, K. Matsubara 1.2 Organic Friction modifiers 1.2.1 Novel Organic Friction Modifiers with Extended Performance Durability 43 Pieter Struelens, Marion Kerbrat, Micky Lee 1.2.2 Effect of Organic Friction Modifiers on Friction and Wear of HDDEO Formulations 45 Gareth Moody, Alexei Kurchan, Sydne Tison 1.2.3 Performance Enhancement of Molybdenum-Based Friction Modifiers 47 David Boudreau Sr, Brian Casey 8 24th International Colloquium Tribology - January 2024 1.3 Nanoparticle-based Friction Additives 1.3.1 Lubricity-improving Additives Based on the Synergy of Nanoparticles and Protic Ionic Liquid 49 Raimondas Kreivaitis, Milda Gumbytė, Artūras Kupčinskas, Jolanta Treinytė 1.3.2 Looking for the Perfect Friction Match in the 2D World * Prof. Dr. Carsten Gachot 1.3.3 In-Operando Formation of Transition Metal Dichalcogenides - Instant Lubrication by Simple Sprinkling of Se Nano-Powder onto Sliding Contact Interfaces 51 Philipp G. Grützmacher, Maria Clelia Righi, Ali Erdemir, Carsten Gachot 1.4 Biobased Lubricants, Greases and Additives 1.4.1 SAPS-free Bio-based Additives for Lubrication in Next-generation Vehicles 53 Xin He, Christelle Chretien 1.4.2 Biobased Ionic Liquid for Conductive Lubricants 55 Pieter Struelens, Yen Yee Chong, Micky Lee 1.4.3 Introducing a New High-Performance Water-Based Rust Preventive Additive for Formulations Demanding Superior Metal Parts Protection in Severe Corrosion Conditions 57 Clifford Pratt 1.5 Base Oils 1.5.1 Production of High VI Base Oils from Full Conversion Hydrocracker Residue with Solvent Refining 59 Dimitrios Karonis, Panorea Kaframani 1.5.2 Base Oil Solvency and High Temperature Deposit Formation in Gas Engine Oils - a Model Study - 61 Thomas Norrby, Marcella Frauscher, Christoph Schneidhofer, Frans Nowotny-Farkas 1.5.3 An investigation of Using Ultra-low Viscous Naphthenic Oil in Lubes and Greases 63 Jinxia Li, Mehdi Fathi-Najafi, Thomas Norrby 1.6 Lubricants and Additives for Cutting and Drawing 1.6.1 Tunable Viscosity of PAG and its Application in Sheet Metal Forming 65 Dominic Linsler, Korhan Celikbilek, Stefan Reinicke, Bernd Aha 1.6.2 Surfactant Systems with Improved Lubricity for Water Miscible Cooling Lubricants 67 Ludger Bösing, Arjan Gelissen 1.6.3 Formulating Next Generation Multi-Metal Wire Drawing Fluids with Multifunctional Amino Alcohols 69 Denis Buffiere, Kathleen Havelka, Amelie Bretonnet 24th International Colloquium Tribology - January 2024 9 1.7 Anti-Oxidation and Anti-Wear Technology 1.7.1 Antioxidative Action and Tribological Performance of CuDTP as a Potential Additive for Hydraulic Fluids 71 N. Ayame, K. Yagishita, T. Oshio 1.7.2 Boundary Lubricant Additive Responses on Steel, Aluminum and Copper Using Twist Compression Tests (TCT) for Multimetal Lubricant Formulation 73 Ted G. McClure, Alexes Morgan 1.7.3 Effect of Phosphonium Ionic Liquid as Lubricant Additive in Gear Oil against White Etching Areas Formation in Bearing Steel 75 Linto Davis, P. Ramkumar 2 Coatings, Surface Interactions and Underlying Mechanisms 2.1 Coatings 2.1.1 Combination of DLC Coatings and Dedicated Lubricants in order to Achieve Supralow Friction in Highly Loaded Sliding Contacts 79 Johnny Dufils, Etienne Macron, Christophe Héau 2.1.2 Numerical and Experimental Analysis of the Tribological Performance of a DLC-Coated Piston Ring-Cylinder Liner Contact 81 Thomas Lubrecht, Nans Biboulet, Antonius A. Lubrecht, Johnny Dufils 2.1.3 The Running-In of a DLC-Metal-Tribosystem - A Study on Multiple Scales 83 Matthias Scherge, Joachim Faller 2.1.4 Influence of Particles on DLC Coated Journal Bearings 85 Alexander Hofer, Manuel Zellhofer, Thomas Wopelka, Andreas Kübler, Andreas Nevosad, Martin Jech 2.1.5 Assessment of Different Coatings on the Friction and Wear Behavior of Differential Shafts for Electric Vehicles 87 Etienne Macron, Johnny Dufils, Christophe Heau 2.1.6 Atomistic Insights into the Behavior of Solid Lubricants Under Tribological Load 89 Andreas Klemenz, Michael Moseler 2.2 Surface Modification 2.2.1 Modification of Surface Properties on Various Mg-Based Alloys for Tribological Applications via Plasma Electrolytic Oxidation Process 91 Ashutosh Tiwari, Jörg Zerrer, Anna Buling 2.2.2 Mechanical Adhesion with Micropatterned Surfaces 93 Marco Bruno, Luigi Portaluri, Luciana Algieri, Stanislav Gorb, Massimo De Vittorio, Michele Scaraggi 2.2.3 Unveiling Extreme Lightweight Potential by PEO Refinement of Innovative Al Alloys 95 Anutsek Sharma, Jörg Zerrer, Genki Funamoto, Anna Buling 10 24th International Colloquium Tribology - January 2024 2.3 Surface Interactions 2.3.1 The Effects of the Lubricant Properties and Surface Finish Characteristics on the Tribology of High-Speed Gears for EV Transmissions 97 Boris Zhmud, Morteza Najjari, Boris Brodmann 2.3.2 Effects of Calcium Detergents on Micro-Pitting of Gear Metals 99 Akira Tada, Dirk Spaltmann, Kazuo Tagawa, Valentin L. Popov 2.3.3 Friction Reducing Effect of Lubricants Applied to Organic Fibres 101 Igor Velkavrh, Nicole Dörr 2.3.4 Lubricant Inerting - a New Era in Lubrication Technology 103 Jie Zhang, Janet Wong, Hugh Spikes 2.3.5 Tribological Behaviour of Polymer Compounds containing Microencapsulated Lubricants 105 Susanne Beyer-Faiss, Regina Wannenmacher, Thomas Witt, Moritz Grünewald 2.3.6 Early Stages of Tribo-Oxidation in Single Crystalline Copper 107 Ines L. Kisch, Julia S. Rau, Vahid Tavakkoli, Lisa T. Belkacemi, Baptiste Gault, Christian Greiner 2.3.7 Effect of Atmospheric Composition on the Friction and Wear of Cobalt-Based Alloys at Elevated Temperatures 109 Tobias König, Philipp Daum, Dominik Kürten, Andreas Kailer, Martin Dienwiebel 2.3.8 Thermal-Elasto-Plastic Hydrodynamic Contact Between Rough Surfaces 111 M. J. Montenegro Cortez, P. Correia Romio, C. M. da Costa Gomes Fernandes, P. M. Teixeira Marques, S. Portron, J. H. O. Seabra 2.3.9 Micropitting in Rolling-Sliding Contacts: Mechanisms and Prevention * Dr. Amir Kadiric 3 Machine Elements and their Application in Tribology 3.1 Efficiency and NVH of engines and power trains 3.3.1 Simulation-Based Evaluation of Drive Cycle Fuel Efficiency Gains in Gasoline Engines through Engine Oil Viscosity Reduction 115 X. Simón-Montero, J. Blanco-Rodríguez, J. Porteiro, M. Cortada-Garcia, S. Maroto 3.3.2 A Study on the Effect of Surface Tension on the Drag Torque of Wet Clutches 117 Nikolaos Rogkas, Vasilios Spitas 3.3.3 Influence of the Steel Disk on the NVH Behavior of Industrial Wet Disk Clutches 119 Patrick Strobl, Katharina Voelkel, Thomas Schneider, Karsten Stahl 24th International Colloquium Tribology - January 2024 11 3.2 Gears and transmission systems 3.2.1 Stick-Slip in Hydraulic Cylinders: New Test Methods & Simulation as a Tool for Selecting Coating Solutions for Piston Rods to Avoid Critical Operating Conditions 121 Giuseppe Tidona, Jürgen Molter 3.2.2 Wear Optimization of Roller Chain Drives with Triboactive Transfer Coatings 123 Martin Rank, Manuel Oehler, Oliver Koch 3.2.3 Investigation of Polymer Solid Lubricated Steel-Bronze Contacts for Worm Gears Applications 125 Konstantinos Pagkalis, Manuel Oehler, Thomas Schmidt, Michaela Gedan-Smolka, Stefan Emrich, Michael Kopnarski, Oliver Koch 3.3 Bearings 3.3.1 Power Loss in High-Speed Angular Contact Ball Bearings 127 Lúcia B. S. Pereira, Justino A. O. Cruz, Pedro M. T. Marques, Stephane Portron, Jorge H. O. Seabra, Carlos M. C. G. Fernandes 3.3.2 Effect of Slip on Piezo-Viscous-Polar Lubricated Multirecessed Hybrid Journal Bearing 129 Vishal Singh, Arvind K. Rajput 3.3.3 Film Formation Evolution in Grease-Lubricated Rolling Contacts 131 Shuo Zhang, Georg Jacobs, Benjamin Klinghart, Florian König 3.4 Condition Monitoring and Damage Mechanisms 3.4.1 Enhancing Reliability and Service Life Predictions through Friction Monitoring and Sensor-Embedded Smart Contacts 133 Michael Gless, Anette Schwarz 3.4.2 The Effect of Electrical Currents and Lubricant Formulation on Rolling Contact Fatigue * Dr. Monica Ratoi 3.4.3 Optimisation of EV Transmission Efficiency Using a Tribological Model * Dr. Amir Kadiric 3.5 Seals and Lubricants for increased Sustainability 3.5.1 Analysis of Biodegradable Lubricants for Radial Shaft Seals Under Critical Conditions 135 Stefanie Haupt, Dr. Florian Johannes Heiligtag, Maria Frackowiak, Tanja Püler, Danijela Grad, Dirk Fabry 3.5.2 Implementing the use of Water Based Enviornmentally Acceptable Lubricants in the Ship Industry 137 N. Espallargas, E. Valaker, H. Khanmohammadi 3.5.3 Enhancing Machining Efficiency and Sustainability of Ti-6Al-4V through MQL with Polymeric Ester Based Metalworking Fluids: A Comparative Study with Conventional Cutting Fluids 139 Ramazan Hakkı Namlu, Kübra Kavut, Hanife Gülen Tom 12 24th International Colloquium Tribology - January 2024 4 Computational Methods and Digital Transformation in Tribology 4.1 Contact Mechanics 4.1.1 Simulation of the Local CoF Development in Dynamically Loaded Contact Surfaces (Fretting) 143 Silvano Oehme, Alexander Hasse 4.1.2 Static and Dynamic Friction of Elastomers in Dry Conditions 145 Fabian Kaiser, Daniele Savio, Felix Meier, Michele Scaraggi 4.1.3 Identification of the Dominant Wear Mechanism in Dry Contacts by Numerical Modeling 147 F. Koehn 4.2 Hydro/ Elastohydrodynamics 4.2.1 EHL Simulation for the Design Workflow of Contacts with Limited Lubricant Availability 149 Pastor Cesar, Solovyev Sergey 4.2.1 A Novel Mortar Multiphysics Computational Method for Thermal Elastohydrodynamic Lubrication 151 Volker Gravemeier 4.2.3 A Full-Scale Numerical Model for the Prediction of EHD Friction in Circular Contacts Lubricated with Pure Glycerol * Dr. Deepak Prajapati 4.2.4 Development of a Digital Twin through Simulation of PVD/ PACVD Coatings 153 Vincent Hoffmann, Emanuel Tack, Nick Bierwisch 4.2.5 Lubrication Mechanism Analysis of Textures in Journal Bearings Using CFD Simulations 155 Yujun Wang, Georg Jacobs, Florian König, Weiyin Zou, Benjamin Klinghart 4.2.6 Investigation of Wear Protection and Friction Losses in Ultralow Viscosity Lubricant Formulations: A Combined FEM-CFD Simulation 157 Javier Blanco-Rodríguez, Jacobo Porteiro and Marti Cortada-Garcia, Silvia Fernández 4.3 Machine Learning 4.3.1 Towards the Prediction of Lubricated Contacts by Machine Learning 159 Max Marian 4.3.2 Detection of Critical Operation in Porous Journal Bearings Using Machine Learning 161 Josef Prost, Guido Boidi, Georg Vorlaufer, Markus Varga 4.3.3 A Machine Learning approach to Tribological Performance Prediction of New Lubricant Formulations 163 Wahyu Wijanarko, Nuria Espallargas 24th International Colloquium Tribology - January 2024 13 4.4 Molecular Dynamics 4.4.1 Per Aspera ad Astra 165 L. B. Kruse, K. Falk, M. Moseler, D. Markert, R. Klein, J. Rausch, R. Luther 4.4.2 Computational Modeling of Tribological Systems: Insights into Grinding Processes, Materials Tribology and Tribofilm Formation through Molecular Dynamics 167 Stefan J. Eder, Philipp G. Grützmacher, Manel Rodríguez Ripoll, Andreas Nevosad, Karen Mohammadtabar, Ashlie Martini, Nicole Dörr, Daniele Dini, Carsten Gachot 4.4.3 Tribochemical Reactions in the Degradation Process of Iron Nitride with Reactive Molecular Dynamics Simulation 169 Mizuho Yokoi, Masayuki Kawaura, Shogo Fukushima, Yuta Asano, Yusuke Ootani, Nobuki Ozawa, Momoji Kubo 4.4.4 Towards a Continuum Description of Lubrication in Highly Pressurized Nanometer-wide Constrictions: the Importance of Accurate Slip Laws 171 Andrea Codrignani, Stefan Peeters, Hannes Holey, Franziska Stief, Daniele Savio, Lars Pastewka, Gianpietro Moras, Kerstin Falk, Michael Moseler 4.4.5 Tribochemical Properties of Glycerol as a Green Lubricant on Ferrous Substrates: Atomic-scale Study by Reactive Molecular Dynamics Simulation 173 Vahid Fadaei Naeini, J. Andreas Larsson, Roland Larsson 4.4.6 Effect of Polar Additives on the Slip and Bulk Shear of Hydrocarbon Oils 175 Seyedmajid Mehrnia, Maximilian Kuhr, Peter F. Pelz 4.5 Multiscale + Multiphysics 4.5.1 Role of Coating Thickness on Static Leakages, Contact Area and Electrical Resistance: A Theoretical and Experimental Study for Randomly Rough Interactions * Prof. Dr. Michele Scaraggi 4.5.2 Numerical and Experimental Analyses of the Multiscale Effects in the Tribological System Rotary Shaft Seals 177 Jeremias Grün, Marco Gohs, Simon Feldmeth, Frank Bauer 4.5.3 Simulative and Experimental Characterization of the Tribo-Electrical Contact of Roller Bearings 179 Stefan Paulus, Simon Graf, Oliver Koch, Stefan Götz 5 Test and Measurement Methodologies 5.1 Greases 5.1.1 Comparison of Different Standard Test Methods for Evaluating Greases for Rolling Bearings under Vibration Load or at Small Oscillation Angles 183 Markus Grebe, Henrik Buse, Alexander Widmann 5.1.2 Panta Rei: Everything Flows 185 René Westbroek, Ben Habgood, Daniel Williams 5.1.3 Enhancing Understanding of Grease-Retention and Lubrication-Mechanisms of Oscillating Sliding Contacts with Long Stroke Lengths 187 Andreas Keller, Markus Grebe 14 24th International Colloquium Tribology - January 2024 5.2 Tribometry 5.2.1 Correlation of MTM Striebeck Curves with Efficiency Data for Predictive Analysis of Coaxial EV Gearbox Performance 189 Dmitriy Shakhvorostov, Mirjam Bäse 5.2.2 LIF Signal Calibration for Bench Simulating Experiments and Engine Oil Film Thickness Investigations 191 Polychronis S. Dellis 5.2.3 Digital Twin Parametrization of a Roller Bearing based on Ultrasonic Film Thickness Measurement 193 Fabio Tatzgern, Boris Gigov, Michal Kracalik, Georg Vorlaufer, Markus Varga 5.3 EV Fluid Testing 5.3.1 Oil Aging on a Test Rig to Introduce Sustainable Lubricants in Electric Vehicle Transmissions 195 Timo Koenig, Marco Kohnle, Luca Cadau, Lukas Steidle, Didem Cansu Gueney, Katharina Weber, Joachim Albrecht, Markus Kley 5.3.1 Copper Wire Resistance Corrosion Test for Assessing Potential Fluids as E-Thermal Fluids in BEVs Immersion Cooling Applications 197 Bernardo Tormos, Vicente Bermúdez, Jorge Alvis-Sanchez, Leonardo Farfan-Cabrera 5.3.1 Shear Stability and Thermal Performance Analysis of Engine Oils for Electric Vehicles 199 Victor Nino, Fabio Alemanno, Deepak Halenahally Veeregowda 5.4 Metalworking 5.4.1 Go Greener by In-situ Characterization of Lubricants for Cold Rolling - Droplet Size Distribution and Physical Separation/ Emulsion stability 201 Arnold Uhl, Stefan Küchler, Sylvain Gressier, Titus Sobisch 5.4.2 Investigtion of Functional Lubricity of Water-Based MWFs by an Innovative Tool 203 Ameneh Schneider, Felix Zak 5.4.3 Tribological Testing for the Assessment of Friction and Metal Transfer in Sliding Contacts between Cemented Carbide and Aluminum during Metal Forming 205 N. Cinca, M. Olsson, M. G. Gee 5.5 Metrology in Tribology 5.5.1 Analysis of Tribo-Films in Industrial Applications 207 Joerg W. H. Franke, Janine Fritz, Daniel Merk 5.5.2 Detection of Wear in Modern Naval Engines 209 Theodora Tyrovola, Fanourios Zannikos 5.5.3 Unveiling the Butterfly Effect in Tribology: The Impact of Surface Profile 211 Yulong Li, Nikolay Garabedian, Johannes Schneider, Christian Greiner 5.5.4 Soft and Highly Sensitive Contact Pressure Sensors Based on Randomly Rough Surfaces 213 Luciana Algieri, Luigi Portaluri, Marco Bruno, Massimo De Vittorio, Michele Scaraggi 5.5.5 The Importance of Inocula for Biodegradation Testing of Lubricants 215 Dr. Peter Lohmann 5.5.6 Active, Real-Time Friction Control with ElectroAdhesion: Application to Soft Contacts for Augmented Tactile Perception 217 Luigi Portaluri, Luciana Algieri, Massimo De Vittorio, Michele Scaraggi 24th International Colloquium Tribology - January 2024 15 5.6 Lubricant Analysis 5.6.1 Limit Values for the Evaluation of Lubricant Analyses 219 Stefan Mitterer 5.6.2 The European Tribology Centre 221 Franz Pirker, Alberto Alberdi, Xavier Borras 5.6.3 Tribological Investigations under Varying Pressure Atmosphere 223 Felix S. M. Zak, Ameneh Schneider, Gregor Patzer 5.7 Test Methodologies 5.7.1 Efficiency Improvements of In-Situ Hydrogen Permeation Measurements in Lubricated Bearing Steel Contacts Using the Modified Devanathan-Stachurski Cell (MDSC) Method 225 Edward Vernon-Stroud, Ajay Pratap Singh Lodhi, Frederick Pessu, Ivan Delic, Nicole Dörr, Markus Varga, Josef Brenner, Ardian Morina 5.7.1 Parallel Wear Testing - an Update 227 Lais Lopes, Dirk Drees, Pedro Bai-o, Emmanouil Georgiou 5.7.1 Building Tribology Application Testing to Determine Wear and Characterization of Polymer-based Composites 229 Michael Katzer, David Rich, Diarmaid Williams 6 Sustainability and Resource Efficiency 6.1 Sustainability 6.1.1 How Oil Care Can Reduce Oil and Maintenance Costs 233 Steffen D. Nyman 6.1.2 Using Molecular Modelling to Anticipate Future Toxicity Classifications of Anti-oxidants and Identify Safer Structures 235 Siegfried Lucazeau, Grégoire Hervé, Florence Séverac 6.1.3 Viscosity Index Improvers with an Environmental Acceptable design and an Improved Performance * Gerard Cañellas 6.2 Applications 6.2.1 Oxidation Effects on the Rheology and Tribology of Sustainable Lubricants for Electromechanical Drive Systems 237 Didem Cansu Güney, Joachim Albrecht, Katharina Weber 6.2.2 Bio-Lubricants as Metal-Working Fluids: More than an Environmental-Friendly Choice 239 Marco Bellini, Simone Pota 6.2.3 Potential and Performance of Pure Water Lubrication in Gearboxes 241 Andreas Nevosad, Stefan Krenn, Michael Adler, Dominik Cofalka, Siegfried Lais, Uwe Gaiser 16 24th International Colloquium Tribology - January 2024 6.3 Baseoils 6.3.1 A Life Cycle Assement (LCA) to Analzye the Green House Gas (GHG) Emissions for Estolides Produced from Castor Oil * Dr. Matthew Kriech 6.3.2 Sustainability Assessment of Polyol Esters - A Comparative LCA Analysis of a Bio-Based vs. Fossil-Based Product 243 Verena Koch, Denise Haas 6.3.3 How can Esters Improve the Sustainability of Both Intrinsic and Extrinsic Factors? 245 Gareth Moody, Gemma Stephenson 6.4 Additives 6.4.1 Moving towards Sustainable Lubrication - Challenges and Findings for Lube Components from Biobased Sources 247 Marcella Frauscher, Jessica Pichler, Rosa-Maria Nothnagel, Adam Slabon 6.4.2 New Technologies of Antiwear and Antioxidant Additives Used for Designing Nonhazardous Turbine Oils and Sustainable High-Performance Lubricants Including Greases 249 Grégoire Hervé, Florence Severac 6.4.3 The Effects of Applying the Tribological Compound TZ NIOD 251 Philipp Harrer, Dmitrii Svetov, Patrick Eisner, Maximilian Lackner, Erich Markl 6.5 Recyling/ Waste 6.5.1 Innovative Lubricant Components with Lower Greenhouse Gas Emissions 253 Dr. Sabrina Stark, Edith Tuzyna, Rene Koschabek 6.5.2 High Quality Sustainable Base Oils from Plastic Waste and Biomass 255 Matias de Tezanos, Boris Zhmud 6.5.3 Hybrid Lubricating Grease Formulations: A Sustainable Approach for Utilizing Renewable Resources within a Circular Economy Model 257 George S. Dodos, Mehdi Fathi-Najafi, Christina Dima, Nora Kaframani, Andreas Dodos 7 Young Tribologists/ Various Tribology 7.1 Young Tribologists 7.1.1 Amorphous Carbon Coatings for Total Knee Arthroplasty - a Knee Simulator Evaluation 261 Benedict Rothammer, Kevin Neusser, Marcel Bartz, Sandro Wartzack 7.1.2 On the Relation between Friction and Surface Topography - Models and Challenges 263 Charlotte Spies, Arshia Fatemi 7.1.3 Modeling of Shape Deviations for the Development of Predictive Models of TEHD Contacts 265 Klara Feile, Marcel Bartz, Sandro Wartzack 24th International Colloquium Tribology - January 2024 17 7.2 Various Tribology 7.2.1 Estimation of Remaining Useful Life of Greases after Thermo-Oxidative Ageing by Application of New Method DIN 51830-2 267 Markus Matzke, Gerd Dornhöfer 7.2.2 Correct Lubricant Selection for Metal Forming 269 Dr. Richard Baker, Dr. Dirk Drees 7.2.3 Measurable Sustainability enhancemer * Richard Wurzbach Appendix Scientific-Technical Board 275 Index of Authors 277 * not available at the time of publication Weitere Informationen und Anmeldung unter www.tae.de/ go/ maschbau Besuchen Sie unsere Seminare, Lehrgänge und Fachtagungen. Maschinenbau und Feinwerktechnik Fahrzeugtechnik Elemente, Maschinen und Anlagen Entwicklung und Konstruktion Werkstoffe und Betriebsstoffe Qualität, Mess- und Prüftechnik Fertigungs- und Produktionstechnik Verfahrens- und Oberflächentechnik, Korrosion Instandhaltung Betriebliche Organisation Arbeitssicherheit, Umwelt- und Strahlenschutz Ein Großteil unserer Seminare wird unterstützt durch das Ministerium für Wirtschaft, Arbeit und Wohnungsbau Baden-Württemberg aus Mitteln des Europäischen Sozialfonds. Profitieren Sie von der ESF-Fachkursförderung und sichern Sie sich bis zu 70 % Zuschuss auf Ihre Teilnahmegebühr. Alle Infos zur Förderfähigkeit unter www.tae.de/ foerdermoeglichkeiten Maschinenbau, Produktion und Fahrzeugtechnik Bis zu 70 % Zuschuss möglich Plenary Lectures 24th International Colloquium Tribology - January 2024 23 Sustainability in Winter Sports - The Tribological Perspective Matthias Scherge 1 1 Fraunhofer/ KIT MikroTribologie Centrum, Rintheimer Querallee 2b, 76131 Karlsruhe 1. Introduction This contribution highlights the topic of sustainability in winter sports with a focus on tribology, i.e. processes related to friction, lubrication and wear. It will be shown what measures have been taken in skiing and skating to conserve energy and resources. Since snow and ice have unique properties related to gliding, it is assessed how these properties are changed by substitute products. Furthermore, it is explained how the tribological mechanisms change, for example, when switching from the runner - ice system to a runner - polymer system. 2. Example I: Ski Jumping Ski jumps are operated in winter as well as in summer. Therefore, there are various friction partners with which the skis make contact, such as e.g. snow, ice, porcelain, various plastic mats and grass. All the above-mentioned materials result in a wide range of friction coefficients. In the inrun of a ski jump as well as in the landing hill, the friction in the direction of travel must be significantly lower than perpendicular to it. Inside the inrun, this is taken into account in that movement perpendicular to the direction of travel is not possible due to side restraints. In the landing hill, this requirement is implemented by the structure of the mats, which provide a certain degree of resistance when the skis are edging. Naturally, different inrun tracks exist for winter and summer operation. In winter one finds tracks made of snow or ice, while in the rest of the year tracks made of metal or metal with sliding bodies are used. To reduce friction, these tracks are rinsed with water. To further reduce friction, the tracks are given hemispherical, partially flattened knobs. Plastic (POM), ceramic or porcelain are used as material. In some cases, embossing is also used to structure the metal track in such a way that nubs are formed, see Fig. 1. While in the inrun the lowest friction is required, in the landing hill there is the demand for a safe landing, which requires a certain friction value for lateral guidance. Since the 70’s, mainly green mats consisting of a multitude of individual threads have proven their worth, see Fig. 1a. Like the inrun, the mats must also be watered to reduce friction. When magnified it can be shown, how the mats retain water. The used plastic is hydrophilic and wets very well. If this were not the case, the water would run through the mat into the ground and the water demand of the hill would be very high. Friction measurements with a portable tribometer proved the clear difference between dry and lubricated friction. Fig. 1: Different kinds of inruns of a ski jump. The type of mat presented in Fig. 2 was installed on a small hill at Steinbach-Hallenberg in 2018. Due to the combination of plastic fiber and loop shape, smaller friction was achieved with these mats than on their green predecessor. Due to the fiber bundles, the mats hold significantly more water. This reduces the water consumption of the system. Fig. 2: A modern type of mats. As with the inrun, the tribological mechanism can be found in the hydrodynamics. The water stored in the fiber composite of the mats serves as a lubricant. Since contact between the ski and the mat is very rapid during landing, there is not enough time for the water to be forced out of contact and a lubricating effect is created, see Fig. 3. 24 24th International Colloquium Tribology - January 2024 Sustainability in Winter Sports - The Tribological Perspective Fig. 3: Mats and watering system for lubrication. 3. Example II: Ice-Skating and Bobsledding Skating on artificial ice usually refers to activities such as ice skating and ice hockey. Artificial ice in the form of polymer sheets is often used in ice halls, ice rinks or ice parks to provide year-round ice activities. Artificial ice offers the advantage of being less sensitive to temperature fluctuations and does not require any energy for cooling. The ice substitute, like most ski bases, is made of ultrahigh molecular weight polyethylene filled to the limit with oils. The oil diffuses to the surface, where it forms a very thin lubricating film that is imperceptible by hand but efficiently separates the friction partners. A few nanometers are sufficient for this. With additional contact pressure, the oil is pressed out of the sintered granules and improves lubrication. As a result, freshly ground runners glide better because the contact pressure is higher. However, the fact that the coefficient of friction is higher than against ice, can also be read indirectly from the wear. According to users, the skates become dull about twice as fast as against ice. If you compare the friction mechanisms, you will notice that when the skate and ice come into contact, the friction power causes the ice to melt near the surface and forms the lubricating water film. This water cannot be formed by pressure alone. In the case of skate-polymer contact, however, the contact pressure plays the decisive role, because it conveys the lubricant from the interior of the polymer to the surface. For the sport of bobsleigh, it was shown that with polymer sliding surfaces, that do not need to be cooled or watered, training and competition are possible as well. Fig. 4: Bobsled track. As an example, a completely new and innovative push-off training track was developed and already put into use, see Fig. 4. This training track can be set up at any location and does not require a specially cooled building. This achieves several advantages at once. Besides saving cooling power, there is no need for ammonia as the chemical basis of cooling. Since the track can be set up anywhere, realistic training is possible even for smaller clubs that cannot afford to train in a “cold store” and the transportation costs. If the track is used in a social environment, e.g. at city festivals, many new possibilities for recruiting new talent open up. The sliding mechanism is the same as described for skating. Since the bobsleigh runners have considerably more contact surface with the polymer, a higher mass is required to build up the necessary pressure. The total mass of the mono bobsleigh shown above is 248 kg. Thus, the necessary pressure can be built up and low friction can be ensured. 4. Summary Sustainability in winter sports is of great importance to protect the environment in which we enjoy these activities. By conserving resources, protecting nature and promoting responsible behavior among winter sports enthusiasts, we can ensure that future generations will be able to experience the same fascination and enjoyment of winter sports. In addition, mobile facilities can ensure that more people find their way to this sport. By promoting sustainable tourism, we can also support local communities while maintaining the economic benefits of winter sports. Combining fun and responsibility is the key to a sustainable future in winter sports. 24th International Colloquium Tribology - January 2024 25 Minimizing CO 2 Emissions and Maximize ROI: Implementing Known Tribology and Design for Zero Principles for a Carbon Neutral Industry Roland Larsson 1 and Victoria Van Camp 1 1 Division of Machine Elements, Luleå University of Technology 1. Introduction The world is undergoing a significant transformation in the economy and industry to achieve Net Zero CO2 emissions by 2050 [1,2]. Tribology will play a crucial role in this transformation. Machinery, including steel plants, wind turbines, vehicles, and e-motors, will require upgrades and retrofits, with new designs targeting neutral or negative CO2 emissions while minimizing the use of scarce materials. The primary challenge lies not in discovering new technologies (although this is also necessary) but in implementing the technology and knowledge we already possess and doing so quickly. It is often argued that a ‘circular economy’ is the solution to achieving CO2 neutrality, as it involves keeping existing resources in a closed loop within the atmosphere. While the reuse of materials and parts must increase, extending the technical lifespan of machinery well beyond current warranty periods offers a shortcut to improving the financial return on existing assets and justifying new investments. This can be achieved through innovative design, employing ‘Design for Zero’ principles, and through strategic maintenance and upgrades of existing equipment. Here, we explain why ‘circularity’ for industrial machinery is not sufficient and why extending the useful life of equipment to its technical limits is crucial for both minimizing CO2 emissions and improving return on investment (ROI). 2. Circular economy A circular economy is an economic system designed to minimize waste and maximize resource efficiency. Its goal is to depart from the traditional linear “take-make-dispose” model by promoting continuous product use, refurbishment, upgrading and recycling/ reuse. In a circular economy, products are intentionally designed for durability, repairability, and recyclability, while resource use and waste are minimized both through the original design as well as through responsible consumption and production practices. This approach contributes to conserving natural resources, reducing environmental impacts, and establishing a more sustainable and resilient economic system. One approach to implementing a circular economy is to transform the business model in a way that benefits all involved parties by ensuring the long, trouble-free service life of machinery. This can be achieved through methods like leasing contracts or delivering the functionality as a service, often referred to as Product-Service Systems (PSS). 3. Elliptical economy However, Product-Service Systems (PSS) were not initially conceived with the goal of achieving Net Zero emissions but were primarily driven by economic factors, such as cost reduction and increased profitability. Done right, PSS also adds value for customers and gives continuous opportunity for customer feedback and improvements. PSS gained prominence as businesses recognized its potential to align with sustainability objectives and enhance resource efficiency, although the primary motivations may differ among companies. By integrating sustainability considerations into PSS, it becomes evident that profitability is directly linked to extending the machinery’s operational lifespan as much as possible. This extended use-phase significantly contributes to the growth of a well-functioning circular economy, which can be likened to an ellipse rather than a circle [3]. 26 24th International Colloquium Tribology - January 2024 Minimizing CO 2 Emissions and Maximize ROI: Implementing Known Tribology and Design for Zero Principles for a Carbon Neutral Industry 4. The role of tribology The role of tribology in prolonging the use-phase is obvious. Previous research by Holmberg et al. [4,5] and Woydt [6] have demonstrated that the service life of machinery can be significantly enhanced when wear resistance is given top priority during the design process. Traditional linear business models have not actively promoted this concept, but in an elliptical economy business model, there are compelling reasons to leverage existing knowledge in surface enhancement for promoting longevity. Proper maintenance of existing assets has always been important in industry and power plants, mainly driven by high costs for unexpected downtime [7,8]. With connected machinery and machine learning (AI), maintenance practices and the ability to take proactive actions to prolong machine technical life is here to stay. In addition, the sustainability effects of predictive maintenance and data analytics are substantial, with their help, expected life of modern wind turbines are now of the order 30-35 years compared with previous 15-20 years. This substantially reduces lifetime CO2 emissions per produced kWh [9, 10]. Significant changes are required during the design phase in an elliptical economy, with concepts like modularization [11] becoming crucial. Building machinery in modular segments of varying characteristics provides design flexibility and improves maintainability and future (in the design phase yet unknown) upgrades. For instance, surfaces vulnerable to wear can be placed within easily replaceable modules, while parts of the load-bearing structure that remain durable over time can be housed in separate modules. Functionality that may require upgrades to new, as-yet-uninvented technologies can be incorporated into a different module. The climate impact of most mechanical components is largely determined by energy consumption during the use phase, with frictional losses being particularly prominent in components like rolling bearings. Thus, it is imperative to prioritize low-friction solutions when designing machinery for the elliptical economy. Also, maintenance practices such as maintaining shaft alignment and replace worn parts, including seals, before they impact friction losses has substantial impact on energy consumption. 5. Conclusions In the field of tribology, we already possess technologies that can be effectively used to significantly reduce wear rates and frictional losses. The reason these methods have not been consistently applied is twofold: economic viability and a lack of awareness among engineers. A product’s life cycle cost encompasses all phases within a circular (or linear) economy. Typically, these costs are distributed among various stakeholders, with each value chain contributor primarily focusing on their own profitability. When all participants in the cycle - the life cycle value chain -collectively share the total cost, it becomes more economically advantageous to extend the use-phase and employ more costly methods to minimize wear and friction. Moreover, the global shift towards sustainability will inevitably result in higher resource utilization costs and increased emissions fees, further making the adoption of tribological solutions at a higher cost feasible. As a result, the value of tribological solutions will rise, emphasizing the need for even more effective solutions. Finally, the tribologists themselves must prioritize sustainability, using fossil-free and renewable materials in their work. References [1] McKinsey Global Institute summary report (2022). The net-zero transition. What it would cost, what it could bring. https: / / www.mckinsey.com/ capabilities/ sustainability/ our-insights/ the-net-zero-transitionwhat-it-would-cost-what-it-could-bring. [2] Heid, B., Linder, M., Patel, M. (2022). Delivering the climate technologies needed for net zero. McKinsey&- Co, McKinsey Sustainability https: / / www.mckinsey. com/ capabilities/ sustainability/ our-insights/ delivering-the-climate-technologies-needed-for-net-zero [3] S. Jacobson, U. Wiklund, J. Hardell, R. Larsson, “Tribology and the case for an Elliptical economy“, 24th International Conference on Wear of Materials, 2023, 16-20 April, Banff, Alberta, Canada. [4] Holmberg, K., Erdemir, A. (2017). Influence of tribology on global energy consumption, costs and emissions. Friction 5(3), pp. 263-284 [5] Holmberg, K., Siilasto, R., Laitinen, T., Andersson, P., Jäsberg, A. (2013). Global energy consumption due to friction in paper machines. Tribology International 62, pp. 58-77. [6] Woydt, M. (2022) Material efficiency through wear protection - the contribution of tribology for reducing CO2 emissions. Wear 488-489. [7] Almagor, D., Lavid, D., Nowitz, A, Vesely, E. (2019). Maintenance 4.0 Implementation handbook. Reliabilityweb.com. [8] Moleda, M., Malysiak-Mrozek, B., Ding, W., Sunderam, V., Mrozek, D. (2023) From corrective to predictive maintenance - a review of maintenance approaches for the power industry. Sensors 2023, 23, 5970. [9] Cota, E., Garnbratt, A., Jansson, M., Lindh, C., Månsson, K., Sandgren, J. (2022). Livscykelanalys, miljökommunikation och beslutsprocesser, Utvärdering av SR Energys vindkraftsparker ur ett hållbarhetsperspektiv. Candidate thesis in Industrial Economy, TEKX04- 22-06, Chalmers University of Technology, Sweden. [10] Razdan, P., Garret, P. (2019). Life cycle assessment of electricity production from an onshore V150-4.2 MW wind plant. Vestas Wind Systems A/ S https: / / www.vestas.com/ content/ dam/ vestas-com/ global/ en/ sustainability/ reports-and-ratings/ lcas/ LCAV10020 MW181215.pdf.coredownload.inline.pdf [11] Ulrich, K. (1995). The role of product architecture in the manufacturing firm. Research Policy vol 24, issue 3, May pp. 419-440. 24th International Colloquium Tribology - January 2024 27 Dynamic Properties of Lubricants for Electric Vehicles EV fluids Yan Chen 1 and Hong Liang 1,2 * 1 Department of Materials Science & Engineering, Texas A&M University, College Station, TX 77843, USA 2 J. Mike Walker ’66 Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123, USA * Corresponding author: hliang@tamu.edu 1. Summary & Introduction Evolving needs in lubricants requires better understanding and testing. In this presentation, the requirements in lubricants for electric, hybrid, and internal combustion engine (ICE) vehicles will be compared to identify key performance characteristics. Further discussion will be focused on our recent research. Our recent study has revealed that certain fundamental properties of lubricants alter under working and electrified conditions. Specifically, we investigated the properties of working lubricants, their electrical and thermal properties. In establishing relationship between electrical conductivity and a fluid oil film thickness, results indicated the non-ohmic behaviour of a lubricating film in the hydrodynamic regime. In probing thermal performance, we found out that thermal properties of lubricants depended on the shear that are not constant as being widely accepted. These findings are beneficial to design effective EV lubricants. 2. Working Fluids A lubricant becomes a working fluid when a mechan-ical system, such as a vehicle, is in operation. To satisfy the working conditions of an electric vehicle, new challenges arose over the electrical and thermal properties of the fluid. The current understanding about the properties of lubricants has been on the fluidic viscosity, [1-3] film formation, [4-6] and the frictional respond to shear [7, 8]. For the application to EVs, electrical and thermal conductivities are important. 3. Dynamic Properties of Working Fluids In this research, we constructed a system to success-fully examine the electrical conductivity against the oil film thickness. The thickness and electrical re-sistance can be calculated from impedance. We in-tegreate an electrochemical potential state with a disc-on-disc tribomeer. It allowed us to meaure the capacitor and resitor parallel. If we assume that ca-pacitance is fully contributed from the oil film. It thus has a dielectric constant of 2.1 [9]. The our eq-uition is like the following: (1) and (2) where R is the resistance, Z is the impedance sub-tract the impedance of the shorted measuring system Z = Z measured - Z shorted . A is the nominal area of contact, ee r is the dielectric constant, w is the angular frequency of the applied voltage. Then Re and Im take the real and imaginary part of a complex number, respectively. Our data showed that, interestingly, there was a non-ohmic behavior of the fluid in the hydrodynamic regime. Further experiments were conducted, and it showed that the properties of fluids are affected by a few factors. The study on thermal performance of a mineral oil and polyalphaolefin (PAO) was also carried out. Data gathered showed that the thermal properties of fluids are affected by the shear stress that has not been widely understood. 4. Conclusion Fluids‘ behave differently when under a share force than static. In this work, we experimentally studied the non-ohmic behavior of working fluids in they are in the hydrodynamic regime. We electrically measured the oil film thickness against its temperature. Our restuls showed that the „dynamic“ thermal conductivity of a mineral oil was 0.25mW/ K and that of a Poly-alpha-olefin (PAO) oil was 0.2mW/ K, when the speed/ load of the tribometer was set at 100cm/ Ns. These data indicated that commercial lubricants for conventional vehicles could be improved in order for them to be adapted to electric vehicles. Detailed discusison as well as thermal conductivities will be provded during presention. 28 24th International Colloquium Tribology - January 2024 Dynamic Properties of Lubricants for Electric Vehicles Figure 1, The electrical resistance against the thick-ness of an oil film. There are two regions: the linear at the lower left region and non-linear upper right. Figure is adapted from the ref. [10]. References: [1] Hamrock, B. J., and Dowson, D., 1977, “Isothermal Elastohydrodynamic Lubrication of Point Contacts: Part III-Fully Flooded Results,” J. Lubr. Technol., 99(2), pp. 264-275. [2] Okrent, E. H., 1961, “The Effect of Lubricant Viscosity and Composition on Engine Friction and Bearing Wear,” ASLE Trans., 4(1), pp. 97-108. [3] Evans, C. R., and Johnson, K. L., 1986, “The Rheological Properties of Elastohydrodynamic Lubricants,” Proc. Inst. Mech. Eng. Part C J. Mech. Eng. Sci., 200(5), pp. 303-312. [4] Jablonka, K., Glovnea, R., and Bongaerts, J., 2012, “Evaluation of EHD Films by Electrical Capacitance,” J. Phys. Appl. Phys., 45(38), p. 385301.] [5] Jablonka, K., Glovnea, R., Bongaerts, J., and Morales-Espejel, G., 2013, “The Effect of the Polarity of the Lubricant upon Capacitance Measurements of EHD Contacts,” Tribol. Int., 61, pp. 95-101. [6] Johnston, G. J., Wayte, R., and Spikes, H. A., 1991, “The Measurement and Study of Very Thin Lubricant Films in Concentrated Contacts,” Tribol. Trans., 34(2), pp. 187-194. [7] Cann, P. M., and Spikes, H. A., 1989, “Determination of the Shear Stresses of Lubricants in Elastohydrodynamic Contacts,” Tribol. Trans., 32(3), pp. 414-422. [8] Masjedi, M., and Khonsari, M. M., 2014, “Theoretical and Experimental Investigation of Traction Coefficient in Line-Contact EHL of Rough Surfaces,” Tribol. Int., 70, pp. 179-189. [9] Carey, A. A., “The Dielectric Constant of Lubrication Oils,” p. 9. [10] Y. Chen and H. Liang, “Tribological Evaluation of Electrical Resistance of Lubricated Contacts,” ASME J. Trib., 142(11), 2020. Pp: 114502. DOI: 10.1115/ 1.4045578. 24th International Colloquium Tribology - January 2024 29 Supporting Mobility Transition - Alternative Energy Carriers in Tribology Marcella Frauscher 1* , Adam Agocs 1 , Charlotte Besser 1 , Michael Adler 1 , Hannes Hick 2 1 AC2T research GmbH, Wiener Neustadt, Austria 2 Institute for Machine Components, University of Technology Graz, Austria * Corresponding author: marcella.frauscher@ac2t.at 1. Introduction Figure 1: Lab-to-field approach combining artificial ageing and identification of lubricant degradation (e.g., via MS and sensors) to understand the impact of alternative energy carriers on lubricant performance. In addition to e-mobility, which is seen as most promising future technique for passenger cars, fuels with zero-carbon emissions are necessary within the next years or in applications where e-mobility cannot be (easily) realised such as aviation, marine transport, and off-road vehicles. Besides carbon-based fuels from synthetic sources (synfuels), hydrogen and ammonia are considered as emerging sustainable fuels. While the influence on emissions of internal combustion engines was investigated in scientific studies, their impact on lubricants and engine components, and consequently friction and wear are not yet analysed in detail. For this purpose, the so-called Lab-to-Field approach (see figure 1) combines techniques such as artificial ageing and mass spectrometry to investigate the influence of alternative energy carriers on lubricant degradation, lubricant condition, and its influence on lubrication performance [1, 2]. In this presentation, the tribological challenges of alternative energy carriers will be discussed by means of selected examples of synthetic fuels, ammonia and hydrogen. 2. Alternative energy carriers in tribology 2.1 Synthetic fuels To determinate the impact of synfuels on engine components as well as on the friction and wear behavior a fundamental analysis of their application in existing passenger car fleets utilizing a turbo charged single cylinder engine was performed. Periodical oil sampling was done, and oil analysis consisted, amongst others, of fourier-transformed infrared spectroscopy (FTIR) to characterize the degradation of antioxidant and anti-wear additives. Gas chromatography (GC) was applied to measure the accumulation of fuel components in the lubricant. This revealed a higher impact on viscosity due to dilution for the investigated experimental synthetic fuels compared to a conventional fuel. High-resolution mass spectrometry (MS) characterized additive depletion and degradation products on the molecular level, and, hence, the influence of synthetic components, showing remarkable effects on the lubricant. Based on this comprehensive study including tribometry and lubricant condition, new effects of synthetic blends on engine mechanics can be observed, which sup- 30 24th International Colloquium Tribology - January 2024 Supporting Mobility Transition - Alternative Energy Carriers in Tribology ports optimization of fuel and lubricant composition in terms of friction and wear [3]. 2.2 Ammonia based fuels The application of ammonia as prospective zero-carbon emission fuel for use in large marine diesel engines inhibits challenges regarding stability of engine oil and engine components. Thus, a methodology to evaluate this phenomenon was proposed based on an artificial oil alteration (see figure 2) [4,5]. Subsequently, performance tests with the altered oils focussed on corrosion properties, deposit formation, and load-bearing capability. It was shown the application of ammonia resulted in less pronounced thermo-oxidative degradation compared to alterations with air. However, static and dynamic de-posit formation as well as corrosion properties and load-bearing capability were severely impacted by the presence of ammonia [5]. Figure 2. Schematic set-up of artificial ageing under ammoniac atmosphere to determine the influence on engine oil. 2.3 Hydrogen In terms of H2 as energy carrier in mobility, the supply chain is one of the main challenges. For compression, high pressure equipment is needed, which enables easier liquification. However, effects of the pressurised gas and H2-oil interactions must be considered. To exert hydrostatic pressure of up to 1000 bar on hydrogen, a lubricant must fulfil a set of critical requirements such as limited solubility of hydrogen in the fluid. For assessment of the solubility of hydrogen in a potential fluid for high pressure storage, an in-house test rig at AC2T research GmbH was utilized, designed to evaluate the physisorption of gases into fluids at pressures up to 100 bar. Tribological performance was measured using a rheometer with a ball-on-three-pins setup. Promising lubricants from sustainable sources were tested in comparison with conventional ones, and gas absorption was found to be almost negligible [6]. 3. Conclusion Based on the comprehensive Lab-to-Field approach, effects of carbon-free or synthetic fuel blends on lubricants and tribological performance are investigated. The presented examples show how advanced stability assessment and tribotests together with advanced analytical methods lead to deep knowledge used to push investigations on alternative energy carriers and modern lubricant design [7]. Acknowledgements The work presented was funded by the Austrian COMET program (Project InTribology, Nr. 872176) and carried out at the “Excellence Centre of Tribology” (AC2T research GmbH). References [1] Engine oils in the field - comprehensive chemical assessment of engine oil degradation in a passenger car. Dörr N.,-Agocs A.,-Besser C.,-Ristic A.,-Frauscher M. Tribology Letters, Vol 67, Art.Nr. 68. 2019 [2] Improving sustainability by enhanced engine component lifetime through friction modifier additives in fuels. Frauscher M., Agocs A., Wopelka T., Ristic A., Ronai B., Holub F., Payer W. Fuel - The Science and Technology of Fuel and Energy, Vol 358, 130102. 2023 [3] The influence of synthetic fuels on tribology in engine operation. Hick H., Frauscher M., Kopsch P., Agocs A., Plettenberg M., Gell J. European Conference on Tribology 2023 Bari (IT) [4] Generation of engine oils with defined degree of degradation by means of a large scale artificial alteration method. Besser C., Agocs A., Ronai B., Ristic A., Repka M., Jankes E., McAleese C., Dörr N. Tribology International, Vol 132, p 39-49. 2018 [5] The impact of ammonia fuel on marine engine lubrication: An artificial lubricant ageing approach. Agocs A., Rappo M., Obrecht N., Schneidhofer C., Frauscher M., Besser C. Lubricants, Vol 11, 165. 2023 [6] Innovative lubricant solutions for high-pressure hydrogen systems. Adler M., Nagl C., Eder R.M. European Conference on Tribology 2023 Bari (IT) [7] Assessment and design of modern lubricants supported by mass spectrometry. Frauscher M. European Conference on Tribology 2023 Bari (IT) 24th International Colloquium Tribology - January 2024 31 Towards Superefficiency Tribology Solutions Shaping Tomorrow’s Gearboxes Thomas Lohner 1* , Constantin Paschold 1 , Karsten Stahl 1 1 Technical University of Munich, School of Engineering and Design, Department of Mechanical Engineering, Gear Research Center (FZG), Boltzmannstraße 15, 85748 Garching near Munich, Germany * Corresponding author: thomas.lohner@tum.de 1. Introduction The main design criteria of gearboxes are (i) efficiency and heat balance, (ii) power density and load capacity, and (iii) noise, vibration and harshness. These classical design criteria are nowadays superimposed by sustainable aspects of product lifecycles and circular economy, in which the use phase is often dominant. Implementing low-loss technologies like superlubricity, low-loss gearings, and on-demand lubrication methods can push the gearbox efficiency to next levels. This study discusses the potential of low-loss technologies and introduces the term superefficiency. The results are based on the authors’ presentation at ITC 2023 [1]. 2. Methods and Materials This calculation study uses the program WTplus [2] for power loss and heat balance analysis. No-load (index: - 0) and load-dependent (index: - P) gearing (index: G) and bearing (index: B) power losses as well as sealing power losses (index: S) are considered: P L = P LG0 + P LGP + P LB0 + P LBP + P LS (1) The load-dependent gear power loss P LGP is calculated based on the input power P In , tooth loss factor H V and mean coefficient of gear friction µ mz : P LGP = P Ih × H V × µ mz (2) The object of investigation is the gearbox of the FZG efficiency test rig in Figure 1. A complete operating cycle is defined by a fully parametric combination of the wheel rotational speed n 2 -=-{87, 174, 348, 870, 1444, 1739, 2609, 3479}- min -1 , pinion torque T 1 -= {0, 35.3, 94.1, 183.4, 302.0}-Nm and oil temperature ϑ Oil -=-{40, 60, 90, 120}-°C with t-=-5-min per operating point. Figure 1: FZG test gearbox with high toothing gearing A high toothing (HT) gearing with H V -=-0.226 is used as a reference. Moreover, a moderate low-loss (mLL) and an extreme low-loss (eLL) gearing with the same load-carrying capacity as the HT gearing but strongly reduced tooth loss factors of H V -=-0.094 and H V -=-0.049 are considered. Besides different gearings, different gear oils with a viscosity of ≈10-cSt at 100-°C are considered: (i) mineral oil MIN10, (ii) polyalphaolefine PAO10, (iii) polyglycole PG10, (iv) polyether PE10 and (v) water-containing polyglycole PAGW09. The mean coefficient of gear friction µ mz is known from power loss measurements at the FZG efficiency test rig acc. to method FVA345 [3][4] and described by an oil-specific parametrized calculation equation. An oil-specific evaluation of µ mz|FVA345 and its comparison to the value of MIN10 provides values of 0.70 for PAO10, 0.61 for PG10, 0.48 for PE10, and 0.10 for PAGW09. PAGW09 shows superlubricity with µ mz -<-0.01 for a wide range of operating conditions [4]. Furthermore, different lubrication methods are considered: (i) dip lubrication (DL) with an immersion depth of half of the pinion and wheel (e = d a / 2), (ii) DL with a reduced immersion depth of three times the module of the pinion (e 1 = 3∙m n ) and (iii) minimum quantity lubrication (MQL) with an oil volume rate of 28-ml/ h supplied by a continuous air stream. To evaluate the results, the cumulative energy dissipation W L of a complete operating cycle is determined acc. to W L = S i P L,i × t, (3) and divided by a reference W L.ref , which refers to HT, MIN10 and DL with e 1 = 3∙m n . Such energy efficiency index EEI is similar to [5] described by: (4) 3. Results and Discussion Figure 2 shows the calculated EEI for the considered gearing geometries and gear oils for (a) DL with e 1 -=-3∙m n , (b) DL with e = d a / 2, and (c) MQL. Based on Figure- 2a with the reference EEI of 100- %, the variation of the gear oil for HT shows EEI values of 78-% for PAO10, 70-% for PG10, 61-% for PE10, and 37-% for PAGW09. This is mainly due to the reduction of µ mz and, therefore, P LGP . The variation of the gearing geometry for MIN10 shows EEI values of 60-% for mLL and 48-% for eLL. This is mainly due to the reduction of H V and, therefore, P LGP . Hence, choosing an optimized gear oil and gearing geometry offers great potential for reducing the EEI. Switching from MIN10 to PAGW09 with superlubricity provides a more significant potential for reducing the EEI. The maximum potential with 32 24th International Colloquium Tribology - January 2024 Towards Superefficiency an EEI of 32-% is found when combining eLL and PAGW09. However, when using a low-loss gear oil, the energy dissipation is already very small, so the additional reduction of the EEI using a low-loss gearing geometry is therefore slight, c.f. HT+PAGW09 and eLL+PAGW09. Figure 2: Energy Efficiency Index EEI for the complete operating cycle of the FZG test gearbox The influence of the lubrication method can be seen by comparing the results in Figure 2a with DL with e = d a / 2 (Figure 2b) and MQL (Figure 2c). A higher immersion depth increases the no-load power loss and the EEI for all gear oils and gearing geometries. MQL can potentially reduce the EEI to a minimum of 32-% in this study. Note that the limited heat transfer with MQL results in critical gear bulk temperatures of ϑ M ->-160-°C for technologies with an EEI->-50-%. The minimum EEI with the considered low-loss technologies depends on the operating cycle. If operating conditions with T 1 -= {183.4, 302.0}-Nm are removed from the complete operating cycle, the minimum EEI increases to 45-%. This is due to the lower proportion of operating points with high load-dependent power loss in the operating cycle. In terms of gearbox efficiency, the calculated mean efficiency for the complete operating cycle increases from 98.5-% for the reference technology with HT, MIN10, and DL with e 1 = 3∙m n to 99.8-% for eLL, PAGW09, and MQL. For the considered FZG test gearbox complete operating cycle, superefficiency might be defined for operating points with an efficiency of h-> 99.8-%. It should be emphasized that the EEI and superefficiency are specific to a gearbox and the underlying operating cycle. The exemplary application of the presented low-loss technologies to a single-stage industrial gearbox with a gear ratio of 2.23 using an ISO VG 320 gear oil shows a potential to reduce the EEI to 44-% and to lower the oil sump temperature by 33-K for an operation condition of input torque T 1 -=-12-732-Nm, input rotational speed n 1 -=-230-min -1 (P In -= 307-kW) and an operating time of 6000 h/ a. This relates to an increase of h from 99.0-% to 99.6-% and the reduction of W L from 67 GJ/ a to 30 GJ/ a. 4. Conclusion The following conclusions can be drawn from this study: • The energy efficiency index of gearboxes can be reduced to below 50 % with low-loss technologies. • Low-loss gear oils, particularly when enabling superlubricity, and low-loss gearings strongly reduce the load-dependent power loss. • Minimum quantity lubrication reduces the no-load power loss and saves resources. It is enabled by low load-dependent power losses resulting in uncritical component temperatures. • The maximum low-loss potential is found for eLL, PAGW09 and MQL with the EEI-=-32-%. • For the considered FZG test gearbox and complete operating cycle, superefficiency might be classified for operating points with an efficiency >-99.8-%. References [1] Lohner T. et al.: Towards superefficiency in transmissions, 9 th ITC, 25 th -30 th September, Fukuoka, Japan (2023) [2] Paschold C. et al.: Calculating component temperatures in gearboxes for transient operation conditions, doi: 10.1007/ s10010-021-00532-4 (2021) [3] Hinterstoißer M. et al.: Minimizing load-dependent gear losses, doi: 10.30419/ TuS-2019-0014 (2019) [4] Yilmaz M. et al.: Minimizing gear friction with water-containing gear fluids. doi: 10.1007/ s10010-019- 00373-2 (2019) [5] EU Regulation 2017/ 1369 on setting a framework for energy labelling and repealing Directive 2010/ 30/ EU, Official Journal L198/ 1 (2017) 24th International Colloquium Tribology - January 2024 33 The Data Science Frontier in Tribology Nick Garabedian 1* , Ilia Bagov 1 , Malte Flachmann 1 , Nuoyao Ye 1 , Miłosz Meller 2 , Floriane Bresser 1 , Christian Greiner 1 1 Karlsruhe Institute of Technology, Institute for Applied Materials, Germany 2 Helmholtz-Zentrum Hereon, Institute of Membrane Research, Germany * Corresponding author: Nikolay.Garabedian@kit.edu 1. Introduction Tribodigitalization stands as a pivotal process in tribology, representing a transformative journey towards harnessing the full potential of tribological data sets. This process is aimed at developing nuanced and efficient solutions for unraveling the complexities associated with friction and wear. In the ever-evolving landscape of scientific exploration, achieving comprehensive digitalization in tribology demands a shift in the way we conduct research and development. Central to that shift is the responsibility we have to take when we produce, share, store, re-use, or analyze data. To establish the global guidelines of what “good” data management means goes through embracing the FAIR data principles, emphasizing the fundamental attributes of research data being Findable, Accessible, Interoperable, and Reusable. So far, most tribological research data is not shared. The majority of datasets that are used within tribology publications are only “available upon request” - our findings showing statistics on data sharing in tribology will be shown during the presentation at the 24 th International Colloquium Tribology. Not sharing research data puts a limiting barrier on the speed our field can innovate at, as this precludes our ability to collaborate quickly and efficiently, and relies on direct inter-personal relationships. Of course, this also means that a machine-learning algorithm cannot be employed in a big data setting in tribology as most data is residing behind email requests. The reasons why tribologists keep data non-open are numerous. Firstly, in terms of research culture, it is not widely expected that a publication needs to have its raw data shared by default. Secondly, even if a scientist is willing to share her or his data, the technical solutions for doing that might be more confusing than helpful, as there is no widely-agreed framework for doing that within tribology. Lastly, since the questions of sharing data usually become important much later than the point of data collection, essential metadata is impossible to retrieve from the past, and it would be time-prohibitive to annotate existing data according to available schemata. That’s why it is ideal to have data FAIR by design since the first moment it is produced until it is published. To deal with all of these issues, we have designed one possible workflow for production of FAIR data directly from the lab. We have developed software solutions which assist tribologists in their scientific workflows, and make it easy to share the entire set of raw and processed FAIR data with a few clicks. Importantly, the proposed framework integrates with frameworks that other research domains have designed for themselves and utilize. 2. Methods Amidst the complex tapestry of tribological research, intuitive solutions need to illuminate the path toward seamless integration of traditional tribological meth-ods and cutting-edge data science techniques. In our framework, we rely on a knowledge manager which we designed with the goal of creating the schemata necessary for the annotation of FAIR tribological data. VocPopuli [1] is a tool designed to facilitate collaborative creation of FAIR controlled vocabular-ies that are meticulously tailored to the unique speci-fications of individual laboratories. These controlled vocabularies serve as the bedrock upon which FAIR data collection within electronic lab notebooks is structured. Through curation and refinement of these controlled vocabularies, tribologists create a knowledge base with rich, dynamic, and interopera-ble metadata. This structured approach not only en-sures the consistency and harmonization (not stand-ardization) of experiments but also serves as a cata-lyst for elevating the quality and reliability of re-search outcomes Additionally, we rely on the Kadi4Mat [2] to store and share our data internally. For the input of data into Kadi4Mat we designed a tablet-based application which enables lab scientists to enter their data next to any manual process in the lab, thus, removing the need for a paper lab journal. We also have designed integrations with LabVIEW and MATLAB, which further let tribologists preserve their usual workflows, while taking care for the collection of FAIR data and metadata. Finally, we have recently designed a reporting tool which lets users explore the trends in their datasets based on filters derived from the VocPopuli knowledge base. 3. Results The current outcomes of this work constitute a FAIR SKOS controlled vocabulary [3] and a FAIR dataset that was produced as a result of three Master’s Thesis projects [4]. The vocabulary contains 1,067 terms that are hierarchically organized, while the dataset has the following characteristics: • 151,045 RDF triples • 51 Experimental Series, 542 Individual Events • 89 Lab Equipment-Descriptions • 108 Experimental Object Descriptions • 412.1 GB in Total Size. The experiments test the reciprocation sliding of a 10-mm single-crystal sapphire sphere against a polycrystal (average size ~45 µm) copper base body. The range of normal loads is 0-4.5 N, the sliding velocity is always 0.5 mm/ s, and the experiments are performed in ambient 50% RH atmosphere. 34 24th International Colloquium Tribology - January 2024 The Data Science Frontier in Tribology 4. Outlook As we have already published our first results, we are now expanding the coverage of our FAIR data framework. We aim to have a continuous flow of published tribological data, and we are looking forward to having that data reused by other scientists. At the same time, we are looking for external FAIR datasets which we can integrate into our own research, so that we can start to see the scalability which big data approaches promise. References [1] I. Bagov, C. Greiner, and N. Garabedian, “Collaborative Metadata Definition using Controlled Vocabularies, and Ontologies,” Res. Ideas Outcomes, vol. 8, 2022. [2] N. Brandt et al., “Managing FAIR tribological data using Kadi4Mat,” Data, vol. 7, no. 2, p. 15, Jan. 2022. [3] I. Bagov et al., “Vocabulary of Materials Tribology Lab at KIT.” 10-May-2023. [4] M. Flachmann, J. Biesinger, M. Gorenflo, I. Bagov, C. Greiner, and N. Garabedian, “Copper Tribology FAIR Data Experiments - Sapphire Counterbody.” 11-May- 2023. New Trends in Lubricants and Additives 24th International Colloquium Tribology - January 2024 37 Lubricants Technology for Improving the Protection Performance of Reduction Gears in Transaxles for Electric Vehicles D. Takekawa 1* , H. Tatsumi 1 , K. Matsubara 1 , K. Narita 1 1 Lubricants Research Laboratory, Idemitsu Kosan Co., Ltd, Ichihara, Japan * Corresponding author: daisuke.takekawa.7430@idemitsu.com 1. Introduction To reduce carbon dioxide emissions, electrified vehicles, such as battery electric vehicles (BEVs), are expected to become increasingly popular in the automotive industry. Transaxles for electric vehicles (E AXEL) is used for drivetrains in electric vehicles. E AXEL is effective in reducing carbon dioxide emissions. Lubricants for E AXLE require a variety of performance. In particular, the important performance among them is the cooling of the motor and the protection of gears and bearings. In the past, it has been reported that it is important to reduce kinematic viscosity and increase heat transfer coefficient in order to improve motor cooling [1]. We have designed and built a new testing machine to evaluate the performance of lubricants in motor cooling [1]. Using this tester, we investigated the effect of the viscosity on the cooling of the motor. It was found that the lower the viscosity sample, the greater the temperature drop on the copper plate surface and the better the cooling. In addition, reducing the kinematic viscosity of the lubricating oil also has the effect of reducing stirring loss and improving efficiency. However, reducing the viscosity of the lubricating oil will reduce the thickness of the oil film on the contact part of the gear and the bearing, which will reduce the protective performance. Therefore, in this study, we report the results of a detailed examination of low viscosity and protection performance of gears and bearings. 2. Gear and bearing protection performance 2.1 Study on improving gear and bearing protection performance As above-mentioned, applying lubricant with lower viscosity to EVs would system potentially give an advantage for motor cooling to order to achieve a better motor efficiency. However, it is necessary for designing lower viscosity lubricants to consider a negative impact on the durability of gear and bearing, which are the components of the E AXLE. This implies that the role in lubricant additives would be more important for improving lubricity. In this section, we investigated the effects of extreme pressure agents (EP additives) on the seizure resistance and fatigue resistance of gears and the fatigue resistance of bearings. Table 1 shows the samples tested for evaluation. The kinematic viscosity of 100-°C. was adjusted to 3-mm 2 / s, and different phosphorus-based EP agents were blended. Table.1: Formulation of sample fluids and their kinematic viscosity at 100-°C. Oil-A Oil-B Oil-C BO    EP(S)    EP(P)-A  - - EP(P)-B -  - EP(P)-C - -  Others    Kinematic viscosity @100-°C, mm 2 / s 3.0 3.0 3.0 In particular, phosphorus-based additives had a significant impact on the fatigue life of gears and bearings. The results of the FZG gear fatigue resistance test are excerpted and shown in Figure 1. The Figure 2 shows the results of the angular bearing fatigue resistance test. Both FZG gear fatigue resistance test and angular bearing fatigue resistance test, compared with oils A and B, oil C showed better fatigue resistance. Figure 1: FZG gear fatigue resistance test results (C-PT/ 8.3/ 90). Figure 2: Angular bearing fatigue resistance test results (1800-rpm, 3.8-GPa, 100-°C). 38 24th International Colloquium Tribology - January 2024 Lubricants Technology for Improving the Protection Performance of Reduction Gears in Transaxles for Electric Vehicles 2.2 Analysis of factors that improve fatigue resistance Based on the above results, we investigated the reason why oil C exhibits excellent fatigue resistance. First, a friction test simulating the lubrication conditions of the FZG fatigue resistance test was performed using each test oil. The correlation between fatigue life and the amount of elements and surface shape of the tribofilm formed in the wear scar after the friction test was investigated. The correlation with each parameter is shown in Figure 3. Tribofilm surface roughness measured by AFM tended to correlate best with fatigue life of FZG. On the other hand, the correlation between the elemental content of tribofilm and fatigue life was low. Figure 4 shows the measurement results of the nanoscale wear mark surface shape after friction test by AFM. Oil C has a smoother surface shape than oils A and B, disperses contact stress, and is less likely to cause cracks that are the starting point of fatigue damage, so it is considered to exhibit a long fatigue life. This study suggests that additives that smoothly control the friction surface are useful for improving fatigue resistance. Figure 3: Correlation between fatigue life of FZG gear fatigue resistance test and each item. Figure 4: AFM measurement results of wear marks after friction test. 3. Copper corrosion protection performance The compatibility of the copper used in the motor and electronic components, which are components of the E AXLE, is also important. Since there are concerns that some types of EP additives may adversely affect copper compatibility, copper compatibility of each sample was confirmed in a copper plate immersion test. Table 2 shows the amount of copper elution from the copper catalyst after 150 hours at ISOT165-°C and the results of the copper plate corrosion test according to ASTM D130. It was confirmed that oil C, which has excellent fatigue resistance, is compatible with copper having the same performance as oils A and B. In general, sulfur-based additives have low copper compatibility, but the influence of phosphorus-based additives on copper compatibility is considered to be small. Table 2: Copper plate compatibility test results. Oil-A Oil-B Oil-C The amount of copper elution, ppm (ISOT165-°C, 150 h) 66 38 53 Appearance of copper plate (ASTM D130/ 150-°C, 3 h) 2a 2a 1b Conclusion In this study, we investigated the effect of phosphorus additives on the protective performance of gears and bearings, which are anti-performance when viscosity decreases. In addition, the compatibility of copper, which can be adversely affected by the use of extreme pressure additives, was also evaluated, and the following conclusions were obtained: 1. The type of phosphorus-based additive affected the fatigue resistance of gears and bearings. Oil C showed superior fatigue life to oil A and oil B. 2. As a result of investigating the factors that cause the good of specific phosphorus additives in gear fatigue tests and bearing fatigue resistance tests, it was found that the surface shape of the tribofilm measured by AFM had a great influence. 3. Compared to sulfur-based additives, it was found that phosphorus-based additives had a smaller effect on copper compatibility. In this study, we found that by properly selecting base oils and anti-wear agents, it is possible to design lubricants for E AXEL with excellent motor cooling and gear and bearing protection. References [1] Takekawa, D. Narita, K.: Lubricants Technology Applied to Transmissions in Hybrid Electric Vehicles and Electric Vehicles. SAE Technical paper 012338, 2019. [2] Onimaru, S. et.al.: Heat Analysis of the Hybrid Electric Vehicle (HEV) Motor Cooling Structure Using ATF. Denso Technical Review 17 (2008) 1, 19-25. [3] Seki, N. et.al.: Heat transfer engineering, published by Moriita syuppan, 1988. 24th International Colloquium Tribology - January 2024 39 Next-Generation Anti-Wear for EV Lubricants Christelle Chretien 1* 1 SOLVAY, Bristol, PA - USA * christelle.chretien@solvay.com 1. Introduction To protect our health and our environment, cleaner transportation is urgently needed, with an ultimate goal of zero emissions and carbon neutrality. To achieve this goal, solutions must be developed for E-Driveline lubricants providing: - more robust anti-wear/ extreme pressure (AW/ EP) capacities - a low friction for energy efficiency - Metal-free and Sulfur-free features for compatibility with E-driveline materials. The objective of this presentation is to introduce a next-generation anti-wear technology called Polymeric AW and to highlight its potential in E-Driveline Lubricants. The objective of this paper is to: - Introduce the technology of next-generation anti-wear - Describe its main benefits especially in EV Driveline lubricants 2. Purpose of this project The objective of this project is to develop a next generation anti-wear additive presenting the following features: - Ashless, Sulfur-free, and low phosphorus content - Providing benefits in EV driveline lubricants 3. Technology description As the toolbox of chemicals is more and more limited, the idea is to look out-of-the-box and develop a technology of anti-wear brand new to the industry. The technology we propose as a next-generation anti-wear is very unique as: - The phosphorous, which is the anchoring group, is outside of the backbone of the polymer bringing a freer access to the surface vs. technologies containing phosphorus in their backbone - It is based on a co-polymer leading to the use of different monomers to achieve an appropriate balance between surface adhesion and oil solubility. This technology can be depicted as the following: Figure 1: Polymeric Anti-Wear (Polymeric AW) 4. Prior art analysis This analysis of the prior art shows the uniqueness of this Polymeric AW technology as: - It directly coats the surface as a polymer vs. ZDDP which reacts first with the surface to form a polymeric tribofilm on the surface [1]. Figure 2: Tribofilm formation by ZDDP [2] - It has multiple anchoring groups while single molecules contain only one group. - Its anchoring group is easily reachable as it is outside of the backbone. 5. An Innovative solution under development A set of testings was performed on tribology properties as a first step. 5.1 Anti-wear performances In terms of performances, the core of this technology is its anti-wear feature . This was evaluated on one of our lead candidates using the 4 ball wear test (ASTM D4172). EDS (Energy Dispersive X-Ray Spectroscopy) surface analysis was conducted on the wear scar. The wear performance shows that the polymeric anti-wear candidate is equivalent to ZDDP, while being Ashless and Sulfur-free and while providing a concentration of Phosphorus 95% lower than what ZDDP brings in the lubricant: Figure 3: 4 ball wear evaluation 40 24th International Colloquium Tribology - January 2024 Next-Generation Anti-Wear for EV Lubricants By analysing the surface of the wear scar, the P level on the surface is the same either using ZDDP or the polymeric anti-wear candidate: Figure 4: EDS Analysis In conclusion, even if the Polymeric anti-wear brings in the lubricant 95% less Phosphorus than ZDDP, the Phosphorus level adhering on the surface is the same either using ZDDP or the polymeric anti-wear candidate. The hypothesis is that the Polymeric anti-wear might provide a higher phosphorus adhesion power on the surface than ZDDP. 5.2 Additional benefits Among the different candidates developed at lab scale, one candidate (ref. D.097) has shown leading performances with 11 benefits identified: - Metal-free, Sulfur-free and very low Phosphorus content - Good anti-wear performance - Friction level similar to ZDDP - Tremendous thermo-oxidative stability - Very low sludging tendency - Harmless on viscometrics - Not impacting foam - Equivalent on corrosion protection - Excellent solubility in standard base oils - Very low acidity level It is especially important to highlight the high thermal stability of this candidate illustrated by TGA (Thermogravimetric Analysis). The onset temperature of the Polymeric AW is 61°C higher than. ZDDP. In addition, the Polymeric AW leaves no residue thanks to its ashless nature. In addition, The Polymeric AW lead candidate (ref. D097) shows a good oxidation stability with no impact on the aging and deposit generation after 192 hours at 160°C. In addition to the development of the lead candidate, 9 additional candidates were developed and prepared showing different features. 3 new candidates were added to the first lead candidate as they show potential for good performance. 5.3 Application to EV Among these 3 new candidates, 2 show potential to be used in EV drivelines. - 4 performances were evaluated: - The electrical conductivity showed a 75% and 95% lower level than ZDDP at 150°C. - Cu corrosion protection is also a benefit brought by this technology. The 2 candidates showed a significantly better corrosion rating vs. ZDDP and a polysulfide. - In terms of anti-wear protection, the Polymeric AW technology shows an equivalent performance to ZDDP. - Finally, in terms of stability, the Polymeric AW technology is stable after 1 month at 3 temperatures. 6. Conclusion The Polymeric AW technology shows strong potential as a next-generation anti-wear thanks to the different benefits it provides, on top of anti-wear prevention. Further evaluations are ongoing in fully formulated formulation and specifically in the EV segment to assess its potential. References [1] “ZDDP’s uncertain future” in the TLT Tribology & Lubrication Technology - September 2019 [2] “Overview of automotive engine friction and reduction trends-Effects of surface, material, and lubricant-additive technologies”: DOI 10.1007/ s40544-016-0107-9 24th International Colloquium Tribology - January 2024 41 Impact of Lubricating Oils on the Performance for Liquid-Cooled Motor and Battery Thermal Control System Applied to Electric Transaxles K. Narita 1* , Y. Nakahara 1 and K. Matsubara 1 1 Lubricants Research Laboratory, Idemitsu Kosan Co., Ltd, Ichihara-shi, Chiba, Japan * Corresponding author: keiichi.narita.0440@idemitsu.com 1. Introduction E-axle (transaxle for electric vehicles) is electric drive unit that integrates a motor, an inverter and reduction gears, which provide an excellent fuel efficiency. E-axle is being developed by various manufactures. For further improvement in motor performance and utilization for EVs, these units are expected to become a downsized transaxle in future. In addition, for street use with frequent starts and stops, motor loss is caused by copper loss, which may be affected by coil temperature 1) . Heat transfer property for between motor and coolant, which is called as motor cooling performance is an important issue for E-axle to improve the efficiency and reliability of driving motors. There are three types for motor cooling: air, water, and oil. Although water itself has cooling ability, it has no insulating capacity and is used for cooling through a jacket, and therefore water-cooled system result in a combination of water coolant and a complex structure. Excellent cooling performance can be obtained for oil-cooled system because oils is highly insulating material when the motor is immersed directly into oil. Automatic transmission fluids (ATFs) are used in some cases as lubricating oil for E-axle 2) . ATFs have complex compositions designed to provide lubrication and friction control for shift devises and are not always optimal as fluids for E-axle. Lubricant additive aspect involves the ability of the oil limit corrosion of copper elements, mainly copper wire, and electric sensors 2) . Lithium-ion batteries are commonly used as energy storage devices in EVs because they have advantages in longer lifetime, low self-discharge rates compared to other batteries 3) . Heat thermal energy is produced inside the battery while charging or discharging the batteries, which lead to a temperature increment 4) . This is caused by the internal resistance and electrochemical reactions occurring inside the battery. Temperature increasing in the batteries give an effect on the life cycle, safety, reliability, and efficiency of the battery. There is an increasing interest in the technical approaches for battery thermal management. Battery cells usually do not have direct contact between the cell and the cooling fluids. Heat capacity of fluids is known to be more effective than air. Lubricating oil is a cooling fluid which does not conduct electricity. Battery pack is often submerged in a cooling system designed so that heat is transferred form batteries to oil directly 3) . Therefore, thermal control for the battery system by oil-cooled coolant may be more attractive. Heat transfer characteristics between oil fluid and motor/ battery pack seems to be related with viscosity, thermal conductivity, specific heat, and density of fluids. At present, the base properties of required for EVs fluids have not been systematically studied. In this study, we report the results aimed mainly at improving cooling performance for motor and battery system by lubricating oils. 2. Impact of lubricating oils on the cooling ability for liquid-cooled motor thermal control system A laboratory test method for the cooling at a forced convection was originally designed for screening the cooling performance of oil-type coolant. It is reported that a threephase induction motor is operated at the maximum temperature of 150-°C 1) . The changes in the copper plate temperature were monitored by thermocouples and then the cooling speed was calculated through the temperature by time. Figure 1 shows the effect of viscosity and type of base oil on the cooling speed. Test oils were prepared so that viscosities at 70-°C was from 1.3 to 16.4 mm 2 / s by using different type base oils. It is obvious that the cooling speed increase with lower viscosity oils comparing between hydrocracked mineral and naphthene mineral base oil similar viscosity 8-9 mm 2 / s, hydrocracked base shows a better cooling ability. In addition, synthetic base oil results in an excellent cooling performance. The difference in cooling speed among base oil type may be caused by their molecular structure. Hydrocracked base oil includes more normal chain saturated hydrocarbons rather than naphthene base oil. Figure 1: Experimental result of cooling performance by oils with different viscosity and base oil type Thermal vibration energy will come down to the main chain, and such energy transferred through collisions with neighboring molecules propagates to the end of the main chain through intermolecular heat transfer, leading to a higher heat conductivity. 42 24th International Colloquium Tribology - January 2024 Impact of Lubricating Oils on the Performance for Liquid-Cooled Motor and Battery Thermal Control System Applied to Electric Transaxles 3. Impact of lubricating oils on the cooling ability for liquid-cooled battery thermal control Kakaf et.al 3) explain that the optimum temperature range in the battery pack is 25 to 45-°C. The mainstream of cooling methods for batteries is air cooled and watercooled type. Recently, oil type fluids have been studied as to immersion cooling for batteries because oils do not conduct electricity and a higher heat capacity compared to air-cooling 5) . In this study, we developed the test method assuming immersion cooling for batteries, as shown in Figure 2. This test is aimed to evaluate the heat transfer characteristics between the heating elements and test oil when heating elements are installed in the test chamber and the test oil is flowed in the chamber. Heat flux at 500 W was supplied to the heating elements with a flow rate of 7.5 L/ min. The surface temperature stabilized after 15 minutes elapsed. Comparing at 30 minutes after starting tests, lower viscosity oil shows lower surface temperature with 5-°C than the case of higher viscosity oil. The effect of flow rate and heater surface temperature was evaluated ranging flow rate from 0.5 to 7.5 L/ min. Heating element temperature shows a lower value with higher flow rate, indicating that flow control as well as oil properties would be important for cooling performance. Figure 2: Test method for cooling performance assuming immersion cooling for batteries. It is necessary to understand oil flow behaviors near the heating elements. Particle method 6) is used for fluid simulation, which represents a fluid as a collection of particles, and local flow distribution can be analyzed. Figure 3 shows the simulation results by the particle method. Physical properties of the test oil, inlet flow rate, and heat capacity of the heating element were the same as in the experimental test. The particle size of the fluid was 0.3 mm, and the analysis was performed for 900 seconds, no external force was applied to the fluid in the chamber. From the velocity distribution in the left side of Fig. 3, the inlet speed in the lower right side is higher. The temperature distribution in the right side of Fig. 3 shows that the fluids take away heat when it passes between the heating elements, resulting in a temperature increase of the fluid. The flow velocity near the heating element would be changed depending on the fluid viscosity. Figure 3: Simulation results in the test chamber Conclusion Lubricant oil impact on the cooling performance for motor and battery thermal management system applied to electric transaxle unit. As a result, lowering kinematic viscosity of lubricating oils improved cooling performance at forced convection, and this cooling speed could be greatly influenced by base oil molecular structure. Furthermore, the test method assuming immersion cooling for batteries was developed. The lower viscosity oil could improve the cooling performance with higher flow rate, indicating that flow control as well as oil properties would be important for cooling performance. Simulation using particle method for was conducted for understanding phenomenon of the fluid flow near the heating elements in the chamber. Results revealed the velocity and temperature distribution near the heating elements in the chamber, which might play a role in affecting the cooling performance. References [1] Society of Automotive Engineers of Japan, “Automotive Technology Handbook-Design (EV-Handbook)”, 2016. [2] Onimaru et.al., “Heat analysis of the hybrid electric vehicle (HEV) Motor cooling structure using ATF” Denso Technical Review, vol. 13, no. 1, pp. 13, 2008. [3] O. Kalaf et.al., “Experimental and simulation study of liquid coolant battery thermal management system for electric vehicles: A review” Int. J. Energy Res., vol. 45, pp. 6495-6517, 2021. [4] B. Yan, C.Lim, LL.Yin, LK.Zhu, “Simulation of heat generation in a reconstructed LiCoO 2 cathode during Calvanostatic discharge” Electrochim Acta, vol. 100, pp. 171-179, 2013. [5] C. Roe et.al, “Immersion cooling for lithium ion batteries - A review” Journal of Power Sources 525(2022)231094. [6] Japan Society for Computational Engineering and Science, “Computational Mechanics Lecture Series 5 Particle method” Maruzen Publishing Co., Ltd. 24th International Colloquium Tribology - January 2024 43 Novel Organic Friction Modifiers with Extended Performance Durability Pieter Struelens 1,* , Marion Kerbrat 2 , Micky Lee 3 1 OLEON NV, Evergem, Belgium 2 OLEON SA, Compiègne, France 3 OLEON Port Klang, Sdn Bhd, Selangor, Malaysia * Corresponding author: pieter.struelens@oleon.com 1. Introduction Tighter emission control has accelerated the progress to roll out new engine oil standards. The commitment towards more stringent greenhouse gas emission standards requires a significant improvement over fuel economy and hence the roll out of ILSAC GF-7, tentatively in Q2 2028 [1]. In the light of the upcoming ILSAC GF-7 standard, the quest remains for high-performance friction modifiers maintaining their performance over an extended period of time, ensuring long-term efficiency and effectiveness. In this study, a series of organic friction modifiers (OFM) were designed, comprising polymerized fatty acid glycerol ester and polymeric friction modifiers, to be compared with metallic friction modifier (molybdenum-based). More specific it has been shown that the use of a specific polymerized organic friction modifier allows to achieve a very low friction coefficient at low speed compared to conventional organic or molybdenum based friction modifier, even after prolonged usage. Methodology Friction reduction performance of the materials was studied using an MTM (Mini-Traction Machine, experimental conditions described in Fig. 1.) Fig. 1: Mini-Traction Machine and test condition The benchmarks used in this study are molybdenum dithiocarbamate (MoDTC) and a typical reference OFM (GMO, 50% monoglyceride). 0.5% of friction modifier (except 1.5% was used for MoDTC) was mixed with Group III base stock and was only tested if it’s fully soluble. In this study, potential chemistries of amide, polymerized glycerol ester and polymeric ester were explored (refer to Table 1). Amide 1 and 2 were fatty amides. PFGE 1, PFGE 2, PF 1 and PF 2 are based on ester chemistry with different extents of repeating units. These are designed to enhance the adsorption of the friction modifiers on the metal surface. Table 1: Properties of friction modifiers studied Friction modifiers Chemistry MoDTC Molybdenum di-thiocarbamate Reference OFM Glycerol ester Amide 1 Amide Amide 2 Amide PFGE 1 Polymerized glycerol ester PFGE 2 Polymerized glycerol ester PF 1 Polymeric ester PF 2 Polymeric ester 2. Results and Discussion Friction reduction performance The impact of temperature was studied to investigate the thermal endurance of tribofilm of different chemistries. In this case, the durability of lubrication film was tested under 100- o C and 130- o C after 8 h of ageing. Figure 2 illustrates the impact of temperature on lubricity performance of GMO and MoDTC. At higher temperature (130- o C), MoDTC did not retain its lubricity as good as it was at 100- o C. On the other hand, GMO exhibited opposite behavior at 130- o C, in which it lowered friction further than it was at 100- o C, particularly at low speed region. By using a higher temperature, this distinguishes the performance of GMO from MoDTC. Therefore, it is crucial to employ a harsher condition (such as this case) to evaluate friction modifier over a prolonged time. This could give a better picture on the performance durability of friction modifiers. Fig. 2: Impact of temperature on MoDTC and GMO 44 24th International Colloquium Tribology - January 2024 Novel Organic Friction Modifiers with Extended Performance Durability Few chemistries were synthesized and compared at 100- o C. As shown in Fig 3., the ranking of performance of friction modifiers can be distinguished as follows: PFGE2, PF2->-Amide-1, PFGE 1, PF 1 > Amide 2. This indicates a careful selection of polymeric friction modifiers could significantly enhance the formation of tribo-film on the metal surface. PFGE 2 and PF2 have generally better friction reduction, from high speed to low speed regions at 100- o C. Fig. 3: Different chemistries at 100- o C after 8 h of ageing The durability of friction reduction was further stretched by a harsher 16 h ageing test (the test began with 8 h test at 100- o C, followed by 8 h test at 130- o C). This should illustrate the lubricity retention in corresponding to part of the criteria of a longer oil interval change better. Referring to Fig 4., 16 h ageing test retarded the performance of Amide 1, PFGE 2 and PF 2. On the other hand, PFGE 1 gives a very low COF at low speed region and the performance has been drastically enhanced as compare with 8-h ageing test earlier. However, this attribute diminished when the speed increased (similar to GMO earlier). The possible mechanism that contributes to this phenomenon is currently under investigation. Fig. 4: MTM test on Amide 1, PFGE 1 and 2, and PF 2 after 16-h ageing Conclusion In response to the urge for improving fuel economy, different chemistries were explored in this study to identify more durable friction modifiers. Polymeric friction modifier, PFGE-1, stands out among the friction modifiers with a unique friction reduction behaviour seen only after 16 h ageing. This could lay a foundation for further improvement on lubricity retention in aged oil by the fine-tuning of polymeric friction modifier. References [1] F+L Magazine, ILSAC GF-7: Implementation “tentatively” planned for Q2 2028, 2022. 24th International Colloquium Tribology - January 2024 45 Effect of Organic Friction Modifiers on Friction and Wear of HDDEO Formulations Gareth Moody 1* , Alexei Kurchan 2 , Sydne Tison 2 1 Cargill York, UK 2 Cargill Wilmington DE, USA * E-mail gareth_moody@cargill.com 1. Introduction Heavy duty engine oils remain an important part in transportation and have different requirements to their passenger car counterparts. Whilst viscosity of the oils has significantly reduced, 10W30 and 5W30 grades remain common unlike passenger car where 0W20 oils are popular. The additive package in a HDDEO (Heavy Duty Diesel Engine Oil) is designed with drain interval and component longevity in mind with other considerations such as cleanliness (soot management) and fuel economy also being important considerations. The introduction of the new heavy duty category PC-12 which will aim to improve fuel economy 1 and reduce NO x and CO 2 2 will further test lubricant formulators and OEMs to protect hardware whilst reducing emissions with lower SAPS levels proposed. In many of the future developments of heavy duty architecture engine oil will remain a necessary component of the system and although the technical roadmap for the future of heavy duty vehicles remains uncertain it is important that the oil can work effectively with new fuels and help meet new regulatory requirements. For small to medium goods delivery vehicles electrification and full battery electric vehicle numbers are on the increase but there are several other alternatives to battery electric particularly for very large heavy duty vehicles which include (but are not limited to) biofuels, compressed natural gas, liquified natural gas hybridisation, fuel cell and hydrogen 1 internal combustion engines. This work will evaluate the use of existing and new Polymeric Friction Modifiers (PFMs) with the new friction modifiers designed specifically for heavy duty engine oils with newly developed materials having enhanced oil compatibility without compromising on performance. The concept of a hydrogen fuelled internal combustion engine will be introduced and the potential impact the difference in conditions and in particular water content will have on the oil will be discussed. 2. Results and discussion The methods used for evaluation of the engine oils were MTM and 4-ball (ASTM D4172). The MTM method used stribeck curve conditions of 36N load, 0.5 - 3.0 m/ s speed, a temperature of 80-°C and a slide roll ration of 50%. Stribeck curves were measured after several rubbing stages of 31N load and a speed of 0.03 m/ s to generate a tribofilm. The rubbing stages totalled 120 mins. Figure 1 shows the MTM results of the HDDEO with and without the addition of friction modifiers. Initially, both the GMO and the MoDTC had very little impact whereas the PFM reduced friction immediately without the need for tribofilm development (dotted lines in Figure 1). After 120 mins of rubbing, the GMO reduced friction from 0.14 to 0.10 (boundary friction peak). The MoDTC and PFM after rubbing both reduced friction below 0.04. Figure 1: MTM stribeck curves of HDDEO with and without friction modifiers SLIM analysis was also done post stribeck to assess the tribofilm formation of the oils. After 120 mins rubbing, all of the test runs showed some tribofilm growth (Figure 2). The reference oil appears to have the thickest tribofilm from the darker colouration on the MTM ball but it is known that the correlation between colour and thickness is not always straightforward and that the addition of a PFM can alter the composition of the tribofilm 3 without having a negative effect on wear. Figure 2: Mapper images of the MTM ball 46 24th International Colloquium Tribology - January 2024 Effect of Organic Friction Modifiers on Friction and Wear of HDDEO Formulations Low friction is an indicator of increased efficiency which can potentially result in increased fuel economy and therefore range of the vehicle which is highly desirable in heavy duty vehicles, however these frictional improvements must not come at the detriment of hardware longevity. Figure 3 shows the 4-ball wear scars according to ASTM D4172 of a 15W-40 HDDEO with and without friction modifiers. In this test all of the oils behaved fairly similarly with the GMO, PFM and MoDTC all having no negative effect on performance of the antiwear additives in the engine oil formulation. Figure 3: 4-ball testing of the engine oil with and without friction modifiers. The difference with respect to looking forward to PC-12 style formulations is that lower friction can be achieved by a small amount by GMO and a large amount by either MoDTC or PFM but the PFM is able to achieve this without adding to the SAPS level of the oil making it a useful tool for formulators who wish to lower SAPS and improve fuel economy without impacting on wear. Whilst fuel economy and equipment lifetime will remain the most important factors sustainability and Product Carbon Footprint (PCF) will also undoubtedly increase in importance. Here, things such as ester base oils can be used to improve biobased contents of oils, increase biodegradability and reduce PCF. Looking ahead to new technologies such as hydrogen fuelled internal combustion engines, there will be some modifications required to both engine design and lubrication with things such as water levels of up to 3 times higher than diesel fuelled cases, 4 meaning the oil must be able to function in the presence of higher levels of water than in the past. 3. Conclusions The use of GMO, MoDTC or PFM can reduce the coefficient of friction in a HDDEO Using PFM or MoDTC can reduce friction more than using GMO Using a PFM can achieve high levels of friction reduction without increasing SAPS levels in the oil The use of organic or inorganic friction modifiers allows the creation of a tribofilm but it is different from that of a reference oil. The use of friction modifiers in a HDDEO was not found to have a negative effect on wear when tested in a 4-ball. The additional water content of engine oils associated with using hydrogen as a fuel instead of diesel must be taken into account to prevent potential problems including increased wear caused by moisture delaying development of ZDDP films 5 . References [1] A. Brown, R. Fowler, - Tribology & Lubrication Technology; Park Ridge Vol.-79, Iss.-9, (Sep 2023): 28-29 [2] https: / / www.infineuminsight.com/ en-gb/ articles/ pc- 12-moving-ahead/ [3] J. Eastwood, G. Moody Tribofilm chemistry for engine oils formulated with organic polymeric friction modifiers ITC conference Sendai 2019. [4] Yamada, N. and Mohamad, M.N.A., 2010. Efficiency of hydrogen internal combustion engine combined with open steam Rankine cycle recovering water and waste heat. International Journal of Hydrogen Energy, 35(3), pp. 1430-1442. [5] A. Dorgham., A. Azam, P. Parsaeian,. et al. An Assessment of the Effect of Relative Humidity on the Decomposition of the ZDDP Antiwear Additive. Tribol Lett-69, 72 (2021). 24th International Colloquium Tribology - January 2024 47 Performance Enhancement of Molybdenum-Based Friction Modifiers David Boudreau Sr 1* , Brian Casey 1 1 Vanderbilt Chemicals LLC, Norwalk, CT, USA * dboudreau@vanderbiltchemicals.com 1. Introduction Organomolybdenum friction modifiers reduce friction and enhance wear protection in lubrication formulations through the in-situ formation of molybdenum disulfide tribofilms (MoS 2 ). While these additives were initially developed for a traditional ICE architecture, a shift to hybrid and battery EV brings new challenges. This work investigates additive combinations of three traditional organometallic friction modifiers in EV-focused low viscosity fluids. These friction modifiers are based on molybdenum and boron and have been used extensively in traditional drivetrains. Each has unique benefits and challenges in traditional drivetrain applications. Development of new fluids for electric vehicles includes some of the same challenges as with fluids for ICE, but there are significant differences. First, the average overall operating temperature of a lubricant in a plug-in hybrid engine is lower than that of an ICE, by as much as 25% [1]. In the context of traditional molybdenum and boron additives, that often require a high activation temperature to be functional, lower operating temperatures may pose a significant barrier to their effectiveness. An additional concern is yellow metal corrosion. In the context of EV, there is a significant increase of the possibility of contact of the lubricating fluid with copper and electronics. As such, the issue of fluid conductivity comes into play in EV-based systems. A high conductivity fluid can promote short circuiting and current leaks. A low conductivity fluid in turn acts as an insulator and or capacitor. This can result in the build-up of a large charge gradient, which will eventually equilibrate through electrical discharge [2]. For electrical properties, the use of organo-metallic ligand chemistry is in its relative infancy. Some studies indicate that organoborates may possess very high conductivity [3]. The use of borate-capped dispersants also shows a large contribution to electrical conductivity. In contrast, molybdenum dithiocarbamates might contribute only a small increase to the overall conductivity of a formulation [4]. In this experiment combinations of three friction modifiers, based on molybdenum and boron, are explored to assess their effects on low friction onset, wear, corrosion, and conductivity in an EV style formulation. 2. Experimental A base formulation was prepared using a Group IV PAO 4cSt base stock. The formulation contained additives typically found in an EV or automatic transmission fluid. This formulation was topped with combinations of three friction modifiers as described in [Table 1]. Each additive is charged within a typical concentration range used in ICE, with total molybdenum concentration not exceeding 320 ppm Mo. Table 1: Friction modifiers and concentrations Additive ID Description Concentration Range (ppm metal) Mo-FM1 Molybdenum dithiocarbamate 0-0.3%wt (0-300 ppm Mo) Mo-FM2 Molybdate Ester/ Amide 0-0.3%wt (0-320 ppm Mo) B-FM Borate Ester/ Amide 0-1%wt (0-90 ppm B) The ternary combinations are shown in [Figure 1], with minimum and maximum values of each additive representing the range of 0% to 100% charge. Figure 1: Composition of ternary mixtures In addition to testing freshly prepared samples, lubricant samples were also aged by a bulk oxidation method of 48 hours at 160-°C under an oxygen bubble. This was done to simulate oil after an extended in-use period. Extended Copper Corrosion testing was performed on fresh oils in a modified ASTM D130 test, with separate tests running at 24, 168, and 336 hours at 150-°C, and including ICP analysis of solvated copper from each run. Electrical conductivity was measured using an Epsilon+ dielectric meter in a temperature range of 40-160-°C. Mini-Traction Machine (MTM) Stribeck cuves were obtained using a 35 N load, 50% SRR, and 3-3000 mm/ s rolling speed. Three curves were averaged at temperatures from 40-140-°C, in 20-°C increments. 48 24th International Colloquium Tribology - January 2024 Performance Enhancement of Molybdenum-Based Friction Modifiers 3. Results and Discussion 3.1 Extended Cu Corrosion (Modified ASTM D130) The ICP results of the extended copper corrosion [Figure 2] show the increased solubilized Cu with increasing time. Copper leaching is relatively low (max 136 ppm Cu, average 85 ppm Cu at 336 hours across all formulations). Copper strip ratings at 336 hours range from 1a to 3a. The lowest corrosion being Formula 4, containing only Mo-FM2. Figure 2: Cu Corrosion (Extended D130 @ 150-°C) 3.2 Electrical Conductivity All formulations, fresh and aged, yield electrical conductivities in a range similar to current EV specific fluid technology [5]. Due to the total number of curves, only selected low and high conductivity formulations are shown in FIGURE 3. Figure 3: Electrical Conductivity All aged oils show some increase in conductivity, relative to the fresh oils, including baseline F2. Formula F8 shows the least deviation from baseline in both fresh and aged oils, indicating it may be the best additive combination for longterm/ fill-for-life applications. 3.3 Friction Coefficient (via MTM) While measurements were taken on both fresh and aged oils, only aged data is shown below in FIGURE 4. Values represent the boundary friction obtained from 3 individual Stribeck curves at each temperature observed. For clarity, only select low and high CoF formulations are shown. Fresh oil data generally trended to have slightly lower CoF values than aged oil, independent of formulation. Figure 4: MTM Boundary Friction at Temperature Most formulations of aged oils trended like F11 and showed an increase in friction with ageing relative to baseline F2. F8 was a clear exception, showing a decrease in friction. For F8, friction reduction was not only maintained after ageing, but actually improved. 4. Conclusion In this initial investigation, combinations of B-FM, Mo-FM1 and Mo-FM2 were shown to be effectively used in an EV based fluid without significant detriment to critical concerns such as conductivity and corrosion. Of particular merit was a combination of molybdenum dithiocarbamate and borate ester. In terms of extended oil life, conductivity and friction remained low with this combination. Copper corrosion of this formulation was not the lowest in the series, but all formulations including this combination performed very well in this extended test - giving good coupon ratings and relatively low solvated copper. Future investigations will focus on the effects of these additives on other properties such as extreme pressure, scuffing, and wear. References [1] Chevron Oronite (2023), Hybrid Vehicles are a Bridge to the Future - A Natural Step in the Evolution from Internal Combustion Engines (ICEs) to Full Battery Electric Vehicles (BEVs), https: / / www.oronite.com/ about/ news-activities/ hybrid-solutions.html [2] Lubes n‘ Greases (Jul 01, 2021), EV Fluid Development Challenges, https: / / www.lubesngreases.com/ electric-vehicles/ article/ ev-fluid-development-challenges/ [3] M. Özcan, C. Kaya, F. Kaya (12 July 2023), An Optimization Study for the Electrospun Borate Ester Nanofibers as Light-Weight, Flexible, and Affordable Neutron Shields for Personal Protection, https: / / doi. org/ 10.1002/ mame.202300150] [4] Gao et al (2018) US Patent Application No. 2018/ 0100114 A1 [5] A. Eastwood, S. Patterson, “e-Fluid Technology for Electrified Drivetrains”, LUBRIZOL 360 Webinar Series, Sept. 21, 2021, https: / / 360.lubrizol.com/ Resources/ Webinars 24th International Colloquium Tribology - January 2024 49 Lubricity-improving Additives Based on the Synergy of Nanoparticles and Protic Ionic Liquid Raimondas Kreivaitis * , Milda Gumbytė, Artūras Kupčinskas, Jolanta Treinytė Vytautas Magnus University Agriculture Academy, Kaunas, Lithuania * Corresponding author: E-mail raimondas.kreivaitis@vdu.lt 1. Introduction Environmental requirements become more strict; therefore, new lubricant formulations are needed. Water-based lubricating fluids are widely used in industrial areas due to their nontoxicity, availability, low cost, and fire resistance [1, 2]. However, water as a base fluid must be improved to fit the needs of a particular application. Functional additives and diluents are applied to fulfil lubricity requirements, cooling capacity, corrosion prevention, and low-temperature fluidity [3,4]. Both the base stock and additives must meet biodegradability, toxicity and renewability requirements [5]. For this purpose, halogen-free ionic liquids (ILs) and nanoparticles (NPs) based additives were investigated [6, 7]. Furthermore, researchers confirmed that the synergy between ILs and NPs can develop better tribological responses in significant wear and friction reduction [7, 8]. The aim of this study is to investigate the synergistic effect between protic IL tert-Octylamine oleate and silicon oxide NPs and graphene nanoplatelets as additives in the water/ glycerol-based lubricating fluid. 2. Experimental 2.1 Materials The base lubricating fluid (WGL) comprises deionised water and glycerol at 1: 1 by wt. Protic ionic liquid tert-Octylamine oleate (TO) was synthesised in our lab [9]. Specifications of investigated nanoparticles are as follows: silicon dioxide [SiO 2 ], appearance spherical, porous 5 - 15 nm in size, trace metal - 2111.4 ppm; graphene [G], appearance - nanoplatelets - surface area - 750 m 2 / g. 2.2 Preparation of the lubricating samples The commercially available NPs will be dispersed in water. Then, the dispersions will be functionalised with a protic ionic liquid. Finally, glycerol will be added as the second component of the base fluid. For the preparation of nano lubricants, we used bath ultrasonication. After sonication, silicon oxide and graphene dispersions were centrifuged in Thermo ScientificTM Multifuge X3R centrifuge. Finally, using a magnetic stirrer, six lubricating samples were prepared: a base fluid (WGL), graphene nanoplatelets containing nano-lubricant (WGL+G), silicon oxide NPs containing nano-lubricant (WGL+SiO), PIL modified lubricating sample (WGL+TO), PIL functionalised graphene nanoplatelets (WGL+TO+G), PIL functionalised silicon oxide nanoparticles (WGL+TO+SiO). G and SiO NPs concentrations in lubricating samples were 0.034 and 0.135 wt.%, respectively. 2.3 Physicochemical properties and tribological performance investigation The kinematic viscosity of investigated lubricating samples was measured at 25- °C using Anton Paar Staminger viscometer SVM 3000. DUCOM ball-on-plate reciprocating tribometer was used for the tribological tests. Investigation parameters are as follows: load - 4-N, temperature - 25-°C, duration-- 30 min, reciprocation frequency - 15 Hz, stroke length - 1-mm, amount of sample - 2 ml. After the tribo-test, worn surfaces were analysed with optical microscope, SEM and EDS. 3. Results and discussion The kinematic viscosity is listed in Table 1. TO increased the base fluid viscosity by almost 40 %, G and SiO NPs - by 2-and 6 %. The viscosity of the nanoparticle-loaded base fluid is proportional to the concentration of nanoparticles [10]. Therefore, SiO NPs loaded samples have higher viscosities. Table 1: The values of investigated lubricating samples Lubricating sample Kinematic viscosity @ 25-°C, mm 2 / s COF Wear volume × 10 3 , μm 3 WGL 4.45 0.209 834.3 WGL+G 4.54 0.208 1005.2 WGL+SiO 4.72 0.211 1019.5 WGL+TO 6.22 0.100 91.5 WGL+TO+G 6.36 0.096 35.5 WGL+TO+SiO 6.70 0.090 36.9 The coefficient of friction (COF) and wear volume are summarised in Table 1. The base fluid has a high COF. The addition of NPs did not improve friction. When TO PIL is introduced, friction is reduced. Friction is further reduced when PIL is combined with NPs. According to the COF, the ranked lubricating fluids are WGL+SiO > WGL+G > WGL- >> WGL+TO > WGL+TO+G > WGL+TO+SiO. The samples containing combined additive of PIL and SiO NPs have the lowest COF. Lubrication without additives resulted in extremely intensive wear. Adding NPs alone did not improve the situation. The introduction of PIL reduced wear volume more than 16 times. When combined with NPs, wear reduction was further improved. PIL with graphene NPs resulted in the lowest wear, decreasing it by 23.5 times. Figure 1 shows images of wear traces on the plates and wear scars on the balls taken through an optical microscope. The 50 24th International Colloquium Tribology - January 2024 Lubricity-improving Additives Based on the Synergy of Nanoparticles and Protic Ionic Liquid worn surfaces observed in the tribo-tests using a base fluid loaded with PIL and additives appeared similar. The worn surfaces contain small scratches and some kind of layer. The layer is more evident on the ball surface. It was proposed that the layer was formed during the reaction of PIL with the metal surface. Moreover, the NPs presented in the lubricants were embedded in the layer, giving a better tribological response. Fig. 1: Lubricated worn surfaces on the plate (right) and the ball (left). (a) WGL+TO, (b) WGL+TO+G, (c)-WGL+TO+SiO The lubrication with NPs loaded samples resulted in a smoother, worn surface. The roughness of wear traces on the plate - Ra and Rz, μm, is as follows: WGL - 0.092 and 0.455, WGL+G - 0.053 and 0.280, WGL+SiO - 0.052 and 0.310, WGL+TO - 0.043 and 0.188, WGL+TO+G - 0.008 and 0.042, WGL+TO+SiO - 0.011 and 0.060, respectively. TO-loaded base fluid produced a relatively rough wear trace on the plate. In this study, we observed that G and SiO NPs alone could not improve the lubricity of water/ glycerol base fluid. However, their combination with PIL resulted in outstanding tribological performance. The following explanation schematically is proposed (Fig. 2). 4. Conclusions This study examined the combined effect of NPs and a lubricity-enhancing additive PIL for a water/ glycerol base fluid. The main findings are as follows: ultrasonication of the nanomaterials in water could not fully separate their clusters; the NPs alone did not improve the fluid’s lubricity, but adding PIL improved it. The best results were achieved when the hybrid additives, containing both PIL and NPs, were used; the study suggests that the PIL reacted during friction to form a friction polymer or metal soap layer. The NPs became embedded in this layer, creating a composite tribo-film. This composite film enhanced the lubrication abilities of the fluid. Fig. 2: The proposed synergistic lubrication mechanism References [1] Zheng G., et al. Tribological properties and surface interaction of novel water-soluble ionic liquid in water-glycol. Tribol Int. 2017, 116: 440-448. [2] Yang Z., et al. Amino acid ionic liquids as anticorrosive and lubricating additives for water and their environmental impact. Tribol Int. 2021, 153: 106663. [3] Rahman H., et al. Water-Based Lubricants: development, properties, and performances. 2021. [4] Nune M. M. R, Chaganti P.K. Development, characterization, and evaluation of novel eco-friendly metal working fluid. Measurement (Lond). 2019, 137: 401-416. [5] Bartz W. J. Ecotribology: Environmentally acceptable tribological practices. Tribol Int. 2006, 39: 728-733. [6] Amiril S. A. S, et al. A review on ionic liquids as sustainable lubricants in manufacturing and engineering: Recent research, performance, and applications. J-Clean Prod 2017, 168: 1571-1589. [7] Avilés M. D, et al. Ionanocarbon lubricants. The combination of ionic liquids and carbon nanophases in tribology. Lubricants. 2017, 5. [8] He Z., Alexandridis P. Ionic liquid and nanoparticle hybrid systems: Emerging applications. Adv Colloid Interface Sci. 2017, 244: 54-70. [9] Kreivaitis R., at al. Investigating the tribological properties of PILs derived from different ammonium cations and long chain carboxylic acid anion. Tribol Int. 2020, 141. [10] Sanukrishna S. S, et al. Experimental investigation on thermal and rheological behaviour of PAG lubricant modified with SiO2 nanoparticles. J Mol Liq. 2018, 261: 411-422. 24th International Colloquium Tribology - January 2024 51 In-Operando Formation of Transition Metal Dichalcogenides - Instant Lubrication by Simple Sprinkling of Se Nano-Powder onto Sliding Contact Interfaces Philipp G. Grützmacher 1* , Maria Clelia Righi 2 , Ali Erdemir 3 , Carsten Gachot 1 1 Institute for Engineering Design and Product Development, Tribology Research Division, TU Wien, Vienna, Austria 2 Department of Physics and Astronomy, Alma Mater Studiorum − University of Bologna, Bologna, Italy 3 J. Mike Walker ’66 Department of Mechanical Engineering, Texas A&M University, College Station, TX, USA * Corresponding author: philipp.gruetzmacher@tuwien.ac.at 1. Introduction There are many situations in which liquid lubricants come to their limits, e.g., for space applications [1]. In these cases, the attention turns to solid lubricants, such as graphite or MoS 2 . They have been used by industry for many years due to their excellent lubricating properties stemming from a layered structure. In the layers the atoms are bonded by strong bonds, while between the layers mostly act weak van der Waals forces. If these materials are subjected to shear forces the layers slide easily over each other, resulting in low friction. However, a major drawback of solid lubricants is their short wear life [2]. If solid lubricant layers are worn out their protective function ceases and friction and wear increases drastically. Additionally, the performance of many solid lubricants depends highly on the environmental conditions. Graphite works best in humid conditions, whereas MoS 2 shows lowest friction in dry environment [2]. In contrast, the in-operando formation of a solid lubricant has many advantages. If the lubricant is formed under operation conditions it is selectively formed at the locations where it is most needed, wear life in these cases is basically infinite, and the lubricant is formed only the environment where it is applied, thus reducing the environmental factor [3]. We describe an innovative and very unique approach that results in the formation of a solid lubricant in the form of a transition metal dichalcogenide (TMD), namely MoSe 2 or WSe 2 . The formation of the TMD is verified using Raman spectroscopy, X- ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (TEM). Furthermore, the mechanisms of the in-operando formation are unveiled by ab initio molecular dynamics simulations (AIMD). 2. Results and Discussion To observe the in-operando formation of TMDs, particularly MoSe 2 and WSe 2 , we performed ball-on-disk experiments in unidirectional, rotational sliding mode with an inert Al 2 O 3 counterbody at a load of 1 N and a sliding speed of 15 mm s -1 . The substrates were steel platelets that were coated by a 3-µm layer of either Mo or W. The chalcogen element was added to the sliding surfaces by simple sprinkling of a Se nano-powder (particle size distribution between 40 to 80 nm) before the start of the experiment. Without the application of the Se nano-powder, the coefficient of friction (COF) is relatively high with a steady-state COF after running in of 0.60 and 0.40 for the Mo and W substrates, respectively (Figure 1). This can be expected for non-lubricated sliding of a metal-ceramic friction couple. In contrast, the evolution of the COF changes drastically after adding the Se nano-powder to the sliding surfaces before the tests. While friction is comparable at the start of the tests, the COF continuously decreases over the course of the experiment. The final COF during the last 100 cycles of the experiment comes down to 0.09 and 0.13 for the Mo and W substrates with Se nano-powder addition, respectively. These COFs constitute low friction values, which are typically reached using fully formulated oils, which is remarkable considering the non-lubricated metal-ceramic sliding contact. Figure 1: Friction performance of the (a) Mo and (b) W substrates in unidirectional ball-on-disk experiments. Adapted from [3]. 52 24th International Colloquium Tribology - January 2024 In-Operando Formation of Transition Metal Dichalcogenides - Instant Lubrication by Simple Sprinkling of Se Nano-Powder onto Sliding Contact Interfaces It is worth mentioning that friction increases again if the experiments are continued for a longer time, which can be correlated with the degradation of the formed tribofilms under shear and load. However, sprinkling Se nano-powder again into the contact leads to the re-establishment of the lubricious layers and, hence, once more low friction. To verify that these substantial improvements in friction can be correlated with the in-operando formation of MoSe 2 and WSe 2 tribofilms TEM lamellae were prepared from the wear tracks of the substrates after tribological testing. The TEM analysis clearly shows continuous tribofilms on both surfaces with a thickness of 10 to 20 nm (Figure 2). In these tribofilms layered structures with inter-layer distances close to the ones corresponding to the (0 0 2) planes of hexagonal MoSe 2 and WSe 2 can be found. That these layered structures correspond to MoSe 2 and WSe 2 was further confirmed by Raman and XPS [3]. Figure 2: Transmission electron microscopy images of the tribofilms formed on the sliding interface of the (a,b) Mo and (c,d) W substrates. Adapted from [3]. Finally, our AIMD simulations unveiled the mechanisms responsible for the in-operando formation of the TMDs under sliding conditions. The results demonstrated that the formation of TMD layers from the Mo and W bulk constituents with Se nano-particles is highly favourable, associated with an energy gain of 2.0 eV and 1.1 eV per unit of MoSe 2 and WSe 2 , respectively. Furthermore, AIMD simulations under load and shear demonstrated the formation of TMD layers from metal and Se particles. First, bonds with trigonal prismatic coordination typical of TMDs are established between the metallic and Se particles. Then single metal atoms are detached from the metallic nano-particles and are surrounded and bond to Se atoms, leading to the complete disaggregation of the metallic nano-particle and finally, the formation of MoSe 2 layers. The simulations indicate that the formation of crystalline TMD layers during tribological loading of a Mo (W) substrate in presence of Se nano-powder. However, metallic nano-particles have to present for these reactions to happen. Extracting metal atoms from metal-oxide nano-particles or ideal Mo surfaces could not be observed. Therefore, we conclude that metallic particles are extracted from the surfaces thanks to wear-induced defects and debris, which then react with the Se nano-powder to form TMD layers. While, the direct reaction of metal-oxide nano-particles with the Se nano-powder seems not to be possible, the hard metal-oxide particles might accelerate wear of the surfaces, thus providing more metallic wear debris, which are then converted to the lubricious TMD layers. 3. Conclusion In this study, we have presented an innovative approach to form lubricious tribofilms based on MoSe 2 or WSe 2 TMD layers in-operando. The precursor for this formation, Se nano-powder, is simply sprinkled onto the sliding surfaces in ambient conditions. As a result, the COF is continuously decreasing down to 0.10 or even lower, levels typically reached only with fully formulated oils. The formation of the TMD layers is confirmed by TEM, XPS, and Raman spectroscopy. Ab initio molecular dynamics simulations give fundamental insight into the mechanisms of the TMD formation under sliding conditions. Our study clearly shows that the in-operando formation of highly lubricious TMD layers through tribochemical reactions between Mo (W) coatings and Se nano-powder is feasible. The approach by simple sprinkling of the Se nano-powder onto the sliding interface leads to highly reproducible results. This could be the solution for the low wear life of solid lubricant layers and is particularly interesting for maintenance-critical applications, where commonly used TMD coatings lead to component failure once they are fully worn. Additionally, our approach can inspire the in situ synthesis through tribochemical reactions of new compounds even different from TMD. References [1] Scharf, T. W., & Prasad, S. V. (2013). Solid lubricants: A review. Journal of Materials Science, 48(2), 511-531. [2] Zhang, S., Ma, T., Erdemir, A., & Li, Q. (2019). Tribology of two-dimensional materials: From mechanisms to modulating strategies. Materials Today, 26, 67-86. [3] Grützmacher, P. G., Cutini, M., Marquis, E., Rodríguez Ripoll, M., Riedl, H., Kutrowatz, P., Bug, S., Hsu, C.-J., Bernardi, J., Gachot, C., Erdemir, A., & Righi, M. C. (2023). Se Nanopowder Conversion into Lubricious 2D Selenide Layers by Tribochemical Reactions. Advanced Materials. 24th International Colloquium Tribology - January 2024 53 SAPS-free Bio-based Additives for Lubrication in Next-generation Vehicles Xin He 1* , Christelle Chretien 1 1 Solvay USA Inc., Bristol, United States * Corresponding author: xin.he@solvay.com 1. Introduction All machinery parts consume frictional energy to overcome the resistance to motion due to sliding. This type of energy dissipation and wear caused an economic loss of about 2%-7% of its GDP for countries every year. [1] In order to save energy and be sustainable, lubricants and additives have been developed to minimize friction at the interfaces. Zinc dialkyldithiophosphate (ZDDP) is the most commonly used antiwear additive in boundary lubrication, offering exceptional wear prevention properties. However, ZDDP can present challenges in specific conditions. For instance, it generates deposits during decomposition and is corrosive to certain metals. To achieve sustainable design objectives, it becomes necessary to explore sulfated ash, phosphorus, and sulfur (SAPS)-free alternatives. SAPS-free additives have been proposed to achieve friction modification. Examples include fatty esters and amides, such as glycerol monooleate (GMO) or oleyl amides. However, these SAPS-free additives generally exhibit weaker antiwear properties compared to their metal, phosphorus, and/ or sulfur-based counterparts. In this study, we developed a new family of twin tailed amine derivatives, which not only provide excellent frictional properties but also exhibit antiwear performance. The newly developed additives are fully bio-based and show great potential in the field of engines and electric motors. 2. Methods and Experiments 2.1 Bio-based additives The additives were obtained through a two-step reaction. The initial intermediate was obtained from a decarboxylative ketonization reaction of corresponding fatty acids RCOOH. The twin-tailed intermediate R-(C=O)-R can be further functionalized with amine/ amide groups. The final product will show the following formula: Figure 1: Molecular structure of the SAPS-free bio-based additives Wherein each of R and R’, which are identical of different, is an aliphatic group preferably containing between 5 and 23 carbon atoms. 2.2 Formulation Seven bio-based additives were developed and investigated in this study, including five twin-tailed amine additives and two twin-tailed amides. Four commercial additives: primary-ZDDP, Glycerol monooleate (GMO), molybdenum dithiocarbamate (MoDTC), and oleyl amide were selected as a comparison. The additives were added to the Group III oil (SAE 20, KV @ 100-°C = 8 cSt) with 1 wt% treatment. The formulated oils were mixed at 60-°C for at least 30 min using a mechanical blender. 2.3 Tribology tests Two types of tribology tests were conducted to evaluate the lubrication performance of the newly developed additives. • High-Frequency Reciprocating Rig (HFRR): The friction coefficient was obtained using a ball-on-flat reciprocating configuration in the regime of boundary lubrication. A dead load of 200 g was applied on the ball and the stroke length was 1 mm. The frequency was set as 20 Hz, corresponding to a sliding speed of 20 mm/ s. The friction test was conducted at a constant 40 °C for 15 min and then ramped to 150 °C at 2 °C/ min. The entire process took 70 min. • Four-ball tribometer (ASTM D4172): The wear tests were conducted based on ASTM standard D4172. All candidate oils were studied at 75 °C under a 40 kg load. The instrument was running at a 1200 rpm speed and the test duration was 60 min. An optical microscope with scales was employed to estimate the diameter of the wear scars. 3. Results and Discussions Figure 2 illustrates a comparison of the friction traces between the top contenders (C2, C5, C6) and benchmark additives. Both commercial GMO and MoDTC additives demonstrated a consistent friction coefficient of approximately 0.10 within the designated temperature range. Notably, candidates C5 and C6 exhibited a relatively higher friction coefficient during the temperature ramp-up until reaching 90 °C, after which it dropped. This observation may be attributed to the inferior solubility of the twin-tailed amides in the Group III base oil. However, as the solubility concern became negligible at higher temperatures, the friction coefficients became equivalent to those of GMO and MoDTC starting at 120 °C. Table 1 provides a summary of the friction coefficients at 150 °C, revealing that candidates C5 and C6 achieved lower friction coefficients than GMO and oleyl amide. The remaining products demonstrated comparable or lower values when compared to primary ZDDP. 54 24th International Colloquium Tribology - January 2024 SAPS-free Bio-based Additives for Lubrication in Next-generation Vehicles Figure 2. Friction trace of the selected additives with the temperature ramping from 60 to 150 °C The wear results are presented in Table 1. The wear protection provided by GMO was found to be inferior, resulting in a scar diameter of 0.75 mm. This can be attributed to the ineffective formation of a tribofilm. In contrast, MoDTC, with its sulfur-containing tails, demonstrated quick generation of a protective film during the sliding process, resulting in a significantly lower wear scar of 0.42 mm. It was observed that all the newly developed additives exhibited wear scars ranging from 0.3 to 0.6 mm. Additionally, additives C5 and C6 displayed superior performance compared to all the comparative additives (Figure 3). The wear mechanism of amine or amide-based additives is presumed to involve a passivation effect facilitated by the functional groups. This passivation prevents extensive tribocorrosion at the interface, thereby reducing wear. Table 1: Summary of wear scar diameter (mm) and friction coefficient at 150 °C for the candidate additives and commercial products Tested compound Antiwear: scar diameter (mm) Friction test: coefficient of friction @-150-°C C1 0.53 0.176 C2 0.57 0.127 C3 0.58 0.135 C4 0.54 0.122 C5 0.34 0.085 C6 0.41 0.076 C7 - not tested - 0.171 GMO (COMPARATIVE) 0.75 0.094 Oleyl amide (COMPARATIVE) 0.55 0.105 MODTC (COMPARATIVE) 0.42 0.076 Figure 3. Wear scar diameter of the selected additives Although certain commercial additives have demonstrated excellent lubrication performance, they do not align with the sustainable development trend. Both ZDDP and MoD- TC have ash-related issues, making them unsuitable for various industries. On the other hand, the bio-based GMO additive possesses a hydrolysable nature that tends to degrade under harsh conditions. In contrast, newly developed additives show promising potential in several ways: • The twin-tailed amine/ amide additives are SAPS-free and bio-based • The top candidates showed an equivalent friction to GMO and MoDTC from 120 °C • The top candidates presented superior wear protection compared with ZDDP and MoDTC • The reductive nature of amine and amide group endow the potential low corrosion rate to metals, which expand the application to HEV/ EV field 4. Conclusion The SAPS-free bio-based technology appears to be an excellent alternative for conventional lubricating additives. The leading candidates exhibit remarkable friction and wear characteristics. Considering the growing emphasis on sustainable development and market demands, it is evident that this technology has the potential for broader utilization. Future research will investigate the anti-corrosion and thermal properties, enabling us to explore its potential application in the electric vehicle (EV) industry. References [1] Liu, H., Yang, B., Wang, C. et al. The mechanisms and applications of friction energy dissipation. Friction 11, 839-864 (2023). [2] Robbins, S. J., Nicholson, S. H. Long-term stability studies on stored glycerol monostearate (GMS)-effects of relative humidity. J Am Oil Chem Soc 64, 120-123 (1987). 24th International Colloquium Tribology - January 2024 55 Biobased Ionic Liquid for Conductive Lubricants Pieter Struelens 1* , Yen Yee Chong 2 , Micky Lee 2 1 OLEON NV, Evergem, Belgium 2 OLEON Port Klang, Sdn Bhd, Selangor, Malaysia * Corresponding author: pieter.struelens@oleon.com 1. Introduction In the last years, ionic liquids have gained significant attention as promising lubricant additives due to their unique physicochemical properties. Despite the advantages that ionic liquids bring in terms of performance, ionic liquids are commonly linked to having relatively high toxicity and low base stock solubility, limiting their applications. In this study, a novel, sulphurand halogen-free ionic liquid was developed from renewable materials and carefully designed to possess a tailored molecular structure, enabling enhanced solubility and anti-wear characteristics. The conductivity characteristic of the biobased ionic liquid was also investigated to assess its suitability for applications that require electrostatic discharge protection. 1.1 Properties of ionic liquids The properties of the developed product (labelled as L795) are presented in Table 1, and so are those of two benchmarks (labelled as Benchmark 1 and Benchmark 2). The benchmarks selected are marketed phosphonium ionic liquids that have been actively investigated as lubricant additives, where Benchmark 1 is halogen-containing and Benchmark 2 is halogen-free. While the benchmarks are mineral based, L795 can be produced 100 % biobased. Table 1: Properties of ionic liquids Ionic liquid Chemistry Viscosity @ 25-°C, cSt Biobased carbon content (%) L795 Halogen & Phosphonium free 1498 100 Benchmark 1 Phosphonium (contains halogen) 270 Mineral-based Benchmark 2 Phosphonium (halogen-free) 1230 Mineral-based 2. Results and Discussion 2.1 Solubility of ionic liquids To evaluate the solubility and stability of the ionic liquids in base oils (Ester A, PAO, and GIII base oil), 1 wt.% of each ionic liquid was mixed with the base oils and stirred at room temperature for 30 mins. Then, the mixtures were centrifuged at 40 °C, 15000 rpm, 45 mins. Results presented in Table 2 show that L795 and Benchmark 2 were soluble and stable in all the base oils studied. On the other hand, Benchmark 1 was insoluble in PAO and GIII base oil. Based on this observation, two remarks that can be made are: i) Even though both are phosphorus-based, Benchmarks 1-and 2 have different solubility profiles. ii) L795 is as robust as Benchmark 2 in respect of solubility in different groups of base oils. As Benchmark 1 was not soluble in both PAO and GIII base oils, these mixtures were not selected for further evaluation. Table 2: Solubility results of ionic liquids in base oils L795 Benchmark 1 Benchmark 2 Ester A (ISO VG 100) clear clear clear PAO (ISO VG 100) clear cloudy clear GIII (ISO VG 32) clear cloudy clear 2.2 Wear reduction performance of ionic liquids The wear reduction performance of the ionic liquids was evaluated based on ASTM D4172 by comparing the average diameter of the wear scars generated. Figure 1: Average wear scars As depicted in Figure 1, L795 has significantly better performance than Benchmark 2 in reducing wear of PAO and GIII base oil. The addition of 1 wt.% L795 reduced the wear of PAO and GIII base oil by 42.6% and 27.5%, respectively. Although Benchmark 2 performed better than L795 in Ester A, higher wear scars were generated in PAO and GIII base 56 24th International Colloquium Tribology - January 2024 Biobased Ionic Liquid for Conductive Lubricants oil, which limits the feasibility of Benchmark 2 to be used in these base oils. This observation shows that L795 is a more versatile ionic liquid that can be used as anti-wear additives in different base stocks. 2.3 Copper corrosion test results Copper has become an essential metal in automotives as high-performance copper alloy material for foil (battery), winding wires (electric motor), etc. The use of copper is also common in electric vehicles. Hence, the compatibility of lubricants with copper is critical. A modified ASTM D130 was used in this study with testing conditions of 100-°C for 24-h [1,2]. Figure 2: Copper corrosion test in Ester A with 1 wt.% of ionic liquids A study carried out by Yu, Q et. al. (2020) showed that halogen-containing ionic liquids can cause the copper strips to corrode [2]. Nevertheless, such phenomena were not observed in the current study with Benchmark 1 in Ester A (Figure 2). This may be due to the absence of water in the test to hydrolyse the ionic liquid, which can release halogenic gas that corrodes the copper strip. Figure 3: Copper corrosion test in PAO base oil with 1-wt.% of ionic liquids Figure 4: Copper corrosion test in GIII base oil with 1 wt.% of ionic liquids On the contrary, anticorrosion property of L795 and Benchmark 2 was observed in all base oils (Figure 2 - 4). While Ester A, PAO, and GIII base oil created dark tarnish on the copper strips, both L795 and Benchmark 2 reduced the tarnishing of the copper strips. 2.4 Resistivity results The study on ionic liquids to be used in conductive lubricants have captured the interest of many and are being actively investigated for applications that require electrostatic discharge protection. It is worth noting that the targeted resistivity varies for different applications and may not always be beneficial to have a very conductive lubricant (e.g., leak charge). In this study, the resistivity reduction performance of the ionic liquids in base oils were measured based on ASTM D1169 at 25 °C, where the results are tabulated in Table 3. Table 3: Resistivity results of base oils with ionic liquids in GΩ.cm Base oil only L795 Benchmark 1 Benchmark 2 Ester A (ISO VG 100) 1000 60 0.7 20 PAO (ISO VG 100) 6000 70 Not tested (insoluble) 700 GIII (ISO VG 32) 2000 20 Not tested (insoluble) 100 In terms of resistivity reduction, Benchmark 1 was able to reduce the resistivity of Ester A by 4 magnitudes of order. As for the two halogen-free ionic liquids tested, L795 managed to reduce the resistivity of PAO 10 times more than Benchmark 2 and of GIII, 5 times more. As PAO and GIII are conventional base oils used in lubricants, this makes L795 a more versatile ionic liquid to be used in the industry. 3. Conclusion Based on the tests carried out, it can be concluded that compared to market benchmarks, the developed product (L795) shows the best overall performance and versatility with regard to solubility, wear reduction, anticorrosion property and resistivity reduction. Future work will be carried out to investigate the tunability of the performances of base oils with L795 at various dosages. References [1] Fang, Hongling et al. 2021. ‘Lubricating Performances of Oil-Miscible Trialkylanmmonium Carboxylate Ionic Liquids as Additives in PAO at Room and Low Temperatures’. Applied Surface Science 568: 150922. [2] Yu, Qiangliang et al. 2020. ‘Physicochemical and Tribological Properties of Gemini-Type Halogen-Free Dicationic Ionic Liquids’. Friction 9. 24th International Colloquium Tribology - January 2024 57 Introducing a New High-Performance Water-Based Rust Preventive Additive for Formulations Demanding Superior Metal Parts Protection in Severe Corrosion Conditions Clifford Pratt, Ph.D. 1* 1 King Industries, Inc. Norwalk Connecticut USA * Corresponding author: cpratt@kingindustries.com 1. Introduction Protection of metal parts against corrosion, including everything from small to large components, remains a critical element in the financial investment of parts manufacturing. Most high-performance rust preventive formulations consist of petroleum oil and petroleum derived solvents in combination with rust preventive additives. Solvents typically make up 30% to 95% of the rust preventive formulation. Evaporation of these petroleum derived solvents poses a serious pollution problem in many areas of the world. Given the shift driven by environmental regulation from the market dominant oil-solvent based rust preventives to water-based formulations, a gap has emerged in products that provide adequate protection. For water based formulations, King Industries already has a solid line of rust preventive additves that give either emulsion or clear solution type formulations. Formulations based on these products offer excellent multi-metal protection under high humidity conditions; however, none provide adequate steel protection in high salt environments. A new rust preventive additive based on King Industries’ unique calcium dinonylnaphthalene sulfonate technology, CP-46, has been developed for use in water-based systems to fill the gap, providing superior performance in high humidity and specifically high salt containing atmospheres. Through a series of ASTM, DIN, and in-house tests, this new product, CP-46, has demonstrated exceptional corrosion protection performance in addition to emulsion stability under stressed conditions. This new additive combines the performance, stability, and versatility expected in traditional RP systems without the concerns of handling volatile solvents, which is better for the environment and eliminates worker exposure to hazardous VOC’s. 2. Sample Preparation CP-46 has been designed to first be added to oil then emulsified with ordinary tap water. For this study, the Standard CP- 46 emulsion refers to the following formulation: 10% CP-46 20% ISO VG 32 Group I Oil 70% Norwalk CT Tap Water This emulsion is extremely stable and has been observed for over 90 days without showing signs of separation. Alternate formulations with different oil types, including some esters, along with different ratios of CP-46 to oil, were also evaluated. The resulting emulsions based on these alternate formulations are also extremely stable, and the results are examined in depth in the presentation. 3. Experimental To evaluate the rust protection performance of CP-46 formulations, a combination of standardized and in-house test methods was used. 3.1 Salt Fog (ASTM B117) CP-46 was designed for high salt environments, therefore, a significant portion of the testing in this study involved Salt Fog (ASTM B117) evaluations. Because ASTM B117 is a practice, only the chamber conditions are specified, and several other test parameters including test pieces, specimen preparation, and failure criteria are left to the user to define. The test specimens were all prepared in a consistent manner defined by King Industries, and the rust preventive formulation was applied by dipping. For this study, failure was defined as rust that extends more than 1.5 cm from the top edge of the panel and/ or 0.5 cm from either side edge. The edges of the panels were not taped. Uncoated panels fail very quickly in this test. Figure 1 shows significant rust formation on an uncoated panel after 1 hour of exposure. Figure 1: Salt Fog (ASTM B117) Testing on Uncoated 1010 Steel Q-Panel. 30 minutes (left) and 1 hour (right) of exposure Salt Fog testing was reviewed on many different metal alloys showing the exceptional protection obtained by using CP-46. Figures 2 and 3 show examples of the Salt Fog testing results of the Standard CP-46 Formulation on steel and aluminum panels, respectively. 58 24th International Colloquium Tribology - January 2024 Introducing a New High-Performance Water-Based Rust Preventive Additive for Formulations Demanding Superior Metal Parts Protection ... Figure 2: Standard CP-46 Formulation. Salt Fog (ASTM B117) Testing on 1010 Steel Q-Panels. Average hours to failure: 140 (polished, front), 72 (matte, rear) Figure 3: Standard CP-46 Formulation. Salt Fog (ASTM B117) Testing on 2024/ T3 Aluminum panels. Average hours to failure: 85 - 90 (polished) 3.2 Humidity (ASTM D1748) High humidity testing on multi-metal alloys was also evaluated per ASTM D1748. Unlike ASTM B117, the humidity cabinet method clearly defines cabinet conditions, panel preparation steps, and failure criteria for the panels evaluated, and those parameters were followed for this study. Similar to the Salt Fog testing, panels were coated with the rust preventive formulation by dipping. CP-46 provides excellent rust protection under high humidity conditions to a variety of metal alloys with and without added oil. Figure 4 highlights the protection provided by the previously defined Standard CP-46 Formulation on steel panels. Figure 4: Standard CP-46 Formulation. Humidity Cabinet (ASTM D1748) Testing on 1010 Steel Panels. Average hours to failure: 2892 3.3 Acid Atmosphere (In-house) King Industries has also developed an in-house test for Acid Atmosphere to help evaluate which rust preventive additives would work well for protecting metal stored near pickling lines or anywhere hydrochloric acid fumes are present. In the test, coated panels are hung from the lid of a test chamber that has 200 mL distilled water in the bottom as well as a beaker containing 50 mL of a 6% HCl solution. The chamber assembly is placed in an oven at 40°C, and each side of the test panel is monitored for corrosion. Failure has been defined as more than 20% rust per side. CP-46 was evaluated in this in-house acid atmosphere chamber in three formulations to examine the effects of varying the amount of oil on the level of protection provided. Average Hours to Failure 10% CP-46 5% ISO VG 32 Group | Oil 85% Tap Water 306 10% CP-46 10% ISO VG 32 Group | Oil 80% Tap Water 666 10% CP-46 20% ISO VG 32 Group | Oil 70% Tap Water 818 4. Summary CP-46, King Industries’ new water emulsifiable rust preventive concentrate, has many attributes that make it well suited for formulating water-based rust preventives: • Exceptional multi-metal rust protection performance under severe conditions including salt fog, high humidity, and acid atmosphere. • Compatibility with a variety of oils that can then be easily blended with water to make very stable emulsions. • Low foam emulsions leading to versatility of application methods (brush, spray or dip). • Excellent Humidity Cabinet (ASTM D1748) corrosion protection with or without added oil. • Rust protection booster of semi-synthetic and oil soluble metal working fluids. • Easily removable thin films using common alkaline detergents. • VOC solvent free. 24th International Colloquium Tribology - January 2024 59 Production of High VI Base Oils from Full Conversion Hydrocracker Residue with Solvent Refining Dimitrios Karonis 1* , Panorea Kaframani 1 1 National Technical University of Athens, School of Chemical Engineering, Laboratory of Fuels Technology and Lubricants, Athens, Greece * Corresponding author: dkaronis@central.ntua.gr 1. Introduction The production of low sulfur base oils with enhanced properties has gained attention compared to the classic base oils produced from classic solvent refining process of vacuum distillates. In a previous work, low sulfur refinery stock, the residue of a hydrocracker unit was evaluated as feedstock for the production of base oils with classic solvent treatment process. Some of the results were very promising. For this reason, the solvent refining process has been used for the upgrade of an ultra low sulphur content unconverted oil (UCO) from a full conversion fuel producing hydrocracker unit. The treatment used was focused on the removal of aromatics with polar solvent and the improve of pour point by solvent dewaxing. The results showed that the treated products have properties that can be classified as Group II base oils. The properties of the base oils were further improved by the addition of commercial additives (VI improver, pour point depressant). 2. Experimental Section 2.1 Feedstocks The feedstock used in this series of experiments was an ultra low sulfur residue derived from a full conversion severe hydrocracking process, used for transport fuels production; this residue is characterized as the unconverted oil (UCO) of the process. Table 1: Properties of the Feedstock Property UCO Method Density, 15 °C kg/ m 3 863.6 ASTM D4052 Viscosity mm 2 / s ASTM D445 40 °C 38.78 100 °C 6.184 Viscosity Index 104 ASTM D2270 Sulfur mg/ kg 22.1 ASTM D5453 Distillation °C ASTM D1160 IBP 329 10% 403 50% 449 90% 529 FBP 568 Pour Point °C 34 ASTM D97 The main properties of the feedstock are given in Table 1. UCO has a high VI as it is, a very low sulfur content, but high pour point. 2.2 Fractionation UCO was evaluated as received, but also after fractionation. The fractionating distillation employed was the ASTM D2892 under reduced pressure, using a fractionating column with no internals. UCO was separated into 4 fractions. The aim of the distillation was to remove the more volatile part of the UCO, and more precisely the fraction that boils below 400-°C. The cut point temperatures were 450 and 500-°C, in an attempt to produce cuts with different viscosity grades. The heavier part (+500-°C) wase the residue of the distillation. The properties of the distillation fractions from the UCO are given in Table 2. Table 2. Properties of UCO cuts Cut Cut Temp. °C Yield % m/ m v 40 mm 2 / s v 100 mm 2 / s VI I 400 14.3 13.30 3.182 102 II 450 41.7 25.24 4.714 104 III 500 29.8 54.71 8.038 109 IV +500 12.4 166.6 16.34 102 Loss 0.2 Cut I was not evaluated as is very volatile for a base oil (FBP at 400-°C), as well as Cut IV, due to its very high pour point (higher than 45-°C). Cuts II and III are satisfactory, with different viscosity grades and they were treated with solvents in order to improve their properties. 2.3 Solvent Extraction As shown in Tables 1 and 2, the fractionation produced two cuts for evaluation, one with VI similar and one with VI higher than that of the feedstock, but with different viscosity grades. The next step in the processing was to apply the classic solvent extraction process in order to remove aromatic hydrocarbons from the cuts and improve thus further the VI of the cuts. The extraction solvent used was NMP. Solvent/ Feed ratios used were 1.5/ 1, 2/ 0/ 1, and 2.5/ 1 m/ m. The contact time was 30 min, and the extraction process took place at 50-°C. The results of the VI improvement for Cut II and Cut III are shown in Figure 1. There was a significant improvement of VI for Cut II with increasing solvent/ feed ratio, while for Cut- III no significant improvement was noticed. The extraction yields were considerably high. 60 24th International Colloquium Tribology - January 2024 Production of High VI Base Oils from Full Conversion Hydrocracker Residue with Solvent Refining Figure 1: Impact of NMP extraction on VI and yields of the distillation cuts 2.4 Solvent Dewaxing The most common process in a solvent treating base oils production facility for the improvement of the pour point is solvent dewaxing. This process uses a mixture of methyl ethyl ketone (MEK) with toluene, in order to remove the wax that is formed during the cooling of the process feedstock, as the solvent mixture facilitates the separation of paraffins wax from the liquid phase. The improvement of the VI of the two distillation cuts had as result a slight increase of their pour points, which is normal since the extraction yields were very high, so the amount of aromatic compounds that were removed was small. The solvent extraction process while improves the VI of the feedstock, has a disadvantage on the cold flow characteristics of the product. The removal of the aromatic compounds results to the increase of the pour point of the extraction products. For Cut II, the extraction process with NMP increased the pour point from 24 °C to 26 - 27-°C, and for Cut III from 38-°C to 41 - 42-°C. The impact of solvent to feed ratio on the pour point was negligible (within the accuracy of the test method). The most common process to improve the pour point is the solvent dewaxing process, where the feed is mixed with the proper solvent, the mix is cooled, and the paraffins are separated by filtration. Based on results from previous similar experiments, the dewaxing solvent was a 1÷1 (on mass basis) mixture of methyl ethyl ketone (MEK) and toluene). The solvent to feed ratio was 3÷1 (on mass basis). The cooling temperature was -20-°C. The results of the dewaxing process are depicted in Figure 2. As shown in this figure, the pour point of the dewaxed products was significantly reduced, compared to the relevant values of the treated with NMP oil. Regarding the two distillation cuts, the pour point of Cut-II was reduced to values below zero, which is considered as a positive result, but a more intense reduction is required in order to achieve products that can be used as lubricating oil blending components. For Cut III the pour points were higher than 10-°C, at not acceptable level for lubricants. Figure 2: Pour point of extracts and dewaxed cuts 2.5 Pour Point Depressant In order to improve the pour point of the dewaxed oils, a commercial pour point depressant was added (polyalkyl-methacrylate (PAMA) type), at the level of 0.3% (m/ m). The results are shown in Figure 3. It is clear that pour point is reduced at all cases, but the reduction was not at the desired level. The decrease in pour point was higher for Cut II. Figure 3: Impact of PPD addition on pour point 3. Conclusions The upgrade of a low sulfur full range hydrocracker UCO with solvent refining was evaluated. The results showed that the production of high VI base oils, close to the requirements of Group II base oils can be achieved. The main problem of the base oils is their high pour points, a characteristic that must be further improved in order to produce acceptable quality products. References [1] T. R. Lynch: “Process Chemistry of Lubricant Base Stocks”, CRC Press, 2008, ISBN 978-0-8493-3849-6. [2] R. M. Mortier, M. F. Fox, S. T. Orszulik (Editors): “Chemistry and Technology of Lubricants”, 3 rd Edition, Springer, 2010, ISBN 978-1-4020-8661-8. 24th International Colloquium Tribology - January 2024 61 Base Oil Solvency and High Temperature Deposit Formation in Gas Engine Oils - a Model Study - Thomas Norrby 1* , Marcella Frauscher 2 , Christoph Schneidhofer 3 , Frans Nowotny-Farkas 4 1 Nynas AB, MS Technical Support & Development, Nynashamn, Sweden, 2 AC2T, Lubricants & Interface Mechanisms, Wiener Neustadt, Austria 3 AC2T, Oil Condition Sensors, Wiener Neustadt, Austria 4 LubEx Consulting, Schwechat, Austria * Corresponding author: E-mail thomas.norrby@nynas.com 1. Introduction The ever-growing demand for electric power results in the need to further improve engine efficiencies by increasing the power density of stationary gas engines. Thus, newer generations of gas engines feature an innovative design, e.g., including steel pistons, and operate at higher BMEP (Break Mean Effective Pressure). Due to the resulting higher operating temperatures, gas engine oils are subjected to higher stress, leading to increased deposit formation and more intense thermo-oxidative degradation. Both factors, among others, promote undesirable consequences like shorter oil change intervals, more engine downtime, and even engine failure [1, 2]. Recent trends in engine oil formulation, i.e., the use of highly refined paraffinic base oils also in gas engine oil formulation have not always been successful, if care is not taken to ensure sufficient solvency of the base oil blends employed. This study focuses on investigating the tendency of various blends of paraffinic and naphthenic base oils to form high-temperature deposits using a novel test rig design that we have called the „oil chute“ laboratory test for deposit formation tendency [3]. The laboratory investigations give clear indications on how base oil solvency, as described by standard test methods e.g., Aniline Point (ASTM D 611), correlated to deposit formation when the oil passes over hot metal. 1.1 Experimental methods 1.1.1 The Oil Chute The Oil Chute setup, Figure 1, circulates oil through two temperature zones, where deposit formation is triggered in a heated aluminium metal U-shaped profile, the “Oil Chute”. A pump circulates the oil which flows under gravity down the slope of the U-profile. The mass of deposits formed was determined gravimetrically. The detailed test parameters are given in the following: • U-channel temperature: 300-°C • Water cooling temperature: 40-°C • Sample amount: 85 g • Oil flow rate: ~ 7 mL/ min • Test duration: 21 h The temperatures of the hot and cold zone were chosen following the real-life conditions in a gas engine. Figure 1: Oil chute setup Typical sump and hot zone temperatures of gas engines with aluminium pistons are in the range of around 85-°C and 260- 275-°C, respectively. Modern gas engines with steel pistons reach temperatures of approximately 95-°C in the sump and about 265-290-°C in the upper piston ring area. 1.1.2 Oil test matrix A selection of base oil properties is given in Table 1. P-1 is a Group I paraffinic base oil; P2and P-3 are Group II; P-4 is a Group III; and N 1 and N-2 are Group V Naphthenic base oils. Note the very significant differences is VI, and solvency as represented by the Aniline Point. Table 1: Base oils used for model blends Designation KV 40 (cSt) ASTM D 7042 VI (-) ASTM D 7042 Aniline Point (°C) ASTM D 611 P-1 340 91 118 P-2 42.2 107 116 P-3 104.1 106 127 P-4 48.2 135 129 N-1 358 40 98 N-2 607 -20 92 In Tabe 2, a selection from the full test matrix is reported. Note that pure Paraffinic blends, or Naphthenic/ Paraffinic 62 24th International Colloquium Tribology - January 2024 Base Oil Solvency and High Temperature Deposit Formation in Gas Engine Oils - a Model Study - bends, and varying treat rates of the gas engine oil additive package (GEO-AP) have been utilized for the model gas engine oil candidates. Table 2: Base oil and additive selection for GEOs 1.1.3 Thermo-oxidative stability Utilizing the Oil Chute, the thermo-oxidative stability of the oil blends was examined using an artificial alteration method. Oxidation stability (acidity, viscosity change) varies, with the more highly paraffinic blend having an edge, but where the paraffinic/ naphthenic blends still show acceptable behaviour. A correlation was found between solvency, or oxidation stability, and the resulting deposit formation at end of test. Regarding high-temperature deposit formation, Figure 2 shows photos from above of the oil chute profiles at end of test. The oil blends containing naphthenic components (GEO Exp 11 & 12) yielded very low deposit levels and, thus, represent a significant improvement compared with purely paraffinic oil blends (GEO Exp 5). Moreover, their deposit-formation tendency accords with that of highly rated reference gas engine oil (GEO REF 7), meaning that they offer an advantage over many conventional gas engine oils. Figure 2: Photos showing extent and nature of deposits formed at end of test. 2. Conclusion Thus, adding selected high viscosity, high solvency naphthenic base oils to a formulation offers a direct route to gas engine oils with improved technical performance. It is also cost-effective in terms of addressing the cleanliness and deposit formation issues via base oil selection compared with using higher additive pack treat rates or boosters. However, the model blends comprising a naphthenic base oil exhibits somewhat faster degradation under thermo-oxidative stress. Consequently, achieving an optimal balance between paraffinic and naphthenic base oil components will enable the formulation of gas engine oils that possess the necessary properties to withstand the elevated stress levels experienced in modern gas engines. Funding: This research was funded by the “Austrian COM- ET-Program” (project InTribology1, no.-872176) via the Austrian Research Promotion Agency (FFG) and the federal states of Niederösterreichand Vorarlberg. References [1] Garcia, L.; Reher, J. Latest Generation of High Performance Gas Engine Oils - Tackling Reliability Challenges and Extending Oil Life in Modern Highly Efficient Gas Engines. In Proceedings of the CIMAC Congress; Vancouver, Canada, June 2019. [2] Hughes, J. Development of a New Lubricating Oil for Use in Modern High Efficiency Gas Engines. In Proceedings of the CIMAC Congress; Vancouver, Canada, June 2019. [3] Ronai, B.; Schneidhofer, C.; Novotny-Farkas, F.; Norrby, T.; Li, J.; Pichler, J.; Frauscher, M. ‘Assessing the High-Temperature Deposit Formation of Paraffinic and Naphthenic Oil Blends Using the Oil Chute Method‘. Lubricants 2022, 10, 327. https: / / doi.org/ 10.3390/ lubricants10120327 24th International Colloquium Tribology - January 2024 63 An investigation of Using Ultra-low Viscous Naphthenic Oil in Lubes and Greases Jinxia Li * , Mehdi Fathi-Najafi, Thomas Norrby Nynas AB, Raffinadervagen 21, SE-14982 Nynäshamn, Sweden * Jinxia.li@nynas.com 1. Introduction Ultra-low viscosity naphthenic oil (ULVN) has some well-appreciated properties such as excellent solvency and superb fluidity at low temperature. However, due to a relatively low flash point, it is more recommended to be used for the low temperature applications such as hydraulic fluids that are targeting low to artic conditions, sometimes in combination with other type of oils. The aim of this study was to investigate the performance of the ultra-low viscosity naphthenic oil in three different types of applications namely, I) Metal Working Fluid, MWF, II) Hydraulic Fluids, HF, and III) Lubricating Greases, LG. 2. Experiment part Three commercial ULVNs have been used in this study: Base Oil nr.1 (BO1), Base Oil nr.-2 (BO2), and Base Oil nr.-3 (BO3). Some of the typical characteristics are shown in table 1. Table 1: Basic characteristics of the three ULVNS Specification- BO1 BO2 BO3 Viscosity 40-°C, (mm 2 / s) 2.8 2.9 3.7 Viscosity 100-°C, (mm 2 / s) n/ a 1.2 1.3 Flash Point PM, °C 84 105 104 Aniline Point, °C 66 72 68 Pour Point, °C <-70 <-70 <-70 It is well known that one of the most critical properties of a hydraulic oil is to have good to excellent viscosity index (VI) which cannot be fulfilled by the ULVN’s due to low VI. Hence, the use of viscosity index improver was necessary. In total, three hydraulic fluid models were formulated in which the same VI improver and additive package were used. For Metal Working Fluids, two different types of concentrates for water-based fluids have been prepared based on BO1: Emulsifier package Span80/ Tween80 for Conventional soluble oils concentrate and Semi-synthetic Model A. The emulsions were formulation by 5 (wt%) concentrate and 95 (wt%) distilled water, and the particle sizes have been investigated by Mastersizer 3000E at Day 0, Day 1 and Day 7. In the case of lubricating greases, BO2 has been used to investigate the possibilities of incorporating ULVN in the formulation of lithium greases. Due to the low flash point, BO2 has been used as cooling oil while one heavy naphthenic base oil has been used as cooking oil. In addition to the above experiment, BO2 in the grease formulation was replaced by a low viscous API group II oil. The purpose of this part was a comparative study between the two types of low viscous oils in grease application. 3. Results and discussion 3.1 ULVN in Hydraulic fluid The final compositions and basic properties of the HF Iso VG 15 (HF15) are shown in table 2. Table 2: formulations and some properties of three hydraulic fluids Specification Base oil (wt%) VI I (wt%) Additive package (wt%) Viscosity 40-°C, mm 2 / s) VI BO1_HF15 83.2 15.9 0.85 14.4 406 BO2_HF15 83.1 16.0 0.85 14.8 406 BO3_HF15 86.1 13.0 0.85 15.0 348 For the Iso VG15, the Swedish Standard 15 54 34 [1] has a low temperature limit of 1600 (mm 2 / s) at -30-°C, which means all three ULVN (BO1, BO2 and BO3) could meet this standard requirement, shown in Figure1. However, according to the requirement of the British Defense Standard 91-48 [2], the low temperature kinematic viscosity limits of 500 (mm 2 / s) at -40-°C, and 3000 (mm 2 / s) at -54-°C are indicated in Figure 1 by the red horizontal lines. As shown, only BO1 and BO2 do meet this requirement. The two formulated Hydraulic fluids BO1_HF15 and BO2_ HF15, which meet both Swedish standard and British Defense Standard, have been proceeded with some essential physical chemical properties of hydraulic fluids. The results demonstrate low foam, fast air release and good demulsibility. Figure 1: Kinematic viscosity of Hydraulic fluids at lower temperatures. 64 24th International Colloquium Tribology - January 2024 An investigation of Using Ultra-low Viscous Naphthenic Oil in Lubes and Greases 3.2 ULVN in Emulsions BO1 has been selected to study of the stability of Emulsions. The results are shown in Figure 2. For Span80/ Tween 80 system, the Dx (50) of the emulsion shows ‘U’ shape from HLB 11 to HLB 15, gives the minimum value at HLB 13 and keeps stable from day 1 to day 7. For semi-synthetic model fluid, there was warning about the data quality, which can be explained by the smaller particles less than 10 nano meter in the semi-synthetic emulsion which cannot be detected by Mastersizer with red laser. Figure 2: Median droplet Size (Dx (50)) for Span/ Tween system and Model A 3.3 ULVN in Lubricating Greases The final viscosity of the high viscous naphthenic oil and BO2 (alternatively with the Low viscous API Group II) is determined to be about 107 (mm 2 / s) which is in the same range of a straight naphthenic base oil called T110 (which is recognised by the grease industry an excellent oil for the preparation of e.g., lubricating greases). The results of the grease shown in Table 3. Table 3: some characteristics of the conventional lithium greases based on 107 mm2/ s in base oil viscosity Properties; Method Grease A Grease B Grease C Base oil - type NSP + BO2 NSP + Gr.II T110 Thickener - type Li Li Li Thickener Content; wt% 9.0 8.7 7.6 Dropping Point; °C; IP 396-2 194 199 196 Pen (after 60 str.); mm -1 ; ASTM D217 271 268 269 Pen (after 105 str.); mm -1 ; ASTM D217 309 312 295 Diff after 105 str.; mm -1 +38 +44 -+26 Roll stab.; mm -1 ; ASTM 1831 +16 +18 +14 Oil separation; wt%; IP121 5.3 6.1 4.9 Flow pressure -20-°C; hPa; DIN51805 345 320 220 Flow pressure -25-°C; hPa; DIN51805 495 345 420 Flow pressure -30-°C; hPa; DIN51805 645 570 N/ A Flow pressure -35-°C; hPa; DIN51805 945 820 N/ A From table 3, by using blends of heavy naphthenic base oil and BO2 or API Group II, more thickener is required comparing with grease produced using T110. The shear stability of the greases remains almost the same despite of the differences in the thickener content. The oil separation increases for the greases using BO2 and API Group II, compared with Grease C which is based on T110. 4. Conclusions This multitasks study has demonstrated that - Hydraulic fluids based on ULVN, can meet different standards specially with regards to the low temperature properties; both pour point and viscosities at low temperatures. - MWF, concentrates for water-based fluids formulated with ULVN results to stable emulsion with conventional soluble oils formulation as well as with semi-synthetic formulation. - Lubricating Greases, ULVN can be used in conventional lithium greases as cooling oil. The greases show slightly better properties at temperatures below -20-°C. However, this grease model doesn’t show any significant drawback when compared with the T 110 based grease model. In summary, the outcomes of this study suggest that Ultra Low viscous Naphthenic Oil can also be used successfully in the various types of formulations with added values such as in MWF, Hydraulics Fluids and Lubricating Greases. References [1] Swedish Standard SS 15 54 34: 2015, “Hydraulic Fluids - Technical requirements, environmental properties and test methods”. [2] British Defense Standard 91-48/ 2. 24th International Colloquium Tribology - January 2024 65 Tunable Viscosity of PAG and its Application in Sheet Metal Forming Dominic Linsler 1,2* , Korhan Celikbilek 1,2 , Stefan Reinicke 1,3 , Bernd Aha 4 1 Fraunhofer CPM, Freiburg, Germany 2 Fraunhofer IWM MikrotribologieCentrum µTC, Freiburg, Germany 3 Fraunhofer IAP, Potsdam, Germany 4 Zeller+Gmelin GmbH & Co KG * Corresponding author: dominic.linsler@iwm.fraunhofer.de Abstract Sheet metal forming by deep drawing is one of the most important forming processes in industry, which is mainly used in large-scale production because of the low running costs. Due to higher demands on formed products and smaller batch sizes, controlled process management is becoming increasingly important. This controlled process management can be significantly improved by actively influencing the friction between the sheet metal and the tool, because the forming result depends on the material flow in several respects (folds, crack formation, springback). State-of-the-art is the local application of lubricant on the metal sheet before the forming process. Still, there are other ways of controlling friction between the metal sheet and the tool. Actively controlling the viscosity of the lubricant in the contact is a feasible approach that has already been demonstrated via UV-triggered anthracene dimerization in respectively functionalized lubricants [1]. The characterization of the tunability of the viscosity of different PAG and fatty acids is carried out in-situ in a rheometer. The effectiveness of the viscosity change on the drawing forces could be demonstrated in the Erichsen cupping test and strip drawing tests. With the results, the development of a higher technology readiness level for the application in sheet metal forming is promising. References [1] C. Gäbert et al.: ACS Appl. Polym. Mater. 2020, 2, 12, 5460-5468 doi: 10.1021/ acsapm.0c00794. 24th International Colloquium Tribology - January 2024 67 Surfactant Systems with Improved Lubricity for Water Miscible Cooling Lubricants Ludger Bösing 1 und Arjan Gelissen 1 1 Sasol Germany GmbH, Marl, Germany 1. Introduction Surfactant lubrication systems with thickening properties at higher temperatures and improved lubrication were investigated. The study focused on observed thickening phenomenon in nonionic surfactant-based lubricants. Experimental analysis reveals temperature-induced structural changes in the nonionic surfactant molecules, forming a gel-like network within the lubricant. This network enhances viscosity and load-carrying capacity. Comparative tests demonstrate superior lubrication properties of the nonionic surfactant system, including reduced friction, wear, and improved film formation at higher temperatures. The three-dimensional network provides boundary lubrication and prevents metal-to-metal contact, while increased viscosity ensures effective surface separation. This research contributes to the understanding of nonionic surfactant-based lubricants, facilitating the development of tailored rheological properties for high-temperature applications, improving machinery efficiency and durability. 2. Conclusion It has been found that surfactants, when aggregating in their lamellar liquid crystalline form (L α ), provide good lubrication of surfaces. The conditions for obtaining lamellar liquid crystalline phases at surfaces depends besides the surfactant molecular structure itself mainly on the concentration and temperature. These dependencies are depicted in binary phase diagrams. Alcohol ethoxylates are typically in their L α -form at elevated concentrations of 50 to 70 wt% (Picture No. 1). The target to achieve a maximum area of lamellar phases (lubricating areas) was investigated on different nonionic surfactant systems. To be more effective, focus was placed on lower application concentrations. One phase behaviour is shown in picture No. 2. Picture 1: Example phase behaviour of a standard nonionic surfactant Picture 2: Phase behaviour of a standard nonionic alkoylate References [1] Kronberg B. How to Design a Surfactant System for Lubrication. In: Biresaw G, Mittal KL, editors. Surfactants in Tribology. Boca Raton, FL, USA: CRC Press; 2008. P.327-329 [2] Carrion F-J, Martinez-Nicolas G, Iglesias P, Sanes J, Bermudez M-D. Liquid Chrystals in Tribology, International Journal of Molecular Sciences. 2009 24th International Colloquium Tribology - January 2024 69 Formulating Next Generation Multi-Metal Wire Drawing Fluids with Multifunctional Amino Alcohols Denis Buffiere 1* , Kathleen Havelka 2) , Amelie Bretonnet 1) 1 Advancion Corporation, Paris, France 2 Advancion Corporation, Buffalo Grove, Illinois USA * Corresponding author: dbuffiere@advancionsciences.com 1. Introduction Vehicle electrification and the growing need for energy have significantly increased the demand for improved copper and aluminum wire drawing fluids. Wire drawing is a difficult operation requiring fluid formulations that provide several important characteristics. This presentation highlights the use of multifunctional amino alcohols with other specialty chemistries in wire drawing fluid formulations to improve fluid performance and longevity, enabling formulators to develop high-performing fluids with an improved sustainability profile. This study demonstrates the use of specialty amino alcohol chemistries in wire drawing fluid formulations to provide excellent lubricity, good cleanliness, low foaming, and extended fluid life. The unique multifunctionality of amino alcohols offers new potential in developing next generation wire drawing fluids that can produce defect-free wires while minimizing operating costs and reducing waste. 2. Experimental 2.1 Raw Materials The following raw materials were evaluated in this study: 2.1.1 Amines N-Methyldiethanolamine, 99% (MDEA), Dow Chemical Tris(2-hydroxyethyl)amine, 99% (TEA), Dow Chemical 3-amino-4-octanol, 85%, with 15% water (3A4O), Advancion Corporation 2.1.2 Esters Trimethylolpropane trioleate ester (TMPTO) Trimethylolpropane C8-C10 triester (TMP C8-C10) Polymer ester 2.2 Tests Performed 2.2.1 Aluminum and Copper Staining Aluminum 2017, aluminum 6082 and copper were freshly abraded using coarse grit followed by fine grit sandpaper to remove the oxidized layer. The freshly abraded coupons were then placed into metalworking fluids freshly diluted to 5% in soft water (~40 ppm hardness). The fluids containing the freshly abraded coupons were then placed at 20-°C for 72 hours. After 72 hours, the coupons were removed and evaluated for staining. 2.2.2 Lubrication Test The lubrication tests on aluminum and copper were carried out with a Reichert machine. A test cylinder is fixed and then placed under load upon a rotating ring wheel which is immersed to its lower third in the freshly diluted fluid. The wheel turns over a distance of 100 meters. The wear scar is noted for each try: the smaller the wear scar, the better the lubrication power. 2.2.3 Fluid Stability in Presence of Copper Diluted laboratory formulations were placed at 60-°C in presence of copper powder for several weeks to assess the fluid stability and soap formation. Fluid appearance and deposit were evaluated every week. 2.2.4 Microbiological Challenge Test Diluted laboratory formulations were subjected to a challenge of mixed field isolates (10 6 colony forming units (CFU)/ mL bacteria and 10 5 CFU/ mL fungi). Solutions were shaken for five days, stopped for two days and repeated until failure (>10 5 CFU/ mL bacteria or >10 3 CFU/ mL fungi for two consecutive weeks). 3. Results & Discussion Both aluminum staining (figure 1) and lubrication (figure-2) performances were enhanced with the addition of 3A4O. Figure 1: Aluminum staining test results Figure 2: Lubrication results on aluminum 70 24th International Colloquium Tribology - January 2024 Formulating Next Generation Multi-Metal Wire Drawing Fluids with Multifunctional Amino Alcohols 3A4O helps to maintain excellent emulsion stability with copper powder over time. Figure 3: Fluid stability in presence of copper 3A4O blended with MDEA improved pH stability of low-purity TMPTO ester and increased biostability. Figure 4: Microbial challenge test 4. Conclusion 3-amino-4-octanol shows strong versatility and benefits in multi-metal, semi-synthetic wire drawing fluids. 3A4Os structural characteristics, e.g., amine with branching and linear eight carbon chain provides excellent neutralization, lubricity, compatibility, and fluid life. Performance can be further optimized through more research and collaboration with formulators. 24th International Colloquium Tribology - January 2024 71 Antioxidative Action and Tribological Performance of CuDTP as a Potential Additive for Hydraulic Fluids N. Ayame 1* , K. Yagishita 1 , T. Oshio 1 1 ENEOS Corporation, Yokohama, Japan * Corresponding author: ayame.noriko@eneos.com 1. Introduction In order to contribute to the achievement of the United Nations’ Sustainable Development Goals (SDGs), the reduction of environmental impact and the prevention of environmental pollution are being considered in various industrial sectors. Lubricants are also expected to provide solutions to these challenges, and environmentally friendly lubricants have been developed. For example, efforts in hydraulic fluids are shown in Figure 1. Hydraulic fluids are lubricants used in hydraulic systems such as hydraulic excavators, which are responsible for preventing wear in hydraulic equipment and for transferring the hydraulic energy generated in a hydraulic pump to the individual components such as a drive motor and cylinder. Figure 1: Efforts to reduce the environmental impact with hydraulic fluids For CO 2 reduction, very high viscosity index fluids, which present small change of viscosity with temperature, are effective for improving fuel efficiency. Using such fluids allow to keep sufficiently low viscosity at low temperature and sufficiently high viscosity at high temperature in order to obtain the best compromise in terms of lubrication capability and fuel economy. For preventing environmental pollution, lubricant base oils with excellent biodegradability are used for hydraulic fluids. For waste oil reduction, antioxidant technologies are important for improving oxidation stabilities of hydraulic fluids. From these considerations, the most extensive studies have been antioxidants, which lead to reduce waste oil. Zinc dialkyl dithiophosphate (ZnDTP) has been used as a useful additive in various lubricants because of its multifunctional characteristics providing antioxidative, antiwear, and extreme pressure properties, together with its availability at low cost. For this reason, ZnDTP has long been utilized as an important additive of hydraulic fluids (HFs), and thus HFs with ZnDTP (Zn HFs) have been the mainstream formulation in practical hydraulic applications. On the other hand, recent hydraulic systems have required the operation under severer conditions with increasing pressure and also with decreasing oil volumes, resulting in higher oxidation and heat loads on HFs. It is known that ZnDTP decomposes by oxidation and heat, this eventually causes insoluble sludge in fluids, and it makes trouble such as filter blockage and hydraulic pressure failure. Under the circumstances, HFs without ZnDTP tend to be used in hydraulic systems. But they are less cost competitive than Zn HFs because they need several ashless additives to meet demanded performance. So, Zn HFs still account for the majority of the market. 2. Results As an alternative to ZnDTP, this study investigated the potential of copper dialkyl dithiophosphate (CuDTP) shown in Figure 2. CuDTP was reported not only to have antiwear characteristics equivalent to those of ZnDTP but also to deactivate ROO • and ROOH, the active species in oxidative degradation. The tribological properties of CuDTP were evaluated using the FZG test, four-ball test, and V104C pump test, and the evaluation results demonstrated HFs with CuDTP (Cu HFs) had the performance equivalent or superior to that of Zn HFs. Further, the A2F pump test showed that the oxidation stability of Cu HFs was more than twice as much as that of Zn HFs. This fact suggests that CuDTP, compared with ZnDTP, can achieve the satisfactory antioxidative action with a smaller addition amount, resulting in lower cost and less sludge. Figure 2: Structure of CuDTP 24th International Colloquium Tribology - January 2024 73 Boundary Lubricant Additive Responses on Steel, Aluminum and Copper Using Twist Compression Tests (TCT) for Multimetal Lubricant Formulation Ted G. McClure 1 , Alexes Morgan 1 1 Sea-Land Chemical Co./ SLC Testing Services, Cleveland Ohio, USA 1. Introduction Materials and manufacturing processes continue to evolve quickly in response to changing industry requirements. Environmental pressures and sustainability are driving rapid vehicle electrification and lightweighting initiatives. This necessitates changes in the way vehicles are manufactured, the lubricants used in production, and fluids used in operation. Electric current and heat management are important considerations, which along with lightweighting, has led to an increase in aluminum and copper content of vehicles. [1][2] Lubricant additive availability is being altered by supplier consolidations, local regulations, global registration requirements, sustainability considerations, and supply chain disruptions. A trend towards multi-metal metalworking fluids has been driven by a desire by users for improved lubricant inventory management, efficient use of manufacturing assets, and an increase in multi-metal components being machined. [3] 1.1 Twist Compression Test (TCT) The Twist Compression Test (TCT) is a bench test that creates boundary conditions and lubricant starvation under high pressure and sliding contact; conditions leading to failures in many applications. It is used to evaluate the boundary lubrication performance of lubricants, including galling resistance of material couples. Figure 1: Twist Compression Test Schematic 2. Experimental TCT was used to evaluate the responses of boundary additives on AA5182-0 aluminum, 110-H02 copper, and AISI 1008 steel, when tested with D2 tool steel, according to ASTM G223-23 [4]. Boundary additive categories evaluated include conventional EP additives, esters, fatty acids, and amines. Specimens were cleaned with odourless mineral spirits, followed by n-heptane. An excess of lubricant was applied with disposable pipettes immediately before testing. All tests included three repeats. Data was collected at 50Hz. The TCT responses used in this analysis were the time until lubricant film breakdown (TBD), and average coefficient of friction (COF). TBD is used to rank lubricants in their ability to survive the test conditions and prevent adhesive wear and galling. Average COF ranks lubricants by friction level during the test. 2.1 Twist Compression Test Conditions Flat Specimens: AISI 1008 Cold Rolled Steel Tensile strength: 303MPa (44,000psi) AA5182-0 Tensile strength: 281MPa (40,756psi) C110 H02 Copper Sheet (99.9% Cu) Tensile strength: 260MPa (37,700psi) Annular Specimen: D2 Tool Steel, Approximately 62 Rc Hardness Lapped contact surface Interface Pressure: Cu: 48.3MPa (7ksi), AA: 20.7MPa (3ksi), 1008 Steel: 103.4MPa (15ksi) Speed: 10rpm (1.2cm/ sec) Dynamic tests (rotation before contact) run to failure: three repeats per test. 2.2 Test Lubricants Boundary additive categories evaluated include conventional EP additives, esters, fatty acids, and amines. Table 1 lists the additives and concentrations in naphthenic base oils that were tested. The viscosity of all lubricants was controlled near 38cSt at 40-°C by base oil blending. 74 24th International Colloquium Tribology - January 2024 Boundary Lubricant Additive Responses on Steel, Aluminum and Copper Using Twist Compression Tests (TCT) for Multi-metal Lubricant Formulation Table 1: Test Lubricants Code w/ w% Additive N 100.00 Naphthenic base oil blend: 38cSt @ 40°C Esters-Alcohol OAL 20.00 Oleyl alcohol ML 20.00 Methyl lardate EAR 20.00 Aromatic monoester EHS 20.00 2-ethylhexyl stearate GE1 5.00 Glycerol ester VO 20.00 Vegetable oil (22% erucic acid) VOB 20.00 Blown Vegetable oil (22% erucic acid) LO 20.00 Lard Oil EWS LAN 5.00 Lanolin POLY 10.00 Polymeric ester (saturated) Acids-Amines EA50 10.00 Poly fatty acid (AV=50) WGFA 5.00 WGFA(AV=46) TOFA 10.00 Tall oil fatty acid (AV=190) ISA 10.00 Isostearic acid (AV=185) FAH 10.00 Diacid (AV=271) OLAM 10.00 Oleyl amine OLDM 10.00 Oleyl diamine EP-Antiwear CP 10.00 Chlorinated paraffin; MCCP (55%Cl) SI 10.00 Sulfurized olefin (20%S inactive) SA 10.00 Sulfurized olefin (20%S active) P1 5.00 2-EH phosphate ester (AV=320) P2 5.00 C-18 phosphate ester, ethoxylated (AV=150) ZDDP 5.00 ZDDP (secondary) CSA 10.00 OB Calcium sulfonate, amorphous (TBN=400) CSC 10.00 OB Calcium sulfonate, crystalline (TBN=300) 3. Results and Conclusions Figure 2 is a graph of the TBD and average COF of each boundary additive, grouped by category, in the steel tests. Chlorinated paraffin (CP), ethylhexyl stearate (EHS), and tall oil fatty acids (TOFA) resulted in the longest TBD on steel, while the blown vegetable oil (VOB) and diacid (FAH) gave the lowest COF results. Figure 2: AISI 1008 Steel Test Results Summary Figure 3 compares the TBD and average COF for the aluminum tests, grouped by additive category. As a class, esters/ alcohol resulted in longer TBD results than the acids/ amines or EP/ AW additives. The longest TBD were for the long chain unsaturated esters (VO, ML, and LO). Methyl lardate and lard oil also resulted in the lowest average COF with aluminum. Figure 3: AA5182-0 Test Results Summary Figure 4 compares the TBD and average COF for the copper tests, grouped by additive category. The overbased calcium sulfonates resulted in the longest TBD results, while the diacid (FAH) gave the lowest average COF. Figure 4: C110 H02 Test Results Summary References [1] DuckerFrontier, 2020 North American Light Vehicle Aluminum Content and Outlook. [2] International Energy Agency (IEA), The Role of Critical Minerals in Clean Energy Transitions. [3] Canter, N (2022), “Metalworking Fluids: Current Options for Machining Multi-metal Alloys”, TLT March, 2022, pp. 44-56. [4] ASTM G223-23; Standard Test Method for Measuring Friction and Adhesive Wear Properties of Lubricated and Nonlubricated Materials Using the Twist Compression Test (TCT). Book of Standards Vol. 3.02. 24th International Colloquium Tribology - January 2024 75 Effect of Phosphonium Ionic Liquid as Lubricant Additive in Gear Oil against White Etching Areas Formation in Bearing Steel Linto Davis and P. Ramkumar * Advanced Tribology Research Lab (ATRL) Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai, India, 600036 * Corresponding author: ramkumar@iitm.ac.in 1. Introduction Wind energy is a renewable source of energy and a viable alternative to the current restricted supply of fossil-fuel-generated electricity. While there is a strong push to employ wind energy, there is a disadvantage in terms of the expense of maintaining wind turbines, notably the gearboxes. Bearing components frequently fail prematurely owing to contact fatigue due to microstructural decay, resulting in costly repairs and turbine downtime. These premature bearing failures are allied with the formation of white etching areas and white etching cracks in the bearing subsurface [1, 2]. Wind turbine operations are disrupted by premature bearing damage, which adds significant maintenance charges in the form of bearing up-tower replacement. Therefore, it is critical to advance research that will allow the creation of bearing components and lubricants less prone to white etching areas (WEAs)/ white etching cracks (WECs) failure. Bearing manufacturers and tribologists have made significant efforts in recent years to understand the causes of premature bearing failures and overcome this issue. To attain this purpose, several approaches can be adopted. Considering the industrial challenge of optimising the tribological behaviour of material pairs, one of the most promising ways is the production of high-performance gearbox oils with new additives. In the last decade, ionic liquids (ILs) have attracted attention in the tribology community as potential lubricants and lubricants additives for challenging contacts. The researchers have investigated two approaches to the use of ILs in lubrication: as pure lubricants or base stocks, which has the disadvantage of being more expensive than hydrocarbon oils, and as lubri-cant additives , which is a cheaper solution despite the low solubility of ILs in non-polar hydrocarbon oils. This work investigates the performance of phosphonium phosphinate IL additive in poly alpha olefin (PAO) against WEAs formation under dynamic loading and boundary lubrication regime. Further, the performance is compared with the base stock PAO based on the metallographic inspection and diffusible hydrogen measurement. 2. Experimental Methodology 2.1 Test materials The phosphonium cation-based ionic liquid, trihexyltetradecylphosphoniumbis (2,4,4-trimethylpentyl) phosphinate ([P6,6,6,14][(iC8)2PO2] CAS No: 465527-59-7, supplied by Sigma Aldrich is used in this work as lubricant additive in PAO 6. The AISI 52100 bearing balls (diameter 10 mm) and AISI 52100 washers were used as the counter surface. Posttest analysis were carried out on the test worn pin samples. 2.2 Dynamic load PoD tests The WEAs replication tests for the current research were performed in a dynamic load PoD tribometer. The dynamic PoD is created so that the compressive force and severe slippage (± 200%) of the bearing ball sample occur at the point of contact. The schematic diagram of the dynamic load PoD tribometers is shown in Figure 1. The test parameters are shown in Table 1. Figure 1: Schematic diagram of dynamic load PoD tribometer Table 1: Test parameters Parameter Value Maximum contact pressure 2.0 GPa Loading frequency 4.5 Hz Sliding velocity 0.2 m/ s Film parameter (λ) 0.2 Following the dynamic PoD tests, the tested bearing steel ball was sectioned through the wear scar into equal hemispherical halves and then subjected to polishing and nital etching. Further, the samples were analysed using optical microscopy, SEM and EDS for WEAs presence. For the validation of WEAs, microhardness tests were carried out on the WEAs and the surrounding matrix. Consequently, the tests are repeated to measure the amount of diffusible hydrogen ingressed into the samples. The tests were carried out with PAO oil till the formation of WEAs in the bearing steel samples. Afterwards, tests were repeated with optimised PAO+IL as the lubricant. Some of the preliminary results are discussed in the following section. 76 24th International Colloquium Tribology - January 2024 Effect of Phosphonium Ionic Liquid as Lubricant Additive in Gear Oil against White Etching Areas Formation in Bearing Steel 3. Result and Discussion 3.1 Subsurface metallographic analysis Figure 2 (a) shows the subsurface microstructure cut sectioned AISI 52100 steel ball sample in as-received condition. The microstructure consists of uniformly dispersed globular-shaped cementites in the tempered martensite matrix formed by the quenching of steel from the austenitisation temperature. Figure 2: (a) SEM image of the bearing steel subsurface microstructure in as-received condition showing uniformly spread spherically shaped cementites, (b) Test 1 SEM image: irregularly shaped WEA (c) and (d) OM image of Test 1: showing WEA Experiments were conducted using AISI 52100 steel on the dynamic load PoD tribometer until the formation of WEAs. After several trial-and-error tests followed by extensive metallographic analysis, the WEAs formation was observed in AISI 52100 steel pin samples after 13 hours (Test 1), corresponding to 2.1 × 10 5 load cycles. The microstructural analysis of the tested sample showed long irregular WEAs spreads more than 200 μm long, as shown in Figures 2 (b-d) OM and SEM images. The tests were repeated for the same duration with PAO+IL (Test 2) as the lubricant with the same test parameters. However, there was no microstructural alteration observed in the sample subsurface. Wear scar analyses and diffusible hydrogen testing were carried out to comprehend the delay in WEAs formation, while PAO+IL was used as the lubricant. 3.2 Wear scar and diffusible hydrogen analysis The amount of diffusible hydrogen content in the PAO+IL tested sample was less compared to the sample tested with PAO for the same test duration, as shown in Table 2. In addition, there was no formation of WEAs observed in the metallographic analysis. The presence of phosphorus content was observed in the EDS analysis and Raman analysis (not shown), as shown in Figure 3. Table 2: Test results Test No Number of load cycles (in million) Lubricant Diffusible hydrogen content (ppm) Observations 1 0.216 PAO 0.19 WEAs formation 2* 0.216 PAO+IL 0.05 No microstructure alteration * More tests havet to be conducted for the confirmation, and till the formation of WEAs Figure 3: SEM and EDS analysis result showing the presence of phosphorus in the tribofilm. 4. Conclusions The preliminary outcomes of this study are: • Diffusible hydrogen content in the PAO+IL tested sample is seen to be lesser than that of the PAO base stock. • The WEAs formation in the steel subsurface is delayed when PAO+IL is used as the lubricant. References [1] Richardson A D, Evans M H, Wang L, Wood R J K, Ingram M, Meuth B. The Evolution of White Etching Cracks (WECs) in Rolling Contact Fatigue-Tested 100Cr6 Steel. Tribol Lett 2018; 66: 1-23. https: / / doi. org/ 10.1007/ s11249-017-0946-1 [2] Evans M H. White structure flaking (WSF) in wind turbine gearbox bearings: Effects of “butterflies” and white etching cracks (WECs). Mater Sci Technol 2012; 28: 3-22. https: / / doi.org/ 10.1179/ 02670831 1X13135950699254 Coatings, Surface Interactions and Underlying Mechanism 24th International Colloquium Tribology - January 2024 79 Combination of DLC Coatings and Dedicated Lubricants in order to Achieve Supralow Friction in Highly Loaded Sliding Contacts Johnny Dufils 1* , Etienne Macron 1 , Christophe Héau 1 1 Institut de Recherche En Ingénierie des Surfaces (IREIS) ICE-T platform, HEF GROUPE, Andrézieux-Bouthéon, France * Corresponding author: jdufils@hef.group 1. Introduction The on-going mutation of the automotive sector towards more environmentally compatible technologies has also an impact on automotive lubricants. Indeed, the development of new lubricants becomes mandatory to satisfy both growing performance standards and increasingly stringent environmental regulations. Diamond-like carbon coatings have become widespread in the automotive industry. These coatings are used mainly for their resistance to abrasive wear, scuffing and seizure. However, modern engine oils do not take fully advantage of having DLC coated components as oils are mainly designed for uncoated steel components. In the literature, DLC coatings combined with specific lubricants or additives are known to exhibit supralow friction [1][2]. However, most of these results were not taken up industrially probably for multiple reasons. One of these reasons may be a lack of tribological relevance of the tests performed at the laboratory scale. In the following, alternative lubricants to conventional engine oils were combined with DLC coatings and tested in highly loaded sliding contacts compliant with valvetrain applications. 2. Materials & Methods 2.1 Materials & Lubricants Three alternative lubricants, designed as Lub A, Lub B and Lub C, were studied in DLC/ DLC and steel/ DLC contacts. Lub A has lower viscosity than lub B and lub B has lower viscosity than lub C. The tribological performance of these alternative lubricants were compared to three engine oils: a 5W30 grade oil which does not contain friction modifiers, a 0W30 grade oil containing a MoDTC friction modifier and a 0W12 grade oil also containing a MoDTC friction modifier. These lubricants were tested on a ring-on-flat tribometer in three material configurations: • Grinded steel ring (Ra = 0.2 mm)/ DLC coated flat • Polished steel ring (Ra = 0.03 mm)/ DLC coated flat • DLC coated ring (Ra= 0.03 mm)/ DLC coated flat The DLC coatings were all a-C: H coatings. The roughness parameter Ra of the flat sample was 0.02 mm. 2.2 Tribological testing Various tribological tests were performed on the HEF homemade ring-on-flat tribometer. The ring is set in rotation thanks to an electric motor. The ring is 35 mm in diameter and 8 mm in height leading to a line contact of 8 mm in length when the ring is rubbed against the flat sample. The contact maximum Hertzian pressure is set to 500 MPa. A key feature of the tribometer is the contact pressure being kept constant during the test even if there is wear on the flat sample. This is achieved thanks to a slow reciprocating motion of the flat sample. The sliding speed is varied between 0.6 and 2.7 m/ s. The loading and sliding conditions are typical of that of valvetrain components. The tests were performed at various lubricant temperatures between room temperature and 100-°C. Lub A was also tested in a motored valvetrain with a DLC coated camshaft and DLC coated cam tappets. The 0W30 grade oil was used for comparison. 3. Results & Discussion 3.1 Ring-on-flat testing Figure 1 shows the evolution of the coefficient of friction as a function of the sliding speed for the three alternative lubricants in a DLC/ DLC contact. Coefficients of friction of 0.005 are achieved for the three alternative lubricants but the speed at which supralow friction is achieved, depends on the lubricant. The viscosity of Lub C was optimized in order to get supralow friction in the whole range of speed investigated here. In these tribological conditions, conventional engine oils show coefficients of friction of 0.01 at best (see Figure 3(a)). Figure 2 shows the evolution of the coefficient of friction as a function of the sliding speed for Lub B in the three material configurations tested. It is shown that it is not necessary to have both surfaces coated with DLC in order to get supralow friction. Indeed, the polished steel/ DLC contact with Lub B shows coefficients of friction between 0.01 and 0.006. The friction reduction obtained with Lub B compared to conventional engine oils is even higher in a polished steel/ DLC contact than that obtained in a DLC/ DLC contact (see Figure 3(b)). However, increasing the roughness of the ring leads to higher friction coefficients due to contacting asperities. Figure 1: Evolution of the coefficient of friction as a function of the sliding speed for the three alternative lubricants in a DLC/ DLC contact tested 80 24th International Colloquium Tribology - January 2024 Combination of DLC Coatings and Dedicated Lubricants in order to Achieve Supralow Friction in Highly Loaded Sliding Contacts Figure 2: Evolution of the coefficient of friction as a function of the sliding speed for Lub B in the three material configurations tested 3.2 Tribological performance assessment on a motored valvetrain Figure 4 presents the evolution of the power dissipated by friction as a function of the camshaft rotation speed on the motored valvetrain for three configurations. The addition of a DLC coating on the camshaft led to a relatively small decrease in the power dissipated by friction (approx. 5W) with the 0W30 grade oil. Replacing the 0W30 grade oil by lubricant A and adding a DLC coating on the camshaft led to a large reduction of the power dissipated by friction (up to 70%). Figure 3a: Evolution of the coefficient of friction as a function of the sliding speed for Lub B and conventional engine oils in (a) a DLC/ DLC contact Figure 3b: Evolution of the coefficient of friction as a function of the sliding speed for Lub B and conventional engine oils in a polished steel/ DLC contact Figure 4: Evolution of the power dissipated by friction as a function of the camshaft rotation speed on the motored valvetrain for the 3 tested configurations 4. Conclusions The tests performed on the ring-on-flat tribometer showed that it is possible to strongly reduce the coefficients of friction of highly loaded sliding contacts by using DLC coatings with alternative lubricants compared to conventional lubricants. The excellent tribometer results are here validated on a valvetrain test bench with real components. These results illustrate that the use of DLC coatings may pave the way to alternative lubricants to conventional oils. References [1] Bouchet, M.I.D.B. et al. Scientific Reports 7 (2017). [2] Björling, M. et al. Tribology Letters 67: 23 (2019). 24th International Colloquium Tribology - January 2024 81 Numerical and Experimental Analysis of the Tribological Performance of a DLC-Coated Piston Ring-Cylinder Liner Contact Thomas Lubrecht 1,2 , Nans Biboulet 1 , Antonius A. Lubrecht 1 , Johnny Dufils 2* 1 Univ Lyon, INSA-Lyon, CNRS UMR5259, LaMCoS, Villeurbanne, France. 2 Institut de Recherche En Ingénierie des Surfaces (IREIS) ICE-T platform, HEF GROUPE, Andrézieux-Bouthéon, France * Corresponding author: jdufils@hef.group 1. Introduction According to the “Fit for 55” European package, greenhouse gas (GHG) emissions should be reduced by 55 % in 2030 compared to 1990. In particular, CO 2 emissions related to transportation should be drastically reduced. In this respect, electric battery vehicles seem to emerge as a reference solution for passenger cars however it is poorly adapted for heavy duty transportation. Internal combustion engines, either hybridized or H 2 fueled, will seemingly play a role in the future of transportation. Therefore, the quest for reducing friction within internal combustion engines is still meaningful for efficient transportation. Surface coatings, such as Diamond-Like Carbon (DLC), may help improving the efficiency, reliability and sustainability of future internal combustion engines since they present excellent tribological properties. The application of DLC coatings to all parts of the crucial Piston-Ring/ Cylinder-Liner (PRCL) contact has not been widely studied yet. In this work, both experimental and numerical methods were developed in order to analyze the tribological performance of the DLC coated PRCL contact. 2. Materials and methods 2.1 Semi-analytical model The PRCL contact operates mainly in mixed and hydrodynamic lubrication regimes. A semi-analytical, time-dependent, line-contact, mixed lubrication solver was developed. The fluid and asperity load carrying capacities are computed separately (Figure 1). The full film lubrication model is based on a fast and simple line-contact, time-dependent, semi-analytical solver developed by Biboulet et al. [1]. The asperity contact part of the model is based on a “load-distance” curves computed from measured surface topographies. The load-distance curves are obtained from a numerical tool developed by Sainsot [2]. Contrary to the regular stochastic theories, this method relies on deterministic contact mechanics results using measured topographies. Figure 1: Illustration of the asperity and hydrodynamic load carrying capacities in the mixed lubrication modelling of the piston ring/ cylinder liner contact The solver allows for a rapid prediction of the contact friction forces accounting for oil starvation (geometrical or by lack of lubricant) as well as oil transport. The solver was validated using fully numerical solutions and measurements obtained on a line contact reciprocating tribometer. To overcome the model limitations related to the sample macro-geometry defects, analytical correction coefficients evaluated using the real geometry were introduced. 2.2 Experimental methods In order to validate the semi-analytical model, friction measurements were performed on a linear reciprocating tribometer in a cylinder on plane configuration. Both the cylinder and the plane were DLC coated (a-C: H). The upper specimen is a 20-mm long roller with a diameter of 14 mm which is fixed. The cylinders underwent an additional machining to mimic the limited geometry of a piston ring. A 24N normal load was applied by deadweights leading a maximum Hertzian pressure of 82 MPa. The lower specimen is mounted into a lubricant reservoir, which is set in reciprocating motion by means of a crank/ conrod mechanism driven by an electric motor. The mean sliding speed was varied between 5 mm/ s and 100 mm/ s. In order to be close to the PRCL operating conditions and because the sliding speed was low, a 10W-60 grade oil with a high viscosity at room temperature was selected. 82 24th International Colloquium Tribology - January 2024 Numerical and Experimental Analysis of the Tribological Performance of a DLC-Coated Piston Ring-Cylinder Liner Contact Additional experiments were performed on a PRCL testbench using real engine parts. Piston rings mounted onto a piston are set in reciprocating motion inside a full cylinder liner thanks to a crank/ conrod system powered by an electric motor. The piston underwent an additional machining in order to remove the skirt and only measure the piston rings/ cylinder liner friction. The liner is assembled into a liner holder itself mounted onto a piezoelectric force sensor centered onto the piston axis in order to measure the piston rings/ cylinder liner friction force. The PRCL contact is lubricated on the inner surface of the liner by means of a nozzle located at the bottom end of the liner. The experiments were carried out at low rotational speed (from 100 to 400 RPM) with a 10W-60 grade oil at 50-°C in order to mimic the piston ring lubrication at higher speeds and higher oil temperatures. The experiments were performed with only one piston ring (a compression ring) and with and without DLC coatings on the ring and the liner. 3. Results and discussion 3.1 Validation of the semi-analytical model The semi-analytical model was able to efficiently predict the friction force both in mixed and hydrodynamic lubrication regimes. Figure 2(b) shows a comparison of the friction force predicted by the model and the friction force experimetal measurement. At high speeds, the transient term induces a damping effect of the lubricant which generates load carrying capacity and thus a non zero film thickness (squeeze effect). Friction and film thickness predictions with and without the correction coefficients are distinctly different. Systematically, a lower film thickness (Figure 2(a)) is predicted for the real geometry, which is consistent with the coefficient theory. 3.2 Effect of DLC on the PRCL contact and comparison with the model In the experiments performed on the PRCL testrig, a 10 to 60% reduction in friction at the top and bottom dead centers was obtained compared to the uncoated case as well as an improved wear resistance. The experimental results were compared to results from the semi-analytical model. There is good fit in the hydrodynamic regime but not in the mixed lubrication regime (at the top and bottom dead centers) probably due to the waviness defects on the rings and the liner which were not considered in the PRCL modelling so far. Figure 2: (a): film thickness (black dotted line: Moes and Venner solution, blue: 1d transient model, red: upgraded model with correction coefficients); (b): friction force (plus signs: measurement, black dotted line: Moes and Venner solution, blue: 1d transient model, red: upgraded model with correction coefficients) 4. Conclusions A solver allowing for a rapid prediction of the PRCL contact friction forces accounting for oil starvation and oil transport in mixed lubrication was developed and validated using a linear reciprocating tribometer in a cylinder on plane configuration in which the cylinder was machined to mimic a piston ring shape. In the experiments on the PRCL testrig, an excellent tribological performance of the DLC/ DLC compression ring/ liner contact was observed compared to the uncoated contact. However, the model prediction did not match well with the experiments in the mixed lubrication regime probably because of the waviness defects of the parts not being considered in the model. References [1] N. Biboulet and A. A. Lubrecht, Tribology Letters, vol. 70, 2022. [2] P. Sainsot and A. A. Lubrecht, Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, vol. 225, no. 6, pp. 441-448, 2011.N. 24th International Colloquium Tribology - January 2024 83 The Running-In of a DLC-Metal-Tribosystem - A Study on Multiple Scales Matthias Scherge, Joachim Faller Fraunhofer/ KIT MikroTribologie Centrum, Rintheimer Querallee 2b, 76131 Karlsruhe 1. Introduction This work deals with the running-in behavior of amorphous carbon coatings in a lubricated tribosystem with a metallic counterbody. The extent to which friction and wear develop as a function of stressing sequence and final machining of the coatings was investigated. For this purpose, experiments were carried out in a pin-on-disk tribometer with online wear measurement (RNT) in the mixed lubrication regime. The measurements show a pronounced running-in, comparable to a metal-metal-tribosystem, which benefits from high, initial loading. For the final machining, there is a narrow corridor in which the smallest friction coefficients and wear rates occur. By means of several analysis methods, it could be shown that the running-in leads to an enrichment of the sp 2 content in the near surface zone. 2. Results The goal of this work was to generate a fundamental understanding of the process of running-in of an amorphous carbon (ta-C) coating paired with an iron-sprayed layer (LDS, 13Mn6). For this purpose, systems with different coatings were run in, systematically tested for their sensitivity and the chemical change investigated (XPS, EELS). The tribological system is subject to a topographical as well as tribochemical running-in, which was quantified on the sprayed layer using RNT and on the DLC coating using in situ topography measurements. Fig. 1: Running-in with optimized stressing sequence. The running-in behavior was studied by means of a designated test program and optimized by skilful sequencing of load and speed in the direction of smaller friction values and wear rates, see Fig. 1. Furthermore, by varying the finishing of the ta-C coatings, a narrow roughness corridor in which the smallest friction values occur as a function of the initial peak roughness was discovered, see Fig. 2. This was accompanied by the realization that the friction is largely determined by the operating roughness. Fig. 2: Roughness corridor. The coefficients of friction shown here were taken from the minimum of a Stribeck test run. In addition to the change in topography, a change in the microstructure and chemistry of the near-surface area could be observed on the sprayed layer as a function of the running-in behavior and the counterbody. The amorphous carbon coatings are also subject to a mechanochemical running-in, in which lubricant constituents are incorporated into the near-surface area and the uppermost nanometers are enriched with sp 2 hybridized carbon. Thus, it can be confirmed for these investigated tribosystems that the running-in behavior is largely determined by the formation of a third body, see Figs. 3 and 4. Fig. 3: Chemical composition of the near-surface area of the sprayed metal layer, determined by XPS depth profiling. The first five nanometers consist mainly of CH x contamination. At a depth of ten nanometers, the iron oxide content is 84 24th International Colloquium Tribology - January 2024 The Running-In of a DLC-Metal-Tribosystem - A Study on Multiple Scales maximum with over 20 at.%. This drops below the detection limit over the next 30 nm. Phosphorus and zinc are only present on the first ten nanometers in significant amount, one and two atomic percent. Sulfur and molybdenum have their maximum at a depth of about 20 nm with 5, respectively 3 at.%. Also sodium, silicon and calcium are present on the first 30nm in detectable amounts. Fig. 4: Chemical composition of the near-surface area of the DLC coating, determined by XPS depth profiling. For the DLC coatings q tribochemical change can only be detected on the first three nanometers. Oxygen, nitrogen, calcium, sodium and zinc are present there. The composition of the ta-C coatings influences the tribology significantly via the sp 3 content. This again forms a corridor, which, however, is sharply limited only in the range of high sp 3 contents. The optimum lies between 60 and 75- at.% sp 3 . The wear rates of both bodies run parallel, albeit in different orders of magnitude, see Fig. 5. Fig. 5: Wear rate as function of sp 3 content for DLC and the 13Mn6 sprayed layer. It should be noted that the hardness of the amorphous carbon coating, determined by means of nanoindentation, leads to a corridor-like distribution of the friction coefficients as well. This exhibits a minimum between 30 and 50 GPa, which is a sufficiently wide corridor for a practical application, see Fig. 6. Fig. 6: Coefficient of friction as function of hardness of the DLC coatings. 3. Summary What all the tribosystems investigated have in common is that - particularly due to the narrow roughness corridor the sensitivity is high and, as a result, a running-in with the smallest friction values and wear rates did not occur in all cases. This makes a practical application, where a certain friction value has to be achieved, a challenge. However, if a coefficient of friction below 0.1 is sufficient for the practitioner, a DLC-metal system is a favorable choice from a wear perspective. For applications requiring stable, lowest friction values, pure ta-C pairings should be used. 4. Outlook On the basis of the results obtained, further investigations are required in several areas. On the one hand, this includes deepening the understanding of the tribochemical running-in of the amorphous carbon coatings. For this purpose, further EELS measurements on the identical material with smaller windowing and on coatings with other sp 3 contents are necessary. Furthermore, the transfer film on the counterbody has to be investigated in more depth. The analysis of the carbon hybridization by means of EELS and the characterization of locally-confined effects by EDX on TEM lamellae are suitable for this purpose. On the other hand, the gained knowledge should be used for the production of new coatings, rendering mechanical preconditioning obsolete. 24th International Colloquium Tribology - January 2024 85 Influence of Particles on DLC Coated Journal Bearings Alexander Hofer 1 , Manuel Zellhofer 1 , Thomas Wopelka 1 , Andreas Kübler 2 , Andreas Nevosad 1 and Martin Jech 1 1 AC2T research GmbH, Austria 2 Robert Bosch GmbH, Stuttgart, Germany 1. Introduction Diamond-like carbon (DLC) coatings provide low friction and superior hardness. Therefore, they are frequently used for wear prevention in crucial components such as journal bearings. Despite their robustness, DLC coatings can suffer critical damage such as delamination. This study focuses on a diesel-lubricated journal-bearing-like tribocontact, consisting of a DLC coated bearing shell with a steel-based shaft. Previous parameter studies, simulating the typical applied stress of this tribocontact in terms of pressure, temperature, and sliding velocity, did not reveal any critical wear conditions leading to the DLC delamination, but rather the presence of particles [1]. This study centers on a particle-focused investigation, examining this potentially critical parameter by thoroughly investigating the entire tribocontact in a close to application bench tests as well as in a model tribometer environment. The investigation of the influence of third body abrasives on DLC wear is not widespread. Haque et. al. [2] studied the influence of rather big sand particles (10-to 650-µm diameter) in a ring on block tribometer, with the result that the 10-µm particles lead to the highest wear increase in this setup. However, for the small lubricant gap in the journal bearing, lubricated with the high-quality diesel fuels, these particle sizes are too big for conclusions regarding this type of application. Therefore, investigations with smaller particle sizes were conducted. 2. Material and Methods The subject of the investigations in this work was a journal bearing-like tribocontact with a steel-based shaft and a DLC coated shell as counterpart. Wear of the components was measured continuously, employing the Radio-Isotope Concentration (RIC) method, which is based on radioactive tracer isotopes that are generated in the surface of the investigated specimen previously to the test with a particle accelerator in a so-called activation process. The first test series for the tribosystem was conducted in a test bench equipped with the whole aggregate containing the tribocontact of interest and applying the loading conditions of the real system. The goal of this test series was to determine the positions in the tribocontact where the wear process is initiated as well as the progress of wear. In this test series the shaft was activated as for DLC coatings there is only one tracer isotope available, which does not allow to distinguish wear from different positions. Therefore, the shaft was marked with two different radioactive tracer isotopes, 57 Co and 56 Co, in two different positions. The former was used to label the centre position of the shaft in axial direction while the latter was applied at the outer zones (see inlay in Figure 1). The second test series was carried out on a journal bearing tribometer with a unidirectional rotating motion which was connected to a closed lubricant circuit. This setup was applied to systematically investigate the influence of particles (that enter the tribocontact) on the wear behaviour and delamination of the DLC coating. The DLC coating was marked with 7Be as tracer isotope, which allows to directly measure the wear of the DLC coating. In the tribometer configuration, the shaft was pressed downwards onto the shell with a normal load of 200-N (resulted in ~30 MPa initial Hertzian pressure). The rotational speeds were varied perpetually in ramps between 0 and 240 rounds per minute (equivalent to 0 to 130-mm/ s sliding velocity) simulating boundary lubrication at start-stop conditions as well as hydrodynamic lubrication at high velocities. Diamond particles of 1, 6 and 9-µm diameter were introduced in a wide range of concentration in the lubricant circuit to study the particles’ influence on the wear behaviour. The DLC surfaces of selected test runs were investigated by scanning electron microscopy (SEM) for the analysis of wear tracks. 3. Results and discussion 3.1 Bench test During regular operation of the journal bearing, a typical running-in behaviour has been observed for the central zone for 100 minutes of the experiment, as shown in Figure 1. In some experiments, the outer zones showed a sudden tremendous increase of wear (blue curve) while wear increase of the central zone (red curve) was smaller by orders of magnitude (Figure 1). Figure 1: Wear depth curves of outer zones (Co-56, blue, primary y-axis) and central zone (Co-57, red, secondary y-axis) of bearing shaft (inlay top left corner). 86 24th International Colloquium Tribology - January 2024 Influence of Particles on DLC Coated Journal Bearings Figure 2: Offset (upper row) and wear rates (lower row) of DLC bearing bushes with addition of particle of different concentration and diameters. Upon conducting a thorough inspection of the components following the tests, it was discovered that wear particles were generated in another tribological contact near the end of the shaft. These particles were then transported into the journal bearing gap by the lubricant. This finding is in accordance with the RIC results showing a very steep increase for the outer zones followed by a flatter signal raise in the centre. Therefore, the incorporation of particles into the tribocontact as a significant wear mechanism for DLC was thoroughly examined in tribometer tests. 3.2 Tribometer tests Several tests were carried out with diamond particles of different diameters (1, 3, and 9-µm) and for reference without additional particles. Throughout the experiment, particles of increasing concentration (within the range between 10 3 to-10 9 ) were introduced into the lubricant circuit in discrete time steps of a few hours. The resulting wear behaviour was assessed by two quantities using the RIC method: Abrupt wear increases and following constant wear rates. Figure 2 shows the abrupt increase/ offset in the first phase after particle addition (upper row) and the constant wear rate during the last 40% of the duration of each particle concentration step (lower row). The 1-µm particles showed no influence on both quantities regardless of their concentration except for the-10 5 -ml -1 step, which is considered to be an outlier. For the 6-µm particles, both the offset and the wear rate increase significantly with admixture, with the values tending to rise at higher particle concentrations. For the 9-µm particle diameter, the offset shows similar behaviour as for the 6-µm species (note the values for 10 5 -ml -1 concentration). For the wear rate no significant increase could be observed compared to the diesel without added particles. Figure 3 shows a SEM picture after a wear test with the admixture of 9- µm particles. The image displays a distinct scratch in the DLC coating with an embedded particle. An Energy Dispersive X-Ray spectroscopy (EDX) measurement revealed a >99% (atomic percentage) concentration of carbon in the particle, which confirms that it is an added diamond particle and not a wear particle of, for example, the steel counterpart. Figure 3: SEM image after a test with addition of 9 µm particles. Thus, we conclude that there is a critical particle diameter between 1 and 6-µm for the investigated tribocontact above which sudden failure and increased wear rate of the DLC coating can occur. Furthermore, this behaviour also depends on the particle concentration, which can be explained by the fact that the introduction of particles into the lubrication gap is a statistical process and therefore more likely at higher concentrations. It was found that critical wear events can be provoked by the insertion of abrasive particles into the lubrication circuit. 4. Acknowledgement This work was funded by the Austrian COMET Program (project K2 InTribology1, no. 872176). The work has been carried out within the “Excellence Centre of Tribology” (AC2T research GmbH). References [1] Zellhofer, M., Jech, M., Wopelka, T., Hofer, A., Mayrhofer, P. H.,), Kuebler, A., Weckenmann, F., Experimental approach for investigating critical loading conditions leading to delamination of DLC coatings, 7th World Tribology Congress, WTC 2021, September 5-10, 2021, Lyon, France. [2] Haque, T., Ertas, D., Ozekcin, A., Jin, H.W., Srinivasan, R., 2013. The role of abrasive particle size on the wear of diamond-like carbon coatings. Wear 302, 882-889. https: / / doi.org/ 10.1016/ j.wear.2013.01.080 24th International Colloquium Tribology - January 2024 87 Assessment of Different Coatings on the Friction and Wear Behavior of Differential Shafts for Electric Vehicles Etienne Macron 1* , Johnny Dufils 1 , Christophe Héau 1 1 IREIS, HEF Group, ZI Sud, Rue Benoit Fourneyron, 42162 Andrézieux-Bouthéon, France * Corresponding author: emacron@hef.group 1. Introduction Electric engines for vehicles generate high torques. Compared to ICE engines, this leads to higher torque transmitted to the differential and therefore to higher tribological stress on the components. In particular, the normal load applied to the contact between the differential shaft and the planet gear is increased. Conventional surface treatments are showing some limitations. Therefore, a study combining numerical and experimental approaches was conducted to qualify the performances of hydrogenated amorphous carbon (a-C: H) coating in terms of friction and wear behaviour for this specific application. 2. Preliminary simulations In order to evaluate the influence of the different contact parameters on the lubrication regime, a preliminary numerical simulation work was undertaken. Figure 1: Simplified illustration of a differential assembly First, the normal load between the differential gear and the shaft was calculated on the basis of the characteristics of a dual motor electrical vehicle especially max torque T motor (967 Nm), reduction ratio λ (9.73: 1) and lever arm between the middle of the differential shaft and the middle of the oil cut set to 38 mm. Assuming that torques applied to the two differentials are equal for this dual motor configuration and that the load was equally split between the two pinions, the normal load was estimated to 62 kN max. using the following formula: The estimated oil film thickness was calculated using the Dowson-Higginson formula for a 2D line contact. Various velocity, load and oil temperature conditions were computed using the components characteristics described in section 3. The data were plotted as L parameter combining the oil film thickness with the initial composite roughness criteria of the contact as a function of the differential rotation speed of the wheels (see Figure 2). Figure 2: Lambda ratio as a function of the differential rotation speed of the wheels (oil temp. 50-°C) From the data available and the assumptions made, these calculations showed that: i) the lubrication regime was mainly affected by the differential rotation speed; ii) the pinion/ shaft contact seemed to be initially stressed in the boundary and mixed lubrication regime except at very low motor torque and high differential rotation speeds. These theoretical outputs provided also some useful data in order to define the experimental test protocols. 3. Experimental set up on a Shaft/ Bearing tribometer A specific test setup (Figure 3 & Figure 4) was designed in order to assess the tribological behaviour of hydrogenated amorphous carbon (a-C: H) coatings applied on the shafts. The shafts were rubbed against a steel ring which inner surface simulating the gear inner surface. Rotation speed up to 10 000 rpm Normal load from 500 to 20 000 N Shaft diameter up to 30 mm Oil temperature up to 120-°C Oil volume ~1.2 L Controlled parameters Possible control of test sequence with variation of speed and/ or normal load 88 24th International Colloquium Tribology - January 2024 Assessment of Different Coatings on the Friction and Wear Behavior of Differential Shafts for Electric Vehicles Figure 3: Shaft bearing tribometer and its main features (IREIS Design) Figure 4: Specific test set up for differential shaft-gear contact (IREIS Design) Test specimens were made out of real differential shafts re-designed to fit into the experimental set up presented in Figure 4 . The shafts have an outer diameter of 22.2 mm. The rotating rings standing for the pinion were designed to obtain an oil clearance ≈ 0.14 mm and a line contact length of 16 mm for the short-term test to determine friction coefficient and 2 mm for the endurance test to evaluate wear behaviour. The ring material was a typical gear-type steel (20MnCr5) with a hardness of ≈ 60 HRC and an initial surface roughness of Ra ≈ 0.3 mm (inner diameter). Finally, a commercial lubricant (Gearbox oil 75W80 Tranself NFP) was used at a temperature of 50-°C at the beginning of each test sequence. The oil temperature at the end of each test sequence was dependent on how much power was dissipated by friction in the contact. 4. Experimental results To quantify the friction coefficient, a test sequence at a constant load of 15 kN and a decreasing rotation speed from 300 rpm to 50 rpm by 50 rpm steps of 2 min each was repeated until friction stabilization and at least 5 times for shafts with two surface treatments: e-nickel (reference) and a-C: H DLC coating. The results are presented in Figure 5 with data plotted as a function of (viscosity × entrainment speed)/ (contact pressure × roughness) in order to consider the variability between the different material combinations in terms of i) final roughness of the steel ring (1st order of magnitude parameter) and ii) oil temperature and contact pressure (both being friction and wear dependent). Figure 5: Comparison of the friction coefficient between e-nickel and a-C: H DLC coated differential shafts Complementary tests with a longer duration (8h) were undertaken to better compare the wear behaviour of the electroless nickel treatment and the a-C: H DLC coating (see Figure 6 ). Figure 6: Wear rate obtained after a 8-hour test at constant speed (100 rpm) and constant load (8 kN) with an oil temperature of 50-°C 5. Conclusion In terms of friction coefficient, the entrainment product and the composite roughness of the rubbing surfaces turned out to be first order parameters in order to determine whether a transition from mixed lubrication to hydrodynamic lubrication was achieved or not. Both for a-C: H and e-nickel coated pins, a transition from mixed lubrication to EHL was observed because a low composite roughness is achieved. Out of the endurance test protocol, DLC-coated shafts (a-C: H) showed almost no wear when the electroless nickel was subject to abrasive wear despite a stabilised friction coefficient equivalent in both cases. Well known for their low friction properties, DLC coatings (a-C: H in this case) demonstrated here under realistic test conditions a strong durability performance in this differential assembly application for electric vehicles compared to the standard electroless nickel solution. 24th International Colloquium Tribology - January 2024 89 Atomistic Insights into the Behavior of Solid Lubricants Under Tribological Load Andreas Klemenz 1* , Michael Moseler 1,2 1 Fraunhofer Institute for Mechanics of Materials IWM, Micro Tribology Center µTC, Wöhlerstraße 11, 79108 Freiburg, Germany 2 Institute of Physics, University of Freiburg, Hermann-Herder-Straße 3, 79104 Freiburg, Germany * Corresponding author: andreas.klemenz@iwm.fraunhofer.de 1. Introduction The optimization of technical lubrication systems often follows the trial-and-error approach, which frequently reaches its limits. A fundamental understanding of the underlying physical processes can simplify this task or even open up completely new perspectives. Atomistic simulations have proven to be a valuable tool in this regard over the last decades. This contribution presents two examples of the investigation of fundamental tribological processes using a combination of experiments and molecular dynamics simulations. 2. Mechanisms of Graphite Lubrication Graphite is one of the oldest dry lubricants in technical use. A popular explanation for its lubricating properties is based on the lamellar structure of graphite. Since the layers interact with each other only by weak van der Waals forces, it is assumed that they can move against each other like in a deck of playing cards. This model provides a simple explanation for the lubricating properties of graphite. However, it has been known since the 1930s that graphite loses these properties in dry environments. Various subsequent approaches to explain this behavior, such as the saturation of dangling bonds on graphite planes or the increase of the interlayer distance due to water intercalation have failed to provide a satisfactory explanation, and the mechanisms underlying graphite lubrication are still not completely understood. Tight-Binding Molecular Dynamics simulations are employed to investigate the influence of water on the lubricating properties of graphite. In an extensive parameter study, the amount of water in the gap and the pressure on the systems are systematically varied. The behavior of the system is dominated by two different effects: When there are only small quantities of water in the gap, the surfaces tend to cold weld, resulting in high friction (Fig. 1). When the amount of water is sufficiently high, the system behavior changes and continuous water films form between the surfaces, accompanied by low friction. An estimate of the typical amount of water present on the surfaces due to condensation from ambient humidity suggests that graphite will be predominantly in the low friction regime under normal laboratory conditions. Occasionally, the contacts may run dry and cold weld locally. These results explain the good lubricating properties of graphite under normal laboratory conditions. The results also provide an explanation for experimentally observed structures of graphite layers subjected to tribological load. TEM investigations revealed the formation of layers of turbostratic carbon at the sliding interfaces, which are not included in conventional models of graphite lubrication and can be explained by the occasional cold welding of the surfaces. Fig. 1: Typical atomic scale structures of graphite under tribological load. Under high pressure and with a low amount of water in the gap, the surfaces typically cold-weld. With increasing amounts of water, aromatic structures can form at the contact, leading to a drastic decrease of friction forces. If the amount of water becomes large enough, continuous water films can form, separate the surfaces and lead to low friction. 3. Degradation of Carbon Nanotubes Under Tribological Load Carbon nanotubes (CNTs) are not among the typical dry lubricants. However, by coating surfaces with CNT films, a reduction in friction can be achieved in many cases. In tribometer experiments with CNT coated iron surfaces and subsequent TEM investigations, degradation of the CNTs can be observed after a short period of time. Since CNTs are mechanically very stable, the question arises by which mechanisms this degradation takes place and what influences it. Fig. 2: SEM images of CNT coated iron surfaces. (Images taken from Ref. [2]) 90 24th International Colloquium Tribology - January 2024 Atomistic Insights into the Behavior of Solid Lubricants Under Tribological Load To investigate these questions, layers of CNTs were simulated under tribological load using classical molecular dynamics. Both, the structure of the CNTs (diameter, number of inner walls) and the structure of the coatings are varied. The degradation of CNTs clearly depends on their structure. CNTs with a large number of inner walls are more resistant to mechanical deformation than CNTs with a small number of walls, which is reflected in higher pressures required to induce damage in a layer of CNTs. In addition to the structure of the individual CNTs, the structure of the film as a whole also influences its stability under tribological load. Experimental CNT coatings often have irregular structures with randomly oriented CNT axes and a significant amount of empty space between the tubes (Fig. 2). Different CNTs therefore only contact each other at a small number of points. The contact pressures at these points are considerably higher than the macroscopic pressures applied to the coatings. Therefore, the pressures necessary to induce the first damages in the CNTs depend on the structure of the coatings and the resulting density of CNT contact points. Atomistic simulations with model systems for the coating structures show that these pressures can be easily reduced by more than one order of magnitude compared to a close packing of CNTs (Fig. 3). Fig. 3: Atomistic simulations show that a reduced density of contact points between CNTs can result in a decrease of the pressures necessary to damage the CNTs by more than one order of magnitude. The results obtained from the simulations are used to develop an analytical model that describes the onset of film degradation as a function of the CNT radius and the number of inner walls. Comparison of the model with experimental data shows good agreement between the model and the degradation of CNT films in tribometer experiments. References [1] C.E. Morstein, A. Klemenz, M. Dienwiebel, M. Moseler, Nature Communications 13, 5958 (2022) [2] T. MacLucas, A. Klemenz et al., ACS Applied Nano Materials 6, 1755 (2023) 24th International Colloquium Tribology - January 2024 91 Modification of Surface Properties on Various Mg-Based Alloys for Tribological Applications via Plasma Electrolytic Oxidation Process Ashutosh Tiwari 1* , Jörg Zerrer 1 , Anna Buling 1 1 ELB - Eloxalwerk Ludwigsburg Helmut Zerrer GmbH, Ludwigsburg, Germany * Corresponding author: buling@ceranod.de 1. Introduction Magnesium (Mg) and its alloys have become popular for material selection in different fields such as aerospace, automotive, biomedical, and robotics. Due to their low density and high weight-to-strength ratio, they enable better energy efficacy and, thus, reduction in CO 2 emission which supports overcoming recent ecological challenges [1]. However, poor wear resistance and rapid corrosion in challenging conditions greatly limit the application in the transportation and medical sectors [2]. To overcome such issues, a ceramic surface modification on various Mg-based alloys by the PEO (plasma electrolytic oxidation) process is developed, which ensures superior, robust, and durable CERANOD ® surfaces under harsh tribological and corrosive conditions. Moreover, the PEO process is an environmentally friendly technique in which alkaline electrolytes and no toxic byproducts or waste occur during the process as well as while the recycling of the Mg material. The current research exemplifies the quality and versatility of CERANOD ® surfaces created on differently manufactured Mg alloys such as 3D printing, casting, or extruding processes, irrespective of their dimensions and geometries. A comparative investigation of surface properties between differently manufactured Mg alloys treated with our PEO process was performed. Additionally, hybrid surfaces in the combination of PEO and a polymer (doped PEEK: poly-ether-ether-ketone) were fabricated on Mg-based alloys to further reduce the coefficient of friction of the surfaces without using any lubricants or oils. 2. Materials and Methods As substrate material various Mg-based alloys were used to modify their surfaces via the PEO process. All the treated samples have coupon geometry with 31 mm diameter and 6-mm height. Alloys, their manufacturing process, and surface treatment conditions are described in Table 1 below: Table 1: description of various Mg-based alloys Alloy Manufacturing process Surfaces treatment condition/ Alias WE43 Additive Manufacturing PEO/ (A) AZ31 wrought PEO/ (B) ZK60 wrought PEO/ (C) AZ31 wrought PEO + PEEK/ (D) AZ31 wrought PEO + doped-PEEK/ (E) All the substrates mentioned were refined with our CERAN- OD ® surfaces. Surfaces D & E were modified by the PEO process and further treated by in-house produced PEEK and doped-PEEK dispersion on the CERANOD ® surface by laser melting technique. The PEO process is optimized in alkaline-based electrolyte under high pulsed voltage which leads to plasma discharging on the Mg alloy surface and forms a dense ceramic-like layer on the substrate. A schematic setup of the PEO process is shown in Figure 1: Figure 1: illustration of PEO process on Mg substrate Surface roughness parameters R a (mean thematic roughness) and R p (maximum profile peak) after surface modification and wear tracks (or: wear volume) were analysed with the LSM (Laser Scanning Microscope; Keyence, VK-X100) of all the Mg-based alloys. The tribological performance of modified CERANOD ® surfaces (A, B, and C) was analysed using a pin-on-disc tribometer (TRB MZKO; Anton Paar) in linear reciprocal mode with a 6-mm WC (tungsten carbide) ball as a static partner. The testing parameters were set as: v max -=-4 cm/ s, F N (normal force) = 5 N, l (path length) = 10-mm, and total sliding distance = 70 m. The tribological tests of CERANOD ® surfaces D & E were performed with 100Cr6 ball with 6 mm diameter under test conditions: v max -=-0.64 and 29.7 cm/ s, F N- = 4 & 10 N, l = 3 and 40 mm, and total distance = 80 and 1000 m, respectively. The specific wear coefficient after tribological analysis was calculated using the equation 1: (1) where K n is the specific wear coefficient, F N is the applied normal force and d is the total distance. The tribological traces on the surfaces (refined and unrefined) were investigated using SEM (Scanning Electron Microscope; FlexSEM 1000, Hitachi) and EDS (electron dispersive spectroscopy; with Aztec 4.2 software). The wear depth and surface pro- 92 24th International Colloquium Tribology - January 2024 Modification of Surface Properties on Various Mg-Based Alloys for Tribological Applications via Plasma Electrolytic Oxidation Process file from BSE images of different angles after tribological analysis were constructed using Hitachi 3D Map software. Hardness measurements were performed using a universal hardness tester (HM2000; Helmut Fischer). The HV results shown are an average value of 10 random points across all the CERANOD ® surfaces. Results and Discussion Surface roughness parameters R a / R p of reference WE43 surface (polished) and surface A were recorded as 1.2 ± 0.06/ 5.33 ± 0.3 μm and 0.8 ± 0.06/ 5.2 ± 0.9 μm respectively. The developments of friction over testing time are shown in Figure 2. The unmodified additively manufactured WE43 reference surface shows high deviation and unstable μ (coefficient of friction, COF). Nevertheless, CERANOD ® surfaces exhibit slightly increased but stable μ after running-in. It was observed after tribological testing that the reference WE43 surface exhibits a high rate of wear under 5 N load, whereas the modified surface A exhibits no wear and damage as shown in Figure 3. Surfaces B and C show similar behavior with no sign of wear on the surface. However, a sudden increase in μ after 20 m in the case of surface C is due to the presence of asperities on the surface. These asperities break when they come into contact with the WC ball and redeposit on the surface. Hence, an increase in the contact area between the WC ball and the surface leads to an increased coefficient of friction. Figure 2: coefficient of friction ( μ ) of various modified CERANOD ® surfaces (A, B, C, and reference WE43 surface) with WC ball Figure 3: surface overview after tribological test with WC ball: a) reference WE43 b) modified CERANOD ® surface A on WE43 substate Furthermore, hybrid surfaces such as D and E were analyzed by pin-on-disc test with 100Cr6 ball under different testing conditions. The obtained results are shown in Figure 4. A clear difference in μ occurs between modified Mg PEO surfaces and hybrid surfaces D & E. Evidently, the COF further improved with doped PEEK due to minimized inhomogeneity on the surface. It can be inferred from Figure 4 that hybrid PEEK surfaces provide solid lubrication by exhibiting a low coefficient of friction. Figure 4: coefficient of friction of surfaces D & E during the tribological test with 100Cr6 ball: a) slow b) fast testing regimes [3] The measured hardness value of Mg-PEO at 50 mN was 518-±-90 HV 0.05 . Surfaces D & and E were measured with 5-mN force. They exhibit a value of 40 ± 3 HV 0.005 due to their polymeric properties [3]. Conclusion Preliminary outcomes show great improvements in the wear resistance of various Mg-based alloys after surface modification with CERANOD ® surfaces via the PEO process. Further, surface modification lowers the coefficient of friction with hybrid coatings which can act as a solid lubricant and open numerous potential applications. References [1] Dong, H. (2010). Surface Engineering of Light Alloys: Aluminium.-Magnesium and Titanium Alloys, 243-247. [2] Vignesh Kumar, M., Padmanaban, G., & Balasubramanian, V. (2018). Sliding wear characteristics of friction stir processed CAST ZK60 magnesium alloy under different applied loads.- Transactions of the Indian Institute of Metals,-71, 1223-1230. [3] Buling, A., & Zerrer, J. (2020). Lifting lightweight metals to a new level-tribological improvement by hybrid surface solutions on aluminium and magnesium.- Lubricants,-8(6), 65. 24th International Colloquium Tribology - January 2024 93 Mechanical Adhesion with Micropatterned Surfaces Translating Friction and Elastic Energy to Adhesive Forces Marco Bruno 1,2,3 , Luigi Portaluri 1,2 , Luciana Algieri 1 , Stanislav Gorb 4 , Massimo De Vittorio 1,2 , Michele Scaraggi *1,2,5 1 Center for Biomolecular Nanotechnologies, Italian Institute of Technology, Arnesano (LE), IT 2 Department of Engineering for Innovation, University of Salento, Lecce, IT 3 DIBRIS, University of Genova, Genova, IT 4 Zoological Institute, University of Kiel, Kiel, DE 5 Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, UK * Corresponding author: michele.scaraggi@unisalento.it 1. Interlocking as a new tribological route to enhance and tune adhesion Controlling adhesion in harsh environmental conditions is of primary importance in many fields: From suture in surgical operations to wound healing tapes or underwater adhesives and grippers for robotics applications. [1] Surface interactions happen at all scales and almost at any time in any physical system and nature provides many examples of morphological features of animals, plants and micro-organisms (mosquitos, parasites proboscis or plant diaspore structures) [2] [3] [4] that can translate friction and elastic energy stored through interlocking mechanisms in adhesion forces, [5] any of these examples involves a compliant surface to store elastic energy. Since highly soft materials are becoming more and more common and easy to produce it is possible to use and tune their overall compliance by shaping their structure and changing their mechanical properties. This kind of mechanism is capable to operate in conditions that can compromise or completely negate the effect of standard adhesives: surfaces exposed to water, solvent vapors, lubricants [6], high temperatures, dusty environments, surfaces operating in high vacuum or aerospace applications. In this work we present various micro-patterned surfaces that can adhere to each other and once they are locked they can sustain high mechanical loads in normal direction. A simplified theoretical framework based on Hertz theory has been developed to predict the behavior of such systems. Through stereolithographic (SLA) 3D printers and soft lithography techniques we fabricated soft samples of the proposed geometries and tested them using a custom made opto-mechanical tribometer. Figure 1: (A) Soft-soft interlocking surfaces after printing and curing; (B) Detail of square-lattice patterned soft surface; (C) Schematic representation of the interlocking process indicating the main geometrical parameters of the structures. 2. Theoretical framework In order to build a model we will consider a single microstructure constituted by an elastic sphere and a rigid element that connects the sphere to the substrate as in Figure-1 (C) If we write the equilibrium equation in y direction and we couple it with the force vs penetration relation for Hertzian contact we obtain: Where F n is the normal force at the contact, F t is the friction force at the contact, F pull is the force in the pull-off direction, δ is the penetration, I is the distance between two neighboring structures, R is the radius of the spheres, E* is the effective Young modulus, y is the distance in the pull-off direction and m is the friction coefficient. We can then solve it for F pull as a function of y: We can then integrate this quantity with respect to y from first to last contact point to obtain the predicted (approximated) work of adhesion to be compared with the experiments, and multiply by the density of microstructures per unit area ρ. Figure 2: Effective normal force against displacement (see Fig.-1(C)) in the pull-off direction for a single microstructure and for various values of friction coefficient. 94 24th International Colloquium Tribology - January 2024 Mechanical Adhesion with Micropatterned Surfaces 3. Experimental section In order to evaluate the work of adhesion of patterned surfaces we opted for hexagonal patterns since the packing factor of this interlocking geometry is optimal and more robust against slight rotational tilts. The samples were fabricated through 3D printing techniques by means of a Formlabs 3B printer, using a stiff resin (E ~ 2.2-GPa) for the top approaching surface and a soft resin (E ~1-MPa) for bottom reference surface. The top sample was designed to be dome shaped in order to avoid boundary effects. A compliant shaft to hold the top sample was designed to allow slight adjustments needed for the interlocking of the surfaces. A custom made opto-tribometer was used to assess the performance of these surfaces. A typical test is divided into an approaching first part in which the samples are brought into contact by means of a linear actuator, when the force between the samples reaches a given threshold (2.5- N) the system holds the position allowing some relaxation of the polymer for 5-s, in the last part the linear actuator reverts its direction and a load cell measures the force during detachment. The tests were performed in dry and wet environment (surface completely submerged in distilled water). Figure 3: Adhesion tests performed on the same sample in dry and wet conditions. As expected, pull-off force is higher in dry than in wet conditions due to a higher friction coefficient. 4. Conclusions By introducing the actual material properties and geometrical parameters in our model, an equivalent work of adhesion of 1.51-N/ m was calculated in dry conditions and 0.35-N/ m in wet conditions while tests give us 1.78-N/ m and 1.31-N/ m respectively for dry and wet, showing that the model is robust when friction is high, this comes from the discrepancy between our model and the theory of JKR by which we calculated the work of adhesion from the pull-off force in the experiments. There is a noticeable increase in adhesion with respect to the smooth surfaces, even more in the case of smooth wet contact. This leads us to believe that an optimal set of parameters can be selected to further enhance the adhesion performances (radius of the spheres, overall shape of the interlocked structures, lattice shape, material stiffness and work of adhesion). A tailorable adhesion, effective even in wet conditions, can be disruptive in multiple fields ranging from medical suture and drug-delivery technologies, to industrial applications with adhesive-suppressing environmental conditions (which represent a major limitation to the adoption of van der Waals intermolecular interaction in micropatterned adhesives), to space applications where vacuum, dust, and other degrading factors could strongly impede adhesion. References [1] S. Baik, H. J. Lee, D. W. Kim, J. W. Kim, Y. Lee and C. Pang, “Bioinspired adhesi-ve architectures: from skin patch to integrated bioelectronics,” Advanced Materials, vol. 31, p. 1803309, 2019. [2] E. Gorb and S. Gorb, “Contact separati-on force of the fruit burrs in four plant species adapted to dispersal by mechanical interlo-cking,” Plant Physiology and Biochemistry, vol. 40, p. 373-381, 2002. [3] S. Y. Yang, E. D. O’Cearbhaill, G. C. Sisk, K. M. Park, W. K. Cho, M. Villiger, B. E. Bouma, B. Pomahac and J. M. Karp, “A bio-inspired swellable microneedle adhesive for mechanical interlocking with tissue,” Nature communications, vol. 4, p. 1702, 2013. [4] M. Zhu, F. Zhang and X. Chen, “Bioin-spired mechanically interlocking structures,” Small Structures, vol. 1, p. 2000045, 2020. [5] Y. Wang, Z. Mu, Z. Zhang, W. Song, S. Zhang, H. Hu, Z. Ma, L. Huang, D. Zhang, Z. Wang and others, “Interfacial reinforced carbon fiber composites inspired by biological interlo-cking structure,” Iscience, vol. 25, 2022. [6] H.-H. Park, M. Seong, K. Sun, H. Ko, S. M. Kim and H. E. Jeong, “Flexible and shape-reconfigurable hydrogel interlocking adhesives for high adhesion in wet environments based on anisotropic swelling of hydrogel microstruc-tures,” ACS Macro Letters, vol. 6, p.-1325- 1330, 2017. [7] D. Naik, G. Balakrishnan, M. Rajagopa-lan, X. Huang, N. Trivedi, A. Bhat and C. J. Bettinger, “Villi Inspired Mechanical Interlo-cking for Intestinal Retentive Devices,” Advan-ced Science, p. 2301084, 2023. [8] H. Han, L. E. Weiss and M. L. Reed, “Micromechanical velcro,” Journal of Micro-electromechanical Systems, vol. 1, p. 37-43, 1992. [9] B. Cheng, J. Yu, T. Arisawa, K. Hayashi, J. J. Richardson, Y. Shibuta and H. Ejima, “Ultrastrong underwater adhesion on diverse substrates using non-canonical phenolic groups,” Nature Communications, vol. 13, p.-1892, 2022. 24th International Colloquium Tribology - January 2024 95 Unveiling Extreme Lightweight Potential by PEO Refinement of Innovative Al Alloys Anutsek Sharma 1 , Jörg Zerrer 1 , Genki Funamoto 2 , Anna Buling 1* 1 ELB - Eloxalwerk Ludwigsburg Helmut Zerrer GmbH, Neckartalstrasse 33, DE-71642 Ludwigsburg - Neckarweihingen, Germany 2 Advanced Composite Corporation, 2259-9 Oobuchi, Fuji, Shizuoka, 417-0801, Japan * Corresponding author: buling@cceranod.de 1. Introduction Required component weight reduction often competes against wear and corrosion issues when it comes to lightweight metals. Aluminum (Al) and its alloys are widely used in different industrial applications, e.g., aerospace, automotive and machinery. Further weight reduction of components can be achieved with additively manufacturing methods of Al-alloys. Promising candidates for highly improved mechanical properties can be the aluminum matrix composites (AMCs), where ceramic particles are reinforced in an aluminum matrix. While powder-based 3D printing techniques are limited to the existing powder material and mostly bear high machinery costs by producing high amount of powder waste, the novel liquid metal printing (LMP) technique, developed by GROB Werke GmbH, enables 3D printing using any existing Al wire material without any material waste accompanied by low machinery costs. Unique mechanical properties like high strength and thermal stability of the AC-Albolon ® (AMC) material from Advanced Composite Corporation enable lightweight materials applications, where high rigidity and shape control are required. Plasma Electrolytic Oxidation (PEO) is known as an environmentally friendly technique leading to highly wear and corrosion resistant surfaces on lightweight metals [1]. To unveil further potentials of novel material groups and increasing their use case range, ULTRACERAMIC ® PEO (refinement was applied to LMP and AMC materials. A comparison to the selective laser melting (SLM) produced AlSi10-alloy is provided as well. PEO refined advanced Al alloys were tested under dry sliding conditions using a pin-on-disc system. The coefficient of friction and the wear rate of refined and blank samples were investigated. Initial studies reported a 1000x decrement in wear volume of PEO protected surfaces in comparison to blank samples, which might lead to an extended range of application of these novel lightweight materials. 2. Material and Methods LMP AlSi12 alloy used in the present work is provided by GROB Werke GmbH. LMP has emerged with higher building rates, reduced costs, a possibility of fast scaling up, reduced thermal distortion and increased degree of freedom. For enhanced mechanical and thermal stability, Advanced Composite Corporation developed AC-Albolon ® which is an aluminum matrix composite (AMC) with aluminum borate incorporated particles produced by liquid forging technology, resulting in highly homogeneous and finely dispersed composites. The wear results from pin on disc tests on LMP and AMC produced samples were then compared with additively produced AlSi10Mg samples from Selective Laser Melting (SLM), which is a widely used additively manufactured technique. The coupons manufactured by LMP and AMC had a diameter of 31 mm and were 6 mm in height. To analyze different surface conditions, one side was left in as-built state while the other side was polished to 2500 grit. The samples were refined by ULTRACERAMIC ® PEO process in a low alkaline electrolyte using stainless steel as counter electrode. Figure 1 shows schematically the process with high potentials by a pulsed source. Figure 1: ULTRACERAMIC ® process curve The previously polished side was used to perform tribological tests eliminate the influence of roughness of th manufacturing process. The tribological tests were performed using pin on disc testing setup with tungsten carbide balls as counter bodies. The present work discusses the tests performed at 5 N normal load in linear reciprocal mode at a maximum speed of 4 cm/ s. Sample terminology that will be used in later sections can be seen in Table 1. Table 1: sample terminology Sample Terminology AMC- PEO refined AMC- PEO LMP- PEO refined LMP- PEO SLM- PEO refined SLM- PEO AMC uncoated AMC LMP uncoated LMP SLM uncoated SLM PEO refined samples and uncoated samples exhibit average roughness (R a ) values ranging from 1.2 µm to 7.5 µm. An 96 24th International Colloquium Tribology - January 2024 Unveiling Extreme Lightweight Potential by PEO Refinement of Innovative Al Alloys overview of varying roughness values on polished side is shown in Figure 2 below. Figure 2: overview of average roughness (R a ) values on polished side 3. Results and discussion Development of the coefficient of friction (µ) for reference and PEO refined samples is shown in Figure 3. It can be seen in Figure 3 (a) that the AMC surface shows stable and constant µ while the AMC - PEO showed the lowest friction in comparison to LMP - PEO and SLM - PEO. In Figure 3 (b), a degrading µ for LMP sample can be seen. This surface suffers from increased wear and material transfer to the ball. However, LMP - PEO sample shows a clear running-in behavior. At the start, µ increases and as the testing continues the friction becomes stable. Figure 3 (c) shows µ over testing distance for SLM samples. SLM sample shows similar trend as LMP sample due to the softness of the material. SLM - PEO sample showed steep rise in friction at the start of testing with a subsequent stabilization. Figure 3: Coefficient of Friction (µ) over testing distance for (a) AMC, (b) LMP and (c) SLM produced surfaces It can be seen from these curves that R a has direct impact on coefficient of friction values. The samples with higher average roughness values showed increasing trends in Figure 3 (a), (b) and (c). Figure 4: surface wear coefficients (K surface ) with respect to corresponding samples Figure 4 shows varying coefficients of surface wear (K surface ) for corresponding samples. It can be clearly seen that refining Al samples with PEO provides greater surface protection against wear. AMC - PEO showes the lowest wear followed by LMP - PEO and SLM - PEO. Reference samples, on the other hand, suffered from harsh wear and extensive material transfer to the counter body. 4. Conclusion It can be concluded that refining Al alloys with ULTRAC- ERAMIC ® leads to enhanced wear protection. With above discussion, it can be learned that a wide variety of Al alloys fabricated using various manufacturing methods can be successfully refined with PEO. Following conclusions can be drawn: • The coefficients of surface wear showed a comprehensive increase in wear resistance of PEO refined samples in comparison to reference or uncoated samples. • Unprotected Al surfaces suffer from extensive tribological wear. • Reduced coefficients of surface wear of approximately 10 -3 can be observed on PEO refined surfaces. • Average roughness values have a direct impact on friction. • The PEO process is completely sustainable without usage of harmful chemicals, leading to enhances material wear and corrosion resistance. 5. Acknowledgement ELB - Eloxalwerk Ludwigsburg Helmut Zerrer GmbH thanks GROB Werke GmbH for providing samples. References [1] M. e. al., „Improving the wear resistance of plasma electrolytic oxidation (PEO) coatings applied on Mg and its alloys under the addition of nanoand microsized additives into the electrolytes: A review „Journal of Magnesium and Alloys“, pp. 1164-1186, 2021. https: / / doi.org/ 10.1016/ j.jma.2020.11.016 24th International Colloquium Tribology - January 2024 97 The Effects of the Lubricant Properties and Surface Finish Characteristics on the Tribology of High-Speed Gears for EV Transmissions Boris Zhmud 1* , Morteza Najjari 2 , and Boris Brodmann 3 1 Tribonex AB, Uppsala, Sweden 2 Xtrapid Innovations, Detroit, USA 3 Optosurf, Ettlingen, Germany * Corresponding author: boris.zhmud@tribonex.com 1. Introduction As more and more electric cars are coming to the market, some new unique tribological challenges for electric vehicle transmissions become apparent. Proper selection of fluids for EV transmissions is critical yet complicated by a wide diversity of EV hardware. In general, EV transmission fluids call for a different spectrum of properties compared to conventional ATFs. High-speed gear drives also pose higher demands on gear accuracy and surface finish quality. At the moment, automatic transmission fluids (ATFs) are often used in EV reduction transmissions. Common ATFs such as MERCON ® LV, DEXRON ® VI, Toyota T4, Honda DW-1, etc have KV100 in the range 6 to 8 cSt. Ultralow viscosity (ULV) ATFs go down to KV100 3.5 to 4.5 cSt and are formulated using synthetic base oils, such as poly alpha olefins (PAO), esters and oil-soluble polyalkylene glycols (OSP). The market demand for ULV ATFs has been very low so far. At the same time, viscosity wise, ULV ATFs are a better fit for high-speed EV reduction transmissions. Even though being called “high-speed,” gear in an EV transmission operate at variable speeds from zero to the maximum speed of the motor, with a high torque instantly available already at low speeds. This complicates the lubricant selection as the lubricant should be able to provide both adequate gear protection against scuffing and wear at low speeds and good efficiency and heat removal at high speeds. Wear on the teeth can be a limiting factor at low speeds. Wear particles may affect other transmission components, such as bearings and actuators. To understand the wear behavior, a whole system approach is essential. In general, EV transmission fluids call for a different spectrum of properties compared to conventional ATF: some properties being universally important for both: efficiency, durability, seal compatibility, wide operating range, environment, health and safety profile, with some other properties such as oxidation stability, copper corrosion and electrical conductivity gaining increased significance. When it comes to improving gear tribology specifically for EVs, everything basically boils down to proper gear design and geometric optimization, which includes selecting right materials and methods for gear manufacture and post-processing. There is a delicate balance between gear accuracy and surface finish quality. In practice, we can never get perfect gears - good enough is the best. Gear accuracy is regulated by ISO 1328 and a number of national standards. The adequate accuracy for gears used in electric vehicles is around ISO 1328 Grade 6, but high-speed gears rated for speeds over 20,000-rpm have higher quality requirements. Assuming that gears have been machined with desired accuracy - which is normally accomplished by conventional grinding - additional surface finishing techniques can be applied in order to further optimize the surface roughness and waviness profiles. These include a variety of abrasive and non-abrasive processes, such as shaving, lapping, honing, abrasive flow machining, turbo-abrasive machining, stream finishing, accelerated surface finishing, electropolishing, burnishing, etc. Recently developed mechanochemical surface finishing methods such as Triboconditioning ® CG can be used as the final finishing operation bringing about a triad of effects: (i) surface roughness profile optimization, (ii) compressive stress buildup, and (iii) tribofilm priming, which greatly improves the tribological and NVH behavior of gears [1]. In the present communication, we demonstrate the effects of different lubricants and surface finishing technologies on the tribology of gears using tribological tests and advanced thermal elastohydrodynamic simulations. Important roles of lubricity additives and surface finish optimization are highlighted in conjunction with a move towards ultralow viscosity fluids. 2. Results and Discussion Transmission fluids used in this study were blended by using a mixture of API Group II 600N and PAO3.5. Additional experiments were carried out using sustainable while oils produced from waste plastic instead of PAO. A commercial ATF additive package was used with the same treat level for all viscosity grades, see Table 1. For scuffing resistance evaluation, an FZG back-to-back gear test rig with FZG type A gears was used according to the standard ASTM D5182 procedure (A/ 8.3/ 90). Low-speed wear measurements were carried out according to ASTM D4998. For efficiency studies, an FZG back-to-back gear test rig with FZG type C gears with tip relief was used. Some gear pairs were additionally superfinished using the Triboconditioning ® CG method. 98 24th International Colloquium Tribology - January 2024 The Effects of the Lubricant Properties and Surface Finish Characteristics on the Tribology of High-Speed Gears for EV Transmissions Table 1: The properties of gear oils in study Viscosity grade KV40, cSt KV100, cSt Density, g-cm-3 ISO VG 100 98.1 11.2 0.87 ISO VG 46 46.2 7.0 0.85 ISO VG 22 23.4 5.0 0.83 ISO VG 15 14.5 3.5 0.82 The surface of gears was characterized using using a Form Talysurf Intra stylus-type surface metrology system and an Optosurf OS500 scattered light system. In the FZG A/ 8.3/ 90 scuffing test, all four oils managed to reach load stage 12 without scuffing as was expected based on the add-pack specifications. The low-speed wear data show relatively large variations, but in general, mechanochemically finished gears demonstrate a significant reduction in wear, 3 to 5 times on the average, see Figure 1. Figure 1: The effects of surface finishing technique and lubricant viscosity on pinion wear The total losses for the four oil viscosity grades in study are shown in Figure 2. The lowest viscosity grade is associated with the lowest loss. Figure 2: Total friction loss measured for different oil viscosity grades (LS 9, 60- o C) It should be noted that, as high-speed gear applications are concerned, PLVs in excess of 100-m/ s can be encountered in extreme cases. Such conditions cannot be emulated by the current FZG efficiency test, highlighting the need for a dedicated high-speed test rig. One would expect that a reduction in surface roughness should automatically lead to a reduction in friction torque. However, in practice, this is not always the case as there is an intimate interplay between the gear microgeometry and the surface roughness. With mass-finishing techniques, it is virtually impossible to modify surface roughness without incurring some subtle modifications of the microgeometry. The surface roughness profiles of conventionally finished gears and mechanochemically finished gears are compared in Figure 3. Figure 3: Typical surface roughness profiles of conventionally ground (GR) and mechanochemically finished (MC) gears The TEHD simulations indicate a significant reduction in friction shear stress and the asperity contact ratio, which is beneficial for gear tribology. The type of the base oil and the addpack have a significant effect on the result. For instance, polyalkylene glycols (PAG) have lower asperity-asperity friction than conventional hydrocarbon-based products. Figure 4: Calculated friction torque for conventionally ground and mechanochemically finished low-loss helical gears with VG-15 and VG-46 lubricants References [1] B. Zhmud, M. Najjari, B. Brodmann, L. Everlid, Proc. 64 th German Tribology Conference, September 25-27, 2023, Göttingen, Germany. 24th International Colloquium Tribology - January 2024 99 Effects of Calcium Detergents on Micro-Pitting of Gear Metals Akira Tada 1,2,3* , Dirk Spaltmann 2 , Kazuo Tagawa 3 , Valentin L. Popov 1 1 Technical University of Berlin, Berlin, Germany 2 Federal Institute for Materials Research and Testing (BAM), Berlin, Germany 3 ENEOS Corporation, Tokyo, Japan * Corresponding author: akira.tada@campus.tu-berlin.de 1. Introduction For achievement of carbon neutrality by reducing carbon dioxide emissions, electrified vehicles (EV) such as hybrid electric vehicles (HEVs), plug-in HEVs, and battery EVs attract much attention. To increase range of EVs, reducing transmission torque loss in transaxles is effective. For reducing churning and drag loss, viscosity reduction of lubricants in transaxles is one promising method [1]. However, it generally leads to thin oil films and makes the lubrication condition severe. This sometimes results in fatigue such as micro-pitting, especially at gear contacts. Because micro-pits are known to be initiated on metal surface, lubricant additives which form tribo-films on metal surfaces are considered to affect formation of micro-pits [2]. Some of such additives are calcium detergents. Recently, it was reported that an optimal choice of calcium detergents could lead to high anti-fatigue performance, even if lubricant viscosity is low [3, 4]. However, the mechanism about how tribo-films of calcium detergents affect the formation of pits requires further investigation. In this study, to clarify influence of calcium detergents on micro-pitting, slip-rolling tests were carried out using twin disc tribometers. 2. Experimental The conditions for tribological tests were set so that they were similar to those at gear contacts, i.e., the oil temperature was set to 100-°C, slide-to-roll ratio to -9%, and the maximum Hertzian contact pressure to 2-GPa. SCM420H type of steel was chosen as substrate for the discs because it is widely used as material in transmission gears. Two types of discs were used for experiments: specimen discs and counter discs (Table 1). Counter discs had curvature on their circumference for achieving the high Hertzian pressure above. Since micro-pitting is known to be initiated by roughness asperities, roughness of counter discs was controlled to be relatively large (R a -=-0.3-mm). Furthermore, to maintain the large roughness of counter discs and to promote micro-pitting on specimen discs, softer specimen discs were used. Table 1: Specification of discs Specimen disc Counter disc Material JIS SCM 420H Curvature w/ o w/ (R = 25-mm) Ra, mm 0.04 0.3 Hardness HRC 58 HRC 61 Four kinds of lubricants were created and evaluated to investigate the effect of calcium detergents. Their formulation is shown in Table- 2. They were formulated using poly alpha-olefin (viscosity at 100-°C was 4-mm 2 / s) as base oils. All lubricants included antioxidants (AOs) for prevention of oil degradation during tests. Tricresyl phosphates (TCP), which are typical anti-wear agents, were added for Oil B so that the concentration of phosphorus was 800-ppm. Oil C and D further contained different types of calcium detergents, calcium sulfonates and calcium salicylates, respectively. The concentration of calcium for oil C and D was 600-ppm. Slip-rolling tests were carried out up to 7-million cycles. Each test was stopped at 0.1, 1 and 4-million cycles and discs were detached for surface observation with optical and confocal microscopes. After the observation, they were attached again, and test was continued. Table 2: Formulation of lubricants Oil A Oil B Oil C Oil D Base oil Poly-α-olefin Bal. Bal. Bal. Bal. Antioxidant Phenol type mass% 0.5 0.5 0.5 0.5 Anti-wear agents TCP massppm (P) 800 800 800 Detergents OB Ca Sul. massppm (Ca) 600 OB Ca Sal. massppm (Ca) 600 Kinematic viscosity 100-°C mm 2 / s 4.0 4.0 4.0 4.0 TCP: Tricresyl phosphate, OB: Over-based, Sul.: Sulfonates, Sal.: Salicylates 100 24th International Colloquium Tribology - January 2024 Effects of Calcium Detergents on Micro-Pitting of Gear Metals 3. Results and Discussion In Figure- 1, the friction coefficients of four lubricants are shown up to 7-million cycles. Comparing Oil A, B and C, it was found that friction was increased by addition of TCP and further by addition of Ca sulfonates. On the other hand, Oil D showed almost the same value as the Oil B, indicating that Ca salicylates did not increase friction. Figure 1: Friction coefficient The micrographs of the surface on the specimen discs are shown in Figure-2. In the case of Oil A, there was no micro-pit even after 7-million cycles. Addition of TCP and Ca sulfonates increased the number of micro-pits, while Ca salicylates did not significantly. Figure 2: Micrographs of surfaces of specimen discs Micro-pit area and wear volume was measured using optical and confocal microscope. The results are shown in Figure-3. Oil C, which had Ca sulfonates, was able to suppress wear, but it drastically increased micro-pit area as well. In contrast, Oil D, which included Ca salicylates also showed small wear, but micro-pit area stayed low at the same time. Figure 3: Wear volume and micro-pit area of specimen discs To clarify the influence of the additives on the running-in process, roughness change of the counter discs was measured using confocal microscope. The reduction percentage of R a value is shown in Figure 4. Oil A showed rapid decrease of R a , which could be related to the decrease of friction. TCP suppressed roughness reduction and Ca sulfonates did it further. Ca salicylates also suppressed roughness reduction, but the degree was smaller than that of Ca sulfonates. Figure 4. Roughness reduction of counter discs These results indicate that the tribo-film created by TCP/ Ca sulfonates showed high anti-wear properties, but, with respect to surface roughness, it led to an un-finished running-in process, which resulted in severe micro-pitting. On the other hand, Ca salicylates showed similar high anti-wear properties and simultaneously achieved enough running-in which prevented acceleration of micro-pitting. This difference might be related to the formation rate of tribo-films. Further analyses on the tribo-films are on-going for clarification of mechanisms. 4. Conclusion Slip-rolling tests were carried out to investigate effects of Ca detergents on micro-pitting behaviours on gear metals. The followings were found. 1. By adding Ca sulfonates, wear was reduced, but micro-pit formation was accelerated. 2. Ca salicylates did not accelerate micro-pitting, reducing wear at the same time. References [1] Iino, M. et al., SAE Technical Paper (2021) 2021-01- 1215. [2] Ueda, M. et al, Tribol. Int. 138 (2019) 342-352. [3] Tada, A. et al., SAE Int. J. Adv. & Curr. Prac. in Mobility 5(3) (2023) 1055-1062. [4] Tokozakura, D. et al., SAE Technical Paper (2022) 2022-01-1103. 24th International Colloquium Tribology - January 2024 101 Friction Reducing Effect of Lubricants Applied to Organic Fibres Igor Velkavrh 1* , Nicole Dörr 2 1 V-Research GmbH, Stadtstrasse 33, 6850 Dornbirn, Austria 2 AC2T research GmbH, Viktor Kaplan-Strasse 2/ c, 2700 Wiener Neustadt, Austria * Corresponding author: igor.velkavrh@v-research.at 1. Introduction Organic fibres suffer from wear and hence fibre rupture when sliding against each other and hard surfaces. To achieve a better understanding of the lubrication mechanism of these fibres, better understanding of its tribological behaviour is needed; however, only a limited number of friction and wear studies have been reported in the literature [1]. Within the present study, a similar approach was applied to determine to what extent the use of a lubricant can have a positive effect, i.e., friction reducing effect, on the tribological properties of tribosystems with fibres. Therefore, the aim of the present study was to establish a methodology for identifying the contact conditions at which the friction reducing effect occurs. Focus was put on temperature and load. 2. Experimental methods Dry, i.e., lubricant-free, fibres were coated with bio-based and synthetic lubricants. Tests with dry and coated fibres were performed on an oscillating tribometer (SRV ® 4, Optimol Instruments GmbH, Germany) with organic fibres wrapped around a steel cylinder. Sliding against a tungsten carbide plate was performed at different contact pressures and temperatures. During the tribological tests, coefficient of friction, normal force, contact resistance and surface temperature were continuously measured. After the tests, the appearance of the organic fibres and the tribofilm on the tungsten carbide plate were characterized. In Table 1, parameters applied in the tribological tests are listed. Table 1: Parameters applied in the tribological tests. Parameter Settings Normal force (N) 10 for 2 min (running in), afterwards 45 Hertzian contact pressure (MPa) max. 210/ mean 170 Temperature (°C) 21-°C for 2 min (running in), afterwards heating to 250-°C Stroke (mm) 4 Oscillation frequency (Hz) 1 Sliding velocity (m/ s) max. 0.0126/ mean 0.008 Test duration Until the rupture of organic fibres (observed in an increase of the coefficient of friction and a drop in the contact resistance) 3. Results and Discussion Figure 1: Coefficient of friction, surface temperature and normal force in dependence of testing time. Figure 2: Tribofilm on the tungsten carbide plate after the friction test. In Figure 1, coefficient of friction, surface temperature and normal force in dependence of testing time are presented. During the running-in phase performed during the first 2 minutes of test at ambient temperature under a normal load of 10 N, coefficient of friction was around 0.3. After the increase of normal load to 45 N, coefficient of friction instantaneously decreased to 0.2. Dependence of the coefficient of friction on the nominal contact pressure is typically observed for rubber materials and can be related to (among other) the saturation of the contact area (and friction force) due to high nominal squeezing pressure, non-linear viscoelasticity, adhesion, and frictional heating [2]. Afterwards, as temperature was increased coefficient of friction further decreased and reached a minimum of around 0.06 at a temperature of around 170-°C. 102 24th International Colloquium Tribology - January 2024 Friction Reducing Effect of Lubricants Applied to Organic Fibres This low-friction phase lasted for around 2-minutes, until afterwards coefficient of friction started to increase and became unstable indicating the rupture of the fibres and the removal of the lubricating film from the tribocontact. In Figure 2, a micrograph of the tribofilm on the tungsten carbide plate after the friction test is presented. A relatively thick viscous film formed on the surface. Obviously, the sufficiently high contact pressures and temperatures promoted the extraction of the lubricating fibre components and the applied lubricant from the fibres. 4. Conclusions The lubricating mechanisms of organic fibres depend strongly on load and temperature. Correlations between friction reduction with lubricants and the durability of the fibres (or the prevention of fibre rupture) were found. Furthermore, it was elaborated at which parameters friction reduction can be triggered. Acknowledgements Parts of the presented work were funded by the Austrian COMET Program (Project InTribology, no. 872176) and carried out at the “Excellence Centre of Tribology” (AC2T research GmbH) in collaboration with V-Research GmbH. References [1] N. Ismail, M. B. de Rooij, E. G. de Vries, N. H. Mohd Zini, D. J. Schipper, Friction between single aramid fibres under pre-tension load, Tribology International 137 (2019) 236-245. [2] G. Fortunato, V. Ciaravola, A. Furno, M. Scaraggi, B. Lorenz, B. N. J. Persson, On the dependency of rubber friction on the normal force or load: theory and experiment, Tire Science and Technology (2017) 45 (1): 25-54. 24th International Colloquium Tribology - January 2024 103 Lubricant Inerting - a New Era in Lubrication Technology Jie Zhang 1 , Janet Wong 1 , Hugh Spikes 1* 1 Tribology Group, Imperial College London * Corresponding author: h.spikes@imperial.ac.uk 1. Introduction The concept of blanketing an operating lubricant in nitrogen gas to prevent its oxidative degradation, so-called “inerting”, was first considered by NASA in the 1960s in the context of high temperature aerospace transmission lubrication [1,2]. The principle is quite simple. From the lubricant oxidation cycle as shown in Fig, 1, it is evident that oxygen plays the key role, both in the initiation phase and in sustaining the highly damaging chain-branching hydroperoxide cycle. If there is little or no oxygen present, then there can be little or no hydrocarbon oxidation. Fig. 1: Hydrocarbon oxidation cycle Throughout the 1960s considerable efforts were made to explore this inerting possibility and it was found possible to operate aerospace transmissions lubricated with synthetic hydrocarbons and ester-based oils in prolonged tests at up to 370-°C, with negligible lubricant degradation [2]. Also, to some surprise, it was found that many lubricants showed lower friction and wear in a nitrogen atmosphere than in air, a phenomenon that was eventually ascribed to “oxidative wear” in which oxygen promotes the formation of high friction, easily worn oxides [3]. Eventually it was accepted by the late 1960s that this inerting approach was simply not practicable for aerospace application at that time. High levels of nitrogen leakage from the operating gearboxes could not be prevented and so would necessitate a continual supply of the inert gas during use. However, the storage of nitrogen in aircraft in the quantities needed was simply not feasible [2]. Matters have now changed. In the last decade, low cost, and in some cases portable, membrane-based nitrogen and oxygen concentrators have become commercially available that are able to separate nitrogen and oxygen from air to provide high flow rates of either gas. Oxygen concentrators are now widely used in hospitals, homes and while travelling to support patients with breathing difficulties. Nitrogen concentrators are being employed, for example, in food packaging, laboratory synthesis and for instruments such as mass spectrometers that require an inert gas flow. These can deliver more than 10 litres/ min of almost pure nitrogen, have power consumptions of typically 10 W, and useful life between membrane changes of several years. In principle this technology now makes it quite feasible to operate stationary and mobile, closed lubricated components in an almost pure nitrogen atmosphere, with nitrogen being supplied as needed, e.g. when the machine is operating or when it exceeds a certain temperature. The potential benefits are evident as indicated in Table 1 and clearly offer considerable improvements in sustainability at several levels. Table 1: Potential benefits of lubricant inerting • Much longer lubricant lives (fill for life) • Higher component operating temperature (>100-°C hotter than at present). Reduced cooling needs. • Much wider application of bio-based lubricants • Reduced acid formation and consequent yellow metal corrosion Initial applications envisaged are those that operate at high temperatures, such as hydraulics and steel rolling bearings, and those where lubricant replacement is costly, for example in wind turbine transmissions. However, once the technology is established a much broader range of inerted applications is likely. For lubricant inerting to be introduced safely and robustly, two research questions must be addressed... a. Can lubricants that have been deigned to operate in air function effectively in preventing friction/ wear/ scuffing etc. when little or no oxygen is present? If not, how should lubricants be modified? It should be noted that the solubility of nitrogen in base oils is similar to that of oxygen, so there are no implications in term of hydrodynamic or EHD lubrication of replacing an atmosphere that already has 80% nitrogen to one of almost pure nitrogen. b. What are the predominant lubricant degradation mechanisms in atmospheres containing little or no oxygen, and thus what extension of useful lubricant life can be expected? For example, if we reduce oxygen concentration from 21% (as present in air) to 2%, can we expect a ten-fold increase in lubricant life? The research outlined in this presentation aims to address both of the above questions and thus facilitate the introduction of lubricant inerting technology. 2. Test Methods For both friction and wear tests and high temperature lubricant degradation tests, a PCS high frequency reciprocating rig (HFR) was located in a sealed Perspex chamber with inflow and outflow gas ports and containing an oxygen-level 104 24th International Colloquium Tribology - January 2024 Lubricant Inerting - a New Era in Lubrication Technology sensor (Fig. 2). Base oils used were hexadecane and PAO10 (SpectraSyn 10). Fig. 2: HFR in atmosphere-controlled chamber 3. Friction and Wear Results As was seen in the 1960s, in the absence of lubricant additives, friction and wear of base oils were both found to be considerably lower in N 2 than in air. However, we have shown that this does not originate from oxidative wear, as previously suggested, but rather from the formation of protective carbon-based tribofilms in the absence of oxygen [4]. When O 2 is present these carbon films do not form since oxygen immediately reacts with hydrocarbyl free radicals generated mechanochemically by rubbing and thus prevent the radicals forming tribofilms. In the latter’s absence, friction and wear are dominated by adhesion of metals and metal oxides. Fig. 3 compares the friction of solutions of two friction modifiers in hexadecane at 60- °C, MoDTC and glyceryl monooleate (GMO). MoDTC gives almost identical performance in the two atmospheres, suggesting that molecular oxygen does not participate in its friction-reducing tribofilm formation. However, GMO consistently shows lower friction and wear in nitrogen than in dry air. We believe that this is because GMO adsorbs more effectively on a carbon film than on iron or iron oxide. 4. Lubricant Degradation Results For degradation tests, a high temperature version of the HFR, able to reach 400-°C, was employed. At the end of two hours tests the lubricant was extracted and its viscosity measured. Because a rubbing contact is present, this test measures the ability of the lubricant to withstand tribodegradation. This is recognised to be more severe than autooxidation as measured in glassware-based tests, perhaps because catalytic ferrous ions are generated during rubbing and/ or hydrocarbyl free radicals that initiate oxidation are produced in rubbing contacts. Typical results are shown in Fig. 4. This compares the endof-test viscosity at 25-°C from PAO10 base oil degradation tests in (i) cylinder N 2 (0% O 2 ), (ii) 0.5% O 2 in N 2 (from a nitrogen concentrator) and (iii) cylinder dry air (20% O 2 ). It is evident that there is negligible viscosity increase in N 2 and 0.5% O 2 , even at 300-°C, while in dry air substantial degradation occurs. For the test in dry air at 300-°C, a solid-like brown residue, whose viscosity could not be reliably measured, was left in the lubricant bath at end of test. Fig. 3: Friction of MoDTC and GMO solutions Fig. 4: End of test viscosity of PAO10 tested in cylinder N 2 , dry air and 0.5% O 2 from nitrogen concentrator Conclusion We are currently exploring the use of nitrogen concentrators to blanket operating lubricants in a low or zero oxygen environment to greatly reduce lubricant degradation. Results to date illustrate the overall feasibility of the approach both in terms of both tribology and lubricant life. Acknowledgement We thank ExxonMobil for supply of SpectraSyn TM 10. References [1] Rhoads, W. L. Supersonic transport lubrication system investigation. NASA CR-72424, Sept 1968. [2] Loomis W. R., Townsend, D. P, Johnson, R. L., “Lubricants for inerted lubrication system in engines for advanced aircraft, NASA TN D-5420 1969. [3] Appeldoorn, J. J., Goldman I. B., Tao, F F., Corrosive wear by atmospheric oxygen and moisture, ASLE Trans., 12: 2, 140-150, (1969). [4] Zhang, J., Campen, S., Wong, J. S. S, Spikes, H. A., Oxidational wear in lubricated contacts - or is it? Trib. Intern. 165, 107287, (2022). 24th International Colloquium Tribology - January 2024 105 Tribological Behaviour of Polymer Compounds containing Microencapsulated Lubricants Susanne Beyer-Faiss 1* , Regina Wannenmacher 1 , Thomas Witt 1 , Moritz Grünewald 2 1 Dr. Tillwich GmbH Werner Stehr, Horb-Ahldorf, Germany 2 SKZ - KFE gGmbH, Freiburg, Germany * Corresponding author: susanne.beyer.faiss@tillwich-stehr.com 1. Introduction Worldwide, polymer materials are used in millions of components subjected to friction and wear processes. Especially in applications where additional external lubrication has to be avoided, slip modified polymers are used. For example, PTFE is widely used as an additive in polymers that significantly reduces friction and wear and minimizes stick slip effects. In view of the future restriction of fluorine-containing substances through the planned PFAS strategy of the EU, the development of alternative systems that do not contain fluorine, is very important. Such an alternative can be micro-encapsulated lubri-cants, which are incorporated in the polymer matrix, and which shall have a beneficial effect on the friction and wear behaviour even when running dry, but with-out weakening the mechanical properties too much. To assess the tribological properties, various model testing systems with different load collectives were used to explore the application limits of the newly developed polymer materials. The materials used in this study were produced as part of the project „Tribologically effective pseudo-solids for mechanically and thermally highly loaded thermo-plastic components“ (project ID 21707 BG of the Fördergemeinschaft für das Süddeutsches Kunststoff-Zentrum e.V. FSKZ). The project is funded by the Fe-deral Ministery for Economy and Climate Protection (BMWK) via the Consortium of Industrial Research Associations (AiF) in the frame of the programme to support industrial joint research (IGF). 2. Experimental 2.1 Materials Figure 1: Ester oil filled microcapsules Base polymer material is PP, which has been modified with 10% microcapsules. The microcapsules are built with a skin of melamine resin and filled with an ester oil. They have a spherical shape with a diameter around 20-µm. As reference material the neat PP as well as a modification of PP with 30% glass fibers and 15% PTFE has been tested. All tests have been performed under dry running conditions as well as equipped with an initial minimum quantity lubrication 2.2 Tests in model system sphere-on-plate under oscillating motion Friction and wear tests have been performed using the sphere-on-plate model system under oscillating motion using ½” spheres out of steel 1.3505 and the plates out of the polymer materials. Friction coefficients and linear wear were monitored at two load settings (25N and 50N) with a frequency of 4 Hz and an amplitude of 4-mm for 2 hours at ambient temperature (short term tests). It could be found, that the incorporation of 10 % microcapsules improves the wear rates between 50 to 70 %, whereas the friction values nearly stayed on the same level. The excellent wear rates of an optimized PP blend with GF/ PTFE cannot be reached. They are better by one order of magnitude. 2.3 Tests in model system sphere-on-prism under rotating motion Friction and wear tests have been performed using the sphereon-prism (ISO 7148-2) model system under unidirectional rotating motion using ½” spheres out of steel 1.3505 and the prism out of the polymer materials. Friction coefficients were monitored dependent on sliding speed (0-210 mm/ s) and load (1-3-6 N) at 25 °C (short term tests). Figure 2: Under dry sliding conditions the base material PP shows a typical speed dependent behaviour: static friction values of 0.20 rising up to 0.53 at sliding speed 200-mm/ s. Incorporation of microcapsules shows a tendency to higher friction values than the neat material. The reference 106 24th International Colloquium Tribology - January 2024 Tribological Behaviour of Polymer Compounds containing Microencapsulated Lubricants material with PTFE/ GF filler has lower friction values and a lower dependence on sliding speed. The difference of friction coefficients compared to an initial small volume lubrication with the microcapsules´ oil and the base polymer is remarkably lower around one magnitude. 2.4 Tests in model system plain bearing on shaft under rotating motion The injection moulded polymer plain bearings with an inner diameter of 5mm are checked in long-term tests under application-related conditions in start-stop operation. The tests (ISO 7148-2, alternating rotating motion) are performed in sliding combination with a steel 1.3505 shaft (diameter 5 mm) and a bearing load of 15N and 30 N under an intermittently rotating movement with 240 cycles, each with an increasing and decreasing profile of the rotation speed from zero to max. 100 rpm (continuously accelerated within 120s) and reverse (continuously reduced within 10s). In between the cycles a standstill period of 5s is established, symbolizing 240 start-stop conditions. In addition, the linear wear of the bearing is monitored. Under dry sliding conditions the base polymer PP shows high friction values, even under the low test load configuration with 15N bearing load. At 30N test load friction forces measured exceeded the measuring range of the friction sensor. Figure 3: Incorporation of 10% microcapsules improves the friction behaviour of PP remarkably: at 15 N and 30 N test loads the bearings showed a smooth running in behaviour and friction values stabilizing in between 0.2 and 0.3 dependent on sliding speed. Figure 4: The contacting zone with the steel axis in the test bearing after the tests looks like polished. There is no wear visible. Figure 5: Compared to the reference material PP with glass fibers and PTFE fillers the results are different: there is no pronounced running in of the bearing system visible and friction coefficients vary around 0.25 to 0.28. Figure 6: Within the contact zone with the steel axis, significant wear is produced on the PP/ GF/ PTFE bearing. In turning direction of the axis, abrasive wear is visible. The wear particles are disposed in the running-out zone outside the sliding contact. 3. Conclusion The incorporation of oil filled microcapsules within a polymer matrix does not show in all tested cases a positive influence on the friction and wear behaviour compared to the neat polymer. Dependent on the model system, the type of motion and the load-speed setting chosen, the response of the microcapsules has to be differenciated. In some cases, the friction and wear reduction is nearly as good as tested with the reference material containing glass fibers and PTFE fillers. At higher loads friction coefficients stronger increase with accelerating speeds compared to the neat material. The incorporation of oil filled microcapsules to a polymer cannot reach the effect of an external lubrication. Even an initial lubrication with only a minimum quantity of oil improves friction and wear reduction of the base polymer remarkably. In a next step, also a combination of microcapsules in combination with glass fibers shall be considered. 24th International Colloquium Tribology - January 2024 107 Early Stages of Tribo-Oxidation in Single Crystalline Copper Ines L. Kisch 1, 2 , Julia S. Rau 3 , Vahid Tavakkoli 4 , Lisa T. Belkacemi 5, 6 , Baptiste Gault 6, 7 , Christian Greiner 1, 2, * 1 Institute for Applied Materials (IAM), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany 2 IAM-ZM MicroTribology Center µTC, Karlsruhe, Germany 3 Department of Physics, Chalmers University of Technology, Gothenburg, Sweden 4 Institute of Nanotechnology (INT), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany 5 Leibniz-Institut für Werkstofforientierte Technologien (IWT), Department of Physical Analysis, Bremen, Germany 6 Max-Planck-Institut für Eisenforschung GmbH, Department of Microstructure Physics and Alloy Design, Düsseldorf, Germany 7 Department of Materials, Royal School of Mines, Imperial College London, London, UK * Corresponding author: christian.greiner@kit.edu 1. Introduction Tribological loading on metallic surfaces often leads to accelerated oxidation, which can significantly influence the resulting friction and wear behavior as well as component lifetime. It is therefore important to better understand the underlying chemical and mechanical mechanisms in order to engineer materials with better friction and wear performance. By performing experiments with varying cycle numbers with a copper-sapphire model system, we were able to monitor the sequence of stages in the microstructural evolution and oxide formation, and investigate especially the very early stages of tribo-oxide formation. 2. Materials and Methods Copper single crystals were used in order to eliminate the influence of pre-existing grain boundaries, while sapphire counter bodies were chosen due to their hardness and chemical inertness. The samples underwent heat treatment and were then ground and polished. Details of the procedure are given in [1], [2]. Immediately before the tribological experiment, the samples were electropolished to provide a surface with significantly reduced pre-existing deformation that also exhibits only minimal amounts of native oxidation. Experiments were conducted at room temperature with a low normal load of 1.5 N, in a reciprocating motion with a sliding speed of 0.5 mm/ s. The relative humidity of the surrounding atmosphere was kept at 50 % (± 3 %), while the number of sliding cycles was varied (10 and 100 cycles respectively). In order to assess the oxidative features developed under the tribological loading, two protective platinum layers (first with the electron, then with the ion beam) were deposited onto the region of interest to preserve the surface. Subsequently, scanning (transmission) electron microscopy (S(T)EM) as well as focused-ion beam milling (FIB) were employed to create cross-sections and foils, revealing the oxide evolution into the depth of the material. Additionally, the foils were characterized by energy-dispersive X-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS) and high resolution transmission electron microscopy (HR-TEM). These techniques allowed for the analysis of the chemical composition as well as the crystallographic structure of the oxides. 3. Results Figure 1 shows TEM images from a single crystalline copper sample after ten sliding cycles. The foil was cut parallel to the sliding direction and perpendicular to the sample surface. Figure 1a is using an high-angle annular dark field (HAADF) contrast that gives an elemental contrast. Dark, randomly distributed clusters can be seen below the surface (exemplary ones marked with arrows), these were confirmed by EDS to be rich in oxygen. In order to analyze the clusters further, HR-TEM was performed, the results for one exemplary cluster are depicted in Figure 1b. Fast Fourier transformation (FFT) was performed to reveal the crystallographic structure and showed that the oxide clusters are a mix of amorphous and nanocrystalline areas. Figure 1: Copper sample after 10 sliding cycles. a) HAADF TEM image from a foil cut perpendicular to the surface. The dark contrast shows oxide clusters just below the surface. b) HR-TEM image of an exemplary oxide cluster. FFT shows that there are crystalline as well as amorphous areas within the cluster. We speculate that oxidation initiates at surface defects, where oxygen diffusion into the material leads to the initial formation of supersaturated, amorphous clusters. With the formation of these clusters, a new phase boundary is formed, that subsequently acts as a preferred pathway for oxygen diffusion. This eventually leads to higher levels of oxygen in the outer areas of the clusters, which eventually reach the Cu 2 O stoichiometry and therefore, the formation of crystal- 108 24th International Colloquium Tribology - January 2024 Early Stages of Tribo-Oxidation in Single Crystalline Copper line Cu 2 O can be observed starting from the outer particle borders. In a second experiment, a sample was loaded with 100 sliding cycles, results from the respective TEM analysis are depicted in Figure 2. At this stage, the copper exhibits a fully formed oxide layer with a mean thickness of 0.3-µm. The HAADF image in Figure 2a shows a distinct, line-like feature in a depth of approximately 100-nm below the sureface, which is reminiscent of the dislocation trace line (DTL) observed e.g. in [1]. The darker contrast suggests a higher oxygen content in that area. To confirm this assumption, EELS measurements were performed on the indicated region of interest (ROI), the results are shown in Figure 2b. A line profile measured across the region indicated by the black box shows the atomic percentage of copper and oxygen respectively, plotted over the depth into the material. At the height of the presumed DTL, an elevated oxygen concentration of up to 50 at % is observed. We speculate that a DTL is formed in the early stages of sliding, and that the locally increased dislocation density leads to a locally higher oxygen concentration. Figure 2: Copper sample after 100 sliding cycles. a) HAADF image from foil perpendicular to the surface. At the sample surface, a fully formed oxide layer is visible (dark contrast). On the right hand side, there is a magnified image of the ROI. b) EELS measurement in the ROI. Green indicates copper, while red indicates oxygen. The graph on the right shows a depth profile measured across the indicated area and plots the atomic percentage of Cu and O respectively over the depth into the material. At the height of the DTL, a near 1: 1 composition is observed. The observed Cu and O composition potentially allows for the formation of CuO. The area was subsequently analysed with HR-TEM, and FFT analysis of the images showed a crystalline area with a monoclinic structure, further supporting the hypothesis that CuO has formed locally. Additionally, the EELS measurements show copper rich areas depleted in oxygen just below the surface. These are likely caused by an increased upward diffusion of copper through the oxide. This assumption is supported by the observation of potential Kirkendall pores at the copper-oxide interface. 4. Conclusion The results of this study suggest that there is a distinct correlation between microstructural features that evolved during sliding and the diffusion and local distribution of oxygen within the material. This results in the local formation of different copper oxides. Investigating and understanding the fundamental mechanisms at play will, in the long term, enable the targeted development of frictionand wear-optimized surfaces. References [1] C. Greiner, Z. Liu, L. Strassberger, P. Gumbsch, „Sequences of Stages in the Microstructure Evolution in Copper under Mild Reciprocationg Tribological Loading”, ACS Appl. Mater. Interfaces, Vol. 8, no. 24, pp. 15809 - 15819, 2016. [2] Z. Liu, T. Höche, P. Gumbsch, C. Greiner, „Stages in the tribologically induced oxidation of high-purity copper”, Scr. Mater., Vol. 153, pp. 114 - 117, 2018. 24th International Colloquium Tribology - January 2024 109 Effect of Atmospheric Composition on the Friction and Wear of Cobalt-Based Alloys at Elevated Temperatures Tobias König 1* , Philipp Daum 2 , Dominik Kürten 1 , Andreas Kailer 1 , Martin Dienwiebel 2 1 Fraunhofer Institute for Mechanics of Materials IWM, Microtribology Center mTC, Wöhlerstr. 11, 79108, Freiburg, Germany 2 Institute for Applied Materials - Reliablity and Microstructure at the Karlsruhe Institute of Technology, Straße Am Forum 7, 76131, Karlsruhe, Germany * Corresponding author: tobias.koenig@iwm.fraunhofer.de 1. Today’s challenges Today`s increasing efficiency and emission requirements of internal combustion engines lead to higher combustion temperatures and pressures and require complex exhaust gas systems with multi-stage turbochargers and exhaust gas recirculation. Due to the high control effort in these exhaust systems, many different actuators were used, such as exhaust gas flaps or valves, VTG guide vanes, EGR flaps, brake flaps or wastegates in turbochargers. All these actuators include plain bearings, which are thermally loaded up to 800-°C and exposed to the exhaust gas atmosphere. Most of the tribological material pairings were tested and analysed under lab air atmosphere to investigate their wear behaviour and predict their lifetime. In general, a severe wear section at lower temperatures due to high abrasion or adhesion and a wear and friction reduction based on a glaze-layer formation was observed [1, 2]. But the surrounding atmosphere and the related tribochemical reactions play a major role in the wear behaviour especially for unlubricated tribosystems at elevated temperatures, shown by [3-5]. The aim of this study was to identify the atmospheric effect on the wear and friction of a widely used cobalt-based material combination at high temperatures. A reciprocating model test was used, combining a modified friction measurement with a line contact configuration and an atmospheric admission. 2. Tribological testing The tested material pairing consists of a cast cylinder of Tribaloy ® T400 rubbing on a steel plate (1.4713) coated with a 200-µm thick HVOF sprayed Tribaloy ® T800. An OPTIMOL SRV IV ® test rig with a specially developed cylinder-on-plate contact configuration was used to perform the reciprocating wear tests. A detailed description of the test method can be found in [5]. The Calculation of the energetic coefficient of friction was applied, a proved method for the high friction of unlubricated systems, which was developed by [6].To investigate the atmospheric effects, wear experiments were done in lab air and a CO 2 -N 2 -O 2 atmosphere with 5-vol.% O 2 and 15.2-vol.%-CO 2 , simulating exhaust gas. The tribological tests were performed with a normal force of 50-N, a stroke length of 1.6-mm, a frequency of 15-Hz at constant temperatures between RT and 800-°C for two hours. The wear measurement was conducted with a confocal microscope. Further analysis of the worn samples were conducted with a SEM, Raman spectroscope and XPS. 3. Important Results The measured friction behaviours (Fig. 1) of the tested atmospheres show up to 300-°C significant differences. In air an almost constant level around 0.6 was observed, whereas in the CO 2 -N 2 -O 2 atmosphere higher values up to 0.9 were measured. Above 300-°C both atmospheres show an increasing friction tendency up to 600-°C and at higher temperatures comparatively low values of 0.35. Fig. 1: Friction behaviours in different atmospheres Comparable to the friction, the wear of this material pairing (Fig. 2) also shows atmospheric differences up to 300-°C. In air the wear occurs mainly on the cylinder and shows minimal values for RT and 100-°C. In contrast, the wear in the CO 2 -N 2 -O 2 atmosphere takes place mainly at the plate. At 300-°C the cylinder determines the wear behaviour in both atmospheres, between 400-600-°C mainly the plate is worn out and at higher temperatures only a minimal amount of wear was measured. Fig. 2: Wear behaviours in different atmospheres 110 24th International Colloquium Tribology - January 2024 Effect of Atmospheric Composition on the Friction and Wear of Cobalt-Based Alloys at Elevated Temperatures For the identification of different tribological wear mechanisms due to atmospheric variations and their impact on the wear scar formation selected plates were analysed with confocal microscopy. The optical images of the surface and the topography is shown in Fig. 3. Fig. 3: Wear scars of plates tested at 200-°C In air the plate shows slight abrasive grooves, which can only be seen by high magnifications. Moreover, the porous structure of the HVOF-coating can clearly be seen also in the wear track, where debris sporadically adheres. Contrary, the plate tested in CO 2 -N 2 -O atmosphere shows larger voids in the wear scar, flaking out of the porous coating, but also high amount of adhering debris. At the corresponding position on the cylinder opposite the voids, large material transfer and adhering debris can be found. With XPS analyses a formation of metallic carbides in the debris was detected. Following this, the mainly abrasive wear of the cylinder in air is replaced by adhesive wear of the coating and adhesive debris formation in CO 2 -N 2 -O atmosphere. Between 400-600-°C, where no difference of the wear was documented, adhering debris on the cylinder result in a highly abrasively worn coating. At higher temperatures than 600- °C, a wear and friction reducing glaze layer formation takes place in both atmospheres. This layer is formed by oxidized wear particles, which were compacted and sintered by the high tribological pressures and temperatures in the contact zone. Raman analyses reveal a glaze layer composition out of Co 3 O 4 for 700 and 800-°C, that corresponds to the results of [2]. Following this, the high friction measured at 600-°C is constituted by an incomplete oxidation of the debris and their lower oxidation states. 4. Findings In summary, it can be stated that an exhaust gas-like atmosphere is highly affecting the tribological behaviour of the investigated material pairing (Fig 4). Lower oxidation rates in CO 2 -N 2 -O atmosphere led to a change from abrasion to adhesion at temperatures up to 300-°C. Due to the weaker stability and porous structure of the HVOF-T800-coating, large material flaking occurs and adheres on the cast cylinder out of Tribaloy ® T400. Moreover, tribochemical formed carbide debris groove the coating. However, the dominant tribological mechanisms at high temperatures were not affected by the different atmospheres in this study. It is concluded that the temperatures compensate lower oxygen amount and result in oxidic debris, forming a wear and friction reducing glaze layer. Finally, this research confirms the necessity to test and investigate tribological systems in their application-oriented environment, especially for HT-tribology. Fig. 4: Wear behaviours in different atmospheres References [1] Rynio C., Hattendorf H., Klöwer J. et al. (2014) The evolution of tribolayers during high temperature sliding wear. Wear 315: 1-10. [2] Dreano A., Fouvry S., Guillonneau G. (2020) Understanding and formalization of the fretting-wear behavior of a cobalt-based alloy at high temperature. Wear 452-453: 203297. [3] Velkavrh I., Ausserer F., Klien S. et al. (2016) The influence of temperature on friction and wear of unlubricated steel/ steel contacts in different gaseous atmospheres. Tribology International 98: 155-171. [4] Rahman M. S., Ding J., Beheshti A. et al. (2019) Helium Tribology of Inconel 617 at Elevated Temperatures up to 950- °C: Parametric Study. Nuclear Science and Engineering 193: 998-1012. [5] König T., Kimpel T., Kürten D. et al. (2023) Influence of atmospheres on the friction and wear of cast iron against chromium plated steel at high temperatures. Wear 522: 204695. [6] Fouvry S., Duó P., Perruchaut P. (2004) A quantitative approach of Ti-6Al-4V fretting damage: friction, wear and crack nucleation. Wear 257: 916-929-929. 24th International Colloquium Tribology - January 2024 111 Thermal-Elasto-Plastic Hydrodynamic Contact Between Rough Surfaces M. J. Montenegro Cortez 1 , P. Correia Romio 1 , C. M. da Costa Gomes Fernandes 1 , P. M. Teixeira Marques 2 , S. Portron 2 , J. H. O. Seabra 1 1 Universidade do Porto, Faculdade de Engenharia, Departamento de Engenharia Mecânica, Porto, Portugal, 2 INEGI - Universidade do Porto, Unidade de Tribologia Vibrações e Manutenç-o Industrial, Porto, Portugal 1. Introduction Machine elements often operate under a wide range of conditions, leading to significant variations in interfacial friction due to changes in the lubrication regime. The Stribeck curve provides a comprehensive overview of friction variance across the boundary, mixed, and full-film lubrication [1]. This study used a commercial ISO VG150 polyalphaolefin gear oil in friction measurements to determine the influence of the temperature, Slide-to-Roll Ratio (SRR), and surface roughness on the Stribeck curve. It was observed that all three factors influence the coefficient of friction (CoF) and the lubrication regime. It was observed that the temperature does not significantly impact the lubrication regime when keeping the remaining factors constant, but the surface roughness and SRR do. Using the Stribeck curves resulting from the Ball-on-Disc friction tests and the roughness parameters obtained from the surface roughness assessment, a new parameter was proposed to evaluate the influence of these factors on the Stribeck curve in opposition to the traditional Hersey number [2]. 2. Materials and Methods The Stribeck curves presented in this study were obtained using an EHD2 Ball-on-Disc apparatus (PCS Instruments, England, 2014). One chose a commercial ISO VG150 polyalphaolefin oil for this evaluation, with properties summarized in Table 1 [2]. The balls (19.05 mm diameter) and discs are made of 100Cr6 steel with a measured hardness of 820-HV. For discs and balls, two different isotropic surfaces were chosen, with average roughness (Ra) of 0.02 mm and 0.30 mm, which allows three different roughness combinations (0.02-mm, 0.16-mm and 0.30 mm). The roughnesses were acquired using 3D Optical Profilometer equipment (Bruker NPFLEXTM, Germany, 2013), with acquisitions before and after each test. For this campaign, two different SRRs were applied (0.5 and 1) and three oil temperatures (65-°C, 80-°C and 120-°C). The entrainment velocity ranged between 50- mm/ s and 2500- mm/ s, while the applied load was kept constant and equal to 50 N. Table 2 summarizes the whole campaign based on a factorial Design of Experiments. 3. Stribeck curve results Figure 1 presents the Stribeck curves from the experimental campaign using the Hersey (H) number, Eq. (1), for the horizontal axis. The H number aims to distinguish the three lubrication regimes, promoting a better distribution of the curves. However, it becomes evident from this analysis that the parameter used does not allow a complete differentiation between boundary, mixed, and full-film lubrication. (1) Where, v e is the entrainment velocity in m/ s; h is the viscosity in Pa s; R x is the equivalent radius of curvature in m; and F is the normal load in N. Table 1: ISO VG150 polyalphaolefin oil properties Property Unit Method Value Density @ 27-°C kg m -3 ASTM D 4052 849.8 Thermal expansion °C-1 - 7.53 3 10 -4 Viscosity @ 65-°C Pa s ASTM D 2983 0.0457 Viscosity @ 80-°C Pa s ASTM D 2983 0.0282 Viscosity @ 120-°C Pa s ASTM D 2983 0.0101 Viscosity Index / ASTM D 2270 150 Piezoviscosity @ 65-°C Pa -1 Gold 1.26 3 10 -8 Piezoviscosity @ 80-°C Pa -1 Gold 1.19 3 10 -8 Piezoviscosity @ 120-°C Pa -1 Gold 1.04 3 10 -8 Table 2: Stribeck curves campaign Test ID Roughness [mm] * Temperature [°C] SRR 1 0.02 65 0.5 2 0.02 80 1 3 0.02 120 0.5 4 0.16 65 1 5 0.16 80 0.5 6 0.16 120 1 7 0.30 65 0.5 8 0.30 80 1 9 0.30 120 0.5 * Composite roughness from disc and ball roughness combination 112 24th International Colloquium Tribology - January 2024 Thermal-Elasto-Plastic Hydrodynamic Contact Between Rough Surfaces On the other hand, Figure 2 presents the CoF curves using the proposed dimensionless New Parameter (NP), calculated with Eq. (2) and its constants in Eqs. (3) to (7) [2]. (2) (3) (4) (5) (6) (7) Where, E 9 is the Equivalent Young Modulus in Pa; α Gold is the piezoviscosity in Pa-1; is the composite reduced peak height roughness obtained from the surfaces after the friction tests; and α is a lubricant-dependent exponent, equal to 0.40 at 65-ºC, 0.39 at 80-ºC, and 0.37 at 120-ºC [2]. Figure 1: Stribeck curves with Hersey parameter Figure 2: Stribeck curves with New Parameter (NP) Based on Figure 2, the following limits are proposed to separate the lubrication regimes: NP > 0.15 for full-film, 0.015 < NP < 0.15 for mixed, and NP < 0.015 for boundary. The only exception in this approach is the test condition 8. A practical result from this analysis is that the CoF in the mixed lubrication regime can be estimated as a function of NP, as indicated in Eq. (8). The constants used in Eq. (8) are oil-dependent and valid for the ISO VG150 polyalphaolefin gear oil utilized in this study. (8) 4. Thermal-Elasto-Plastic Hydrodynamic lubrication model (TEPH) Finally, a TEPH model [3] was used to analyze the contact between rough surfaces under full-film, mixed, and boundary lubrication conditions. The CoF predictions by the TEPH model were correlated with the experimental Stribeck curves, as illustrated in Figure 3 for test condition 1. Figure 3: Stribeck curve test 1 versus TEPH model prediction 5. Conclusion Using Stribeck curves from the friction tests and the roughness parameters obtained from the surface roughness measurements after the frictional tests, a new parameter (NP) was proposed to evaluate the Stribeck curve lubrication regimes in a more practical way. A crucial aspect of this approach was using the roughness parameters after the tests to account for the surface modifications during the CoF measurements. Complementary, a numerical model was validated using the experimental results from this study. 6. Acknowledgements National Funds through Fundaç-o para a Ciência e a Tecnologia (FCT) under the PhD grant 2021.05562.BD; - LAETA under project UID/ 50022/ 2020. References [1] Dong Zhu, Jiaxu Wang and Q. Jane Wang. On the Stribeck Curves for Lubricated Counterformal Contacts of Rough Surfaces. ASME. J. Tribol. 2015; 137(2), 021501. DOI: https: / / doi.org/ 10.1115/ 1.4028881 [2] Maria J. M. Cortez, Thermal-Elasto-Plastic-Hydrodynamic Contact Between Rough Surfaces: Influence of Surface Roughness, MSc Dissertation, Engineering Faculty, University of Porto, Portugal, July 2023. [3] P. C. Romio, P. M. T. Marques, C. M. C. G. Fernandes and J. H. O. Seabra, A Simplified Thermal Plasto-Elastohydrodynamic Lubrication Model for Circular Contact With Real Surface Roughness, J. Tribol. Dec 2023, 145(12): 124102. DOI: https: / / doi.org/ 10.1115/ 1.4062898 Machine Elements and their Application in Tribology 24th International Colloquium Tribology - January 2024 115 Simulation-Based Evaluation of Drive Cycle Fuel Efficiency Gains in Gasoline Engines through Engine Oil Viscosity Reduction X. Simón-Montero 1 , J. Blanco-Rodríguez 1 , J. Porteiro 1 , M. Cortada-Garcia 2 , S. Maroto 2 1 CINTECX, Universidade de Vigo, GTE, Lagoas-Marcosende s/ n, Vigo, 36310, Spain 2 Repsol SA, Madrid, Spain 1. Introduction In this study, a methodology for drive cycle performance evaluation combining numerical simulations and experimental lubricant data is presented. This methodology includes a simulation procedure where several simulation models are used to assess lubricants in transient conditions. In the core of this procedure, stationary simulations for each lubricant across a range of engine speed, engine load and oil temperature were run to generate a 4-D map of friction losses. This process significantly helped reduce the run time needed to perform transient real drive simulations. Three OEM approved lubricants for a representative European passenger car vehicle were included in this study, corresponding to three different SAE grades. This study also includes engine protection evaluation based on the Stribeck curve of the average type of lubrication obtained at each timestep of the driving cycle. The simulations were performed for the European standard homologation test WLTC. The results show significant reduction in friction totals when low viscosity engine oils are used. In terms of type of lubrication, the lowest values of specific oil film thickness were achieved for the least viscous of the engine oils, although the lubrication type was mainly hydrodynamic for the three lubricants. Results also include the comparative potential reduction in fuel consumption by using each lubricant. 2. Methodology Three lubricants (Table 1) were modelled in Gamma Technologies GT-Suite, to be ran at a transient model of passenger car, which main features are shown in Table 2. Table 1: Lubricants of the study Lubricant SAE grade Viscosity at 40-°C (cSt) Viscosity at 100-°C (cSt) Oil A- 0W-20- 40.78- 8.26- Oil B- 5W-30- 67.58- 12.00- Oil C- 5W-40- 81.68- 14.23- Additionally, stationary models of the main friction contributors, as bearings and the piston pack, where also created so the lubricants could be tested at specific conditions, and compared to references of similar vehicles and lubricants. Table 2: General features of the modelled vehicle Property Chosen vehicle Model VOLKSWAGEN T-ROC Engine displacement [L] 1.5 Engine configuration 4-inline Turbo Power [CV] 150 Fuel Petrol Weight [kg] 1330 Cx 0.38 Tyre rolling radius [mm] 334 Tyre rolling resisting factor 0.0155 In order to reduce computational time, friction losses were mapped by using the stationary models to create maps within the range of temperature, engine speed a load conditions of the WLTC driving simulations. In the figure below a friction loss map at full load is shown. Figure 1: Oil C Full load Engine Friction Loss Map To take into account the effects of the lubricant in wear and durability, the specific oil film thickness (SOFT) was also mapped, so its instantaneous values during transient simulations could be known. 3. Validation and results 3.1 Validation When performing studies concerning simulations, it is always key not to lose the link to real-world correlation. In this study, no specific test bench data was available, so as it was already mentioned, references of similar engines where taken, such as Taylor’s teardown test [1], featured in Figure-1 116 24th International Colloquium Tribology - January 2024 Simulation-Based Evaluation of Drive Cycle Fuel Efficiency Gains in Gasoline Engines through Engine Oil Viscosity Reduction and other authors [2-3], to cover different temperatures, loads and regimes. The validation process was performed for each of the engine oils, proving reasonable accuracy, thus leading to the driving simulations that led to the results shown on the following section. Figure 2: Oil B Crankshaft FMEP comparison to reference 3.2 Results The results showed that the introduction of Oil A, was an upgrade in terms of efficiency across all the components modelled, as it can be seen in Figure 3, which subsequently, led to a reduction in fuel consumption. Figure 3: WLTC friction energy loss distribution for the 3 oils Figure 4: Pie charts representing the percentage of bearings timesteps spent in each lubrication type The SOFT results (Figure 4), show that using oil A, there were more time steps in the elastohydrodynamic and mixed regimes, that were least common using any of the other lubricants. This was found coherent to the viscosity of the lubricants. 4. Conclusion In conclusion, this research introduced a comprehensive methodology that integrates simulations with experimental lubricant data to assess drive cycle performance. We evaluated three distinct lubricants and found that the low-viscosity oil (Oil A) notably decreased friction losses and fuel consumption. By comparing the simulation results with reference data, this study highlights its value as an effective tool for examining the impact of lubricant property variations on overall vehicle performance. Such insights may help in the development of novel lubricant formulations tailored to achieve optimal properties. References [1] R. I. Taylor, N. Morgan, R. Mainwaring, and T. Davenport, “How much mixed/ boundary friction is there in an engine — and where is it? ,” Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, vol. 234, no. 10, pp. 1563-1579, Oct. 2020, doi: 10.1177/ 1350650119875316 [2] K. Kalogiannis, P. Desai, O. Mian, and R. Mainwaring, “Simulated Bearing Durability and Friction Reduction with Ultra-Low Viscosity Oils,” in SAE Technical Papers, 2018, vol. 2018-September. doi: 10.4271/ 2018- 01-1802. [3] P. Lee and B. Zhmud, “Low friction powertrains: Current advances in lubricants and coatings,” Lubricants, vol. 9, no. 8, Aug. 2021, doi: 10.3390/ lubricants9080074. 24th International Colloquium Tribology - January 2024 117 A Study on the Effect of Surface Tension on the Drag Torque of Wet Clutches Nikolaos Rogkas 1* , Vasilios Spitas 1 1 Laboratory of Machine Design and Dynamics, School of Mechanical Engineering, National Technical University of Athens, 9 Iroon Polytechniou, Athens, 15780 Zografou, * nrogkas@mail.ntua.gr 1. Introduction Wet clutch technology is used to transfer power through friction between two rotating components and many advanced power transmission systems incorporate it to ensure high torque capacity and smooth gear shifts [1, 2]. However, wet clutches are characterized by power loss in their disengaged state due to the viscous shear stresses of the lubricant caused by the rotation of the discs, reducing considerably the overall efficiency of the transmission [3]. Therefore, decreasing drag torque is a primary goal of designers of wet clutches. Drag torque depends on the operating conditions [4, 5] (i.e., rotating speed, film thickness, flow rate), the texturing characteristics of the friction surface (i.e., grooves, dimples etc.) [6] and finally the properties of the lubricant [7]. Usually, researchers tend to use the drag torque-speed relation to investigate the effect of certain parameters. Under usual conditions, drag torque increases linearly in the low-speed region, reaching a maximum value, in the order of 1 Nm. However, when the rotating speed reaches a critical value, drag torque drops rapidly due to aeration effects. In wet clutches, aeration is promoted when sub-atmospheric pressure is developed near the outer radius leading to the suction of air and the development of a two-phase air-ATF flow [3, 8, 9]. The calculation of the critical rotating speed and the overall estimation of the drag torque-speed relation for various operating conditions or grooves characteristics has been pursued by researchers over the last decade by developing simplified analytical models [3, 8, 10], by attempting 2D CFD two-phase simulations for flat discs [11, 12], or by conducting single-disc experiments [9, 13]. More recently, the use of comprehensive 3D CFD two-phase simulations has also been attempted, however, the associated simulation cost is considered extensive [9, 13]. Concerning the properties of the lubricant, besides the well-understood effect of viscosity, most of the available studies either neglect the effect of surface tension or consider a fixed value of this parameter which limits the potential to derive generalized conclusions. The scope of this paper is to address this issue conducting experiments considering the case of non-grooved discs. The CFD simulations are performed utilizing the commercial software ANSYS Fluent, whereas the lubrication experiments are conducted in a prototype test rig. Two in-house developed oils of equal viscosity and density but varying surface tension coefficients are compared regarding their drag-torque measurements. Both the CFD and the experimental results indicate that a deviation of surface tension in the order of 15% may lead to a drop in drag torque even up to 90%. Besides measuring the torque, a high-speed video recording is used to identify the flow characteristics of the two cases. The results of this paper highlight the potential to further improve the technology of wet clutches by controlling a so-far unexplored parameter: the surface tension coefficient of the lubricant. 2. 2D CFD model results Two-phase CFD simulations were conducted using the commercial software ANSYS Fluent. The study adopted a 2D axisymmetric analysis including swirl, considering laminar flow conditions. The Volume of Fluid (VOF) method was employed to accurately track the air-ATF Fig. 1 showcases the results of the CFD simulations, presenting a drag torquespeed graph for the two examined fluids, with surface tension coefficients of 30 mN/ m and 35 mN/ m, respectively. The results suggest that as the rotational speed surpasses a critical value, the drag torque experiences a noticeable drop which is associated with the development of a two-phase flow (suction of air from the outer radius). The drop exhibits a similar slope for both cases under examination. However, it is also important to note that the surface tension coefficient significantly influences the drag torque by affecting the critical aeration speed. Specifically, for σ = 30 mN/ m, the critical speed is recorded at 550 rpm, while for σ = 35 mN/ m, the critical speed almost doubles, reaching up to 1000 rpm. Figure 1: Drag torque-speed relation for two lubricants with different surface tension coefficients. 118 24th International Colloquium Tribology - January 2024 A Study on the Effect of Surface Tension on the Drag Torque of Wet Clutches 3. Experimental results The experiments are carried out using a prototype single-disc test rig, which allows for the adjustment of critical parameters such as rotating speed, discs clearance, and inflow rate. The setup involves the flow between a rotating and a stationary (transparent) disc. Flow patterns are monitored using a high-speed camera (up to 2000 fps). Two images from the flow visualization are displayed in Fig. 2, representing 250 rpm (Fig. 2-a) and 700 rpm (Fig. 2-b) respectively, with a consistent film thickness of 350 μm for both cases. Fig. 2 illustrates the impact of rotating speed on aeration, indicating that as the rotating speed escalates from 250 rpm to 700 rpm, the flow transitions from a single-phase to a two-phase air-ATF flow, marked by a sharp interface of the two phases, positioned near the mean radius. Figure 2: Flow characteristics at two different rotating speeds: a. 250 rpm, b. 700 rpm. The formation of the air- ATF interface in a region close to the mean radius may be observed. 4. Conclusions This study examined the effect of surface tension on the drag torque of wet friction clutches considering non-grooved discs. This was achieved by conducting 2D CFD simulations and single-disc experiments. The findings emphasized the significant impact of the surface tension coefficient on the drag torque, thereby encouraging further exploration of this subject. References [1] Rogkas N., Vasilopoulos L., Spitas V. A hybrid transient/ quasi-static model for wet clutch engagement. Int J Mech Sci 2023: 108507. https: / / doi.org/ 10.1016/ j. ijmecsci.2023.108507 [2] Rogkas N., Vakouftsis C., Vasileiou G., Manopoulos C., Spitas V. Nondimensional Characterization of the Operational Envelope of a Wet Friction Clutch. Computation 2020; 8: 21. https: / / doi.org/ 10.3390/ computation8010021 [3] Aphale C. R., Schultz W. W., Ceccio S. L.Aeration in Lubrication With Application to Drag Torque Reduction. J. Tribol 2011; 133. https: / / doi.org/ 10.1115/ 1.4004303 [4] Iqbal S., Al-Bender F., Pluymers B., Desmet W. Mathematical Model and Experimental Evaluation of Drag Torque in Disengaged Wet Clutches. ISRN Tribology 2013; 2013: 1-16. https: / / doi.org/ 10.5402/ 2013/ 206539 [5] Rogkas N., Spitas V. Investigation of the effect of non-uniform discs clearance on the drag torque of a DCT wet friction clutch. Proceedings of ISMA 2020 - International Conference on Noise and Vibration Engineering and USD 2020 - International Conference on Uncertainty in Structural Dynamics, Leuven: 2020, p.-3799-810. [6] Rogkas N., Almpani D., Vasileiou G., Tsolakis E., Vakouftsis C., Zalimidis P., et al. A comparative study on the effect of disks geometrical features on the drag torque of a wet friction clutch. MATEC Web of Conferences 2020; 317: 04001. https: / / doi.org/ 10.1051/ matecconf/ 202031704001 [7] Kitabayashi H., Li C. Y., Hiraki H. Analysis of the Various Factors Affecting Drag Torque in Multiple-Plate Wet Clutches, 2003. https: / / doi.org/ 10.4271/ 2003-01- 1973 [8] Peng Z., Yuan S. Mathematical Model of Drag Torque with Surface Tension in Single-Plate Wet Clutch. Chinese Journal of Mechanical Engineering (English Edition) 2019; 32. https: / / doi.org/ 10.1186/ s10033-019- 0343-9 [9] Neupert T., Bartel D. High-resolution 3D CFD multiphase simulation of the flow and the drag torque of wet clutch discs considering free surfaces. Tribol Int 2019; 129: 283-96. https: / / doi.org/ 10.1016/ j.triboint.2018.08.031 [10] Iqbal S., Al-Bender F., Pluymers B., Desmet W. Model for predicting drag torque in open multi-disks wet clutches. Journal of Fluids Engineering, Transactions of the ASME 2014; 136: 1-11. https: / / doi. org/ 10.1115/ 1.4025650 [11] Yuan S., Guo K., Hu J., Peng Z. Study on aeration for disengaged wet clutches using a two-phase flow model. Journal of Fluids Engineering, Transactions of the ASME 2010; 132: 111304-1-111304-6. https: / / doi. org/ 10.1115/ 1.4002874 [12] Pardeshi I., Shih TIP. A Computational Fluid Dynamics Methodology for Predicting Aeration in Wet Friction Clutches. Journal of Fluids Engineering, Transactions of the ASME 2019; 141: 1-7. https: / / doi. org/ 10.1115/ 1.4044071 [13] Neupert T., Benke E., Bartel D. Parameter study on the influence of a radial groove design on the drag torque of wet clutch discs in comparison with analytical models. Tribol Int 2018; 119: 809-21. https: / / doi.org/ 10.1016/ j. triboint.2017.12.005 24th International Colloquium Tribology - January 2024 119 Influence of the Steel Disk on the NVH Behavior of Industrial Wet Disk Clutches Patrick Strobl 1* , Katharina Voelkel 1 , Thomas Schneider 1 , Karsten Stahl 1 1 Technical University of Munich, School of Engineering and Design, Department of Mechanical Engineering, Gear Research Center (FZG), Garching near Munich, Germany * Corresponding author: patrick.strobl@tum.de 1. Introduction Wet disk clutches are critical components of, e.g., modern industrial, maritime, and heavy-duty drivetrains. In these applications, friction systems with sinter-metallic friction linings are standard due to their high robustness and their comparatively low production costs. Nevertheless, these friction systems tend to be more critical regarding NVH due to their characteristic friction behavior. An increase of the Coefficient of Friction (CoF) towards lower sliding velocities is the reason for this phenomenon. Since this friction behavior cannot be simulated yet, it requires accurate experimental measurement. Previous investigations mainly focus on the influence of the friction lining and the lubricant on friction behavior. On the other hand, there is still a lack of knowledge regarding the influence of the steel disk on friction behavior. In our investigation, the friction behavior of industrial clutch systems at application-relevant operational modes is analyzed through experimental studies with three different steel disks in combination with serial friction linings and lubricants. The investigations show differences in the occurrence of shudder depending on the tested steel disk variant. To better understand the observed behavior, the surface roughness of the steel and friction disks is determined. These measurements show different reductions in the surface roughness of steel and friction disks depending on the underlying friction system. The methods are adapted from investigations with paper-based friction systems [1] for the application-relevant needs of industrial clutches. The results give valuable insights regarding the influence of steel disk finishing, which is essential for clutch disk manufacturers for both cost-efficient and functionally relevant surface finishing of the steel disks. 2. Materials and Methods To investigate friction behavior, experimental investigations on well-proven test rigs KLP260 and LK-3 are performed in different operational modes. Among other operating modes, KLP-260 allows the experimental testing of wet disk clutches in brake shift and slip mode. These modes are relevant for industrial clutches and brakes. On the other side, LK-3 allows the measurement of the friction behavior at the transition from static to dynamic friction, according to Voelkel, Meingassner et al. [2]. To characterize the surface of the steel disk and the friction disk, optical measurements of the 3D surface topography with the principle of focus variation are performed. Therefore, the measuring device Alicona-Infinite-Focus-G4 is used. This guarantees reasonable insights into the surface topography of the friction surfaces. For our investigations, steel disks with three surface finishes are tested (beltground (bg), belt-ground and nitrocarburized (bg + nc), and cross-ground (cg)). Figure-1: Investigated steel disk surfaces before run-in (750-x-750-µm, objective lens with 50x magnification) These steel disks are paired with two genuine friction materials and two application-relevant lubricants (one typical industrial lubricant and one bio-degradable lubricant). Figure-2: Investigated friction disks ( ⌀ d m -=-162-mm) 3. Results Besides the influence of the system components, the operating conditions influence the friction behavior of wet clutches. Oil temperature’s effect on CoF is exemplified in Figure-3 for low speeds. With low oil temperatures, the slope of CoF over sliding velocity gets more negative. Figure-3: CoF over sliding velocity at stationary slip for three oil temperatures of one friction system (cg) 120 24th International Colloquium Tribology - January 2024 Influence of the Steel Disk on the NVH Behavior of Industrial Wet Disk Clutches Besides this, also the influence of the steel surface on the friction shown in Figure-4 is strong. In this context, a negative slope of the CoF affects the NVH behavior of the clutch system negatively. Figure-4: CoF over sliding velocity at stationary slip with different steel disks (bg/ bg + nc/ cg) The brake shift investigations confirm this steel disk’s influence even clearer for some friction systems. Figure-5 shows a strong influence of the steel surface on the friction behavior at brake shifts. Figure-5: CoF over sliding velocity of a brake shift with different steel disks (bg/ bg + nc/ cg) According to Figure- 6, the steel disks show significant smoothing due to the run-in procedure compared to the new surfaces shown in Figure- 1. After the run-in, all variants showed a directional surface structure in the sliding direction. In the case of the belt-ground surface, almost no recognizable structure of the initial surface texture is left. The initial surface structure is still identifiable after run-in in the case of additionally nitrocarburized steel surfaces. In contrast, the cross-ground surfaces only show deep surface characteristics from the initial texture. Figure-6: Investigated steel disk surfaces after run-in (750-x-750-µm, objective lens with 50x magnification) 4. Summary In this study, the steel disk for three sinter-metallic friction systems is varied. Low oil inlet temperature and low specific surface pressure leads to a higher shudder tendency for all variants. Compared to belt-ground steel disks with or without nitrocarburization, cross-ground steel disks show increased shudder tendency. At the same time, these variants show the highest roughness in the initial state but the strongest smoothing of the steel surface and the sinter-metallic friction lining. 5. Conclusions The choice of the steel disk can affect the NVH behavior of wet disk clutches. In this context, shudder is observed combined with a cross-ground steel disk. To characterize these influences, the steel friction surface and the friction lining surface is analyzed. Due to higher steel surface roughness in the new state, the smoothing of the friction material is influenced. In this context, the static Coefficient of Friction is higher for systems that show negative NVH behavior. Although the static Coefficient of Friction is lower than the corresponding sliding coefficient of friction, this might indicate an influence on the NVH behavior. This study supports a better understanding of the influence of the surface treatment of the steel disks on the friction behavior of wet disk clutches. Nevertheless, the choice of the surface treatment should not only be decided regarding the frictional behavior but should also consider, e.g., the wear and damaging behavior. Therefore, a combined investigation of these factors for future investigations is proposed. Literature [1] Strobl, P., Schneider, T., Voelkel, K., and Stahl, K. 2023. Influence of the steel plate on the friction behavior of automotive wet disk clutches. Forschung im Ingenieurwesen/ Engineering Research 87, 2, 541-554. [2] Voelkel, K., Meingassner, G. J., Pflaum, H., and Stahl, K. 2021. Friction behavior of wet multi-plate disk clutches at the transition from static to dynamic friction. Forschung im Ingenieurwesen/ Engineering Research. Acknowledgment The presented results are based on the research project FVA no. 343/ V; self-financed by the Research Association for Drive Technology e. V. (FVA). The authors would like to express thanks for the sponsorship and support received from the FVA and the members of the project committee. 24th International Colloquium Tribology - January 2024 121 Stick-Slip in Hydraulic Cylinders: New Test Methods & Simulation as a Tool for Selecting Coating Solutions for Piston Rods to Avoid Critical Operating Conditions Giuseppe Tidona 1* , Jürgen Molter 2 1 Competence Center for Tribology, Mannheim University of Applied Sciences 2 Mannheim University of Applied Sciences * Corresponding author: g.tidona@hs-mannheim.de 1. Introduction Hydraulic cylinders play a crucial role as linear actuators and are particularly in demand when high power density is required in applications such as crane booms or in underground mining. Under certain conditions, such as high working pressures and low travel speeds, an undesirable vibration phenomenon known as “stick-slip effect” is often observed [1]. Piston rods of most hydraulic cylinders are provided with a hard chrome coating. This coating not only provides excellent wear resistance, but also meets corrosion resistance requirements, making it an economically attractive option for a wide, technical range of applications. Since 2017, these hard chrome coatings have been subject to the provisions of the REACH regulation [1], which aims to reduce the use of hazardous chemicals to minimize environmental and health risks, creating a need to find coating alternatives that are as technically equivalent as possible but meet current health and environmental requirements. The paper addresses the stickslip behavior of hydraulic cylinder piston rod seals on different piston rod coatings. 2. Used test rig & specimen Table 1 shows the seal designs to be investigated [2]. The primary material of the rod seals used is the plastic polyurethane - a thermoplastic elastomer known for its high abrasion resistance, good resistance to oils and chemicals and good processability. Table 1: Seal designs to be investigated DS101 DS117 DS117R DS121 DS141 In addition to the classic hard chrome coating (HC), which will serve as a comparison reference in the investigations, a total of four other piston rod coatings (Tenifer QPQ-, S3PM-, WC-Co-HVOFand LIC-processed) were selected as mating surfaces for the rod seals. The different manufacturing processes for the surface modifications result in widely differing tribological surfaces, which have a significant influence on the friction and stick-slip behavior and will be considered in the evaluation of the results. A special long-stroke test rig was developed to study the stick-slip behavior of the different rod seal-piston rod pairings with high precision under variation of travel speeds, hydraulic pressure, and oil temperature. Figure 1 shows the pressure chamber used for the experimental investigation in a quarter section view as well as in a test run. Figure 1: Quarter section & full view of the pressure chamber with a rod seal (red), a guide ring (green) and a wiper (orange) 3. Load collective & test results The load collective is changed stepwise during an experiment. The results are presented as a function of pressure (T test -= 0 bar … 300 bar), thermostat temperature ( p test = 30-°C … 70-°C) and velocity (v test = 5 mm/ s … 25 mm/ s). Three characteristic values are introduced for the evaluation of the seal performance. The mean friction force value F RAVG provides information on the general friction level. The friction force is averaged over a stroke length. The maximum friction force F RMax describes the peak (red cross in fig. 2) after a change of direction and is a measure for the static coefficient of friction. The stick-slip range F RSS is introduced as a qualitative measure of the stick-slip effect and is calculated as the difference between the mean value of all maxima (golden line in fig. 2) and all minima (brown line in fig. 2). Figure 2: Friction force of a section of a stroke of the DS121 seal on a hard chrome plated piston rod at p = 100-bar, T = 30-°C and v = 5 mm/ s 122 24th International Colloquium Tribology - January 2024 New Test Methods & Simulation as a Tool for Selecting Coating Solutions for Piston Rods to Avoid Critical Operating Conditions Figure 2 shows an example of the friction force of the DS 121 seal design on a hard chromium-plated piston rod at p-= 100-bar, T = 30-°C and v = 5 mm/ s. After an initial static friction peak, a broad “band” is seen up to stop of the pressure chamber movement, where the friction force exhibits a relaxing behavior. A closer look reveals a sawtooth pattern in the friction signal, which is typical for a stick-slip affected motion [3]. Table 2 shows a section of the comparison of the test results for the DS121 seal design on a selection of piston rods for the pressures p = 100 … 200 bar. Green stands for a low and red for a high characteristic value. Although stickslip can be observed for some seal designs under certain load parameter combinations, the best performance is achieved with the LIC rod as it has similar friction properties and a lower stick-slip tendency than the hard chromium plated piston rod. The worst performance is seen with the QPQ-processed rod. The rough surface results in high characteristic friction values, increased wear of the rod seal and leakage. Table 2: Section of the test results of DS121 on selected piston rods 4. MATLAB & ABAQUS-simulations Parallel to the experimental investigation, simulations with the FEM-program ABAQUS were carried out. These simulations make it possible to determine the deformation of the seals under pressurization and to analyze the resulting pressure profiles of the seal body on the piston rod [4]. With the help of the inverse Reynolds equation, it is possible to calculate the volumetric flow rate of the lubricant transported through the contact as a thin lubricating film which is used to draw conclusions about the stick-slip-tendency of a seal design. The investigations are further supplemented by a MAT- LAB Simulink model of a hydraulic cylinder. It allows the precise analysis of the operating behaviour as a function of speeds, temperatures and pressures. To adequately account for the viscoelastic behavior of the seals, the extended Maxwell model is introduced as a rheological model and stiffness and damping coefficients are defined [5]. To fully analyze the influence of operating conditions, the hydraulic cylinder must be embedded in a hydraulic system and equipped with position and speed control [6]. 5. Conclusion As part of the NoChromeNoStickSlip project, a test rig was developed for the analysis of the stick-slip behavior of rod seals on different piston rods. During the test series, the LIC piston rod emerged as the favorite for the mentioned parameter range. Using the results of the FEM simulations, the friction effects in the tests could be attributed to specific geometric features of the seals. As more knowledge is gained in the project, new designs will be derived that exhibit improved frictional behavior and reduced stick-slip tendencies. The Simulink model provides basic information, but material and friction data sets are needed to make qualitative predictions about the stick-slip behavior of a seal in service. 6. Acknowledgements The authors would like to thank the funding program “Zentrales Innovationsprogramm Mittelstand” of the BMWK for the funds provided to carry out the project. References [1] Skowrońska, J., Kosucki, A., & Stawiński, Ł. (2021). Overview of Materials Used for the Basic Elements of Hydraulic Actuators and Sealing Systems and Their Surfaces Modification Methods. Materials (Basel, Switzerland), 14(6), 1422. https: / / doi.org/ 10.3390/ ma14061422 [2] Elbe-Dichtungen.de. Accessed on 15.05.2023; Available at: https: / / elbe-dichtungen.de/ [3] Popov, V. (2010). Kontaktmechanik und Reibung, Von der Nanotribologie bis zur Erdbebendynamik (3 rd ed.) Berlin, Germany: Springer-Verlag, ISBN: 978-3-662- 45974-4. [4] Nissler, U. (2015). Dichtheit von Hydraulikstangendichtringen aus Polyurethan: Einfluss von Geometrieveränderungen an PU-Nutringen auf deren Dichtverhalten und Vergleich verschiedener Dichtheitsbewertungen. Dissertation. Universität Stuttgart, Institut für Maschinenelemente. Available at: https: / / elib.uni-stuttgart.de/ bitstream/ 11682/ 4633/ 1/ Dissertation_Nissler.pdf. ISBN: 978-3-936100-62-4. [5] Moldenhauer, P. (2014). Modellierung und Simulation der Dynamik und des Kontakts von Reifenprofilblöcken. Dissertation. Technische Universität Bergakademie Freiberg, Fakultät für Maschinenbau, Verfahrens- und Energietechnik, Freiberg. Accessed on 17.07.2022; Available at: https: / / tubaf.qucosa.de/ api/ qucosa%3A22727/ attachment/ ATT-0/ [6] Will, D. and Gebhardt, N (2011). Hydraulik: Grundlagen, Komponenten, Schaltungen (5 th ed.), Dresden, Deutschland: Springer Verlag, ISBN: 978-3-642- 17242-7. 24th International Colloquium Tribology - January 2024 123 Wear Optimization of Roller Chain Drives with Triboactive Transfer Coatings Martin Rank 1* , Manuel Oehler 1 , Oliver Koch 1 1 RPTU Kaiserslautern-Landau, Chair of Machine Elements, Gears and Tribology (MEGT), 67663 Kaiserslautern, Germany * Corresponding author: martin.rank@rptu.de 1. Introduction The resource-saving use of machine elements is becoming increasingly important in the context of social, but also legal requirements. Above all, the optimization of tribological contacts is of great relevance. Studies show that approx. 23% of global CO 2 emissions are due to friction losses and to the replacement of worn components in such contacts [1]. Increasing wear resistance is therefore of particular interest for elements such as roller chains, whose service life is determined by wear. One promising approach is the use of surface coatings. This is already being used in series products. Coatings are usually produced by physical vapor deposition (PVD) or chemical vapor deposition (CVD) processes [2]. However, their in-sight characteristics limit the use of such methods. Economical coating of concealed surfaces such as bores or internal sleeve geometries like chain bushings is hardly feasible. Therefore, usually only the pins of chain joints are coated. The use of triboactive transfer coatings could be one approach to realize wearand friction-reduction in chain bushings. Chemical reactions of lubricant additives with elements of suitable coatings under tribological load results in the deposition of wear and friction reducing transfer layers on the contact partner of the coated component. The use of CrAlMoN coatings and greases additivated with sulfur has already been shown to reduce friction in chain joints-[3]. Pin-on-Disc (PoD) Tribometer tests confirmed these findings and showed also reduced wear with such coating systems [4]. 2. Methods All experiments were performed with a single joint on a Chain Joint Tribometer (CJT) developed at the Chair of Machine Elements, Gears and Tribology (Figure 1). Figure 1: Chain Joint Tribometer (CJT) for wear analysis of single chain joints The CJT allows friction and wear analysis on individual chain joints, but also the bush-roller contact of roller chains. Since only individual joints are analyzed here, parameter and material investigations can be carried out cost-effectively. The test on the CJT promise a significantly higher conformity with real chain drives than model tribometer tests. The load on the joints corresponds to the real loads acting in the chain drive. Multi-body simulations (MBS) are used to calculate the link forces and deflections that are applied as a load spectrum in the CJT. In this work, tests were carried out on 10B1 roller chains with a pitch of P-=-15.875-mm according to DIN-ISO-606. Single prototype chain joints are built from series bushings and three pin variants. In addition to an uncoated series pin, a CrAlN and a CrAlMoN PVD coating of the pins are used (Table 1). Table 1: chemical metallic properties of the used pin coatings Coating Cr / At -% Al / At -% Mo / At -% Coating thickness / µm CrAlN 59 41 - 1.92 ± 0.1 CrAlMoN 17 13 70 1.79 ± 0.04 Two variants of a model grease with different additive packages are used as lubricant. The base grease consists of an ultra-high viscosity mineral oil and an inorganic thickener with antioxidants (AO). One variant is additionally additivated with 6600 ppm sulfur (S), the other with 350 ppm phosphorus (P). The load spectrum used corresponds to a twowheel drive with 120 links at numbers of teeth of z 1 = 17 and z 2 = 45, operated with a braking torque of M 2 = 125 Nm and a drive speed of n 1 = 1000-1/ min. The contact force acting in the joint in this configuration is F z = 1100 N. Wear is analyzed by real-time measurement of the joint elongation on the CJT. In addition, tactile shape measurements and surface topology investigations using a confocal microscope are performed. 3. Results The measurement of the joint elongation on the CJT show differences in wear of the various joint configurations. While the uncoated systems show the significantly highest wear, the CrAlN +P and CrAlMoN +S systems exhibit the most favorable behavior (Figure 2). Both variants show almost no running-in wear and an initially flat wear rate. The rate only increases noticeably at a sliding distance of approx. s = 500 m or a load duration of t-=-46-h. The CrAlMoN +P joint shows 124 24th International Colloquium Tribology - January 2024 Wear Optimization of Roller Chain Drives with Triboactive Transfer Coatings a quite significant run-in of the surfaces but exhibits a similarly low wear rate after. Also, an increase of the wear rate and scatter is exhibited after approx. s = 600 m or t-=-55-h. Figure 2: joint elongation Δl of the different joint configurations over sliding distance s, measured with CJT The wear is also reflected in the surfaces of the specimens. While the as-new specimens show no surface structure, the various combinations show scoring of varying intensity (Figure 3). Figure 3: Surface topology of the new joints and the different worn joint combinations The extent of the surface damage of the different specimen, especially in the bushings, appears to be analogous to the joint elongation. Only the CrAlMoN +S and CrAlN-+P systems show no structures on the surfaces. However, local coating failure and chipping of the pin coating occurs in all the joints. The wear rates, which show an increase towards the end of the runtime for the CrAlN and CrAlMoN systems, support the observation of coating failure. 4. Conclusion The investigations show that the use of suitable coatings with the appropriate greases can realize wear reduction in chain joints. Wear protection of the bushes was achieved by coating the pins. This indicates that transfer coating formation takes place. Investigations of system friction and chemical surface analysis support this impression. The observed wear was high in all specimens. Despite a load selection according to DIN ISO 10823, all coatings seem to have failed. In addition, the use of a model grease with low additive contents could be a further explanation for the generally severe wear. Accordingly, further investigations at other loads will provide further insight into the wear behavior of the chain joints. A detailed chemical analysis of the worn but intact coatings will then be possible. In addition, a single joint test rig was used for the investigations carried out here. For transfer to the real system, further investigations must be carried out on real chain drives. The setup may be supported by using the findings obtained here. 5. Acknowledgment The authors thank the Deutsche Forschungsgemeinschaft (DFG) for funding “Analysis of transfer layer formation in initially lubricated, coated drive chains” SA 898/ 31-1. References [1] K. Holmberg, P. Kivikytö-Reponen, P. Härkisaari, K. Valtonen, A. Erdemir, Global energy consumption due to friction and wear in the mining industry. Tribology International, 115: 116-139, 2017. ISSN 0301-679X. https: / / doi.org/ 10.1016/ j.triboint.2017.05.010 [2] A. Becker, Entwicklung einer Prüfmethodik für Verschleißuntersuchungen an Kettengelenken von Antriebs- und Steuerketten. PhD thesis, TU Kaiserslautern, Kaiserslautern, Germany, 2020. [3] K. Bobzin, C. Kalscheuer, M. P. Möbius, M. Rank, M. Oehler, and O. Koch, Triboactive Coatings for Wear and Friction Reduction in Chain Drives. Tribology International, page 108562, 2023. ISSN 0301-679X. https: / / doi.org/ 10.1016/ j.triboint.2023.108562 [4] M. Rank, M. Oehler, O. Koch, K. Bobzin, C. Kalscheuer, M. P. Möbius, Investigation of the Influence of Triboactive CrAlMoN Coating on the Joint Wear of Grease-lubricated Roller Chains. Tribology Transactions, 2023, https: / / doi.org/ 10.1080/ 10402004.2023.2264908 24th International Colloquium Tribology - January 2024 125 Investigation of Polymer Solid Lubricated Steel-Bronze Contacts for Worm Gears Applications Konstantinos Pagkalis 1* , Manuel Oehler 1 , Thomas Schmidt 2 , Michaela Gedan-Smolka 2 , Stefan Emrich 3 , Michael Kopnarski 3 , Oliver Koch 1 1 RPTU Kaiserslautern-Landau, Chair of Machine Elements, Gears and Tribology Gottlieb-Daimler Str. Geb. 42, D-67661 Kaiserslautern, Germany 2 Leibniz-Institut für Polymerforschung Dresden e. V., Hohe Straße 6, 01069 Dresden, Germany 3 Institut für Oberflächen-und Schichtanalytik GmbH, Trippstadter Str. 120, 67663 Kaiserslautern, Germany * Corresponding author: konstantinos.pagkalis@rptu.de 1. Introduction In drive technology, machine elements are usually lubricated with oil or grease. However, there are cases where these types of lubrication cannot be used. In medical area, in vacuum or in high temperature applications oil and grease lose their efficiency. Hence solid lubrication is essential. PTFE is employed over a wide temperature range (-250°C to +260°C) where it has the advantage that it displays high chemical resistance and very good friction properties. However, its poor mechanical properties and low adhesion to metal surfaces are considered as a drawback. On the other hand, polyamide (PA) exhibits good mechanical properties and wear resistance but has high friction against steel. The advantageous properties are connected via a combination of the polymer matrices. So it made sense to blend these two compounds to get the optimum performance. However, physical blends of PA-PTFE have worse mechanical properties compared to PA [1]. The reason for this is that the polar polyamide matrix and the non-polar PTFE filler are not compatible. Hence, the chemical coupling of PA with irradiated PTFE compounds occurs by reactive melt extrusion. Using high energy radiation PTFE can be modified in the presence of air to obtain perfluoroalkyl radicals and functional groups (-COF and -COOH) while C-C and C-F bonds break. A covalent bond between polyamide and irradiated PTFE can be formed from COOH functional groups, which further improves the adhesion and compatibility between PTFE and hydrophillic/ polar polymer matrix. T. D. Nguygen et. al. found that on block on ring polymer-steel contact, PA66 with chemically bonded (cb) PTFE (MP1100) provided the lowest friction when compared to pure unmodified PA66 while PA66-MP1200-cb provided worse wear behaviour than other PA- PTFE-cb components due to its worse mechanical properties [2]. Moreover, Franke et al. demonstrated that friction and wear behaviour of polyamides PA6, PA66 and PA12 could be enhanced with a mass fraction of PTFE-cb between 3.3 and 50 wt-% [3]. It can be deducted that Polymer-PTFE-cb dry lubricants can enhance tribological behaviour and mechanical properties of polymer-metal contacts. It is therefore investigated if and how the novelty chemically bonded polymer compounds produced by reactive melt extrusion can be beneficial compared to commercial compounds on a steel/ bronze contact in terms of tribological effectiveness. 2. Experimental studies Tests are conducted in 3-disc test rig and worm gearbox setup [1, 4]. The tribological system in both cases includes the polymer compound acting as the sacrificial element in contact with a steel of 16MnCr5, while bronze CuSn12Ni-GC is in contact with the steel. The 3-disc test rig consists of a test unit and three electric motors, which can be independently driven to set an arbitrary slide-to-roll ratio while load is applied vertically. Tests are conducted at a 270 MPa steel/ bronze mean contact pressure, slide-to-roll ratio equal to 50- %, 3- h test duration. The worm gearbox setup (centre distance a-=-32-mm, gear ratio i-=-29, 5000 cycles, 10-h test duration, mean contact pressure 179-MPa and 5-Nm output torque for steel/ bronze contact) has a drive motor connected to the steel worm while the two other motors are connected to the bronze and polymer wheel respectively. The wear of the compounds is determined by measuring mass before and after the experiments. All PA-PTFE compounds have a ratio of 80-wt-% PA and 20-wt-% PTFE. A commercially available physically mixed PA66-PTFE compound (ALCOM) is used as state of the art. Different types of PA (PA12 and PA66) and irradiated PTFE MP1100E were extruded in melt to obtain novel PA-PTFE-cb compounds. There are two different PA12 polymer matrices that are used with the modified PTFE, PA12L and PA12Z. MP1100E type is thermally post-treated during industrial manufacturing to remove the perfluorooctanoic acid (PFOA) it contains to meet the EU requirements regarding the limit values of this compound. This research investigates the effect of those chemically bonded dry lubricants on a steel/ bronze contact in a 3-disc tribometer, on a situation that resembles worm gears. Thus, it can be understood which polymer provides the best results in terms of tribofilm effectiveness (friction, transfer film formation and wear of the components) and then to be tested on the worm gearbox setup. 3. Results Figure 1 displays the results at the three-disc set-up regarding (a) friction coefficient of the steel-bronze contact (b) the wear coefficient of the bronze with the polymer combinations described above while Figure 2 compares the wear mass of the bronze obtained from 3-disc test rig and worm gearbox setup respectively for PA12 and PA12+modified PTFE compounds. 126 24th International Colloquium Tribology - January 2024 Investigation of Polymer Solid Lubricated Steel-Bronze Contacts for Worm Gears Applications Figure 1: (a) COF of steel-bronze contact (b) Bronze wear for the polymer combinations in 3-disc set-up. Figure 2: (a) Bronze wear mass for PA12L and PA12L + MP1100E-cb tested at (a) gearbox (b) at 3-disc setup. Figure 1 indicates that pure unmodified PA12 and PA66 give higher friction and bronze wear compared to the corresponding PA-PTFE compounds. This can be expected as there is no PTFE which acts supportively. Besides, PA12L (NH2 terminated) with MP1100E shows better wear results than PA12Z (non-regulated) with the same modified compound. PA12L has more NH2 groups than PA12Z and so can form more amide groups which can enhance adhesion and hence reduce wear. With regard to PA66, the physically mixed (pm) commercial compound ALCOM provides slightly higher friction and bronze wear compared to PA66-PTFE-cb however the differences are not significant. Measurements of near-surface chemical properties indicate that no PTFE-specific chemical bonds are detected on the tribologically stressed surfaces of the steel and bronze discs when using the commercial dry lubricant PA66+20 wt-% ALCOM. In contrast, the specific CF2bonds are identified in the transfer film of the dry lubricant compound with MP1100E, especially on the bronze discs. The allocation of PTFE on the disc surfaces could explain the comparatively better performance in tribological tests when using the dry lubricant with MP1100E. Moreover, Figure 2 shows that the initial results of worm gearbox setup show that there is a correlation on the wear behavior of the bronze for both setups used. 4. Conclusion The chemical coupling of PA12L with irradiated PTFE MP1100E reduces both the coefficient of friction in the steel/ bronze contact and the wear of the bronze disc in the 3-disc tribometer. Moreover, compared to commercially available polymer compounds made from PA66 and physically bonded PTFE components, the combination of PA66 and radiation-modified PTFE (MP1100E) tends to be tribologically superior in terms of friction and wear. Thus, the reactive melt extrusion process for irradiated PTFE provides better results than the commercial compound. Surface analysis studies show, that on the tribologically stressed surfaces of the bronze discs, transfer films with PTFE-typical CF 2 compounds are formed. References [1] L. Simo Kampa, T-D. Nguyen, S. Emrich, M. Oehler, T.-Schmidt, M. Gedan-Smolka, M. Kopnarski, B. Sauer, The effect of irradiated PTFE on the friction and wear behavior of chemically bonded PA46-PTFE-cb and PA66-cb compounds: Wear 502-503, 2022. [2] T-D. Nguyen, M. Gedan-Smolka, L. Simo Kamga, B.- Sauer, S. Emrich, M. Kopnarski, B. Voit Chemical Bonded Oil-P TFE-PA 66 Composites as Novel Tribologically Effective Materials: Part 1, Solid State Phenomena, 320, pp. 113-118, 2021. [3] R. Franke, D. Lehmann, K. Kunze, Tribological behaviour of new chemically bonded PTFE polyamide compounds, Wear 262, 242-252, 2007. [4] L. Simo Kamga, M. Oehler, O. Koch, and B. Sauer. „Untersuchungen zum thermischen Verhalten eines mit PTFE trockengeschmierten Schneckengetriebes mithilfe vom Experiment und Simulation“ 63. Tribologie-Fachtagung. Gottingen: Gesellschaft fur Tribologie e.V, S. 45/ 1-45/ 5, 2022.- 24th International Colloquium Tribology - January 2024 127 Power Loss in High-Speed Angular Contact Ball Bearings Lúcia B. S. Pereira 1 , Justino A. O. Cruz 2 , Pedro M. T. Marques 2 , Stephane Portron 2 , Jorge H. O. Seabra 1 , Carlos M. C. G. Fernandes 1 1 Universidade do Porto, Faculdade de Engenharia, Departamento de Engenharia Mecânica, Porto, Portugal, 2 INEGI - Universidade do Porto, Unidade de Tribologia Vibrações e Manutenç-o Industrial, Porto, Portugal 1. Introduction The electrification of vehicles brings new challenges to gear transmissions and Angular Contact Ball Bearings (ACBB) become a mechanical solution to fulfil speed and load requirements more efficiently [1]. The focus of this work is to measure the torque loss in highspeed ACBB for different operating conditions of speed, load and temperature, lubricated by a low viscosity Automatic Transmission Fluid (ATF) [2]. The ACBB torque losses are also calculated using the SKF model [3]. The correlation between torque loss calculations and measurements allowed improving the accuracy of the model predictions. An equation is proposed for the coefficient of friction in ACBB. 2. Materials and methods The tests are performed with a pair of spindle ACBBs, from FAG ® , with reference B7206-E-T-P4S-UL, as presented in Table 1. These are super precision single row ACBB with solid outer and inner rings, ball and cage assemblies and solid window cages. They have high load carrying capacity and high rigidity [4]. The ACBBs are lubricated with low viscosity high performance synthetic lubricant, Eni Rotra ATF VI that follows the requirements of GM’s DEXRON ™ VI standard [2], as presented in Table 2. This is an Automatic Transmission Fluid (ATF) that meets the requirements for EV applications, having low electrical conductivity, high cooling capability and low volatility [5]. Figure 1 shows the ACBB test rig [6]. It consists on a test and drive rolling bearing assemblies (1, 5) with their shafts coupled to a torque sensor (2, 3, 4), as shown in Figure 1. The driving transmission (6) is composed by an electric motor and a belt system (transmission ratio 3: 1) connected to the driving rolling bearing assembly (5). A temperature control unit keeps the oil supplied to the test chamber (1) at constant temperature. The oil temperature inside the test chamber (1) is measured by a PT100 thermocouple (6), shown in Figure 2. A type K thermocouple (7) measures the temperature of the oil flowing back [6]. The ACBBs are loaded axially, using waved washers (Borrelly C3P72), allowing axial loads between 510 N and 3-570-N [6]. Tests are performed at low and high speeds, for temperatures between 40°C and 80°C, as presented in Table 3. Temperatures of the ACBB outer ring and of the oil are measured in several locations (see Figure 2). 3. SKF torque loss model The SKF torque loss model considers that the total frictional moment is the sum of four components, as presented in equation (1): Table 1: B7206-E-T-P4S-UL ACBB. Dimension Desig. Value Unit Bore diameter d 30 mm Outside diameter D 62 mm Width B 16 mm Contact angle α 25 / Dynamic load rating C 22 100 N Static load rating C0 9 900 N Fatigue load limit Pu 1 050 N Limit speed oil lubrication n 36 000 rpm Table 2: Eni Rotra ATF VI synthetic lubricant. Property Unit Method Typical value Colour / / red Density @ 15°C kg m -3 ASTM D 4052 850 Viscosity @ 100-°C mm 2 s -1 ASTM D 445 5.7 Viscosity Index / ASTM D 2270 150 Viscosity @ -40-°C mPa s ASTM D 2983 10 400 Thermoviscosity °C -1 (100°C) 0.0179 Piezoviscosity Pa -1 (40°C) 1.9 × 10 -9 Table 3: Axial loads and oil sump temperatures. Temperature Axial Load Fa/ N Low-speed tests High-speed tests T/ °C 2040 2550 3570 2040 2550 3570 40 X X 50 X 60 X X X 70 80 X X X Figure 1: ACBB test rig. Figure 2: ACBB test assembly and temperature measurement locations in the ACBB. 128 24th International Colloquium Tribology - January 2024 Power Loss in High-Speed Angular Contact Ball Bearings M SKF = M rr + M sl + M drag + M seal (1) where M SKF (N mm) is the total frictional moment, M rr is the rolling frictional moment, M sl is the sliding frictional moment, M drag is the frictional moment of drag losses, churning, splashing and M seal is the frictional moment of the seals (in N mm) [3]. The SKF model establishes that the sliding coefficient of friction m sl is defined by equation (2), where f bl is the weighting factor, m bl is the coefficient of friction in boundary film lubrication, and m EHD is the coefficient of friction in full-film lubrication, m sl = f bl ∙ m bl + (1 − f bl ) ∙ m EHD (2) The SKF model also establishes that the weighting factor f bl is defined by equation (3) f bl = {exp [C bl ∙ (n ∙ n ) 1.4 ∙ d m ]} -1 (3) The seals torque losses are calculated using the Simrit model [6], given by equation (4), where T VD is the seal torque loss (N mm) and d sh is the shaft diameter (mm), T VD = C seal ∙ d 2 sh (4) The constants C bl and C seal (= 5.66 × 10 -2 ) are optimized, correlating the measured and calculated ACBB torque losses. 4. Torque loss measurements Figure 3 presents the torque loss measurements at low speeds (between 100 rpm and 2000 rpm). As expected, when the axial load increases the torque loss also increases, even if the oil temperature decreases. At very low speed the torque loss tends to the starting torque which is dependent on the applied axial load. These torque loss measurements are used to optimize the weighting lubrication factor, f bl , of the SKF model, where the constant C bl becomes 4.30 × 10 -8 . For high rotational speeds, n ≥ 3,000 rpm, the value of f bl becomes null (fullfilm lubrication). Figure 4 presents the torque loss measurements (a) and the ACBB temperatures (b) at high speeds (Fa = 3570 N, T-=-80°C). As Figure 4 shows, the correlation between measured and predicted torque losses is quite good (R 2 = 0.93). This correlation is obtained through the optimization of the coefficient of friction under full-film lubrication, m EHD , which can be defined by equations (5) and (6): (5) (6) 5. Conclusion As a general trend, the rolling moment (M rr ) and the drag moment (M drag ) increase when the speed increases, whatever the axial load and lubricant temperature. At higher loads, the sliding torque (M sl ) remains almost constant, independently of the speed and oil temperature. At lower loads, the sliding torque (M sl ) increases slightly when the speed increases. Acknowledgements - FCT PhD programme NORTE-69-2015-15 - supported by NORTE 2020, under European Social Fund - NORTE- 08-5369-FSE-000027; - LAETA under project UID/ EMS/ 50022/ 2020. Figure 3: Torque loss measurements at low speeds. Figure 4: Torque loss (a) and ACBB temperatures (b) at high speeds (Fa = 3570 N, T = 80-°C): test measurements vs. model predictions. References [1] Berker Bilgin et al., Making the case for electrified transportation. IEEE Trans. on Transportation Electrification, 1(1): 4-17. [2] Roy Fewkes et al., General motors DEXRONR-VI globalservice-fill specification. SAE Technical Paper, 2006. [3] SKF. Rolling Bearings Catalogue. October 2018. [4] Schaeffler Technologies AG Co. KG. Super Precision Bearings, November 2019. [5] Tom Hong-Zhi Tang et al., Lubricants for (hybrid) electric transmissions. SAE I. J. of Fuels and Lubricants, 6(2): 289-294, 2013. [6] Lúcia B. S. Pereira, Power loss in high-speed angular contact ball bearings, MSc Dissertation, Engineering Faculty, University of Porto, Portugal, July 2023. 24th International Colloquium Tribology - January 2024 129 Effect of Slip on Piezo-Viscous-Polar Lubricated Multirecessed Hybrid Journal Bearing Vishal Singh 1* , Arvind K. Rajput 2 1, 2 Mechanical Engineering Department, IIT, Jammu, J&K, India -181221 * Corresponding author: vishal.singh@iitjammu.ac.in 1. Introduction In recent years, multirecessed hybrid journal bearing (MHJB) have gain significant attention of researchers to improve the performance of rotary machineries attributed to their inherited advantages, viz., higher load carrying capacity, enhanced stiffness and damping characteristics and rotational accuracy. Nowadays, researchers use coating material on journal or bearing surfaces to improve their tribological properties. Consequently, shear stresses at the solid-liquid interface may results in velocity slip at interface [1-3]. The slip at interface may significantly affect the pressure field and thereby characteristics of the bearing system. Furthermore, the characteristics of a fluid film journal bearing primarily depend on the performance of the lubricant. To achieve a better lubricating performance, several form of long chain polymer additives particles are blended into the oil. The blending of long chain polymer additives particles in oil results in the polar effect due to couple stresses and lubricant no longer behaves as a Newtonian lubricant [4,5]. Further higher pressure may induce the piezo viscosity in the oil. Thus, the nature of the lubricant becomes piezo-viscous polar (PVP) lubricant and may exhibit piezo-viscous behaviour at higher pressures [6, 7]. The present work examines the cumulative effect of velocity-slip and piezo-viscous-polar lubrication on the characteristics of MHJB system. Four different cases of velocity-slip are studied, i.e., (i) no velocity-slip at journal or bearing surfaces, (ii) velocity-slip at journal surface, (iii) velocity-slip at bearing surface and (iv) velocity-slip at both (journal and bearing) surfaces. A modified form of the Reynolds equation governing the flow of the PVP lubricant is numerically solved using FE analysis. The simulated results indicate that velocity-slip at journal and bearing surfaces substantially affect the characteristics of MHJB system. Further, the use of PVP lubricant instead of Newtonian lubricant offers better performance of MHJB system i.e, PVP lubricant may compensate the performance loss caused by slip effect. 2. Analysis The schematic diagram of MHJB operating with PVP lubricant is depicted in Figure 1. The flow of PVP lubricant in the clearance space of MHJB governed by modified form of Reynolds equations which can be expressed as [1, 7, 8]: (1) Here, the behaviour of piezo-polar-slip function ( ) is governed by the piezo-viscous coefficient ( ), couple-stress parameter ( ) and velocity-slip coefficient ( ). Figure 1: Schematic of PVP fluid lubricated MHJB system The oil film thickness ( ) in MHJB can be computed from the following expression [9] (2) Where and are the journal centre coordinates. For computation of unknown pressure field, the oil film domain is discretised using four noded isoparametric quadrilateral element. Incorporating the Galerkin’s orthogonality technique of FE analysis, the governing equation (1) can be converted to matrix form as: (3) In MHJB, the load carrying capacity can be computed from the following expressions [9]: (4) The resultant load carrying capacity in radial direction yields as [9]: (5) The rotordynamic coefficients ( , ) yield as [9]: (6) Where, = Generalised force ; (Journal centre displacement); (Journal centre velocity) 3. Validation of model Based on FE formulation, a computer code is devel-oped in MATLAB. For validation of developed code, the computed results have been compared with the published studies. It may be observed from Figure 2 that the computed results are in close agreement with published results of Lv et al. [2]. The minor deviation between computed results and published results is attributed to difference in solution methodology and mesh size. 130 24th International Colloquium Tribology - January 2024 Effect of Slip on Piezo-Viscous-Polar Lubricated Multirecessed Hybrid Journal Bearing Figure 2: Variation of oil film pressure ( ) against circumferential angle (θ) for validation of velocity slip 4. Results and discussion For the performance analysis of MHJB, the geometric and operating parameters are judiciously chosen from literature. Thereafter, the performance characteristics of MHJB against load ( ) are computed. The variation of minimum oil fil thickness ( ) versus load ( ) is presented in Figure 3. It can be noticed from Figure 3 that for Newtonian and PVP lubricant, the consideration of velocity slip causes severe reduction (0.65-32.01%) in the values of ( ) as compared to ideal no-slip condition. Velocity slip on both (journal and bearing) surfaces causes maximum reduction in the values of ( ) in the range of 1.62-32.01% as compared to ideal no-slip condition. It may be due to reduction in relative velocity in MHJB system. Furthermore, the use of PVP lubricant offers higher values of ( ) in the range of 0.89-35.28% than that of Newtonian lubricant attributed to synergistic effect of piezo-viscosity and couple-stresses for various cases of velocity slip. Figure 3: Variation of versus The variation of rotordynamic coefficient ( ) against load ( ) is depicted in Figure 4. It can be observed that the consideration of velocity slip causes substantial drop in the values of ( ) in the range of 31.64-55.86% as compared to ideal no-slip condition. Likewise to trends of (), velocity slip on both surfaces (journal and bearing) causes maximum drop in the value of ( ) in the range of 52.44-55.86% as compared to ideal no-slip condition. Furthermore, the use of PVP lubricant instead Newtonian lubricant offers higher values of ( ) in the range of 53.75-60.43% of as compared to Newtonian lubricant. The other performance characteristics are not presented here due to space constraints. Figure 4: Variation of ( ) versus 5. Conclusions The present work examine the performance CFV compensated of MHJB system considering the influence of velocity slip and PVP lubrication. The computed results reveal that the consideration of velocity causes degradation in the performance of MHJB, viz. , , , and as compared to ideal no-slip condition. Moreover, PVP lubricant offers higher values of , , , and which may compensate performance degradation attributed to velocity slip at journal and bearing surfaces. References [1] Shukla J. B., Kumar S., Chandra P. Generalized reynolds equation with slip at bearing surfaces: Multiple-layer lubrication theory. Wear 1980; 60: 253-68. [2] Lv F., Rao Z., Ta N., Jiao C. Mixed-lubrication analysis of thin polymer film overplayed metallic marine stern bearing considering wall slip and journal misalignment. Tribol Int 2017; 109: 390-7. [3] Cui S., Zhang C., Fillon M., Gu L. Optimization performance of plain journal bearings with partial wall slip. Tribol Int 2020; 145: 106137. [4] Stokes V. K. Couple Stresses in Fluids. In: Stokes V. K., editor. Theor. Fluids Microstruct. Introd., Berlin, Heidelberg: Springer; 1984, p. 34-80. [5] Lin J.-R., Chu L.-M., Li W.-L., Lu R.-F. Combined effects of piezo-viscous dependency and non-Newtonian couple stresses in wide parallel-plate squeeze-film characteristics. Tribol Int 2011; 44: 1598-602. [6] Rajagopal K. R., Szeri A. Z. On an inconsistency in the derivation of the equations of elastohydrodynamic lubrication. Proc R Soc Lond Ser Math Phys Eng Sci 2003; 459: 2771-86. [7] Singh V., Rajput A. K. Piezoviscous-polar lubrication of capillary compensated hybrid conical undulated journal bearing. Tribol Int 2023; 186: 108588. [8] Mouassa A., Boucherit H., Bou-Saïd B., Lahmar M., Bensouilah H., Ellagoune S. Steady-state behavior of finite compliant journal bearing using a piezoviscous polar fluid as lubricant. Mech Ind 2015; 16: 608. [9] Rajput A. K., Sharma S. C. Combined influence of geometric imperfections and misalignment of journal on the performance of four pocket hybrid journal bearing. Tribol Int 2016; 97: 59-70. 24th International Colloquium Tribology - January 2024 131 Film Formation Evolution in Grease-Lubricated Rolling Contacts Impact of Operating Temperatures Shuo Zhang 1* , Georg Jacobs 1 , Benjamin Klinghart 1 , Florian König 1 1 Institute for Machine Elements and Systems Engineering, RWTH Aachen University, Aachen, Germany * Corresponding author: shuo.zhang@imse.rwth-aachen.de 1. Introduction During the operation of rolling element bearings, the lubricating film is desired to maintain sufficient thickness to avoid direct asperity contact and severe friction losses for extended periods. Nowadays, about 90% of rolling element bearings are lubricated using greases [1]. Therefore, it is benefit to have a model to predict film formation when designing greases and bearings. Grease consists of base oil, additives, and thickener [1]. The thickener has a solid microstructure, which can reserve oil [2] and provide the grease with a specific consistency. In the initial churning phase of a grease-lubricated contact, grease enters the contact zone and separates the contacting surfaces [3]. Simultaneously, most of the grease is pushed to the sides of the contact, which cannot flow back to the track automatically due to its consistency and acts as reservoirs for the base oil [1]. After that, the oil is slowly bled out from these reservoirs due to the repetitive passes of the roller, leading to an enhanced formation of the oil film [4]. At the same time, the remaining thickener deposits onto the track surface gradually, creating a solid-like lubricant layer [5]. Consequently, the film formation of a grease-lubricated contact undergoes evolution in time, even under unchanged operating conditions. Meanwhile, the prediction of this film formation evolution requires a model that addresses the transient formation of both oil film and thickener-rich layer. However, such a model has not yet been established. The objective of this presentation is to present a numerical model, which can be used to predict the film formation evolution in time for a grease-lubricated contact after the churning phase. Therefore, the proposed numerical model, which considers the lubricating film in grease-lubricated contacts as a superposition of the oil film and the thickener-rich layer, will be firstly introduced. As a key factor influencing the grease lubrication, the effects of operating temperature on the film formation evolution will be investigated. 2. Methods and Lubricants Thin layer model assumes that the rolling track is covered by a thin layer of lubricant [6], as shown in Figure 1(a). This lubricant layer is continuously passed by the contacts, resulting in the lateral side flow of the lubricant (the z-direction). Consequently, the thickness of thickener-rich layer decreases due to mass conservation. The stationary greases reservoirs deliver oil through the thickener-rich layer to the contact because of a so-called infiltration effect [7]. In this work, a porous multiphase bleeding model (PMB) is used to simulate this oil infiltration process, see Fig.1(b). This PMB model is based on an open-source toolbox for multiphase flow in porous medium proposed by Horgue et al. [8]. With the subsequent overrolling, the oil, that has infiltrated from the grease reservoirs, is pressed out in front of the contact. Then, the bled oil volume can be determined. Eventually, the oil film thickness can be calculated by Cann’s model, which correlates the film thickness with oil volume [9]. Figure 1: Simulation model, (a) Porous thin layer model (b)-Multiphase bleeding model. A lithium complex grease with PAO as the base oil is chosen for simulations. It has fibrous structures and is represented by PAO-Li-C in this study. The grease parameters are listed in Table 1. Table 1: Grease Parameters Base oil Viscosity 40 o C Viscosity 80 o C PAO 0.0806 Pa∙s 0.0181 Pa∙s Thickener Concentration Fiber radius lithium complex 14.5% 25 nm 132 24th International Colloquium Tribology - January 2024 Film Formation Evolution in Grease-Lubricated Rolling Contacts 3. Results and Discussions The film thickness evolutions under different operating temperatures are shown in Figure 2. The predicted grease film thickness h total is the superposition of the oil film thickness h oil and the thickener-rich layer thickness h th . At the beginning of grease lubrication, the h total for both temperatures mainly consists the thickener-rich layer. The h th decays overtime because the lubricants are squeezed in the contact zone and flow to sides due to the contact pressure gradient. The side flow rate is smaller for a larger viscosity [6]. Therefore, the grease film thickness at 40 o C is higher than the film thickness at 80 o C. This predicted effect agrees well with the experimental observation by Cann and Lubrecht [3]. With the subsequent overrolling, an enhanced oil film formation can be observed in Fig. 2, due to an increasing of bled oil amount [1]. The grease has a higher oil bleeding rate at higher temperature [2]. Therefore, the oil film enhancement at 80 o C has a dominate effect on the grease film evolution, and the h total begins to recover 80 o C. In contrast, the grease film thickness at 40 o C decays monotonically and becomes nearly stable after 4000 overrollings. This film thickness with recovery at higher temperature is also observed experimentally by Cann and Lubrecht [3]. Figure 2: Impacts of operating temperatures on film thickness evolution using PAO-Li-C as lubricant. 4. Conclusion The grease film is desired to maintain sufficient thickness to avoid direct asperity contact and severe friction losses during the operation of bearings. Therefore, it is benefit to have a model to predict film formation when designing greases and bearings. This study provides a porous thin layer model to simulate transient formation of thickener-rich layer, and a multiphase bleeding model to simulate the oil bleeding, respectively. Using this model, the film thickness evolutions are investigated for various temperatures. The findings can be summarized as follows: • The proposed models can describe the transient formation process of the oil film and thickener-rich layer for grease-lubricated rolling contacts. • A grease film thickness with recovery is observed for the contacts working with a higher operating temperature, while the film thickness with lower temperature decays monotonically. This model can provide deeper insights into grease lubrication, as it offers a deeper understanding of two components film thickness evolution. In future work, it is necessary to conduct extensive validations and bleeding tests to ensure the applicability of this model. Acknowledgements This work was supported by China Scholarship Council (No. CSC202006450015) and by the German Federal Ministry for Economic Affairs and Climate Action. Simulations were performed with computing resources granted by RWTH Aachen University under project ID rwth0910. References [1] Lugt P. M. A review on grease lubrication in rolling bearings. Tribology Transactions 2009; 52(4): 470-480. [2] Baart P., van der Vorst B., Lugt P. M., van Ostayen R. A. Oil-bleeding model for lubricating grease based on viscous flow through a porous microstructure. Tribology Transactions 2010; 53(3): 340-348. [3] Cann P., Lubrecht A. A. An analysis of the mechanisms of grease lubrication in rolling element bearings. Lubr. Sci. 1999; 11(3): 227-245. [4] Zhang, Shuo; Jacobs, Georg; Goeldel, Stephan von; Vafaei, Seyedmohammad; König, Florian. Prediction of film thickness in starved EHL point contacts using two-phase flow CFD model. Tribology International 2023; 178: 108103. [5] Cann P. M. E., Lubrecht A., Venner C. H. Grease lubrication of rolling element bearings - A model future; 2000. [6] van Zoelen, M. T.; Venner, C. H.; Lugt, P. M. Prediction of film thickness decay in starved elasto-hydrodynamically lubricated contacts using a thin layer flow model. Journal of Engineering Tribology 2009; 223(3): 541- 552. [7] Komoriya T., Ichimura R., Kochi T., Yoshi-hara M., Sakai M., Dong D. et al. Service life of lubricating grease in ball bearings (Part 1) Behavior of grease and its base oil in a ball bearing 2021; 16(4): 236-245. [8] Horgue P., Soulaine C., Franc J., Guibert R., Debenest G. An open-source toolbox for multiphase flow in porous media. Computer Physics Communications 2015; 187: 217-226. [9] Cann P., Damiens B., Lubrecht A. The transition between fully flooded and starved regimes in EHL. Tribology International 2004; 37(10): 859-864. 24th International Colloquium Tribology - January 2024 133 Enhancing Reliability and Service Life Predictions through Friction Monitoring and Sensor-Embedded Smart Contacts A Comparative Study of Weld, Bolt, and Plug-in Connections Michael Gless 1* , Anette Schwarz 1 ContactEngineering.de, Stuttgart, Germany * Corresponding author: E-mail Contact@ContactEngineering.de 1. Introduction Electrification leads to a high interest in electrical contacts. Battery or Hydrogen Electric Drivetrains, E-Scooters and E-Bikes become increasingly popular. In December 2022, the share of battery-electric and plug-in hybrids in newly registered cars in Germany rose to more than 55%. Every electric vehicle relies on numerous electrical high-current contacts. Therefore, appropriate design with energy efficiency, performance, and the highest reliability, especially in high-current contacts, is essential. However, there remain areas for the use of green synthetic fuels. Hydrogen is considered one of the most promising energy sources of the future. Green Hydrogen generation and Fuel-Cells are based on electrochemical processes and on many electrical contacts. Furthermore, trends toward automation not only aim to enhance energy efficiency but also demand enhanced reliability in contacts. In autonomous systems, these contacts must perform their functions seamlessly throughout their lifecycle or autonomously detect and communicate any deviations. This manuscript delves into systematic decision-making by highlighting and rating different solutions. The focus is on tribology in electric vehicle connections and ensuring robust performance and reliability. Based on these and other requirements, the best type of contact must be chosen. 2. Requirements and Design-Possibilities In engineering, decisions for best design solution have been made in a very early design process. Important electric contacts are shown in Figure 1. Figure 1: Connections and Contacts grouped into Material-Fit, Force-Fit, and Form-Fit. In electrical contacts, Form- Fit Connections need Force as well. Contact Resistance and Reliability of contacts are a common concern, prompting inquiries and extensive discussions. Contact resistance affects temperature, aging and energy efficiency. Power loss P and resulting heating during operation depend on current flow I and resistance R (P = I² · R). Results are shown in Figure 2. Figure 2: Measurements of Contact Resistance and Reliability Rating of Weld, Clinch, and Bolt Connection. Reliability according to MIL-HDBK 2017 and IEC 61709. 3. Focus on tribological failures/ failure mechanisms The main goal of contacts is to fulfill required functions. In an appropriate and optimized design, most failure mechanisms can be avoided. There are electrical, mechanical, and chemical failure and aging mechanisms. Influences which are in this document deeper addressed are frictions and wear. 3.1 Friction monitoring in electrical bolted joints The friction coefficients have a significant influence on the assembly force/ clamping force in the connection, which in turn has a significant influence on the resistance of the connection. Clamping force and friction between contact partners also influence transverse force and sliding, partial 134 24th International Colloquium Tribology - January 2024 Enhancing Reliability and Service Life Predictions through Friction Monitoring and Sensor-Embedded Smart Contacts sliding, or fretting in the contact area. A general rule is to avoid sliding or micro-movements in an electrical contact. Anti-friction coatings on the connecting elements are used to reduce the distribution of coefficients of friction, to set a defined coefficient of friction window, for example from μ-= 0.09 to 0.14. This reduces the spread of the contact force achieved and thus also the spread of the contact resistance/ electrical values of the connection. Determination of the coefficients of friction from the gradients is shown in Figure 3. The big advantage is that this can also be done as part of series production. However, the settlement/ embedding of joints and also the torsional rigidity of the tightening tools are challenging. Therefore, as an alternative, the ratio of tightening and loosening torque was determined, and the coefficient of friction was calculated from these. This approach delivers very satisfactory results. Deviations in the friction values can be identified during series production based on measured torque. Figure 3: Torque vs. Torsion angle for a Bolt Connection with slide coating (black line) and try/ without (blue line). The corresponding clamping force differs from 8 kN to 14-kN 3.2 Relaxation, reduction of spring or contact force Challenging relaxation/ loss of preload force, especially with highly conductive materials. For this purpose, relaxation tests and hot aging tests are usually carried out. For plug-in connections, we explored fretting wear optimization and wear prediction of coatings in electrical contacts. Optical measurement techniques enable the measurement of wear rates after a short test duration, while service lifetime models allow for estimating the service lifetime of the contacts. 3.3 Fretting/ Friction Corrosion and Friction Wear Frictional wear describes the aging of current-carrying connections due to relative movements. Spring forces/ mating forces limit contact pressure and therefore friction force. Micro motions in contact can be caused by thermal expansion, by shocks or vibrational loads. Sliding goes with Oxidation, Corrosion, Wear, and an increasing Contact Resistance. 4. Testing and Monitoring Besides mentioned force, resistance/ voltage drop, electromagnetic field measurements or electronic sensing, temperature is an easily accessible factor in electrical contacts. Monitoring supports: a. Verification of results e.g. at other/ system levels b. Actual operating limits, e.g., to avoid overloading/ overheating of components during operation c. Operating Condition and Anomaly detection. Detection of early signs of failures, early warning, Mitigate potential risks associated with high-current contacts, avoid subsequent errors. d. Derating, Service Lifetime Prediction, Predictive maintenance, planned maintenance Temperature Measurements are implemented in serial production accompanying testing/ prototypes, to further enhance reliability and ensure long-term performance. Thermal expansion/ bimetal-based mechanisms. Positive Temperature Coefficient materials acts as a reversible thermal shutdown mechanism in case of extensive current load e.g. PTCs in consumer cells. Measurements use again temperature-dependent resistors like PT100 or thermoelectric effects are e.g. thermocouples. Thermochromic labels/ materials are a simple and effective way to see if a certain temperature/ maximum temperature that was reached. Additional contactless diagnosis is possible. The microbolometer membrane absorbs emitted infrared radiation, heats up and chances resistance. Infrared sensors or cameras can be used. Infrared is not influenced by electromagnetic/ EM noises. A very elegant and cost-effective solution is to integrate a miniature infrared thermometer on a circuit board/ PCB, shown in Figure 4. Figure 4: Miniature Infrared Thermometer (e.g. MLX90632) stacked on top of Test-Surface. Infrared Thermometer measures surface temperature of an electrical contacts, of a cooling media or lubricant. 5. Conclusion The document gives an overview of important high-current contacts. Furthermore, consideration and results of important influences and reliability rating help in making decisions and designing a robust design. Important influences, which are deeper investigated, are friction and wear. To support testing, automation and increase reliability, monitoring solutions are discussed. By combining friction monitoring, assembly process recording, and sensor-embedded smart contacts, we can significantly enhance the reliability, service life predictions, proactive/ predictive maintenance and mitigate potential risks of tribological contacts. 24th International Colloquium Tribology - January 2024 135 Analysis of Biodegradable Lubricants for Radial Shaft Seals Under Critical Conditions Stefanie Haupt 1* , Dr. Florian Johannes Heiligtag 1 , Maria Frackowiak 1 , Tanja Püler 1 , Danijela Grad 1 , Dirk Fabry 1 1 KLÜBER LUBRICATION MÜNCHEN GmbH & Co. KG/ GPI&GBT, Munich, Germany * Corresponding author: Stefanie.Haupt@klueber.com 1. Introduction Dynamic compatibility tests for radial shaft sealings are becoming increasingly important for matching elastomer and lubricant to the corresponding application. The tests are often time and cost intensive but are more informative than a static test and the correlation with the application is higher. The selected base oil in combination with the additive have a big influence on the performance of the sealing. Lubricant development becomes even more challenging, when users want an innovative bio-based and environmentally acceptable lubricant (EALs), which must meet the following requirements: biodegradability, renewability of the raw materials, low toxicity and non-bioaccumulation. [2] In this work, the tribological performance of elastomers is investigated when lubricated with an EAL under various technical operating conditions. The latter were adjusted iteratively to examine the limits of the respective tribo-system. The measured results and tribological characterization are the basis for the lubricant development that expands the limits of the elastomer sealing performance. 2. Methodology 2.1 Test principle To investigate the dynamical elastomer compatibility, Klüber Lubrication uses a self-developed Dynamical Elastomer Screening test rig (DES). The testing principle is comparable to a ring disk tribometer. [1] The elastomer samples were produced from FKM plates. The elastomer sample is pressed onto the counter surface. The pressure force on the counter surface remains constant for the entire test time. The counterpart material is an austenitic steel compliant to DIN 3761. The test rig also includes an oil reservoir. The oil reservoir can be heated and controlled with a cryostat to a defined oil sump temperature, which is measured during the test. The friction force, the elastomer wear and the temperature in the proximity of the sealing edge are measured. The elastomer wear is determined using a displacement sensor. In case of leakage, the leakage behaviour is observed from the time of first leakage. 2.2 Iterative procedure to determine critical conditions An oil lubricant containing synthetic biodegradable ester with a high renewable content and a special additive package (ISO VG 320) is used for the test series with a biodegradability of over 60-% according to OECD-301-F. The tests were performed at different normal line loads, operational times, sump temperatures (60-°C, 100-°C and 120-°C) and rotational speeds. Each sump temperature variation is measured at least three times. The following Figures 1 and-2 show the temperature behaviour near the sealing edge and the associated friction coefficient, respectively for three test runs and for all considered oil sump temperatures at a normal line load of ~0.85-N/ mm and a circumferential speed of ~1.7-m/ s. The running-in behaviour of approximately 18-hours is not shown. Figure 1: Temperature near the sealing edge at different sump temperatures Figure 2: Friction coefficient at different sump temperatures The sealing edge temperatures are subject to higher fluctuation and higher spread at an oil sump temperature of 60-°C (orange to red lines) compared to 100 °C and 120-°C. After the run-in, it proceeds more stably at an oil sump temperature of 100-°C (green) than by 120-°C (gray to black). The friction coefficient at 60-°C is also higher than at higher oil sump temperatures. At 100-°C, the friction coefficient is on a lower level and is more stable than at 120-°C, where the measurements show a bigger spread. 136 24th International Colloquium Tribology - January 2024 Analysis of Biodegradable Lubricants for Radial Shaft Seals Under Critical Conditions Figure 3: Elastomer wear at different sump temperatures The continuous wear measurement shows a significant increase after the run-in time (Figure 3). At sump temperatures of 100-°C and 120-°C respectively, the wear measurement curves are stable after the run-in. At first glance, the shaft run-in and the elastomer track width do not show any correlations. The visual inspection is also unremarkable. The optical appearance of the counterparts tested at 100-°C show deposits. To further understand the different performances at different temperatures, the samples (elastomer and metal counterparts) were subsequently examined by Secondary Electron Microscope (SEM) with EDX. The samples which were tested at 100-°C clearly show a tribo film and on all samples a transfer film can be observed (data not shown). 2.3 Increase of the loading conditions Using the same parts as before except for the seals from FKM material, the operating conditions are iteratively intensified until thermal damage occurred (Figure 4). Figure 4: Thermal damaged sealing edge with orig. formulation The excessive frictional heat at the sealing edge changes the elastomer boundary layer such that hardening occurs. This hardened layer reduces the elastic compliance and weakens the hydrodynamic sealing mechanism. Furthermore, the contact surface becomes brittle. [3] The sealing edge can tear and partial breakouts from the sealing edge cause a greater spread in the coefficient of friction. Based on these results, the lubricant composition was iteratively adjusted and the new formulations are tested under the same conditions. A formulation was found that increases the thermal load capacity of the sealing system (Figure 5). This formulation also shows a significantly lower friction coefficient than the original formulation and was biodegradable according to OECD-301-F. Figure 5: Sealing edge left: with new formulation; right: combination with ester oil and synthetic hydrocarbon oil 3. Conclusions Environmentally acceptable lubricants in elastomer sealings were tested under dynamic conditions. Critical operating conditions have been identified on an elastomer screening test rig for a biodegradable lubricant. At a certain oil sump temperature, a tribo layer formed on the metal and the elastomer sealing edge as well. The performance limits of the lubricant have been tested iteratively until thermal damage occurred. The additive package in the lubricant has a significant impact on the thermal load capacity of elastomer sealing systems at the considered severe operating conditions. References [1] Burkhart et. al., Development and Optimization of a Tribometer for Radial Shaft Seals, Journal of Tribology Vol 143 (4), 2021, DOI: 10.1115/ 1.4049597. [2] United states Environmental Protection Agency, VES- SEL GENERAL PERMIT FOR DISCHARGES INCI- DENTAL TO THE NORMAL OPERATION OF VES- SELS (VGP), 2018. [3] Bauer, Federvorgespannte-Elastomer-Radial-Wellendichtungen: Grundlagen der Tribologie & Dichtungstechnik, Funktion und Schadensanalyse, Taschenbuch, 2021. 24th International Colloquium Tribology - January 2024 137 Implementing the use of Water Based Enviornmentally Acceptable Lubricants in the Ship Industry On the Frictional and Wear performance of SiC-YAG Composite Coating N. Espallargas 1,* , E. Valaker 2 , H. Khanmohammadi 1 1 Norwegian Tribology Center, Dept. of Mechanical and Industrial Engineering, NTNU, Trondheim, Norway. 2 Corrosion and Tribology, SINTEF Industry, SINTEF, Trondheim, Norway. * Corresponding author: nuria.espallargas@ntnu.no 1. Introduction Maritime transportation is increasing worldwide leading to annual lubricant release into the ports of about 32 to 61 million litres. Norwegian ports account for more than 1.6 million litres ranking Norway as the 12 th world nation with the highest lubricant input to the sea. Lubricants reaching the sea waters can be catastrophic to local marine wildlife and aquatic plants and harmful chemical substances can enter the food chain affecting human health [1-3]. Therefore, the aim of this work is to test a new water-based lubricant for the oil-to-sea interfaces of a thruster system introducing optimized surfaces (e.g. SiC coatings) in contact with polymeric seals. Five different seal materials and four different surfaces have been tested for frictional and wear performance in a water-based lubricant formulated for a thruster application. This work is part of an on-going industrial project in Norway funded by the Research Council of Norway and the partner companies. A methodology based on Stribeck curve parameters fitting real thruster conditions was chosen to study the frictional behaviour of each seal-surface pair. The Stribeck number of three main seals (rotational seal, leap seal and axial seal) was calculated and the contact pressures and speeds of the tests were designed to have the Stribeck curves covering all seals. 1.1 Materials and experimental procedure Five seal materials were tested against the different countersurfaces. Table 1 shows the selected seal materials and their properties. Four countersurfaces were selected: hardened steel as the reference material, WC-CoCr coating, Cr 3 C 2 -NiCr coating and ThermaSiC coating were applied on mild steel using a thermal spray technique. Table 2 shows a summary of the tested counter surfaces. Table 1: description of the seal materials used in this work. Short name Shore Hardness Material type Chemical compatibility and sealing performance PE 67D Ultra-high-molecular-weight polyethylene Moderate swelling below 1% and surface plasticization in mineral oil above RT. Exceptional durability and toughness, self-lubricating in dry-running applications, high abrasion resistance, very low friction. HNBR 90A Hydrogenated acronitrile-butadiene rubber Excellent weathering resistance. High abrasion resistance, outstanding dynamic characteristics, very low compression set. EPR 80A Ethylene-propylene-diene rubber, peroxide-cured Best for aqueous fluids, inferior to mineral oil, excellent weathering resistance. High abrasion and tear resistance in hydrocarbon-free environments. PK 78D Aliphatic polyketone Moderate swelling and water-induced plasticizing. High resilience, excellent dynamic sealing characteristics, low friction, no stick-slip behaviour. SWF - Synthetic woven fabric impregnated with phenolic resin Resistant to water uptake. High strength, flexibility and integrity at extreme pressures, excellent gliding characteristics and thermal expansion stability. Table 2: description of the counter surfaces used in this work. Surface Wt % Metallic matrix Coating technique Hardness (HV) Coating Thickness (µm) Hardened steel 100 - 780 - WC-CoCr 14 HVOF* 1165 254 Cr3C2-NiCr 25 HVOF* 890 221 ThermaSiC 0 HVOF* 540 191 * High Velocity Oxygen Fuel The seal samples were provided by Seal Engineering AS (Norway) in the form of pins with a diameter of 8 mm and a height of 6 mm. All the counter surfaces in Table 2 were prepared as discs with a diameter of 40 mm and a thickness of 6 mm. The cermet coatings (WC-CoCr and Cr 3 C 2 -NiCr) were provided by Wear Solutions AS (Norway) and were plane ground to reach a surface average roughness (Ra) of 0.4 µm. The hardened steel and ThermaSiC (provided by Seram Coatings AS, Norway) samples were ground using sandpapers until reaching the same roughness as the cermet coatings. For the Stribeck curves, seven different conditions giving seven points on the coefficient of friction (COF)-Stribeck number plot were designed. Table 3 shows the seven test conditions used in this work and their assigned Stribeck number and the Stribeck number of three components selected by the final user in the project. 138 24th International Colloquium Tribology - January 2024 Implementing the use of Water Based Enviornmentally Acceptable Lubricants in the Ship Industry Table 3: Test conditions for the Stribeck curves Test number Normal load (N) Contact pressure (MPa) Rotational speed (RPM) Assigned Stribeck number 1 45 0.9 8 0.024 2 7.5 0.15 8 0.145 3 45 0.9 76 0.242 4 45 0.9 300 0.969 5 7.5 0.15 76 1.45 6 7.5 0.15 300 5.81 7 4 0.08 300 10.9 Application - - - - Rotational Seal - - 0.87 Leap Seal - - 4.1 Axial Seal - - 10.9 Each test is repeated at least two times. All seal pins were soaked in the lubricant for 7 days at 80 °C before testing. The test duration was 1800 s and the average friction during the last 400 s was considered as the COF for the test point. The wear investigations were performed using TE88 reciprocating tribometer. The normal load was set to 250 N to measure wear without having any mechanical failure in the pins. The wear tests were performed with a linear velocity of 48 mm/ s with a stroke length of 24 mm and for a duration of 23 hours. The seal pins were weighted before and after the tribotests to measure the material loss. In the case of cermet coatings (WC-CoCr and Cr 3 C 2 -NiCr), the sliding direction was set perpendicular to the grinding lines. 2. Results and discussion Figure 1 shows the Stribeck curves obtained for all surfaces tested against the different seals. Stribeck results showed lower frictional numbers for the plastic seals compared to the rubber seals. Figure 1: Stribeck curves of the seal materials against (a) hardened steel, (b) WC-CoCr coating, (c) Cr 3 C 2 -NiCr coating and (d) ThermaSiC coating. Tests against the WC-CoCr coating showed the highest frictional numbers, followed by Cr 3 C 2 -NiCr. The hardened steel and ThermaSiC showed lower numbers compared to the cermets. Adsorption of the lubricant additives to the metallic surfaces are proposed as the dominant mechanism in the boundary regime of the hardened steel and the cermet coatings. In the case of ThermaSiC the formation of a hydrated film due to the water present in the lubricant is proposed as the main frictional mechanism. For the softer seals, the hydrated film formation mechanism was not activated, resulting in higher friction for these samples against ThermaSiC compared to the other surfaces. For harder seals, the hydrated film formation took place resulting in lower friction against ThermaSiC. In terms of wear, the pair ThermaSiC-PE was the one providing with lowest material loss. 3. Conclusions Using water-based lubricants in a thruster system requires of new material selection to get the optimal friction and wear results. In this case, SiC with PE provides the best surface-seal couple to obtain the lowest friction and wear for the application. References [1] D. S. Etkin. Worldwide Analysis of In-Port Vessel Operational Lubricant Discharges and Leakages. Proceedings of the 33rd Arctic and Marine Oil Spill Program Technical Seminar. New York 2010. [2] https: / / www.tribonet.org/ wiki/ environmentally-accept able-lubricants/ [3] https: / / www.dw.com/ en/ exclusive-cargo-ships-dump ing-oil-into-the-sea-go-unpunished/ a-61201989 24th International Colloquium Tribology - January 2024 139 Enhancing Machining Efficiency and Sustainability of Ti-6Al-4V through MQL with Polymeric Ester Based Metalworking Fluids: A Comparative Study with Conventional Cutting Fluids Ramazan Hakkı Namlu 1* , Kübra Kavut 2 , Hanife Gülen Tom 2 1 Atılım University, Manufacturing Engineering Department, Ankara, Turkey 2 Belgin Oil, R&D Department, Kocaeli, Turkey * Corresponding author: namluramazan@gmail.com 1. Introduction Ti-6Al-4V material is a widely used alloy across various fields, particularly in the aerospace and biomedical sectors, owing to its exceptional corrosion resistance, favorable strength-to-weight ratio, and biocompatibility properties. Machining operations serve as crucial production methods in transforming Ti-6Al-4V into the final product. However, various challenges such as high cutting forces, inadequate surface finish and shortened tool life arising from Ti-6Al-4V‘s very limited machinability due to its low thermal conductivity and high chemical reactivity make its machining challenging [1, 2]. For this reason, it is often referred to as a „difficult to cut“ material. To address these challenges, cutting fluids play a vital role by positively influencing operational efficiency, primarily aiming to reduce friction and dissipate heat. Nonetheless, Conventional Cutting Fluids (CCF) have several disadvantages, such as the requirement for excessive consumption, limited productivity increase, and adverse effects on the environment and operator health [3]. Thus, alternative methods emerged such as Minimum Quantity Lubrication (MQL). The MQL method aims to improve machining performance with advanced penetration into the cutting area through the aerosol delivery of a compressed air and oil mixture, while minimizing the impact on the environment and operator health with significantly reduced fluid consumption [4]. To optimize the benefits of MQL, careful selection of the appropriate fluid for the material used is crucial. This study investigates various polyol and polymeric ester based MQL fluids with distinct properties in the slot milling of Ti-6Al-4V, focusing on differences in cutting forces, surface roughness and surface topography compared to CCF. 2. Methodology The experimental setup can be seen in Figure 1. Four different slot milling operations were conducted with a constant cutting speed of 95 m/ min, a feed rate of 0.07 mm/ tooth, and a depth of cut of 1 mm. Each experiment was repeated three times in order to obtain repeatability. MQL fluids are formulated with three different esters: Trimethylolpropane Ester (TMPE) and Polymeric Ester (PE) with a viscosity index (VI) of 180 and PE with a VI of 221, which have a similar additive structure, contain water at a constant rate to increase the cooling effect in the tough machining of the Ti-6Al-4V alloy. All MQL fluids were applied to the cutting zone through two external nozzles with a flow rate of 50 ml/ h. In order to compare the MQL fluids, vegetable based CCF was utilized as a reference experiment for standard conventional operations with a flow rate of 50 l/ h and oil-water mixture ratio of 8%. The cutting forces are measured by Kistler 9265B dynamometer by using DynoWare software and the surface measurements were taken by Alicona InfiniteFocus optical surface roughness measurement device. Figure 1: The experimental setup 3. Results and discussion The cutting force results can be seen in Figure 2 while surface roughness and topography images are shown in Figures 3 and 4, respectively. The results indicate that all MQL experiments exhibited reduced cutting forces and surface roughness values compared to CCF. The surfaces obtained after MQL application displayed greater homogeneity, uniformity, and lower peak-to-valley values compared to CCF. The aerosolized delivery of MQL facilitated improved penetration between the cutting tool and the workpiece, effectively reducing wear and mitigating heat generation within the cutting zone. Ti-6Al-4V’s poor thermal conductivity challenges cutting due to heat concentration. MQL enhances cutting efficiency by minimizing this challenge [5]. 140 24th International Colloquium Tribology - January 2024 Enhancing Machining Efficiency and Sustainability of Ti-6Al-4V through MQL with Polymeric Ester Based Metalworking Fluids Figure 2: Cutting force results Figure 3: Surface roughness results When comparing different MQL fluids, it was determined that the PE formulation with higher viscosity index exhibited the lowest cutting force and achieved the best surface quality in terms of surface roughness and topography. Based on the results, it has been seen that the implementation of MQL, which demonstrates environmental friendliness and superior machining efficiency, outperforms CCF in the milling of Ti-6Al-4V material, yielding more effective machining outcomes. Furthermore, it has been ascertained that MQL fluids formulated with PE with outstanding thermal and oxidative stability, high viscosity index and excellent lubricity have been found to provide a maximum increase in efficiency. In light of these findings, it is evident that PE MQL fluids exhibiting offer a preferable alternative to CCF usage in terms of promoting sustainability and machining performance. Figure 4: Surface topography of cutting fluids 4. Conclusions In this study, the cutting forces and surface quality of Ti-6Al-4V material in slot milling operation were investigated using MQL with three different types of fluids. The obtained results can be summarized as follows: • MQL application, regardless of the type of oil used, resulted in lower cutting forces and surface roughness compared to CCF, while also providing a more homogeneous and uniform surface topography. • Among the different MQL oils tested, polymeric esters performed better than trimethylolpropane esters. • When examining polymeric esters among themselves, it was observed that polymeric esters with a high viscosity index yielded the best results. 5. Acknowledgement The MQL fluids used in this study were developed within the scope of the project labeled ARISEN, coded S0411, and financially supported by SMART Eureka. The authors express their gratitude to Atılım University Metal Forming Center of Excellence for their assistance in conducting the surface measurements. This research is partially funded by The Scientific and Technological Research Council of Turkey (TÜBİTAK), under grant number 222M381. References [1] E. O. Ezugwu, “Key improvements in the machining of difficult-to-cut aerospace superalloys,” Int J Mach Tools Manuf, vol. 45, no. 12-13, pp. 1353-1367, 2005, doi: 10.1016/ j.ijmachtools.2005.02.003. [2] E. O. Ezugwu and Z. M. Wang, “Titanium alloys and their machinability—a review,” J Mater Process Technol, vol. 68, no. 3, pp. 262-274, 1997, doi: 10.1016/ B978-0-12-801238-3.99864-7. [3] A. Shokrani, V. Dhokia, and S. T. Newman, “Environmentally conscious machining of difficult-to-machine materials with regard to cutting fluids” Int J Mach Tools Manuf, vol. 57, pp. 83-101, 2012 doi: 10.1016/ j. ijmachtools.2012.02.002. [4] R. H. Namlu, O. D. Yılmaz, B. Lotfisadigh, and S. E. Kılıç, “An experimental study on surface quality of Al6061-T6 in ultrasonic vibration-assisted milling with minimum quantity lubrication,” Procedia CIRP, vol. 108, pp. 311-316, 2022, doi: 10.1016/ J.PRO- CIR.2022.04.071. [5] R. H. Namlu, B. L. Sadigh, and S. E. Kiliç, “An experimental investigation on the effects of combined application of ultrasonic assisted milling ( UAM ) and minimum quantity lubrication ( MQL ) on cutting forces and surface roughness of Ti-6AL-4V,” Machining Science and Technology, vol. 25, no. 5, pp. 738-775, 2021, doi: 10.1080/ 10910344.2021.1971706. Computational Methods and Digital Transformation in Tribology 24th International Colloquium Tribology - January 2024 143 Simulation of the Local CoF Development in Dynamically Loaded Contact Surfaces (Fretting) Silvano Oehme 1* , Alexander Hasse 1 1 University of Technology, Chemnitz, GER * Corresponding author: E-mail silvano-giuseppe.oehme@mb.tu-chemnitz.de 1. Introduction In dynamically loaded component contacts microslip movements result in local change of the coefficient of friction (CoF development), which needs to be known in terms of appropriate component design. Due to lack of experimental accessibility to the contact surface, the resulting CoF distribution has to be obtained by simulation. This paper presents an approach to simulate the local CoF development in dynamically loaded component contacts, in dependence of the dissipated friction work. 2. Simulation approach The simulation procedure requires a CoF-friction-workcurve as input data, which shows the friction-work-dependend local CoF development. It has to be obtained experimentally. The Institute’s fretting test rig uses standardised model specimens with flat annular contact surfaces [1]. Defined dynamic contact loads (pressure p F and slip amplitude s a ) lead to an oscillating friction moment, from which the CoF and friction work W fric are calculated. Fig.-1: Exp. obtaining of local CoF development The FE-model containing the component contact then calculates the dissipated friction work at each node and assigns the resulting CoF in an iterative calculation loop (see fig.-2) until the final contact condition is reached. Fig.-2: Iterative calculation loop The dissipated cyclic friction work W fric,cyc is calculated for a determined load cycle (starting with N-=-1). The load cycle numbers relating to the subsequent calculation points are calculated using cycle jump technique [2]. This contains load cycle jumps, that may vary in dependence on particular gradient in tribological stress distribution and predefined critical change in resulting CoF Δµ th . A small value for Δµ th leads to small size of the cycle jumps between calculation points and, therefore, result accuracy and computation time increase, vice versa (see fig.-3). The value for Δµ th has to be chosen in a way that accurate results can be achieved in reasonable computation time. Fig.-3: Cycle jump technique, effect of step size Due to the modular structure of the simulation tool, the module can be connected to any FESystem by adapting only the FESystem-dependent modules. 3. Study on effects of CoF distribution on calculation of machine component contacts 3.1 Performed simulations Simulations were performed in two steps at a model of a pressfit-connection under bending load (fig.-4). Fig.-4: Pressfitconnection under bending load In the first step the simulation method was applied to the model to obtain the CoF distribution in the final state. In the second step the exact same load as in the first simulation was applied to final state of the connection to obtain stress and slip distribution in contact. These were compared with calculation results where CoF had been set constant. 3.2 Results 3.2.1 CoF development Fig.- 5 shows the CoF development of the simulated model. The CoF increases locally starting from a base level of 144 24th International Colloquium Tribology - January 2024 Simulation of the Local CoF Development in Dynamically Loaded Contact Surfaces (Fretting) µ 0 -=-0.2. The CoF has the biggest increase at the hub edge, where slip amplitude and contact pressure and, therefore, the friction work are at their maximum. As a result, the final CoF distribution establishes as shown in Fig.-6. Due to discretization of the geometry and load cycles (fig.-3) chattering effects can be observed in CoF-development, that lead to an alternating increase in CoF of adjacent contact nodes. Fig.-5: CoF distribution and development in contact Fig.-6: Final CoF distribution (green) in contact The impact of including the final CoF distribution in the calculation on result variables are presented in the following section. 3.2.2 Effects on Stress and Slip Fig.- 7 and Fig.- 8 illustrate the results of stress and slip in for distributed CoF and constant CoF throughout the contact surface. Fig.-7: Slip distribution in contact with distributed CoF (green) compared to results with constant CoF The slip distribution in contact (fig.-7) differs considerably when taking CoF distribution into account. The slip value at the edge of the hub as well as the depth into the contact are higher compared to results with constant CoF of µ fin -=-0.8. Fig.-8: Stress distribution in contact with distributed CoF (green) compared to results with constant CoF In contrast CoF distribution has less impact on the stress distribution as shown in fig.-8. A summary of the identified effects of CoF distribution on the considered result variables for the considered shaft-hub-connection is shown in Tab.-1, effects on stiffness and torque transmission added (not presented in particular in this paper). Tab.-1: study summary - influence of CoF distribution on calculation results of a shaft-hub-connection (from green - high influence to grey - low influence) Slip Stress Stiffness Torque transmission When calculating dynamically loaded contact surfaces of a specific application, taking CoF distribution into account can increase the result accuracy of the required result variables considerably. Conclusion A simulation method for calculating CoF development has been introduced which enables to identify the impact of local CoF distribution on calculation results. The simulation method was applied to specific models of shaft-hub-joints. Thus the capability of the simulation method was shown and suggestions for appropriate simulation settings were given. The impact of CoF distribution on slip amplitude and shear stress (commonly considered surface damage parameters) compared to a constant CoF was shown. Using the simulation method, CoF development, resulting from component geometry, load case and assembling conditions (e.g. interference), can be taken into account when calculating component contacts for fretting wear, fretting fatigue and crack initiation location more exactly. References [1] Leidich E, Maiwald A, Vidner J. A proposal for a fretting wear criterion for coated systems with complete contact based on accumulated friction energy density, Wear 297 (2013), S. 903-910. [2] Titscher T, Unger JF. Efficient higher-order cycle jump integration of a continuum fatigue damage model. Int J-Fatigue 141 (2020). 24th International Colloquium Tribology - January 2024 145 Static and Dynamic Friction of Elastomers in Dry Conditions Simulating Commercial Materials and Products Dr.-Ing. Fabian Kaiser 1* , Dr.-Ing. Daniele Savio 1 , Dr.-Ing. Felix Meier 2 , Prof. Michele Scaraggi 3 1 Freudenberg Technology Innovation SE & Co. KG, Tribology, Weinheim, Germany 2 EagleBurgmann Germany GmbH & Co. KG, Research & Development, Wolfratshausen, Germany 3 Istituto Italiano di Technologia, Center for Biomolecular Nanotechnologies, Arnesano (Lecce), Italy * fabian.kaiser@freudenberg.com 1. Introduction Understanding the tribological behavior of elastomers in dry conditions is essential not only for tires but also for sealing applications. One example are the secondary seals in mechanical seals for gases, where rubber O-rings are often used. To ensure the stable function of the whole system, the auxiliary sealing elements need to have smooth and predictable friction properties. 2. Current State of Research In the last decades, Persson, Scaraggi and co-authors have developed a comprehensive theory for rubber contact mechanics [1], adhesion [2], static leakage and dry [3] and lubricated friction [4]. This theory has been thoroughly tested and validated in the literature, e.g., during the “Contact Mechanics Challenge” [5]. 3. Theory According to Tiwari et al. [3], rubber friction without lubricant is due to the viscoelastic losses F visc in the bulk elastomer, and the contribution of the local shear stresses τ f acting in the real contact area A con of the sliding surfaces. The friction force thus is F f = F visc + τ f A con A con is calculated using contact mechanics [1]. The main contributions to τ f are [3]: a. Adhesive bonding-stretching-debonding of rubber molecules or patches at the sliding interface. b. Opening crack propagation at the exit of the contact patches [7]. c. Energy dissipation in wear processes: Hard fillers in the rubber scratching the counter surface or being torn out of the rubber matrix. d. Shearing of thin (nm) transfer film, occurring at temperatures below glass transition [3]. In the present work, the dynamic dry friction model from [3] was implemented with two main modifications: 1. In our experiments with rubber sliding against steel, a transfer film can be observed under all conditions: the corresponding contribution is thus applied at all temperatures. 2. The shear stress τ f for the bonding-stretching-debonding contribution is estimated from the material properties of each elastomer. Furthermore, the model was extended to static friction based on the elastomer relaxation and subsequent increase in contact area with standstill times. The shear stress contribution τ f in the contact spots was described based on the bond-population model of Juvekar et al. [6]. This allows to estimate the lifetime of elastomer-metal bonds during the initial surface motion, thus relating the break-loose friction stresses to the speed at the onset of sliding. 4. Experiments and Validation Validation experiments were performed in a reciprocating tribometer over a wide range of temperatures (-40 - 100-°C), sliding speeds (1 - 300-mm/ s) and waiting times (1 - 5,000-s). The counterface was made of hardened steel ground to a surface roughness of Ra-0.3-µm. The elastomer sample was cut from a 2-mm thick test sheet and installed in a pin to form a curved surface with a radius of approximately 22-mm. The results for many different elastomers are in line with the expected behavior: The general shape of the dynamic friction curves in [3] is reproduced well and the static friction increases with time and break-loose speed. The comparison between experiment and simulation shows a very good agreement: For the tested fully formulated materials (FKM, EPDM and NBR) and under all experimental conditions, the dynamic and static friction can be predicted with an average deviation of only 10% using our parametrization for τ f . Figure 1: Comparison of calculated and measured friction for a 90 ShA FKM material 146 24th International Colloquium Tribology - January 2024 Static and Dynamic Friction of Elastomers in Dry Conditions The analysis of the friction simulation shows that, for the given setup, the contribution from crack propagation is negligibly small and the viscoelastic friction contributes only up to 15% to the total friction. This is considerably less compared to [3] and can be attributed to our smooth steel surface, which significantly lowers the amplitude of oscillations and viscous damping inside the material compared to the road surface used in [3]. Hence, the main contributions to the friction are the shearing of the transfer film and the adhesive bonding-stretching-debonding. Figure 2: Static friction at two different break-loose speeds vs. standstill time 5. Application In mechanical seals, secondary seals (e.g., O-rings) are required to seal alternate leakage paths. To ensure that the main seal rings are exactly parallel, the secondary seals are usually designed to slide some millimeters depending on the conditions (temperature, pressure, etc.). In combination with large standstill times, that lead to increased contact area and breakaway forces, the O-ring may not be able to slide leading to increased leakage and even failure of the whole system. The model was used to explore the relationships between loading conditions, interfacial and material properties and static friction depending on standstill times for a given secondary seal. This allowed to optimize its friction behavior and ultimately improve the function and reliability of the product. 6. Conclusion A comprehensive dry friction model for elastomers was developed and validated to be used for a broad range of applications. No model of similar predictive capabilities is known to the authors. Furthermore, the dry friction model might also be highly relevant for lubricated cases: due to local dewetting some contact patches can slide in dry conditions, even if plenty of liquid is available near the contact zone. References [1] Yang, C., Persson, B.N.J., “Contact mechanics: contact area and interfacial separation from small contact to full contact”, J. Phys.: Condens. Mat-ter, 2008 (20). [2] Persson, B. N. J., Scaraggi, M., “Theory of Adhesion: Role of Surface Roughness”, The Journal of Chemical Physics 141, 124701 (2014) [3] Tiwari, A. et al., “Rubber contact mechanics: adhesion, friction and leakage of seals”, Soft Matter, 2017,13, 9103-9121 [4] Persson, B. N. J., Scaraggi, M., “Lubricated sliding dynamics: Flow factors and Stribeck curve”, Eur. Phys. J. E (2011) 34: 113 [5] Müser, M. H. et al., “Meeting the Contact-Mechanics Challenge”, Tribology Letters, 2017. [6] Juvekar, V. A.; Singh, A. K., “Rate and aging time dependent static friction of a soft and hard solid interface”, arXiv 2016, arXiv: 1602.00973. [7] Persson, B. N. J., “Crack propagation in finite-sized viscoelastic solids with application to adhesion”, 2017 EPL 119 18002. 24th International Colloquium Tribology - January 2024 147 Identification of the Dominant Wear Mechanism in Dry Contacts by Numerical Modeling F. Koehn Research Institute for Innovative surfaces FINO, Aalen University, Beethovenstr. 1, D-73430 Aalen, Germany Investigating the wear behavior of sliding contacts is of paramount importance in numerous industrial applications. A promising approach is focusing on the two-dimensional cross section profile of the wear scar, particularly emphasizing the resulting geometric shape. For this purpose, two mathematical limit cases for wear mechanisms are developed. A third-body-driven process, in which a two-dimensional load occurs constantly during the wear process. Here it is assumed that a homogeneous removal takes place over the contact area due to abrasion particles. In the second process the stress is not homogenous. Here it is assumed that due to particle displacement from the frictional contact, stress occurs excessively at the outer edge. In each case, a characteristic cross-sectional area is created by multichannel analysis, considered that the contact area changes during the wear process. The investigated friction pairings are characterized by both sub-processes. The width and area of the cross-sectional geometry of the generated wear scar are determined in order to identify the respective fractions of the wear mechanisms of a friction pairing that occur. These data are then used to normalize the mathematical models. In this way, they can be offset against each other and the proportion of the processes occurring can be determined. The application of this method allows the comparison of wear scars of different specimens and testing parameters. By using this method, it is possible to quickly and reliably determine which of the investigated wear mechanisms is predominant. Therefore, important conclusions about the role and behavior of wear particles in the investigated friction contact can be drawn from the results. The contact pairs studied comprise flat ceramic wear resistant coatings (Tungsten Carbide Cobalt coatings manufactured by High Power Impulse Magnetron Sputtering (HiPIMS)) with varying surface roughnesses and spherical counterbodies (CB) made of steel (100Cr6). By comprehensively analyzing the geometric features of the wear scar, valuable insights can be gained into the underlying wear mechanisms and the tribological behavior of the sliding interface. Moreover, this investigation aims to examine the influence of the presence or absence of wear particles within the contact. The experimental method involves conducting controlled sliding tests while monitoring the resulting wear scars with optical and interferometric measurements. Subsequently, a precise two-dimensional cross-sectional profiling technique is employed to obtain detailed geometric information from the wear scar. The obtained data is then analyzed and correlated with the operating conditions and the presence of wear particles. The findings from this study have significant implications for understanding the wear processes and enhancing the performance of dry sliding contacts operating under high loads. The analysis of the geometric cross-sectional shape of the wear scar provides crucial information about the effectiveness of the wear-resistant coatings and the presence of wear debris in the contact. Such knowledge can contribute to the development of improved wear protection strategies and the optimization of materials and surface treatments. 1. Introduction Surfaces in direct contact with a friction partner are subjected to significant stresses, particularly in the absence of lubrication. Wear processes, including mechanical, chemical, and thermal actions, occur under varying ambient conditions [1] . Upon initial wear, the formation of particles occurs, either expelled or trapped between the friction partners, significantly influencing the wear behavior [2] . Applying hard coatings, like nitrides (e.g., TiN, TiAlN, MoN) [3] and carbides (e.g., WC) [4] , is a standard wear reduction method. Microstructuring surfaces to trap particles in cavities also minimizes abrasion [5,6] . Wear of component surfaces is pivotal economically, environmentally, and functionally. Understanding wear processes aids in generating models for optimizing surface properties and lifespan. Manual wear analysis predominates, but computer model approaches also exist, necessitating large databases. This work presents an experimental method for qualitative assessment of wear-resistant coating lifespans with minimal data. Diverse industrial methods manufacture hard coatings, each influencing surface wear behavior differently and providing a wide range of coating thicknesses [7] . The maximum wear depth in a tribological contact effectively describes the coatings wear resistance lifespan, with failure occurring when the wear depth equals the coating thickness. Adapting coatings to external conditions and CBs is crucial, as is the role of surface topography and microstructures in reducing friction and wear by capturing and removing abrasive particles from the tribological contact. This study investigates the cross-sectional geometry of wear scars in hard coatings after tribological stress, aiming to quantify wear mechanisms without lubrication. Understanding the wear process facilitates adapting coatings to external loads, extending their lifespans. 2. Experimental Details Thin WC(Co) films are deposited on high-speed steel substrates (1.3343) with different surface roughnesses via magnetron sputtering using a 5-6 at. % cobalt binder. The smooth or textured surfaces are deposited on high-speed steel substrates. Friction and wear analysis is conducted using a SRV3 tribometer under dry conditions with spherical 100Cr6 steel CBs. The wear track’s topography is examined with Zygo ZeGage. Wear track shapes, including Uor W-shapes, are identified, affecting the coating’s lifespan. 148 24th International Colloquium Tribology - January 2024 Identification of the Dominant Wear Mechanism in Dry Contacts by Numerical Modeling 3. Methods The surface topography, obtained through white-light interferometry, allows the extraction of a wear track’s cross-section. Figure 1 illustrates two instances: a wear track on a rough (top) and a smooth (bottom) surface. The extracted profiles serve as the initial data for further analysis. Figure 1: Profiles of wear scars with different geometric shapes. The tribotesting is performed perpendicular to the surface structure. The cross-sectional profile extracts in the x-axis over a range of 834-µm and 1024 data points. The respective height is depicted in the y-axis. The model is created from the measured data as follows. Isolation: The wear scar is automatically isolated from the non-loaded sample by an algorithm. Data extraction: Area, width and maximum depth of the wear scar are determined. Model creation: CB abrasion alters the contact geometry, causing continuous changes. The CB’s surface experiences planar abrasion, leading to a continual shift in the contact area with more cycles. V CB represents the missing volume, signifying the removed spherical cap from the CB, with ‘r’ as the spheres radius and ‘a’ as the caps radius. Using a Taylor series we can simplify this to to describe the contact area with 2*a. Subsequently, limit cases of the wear mechanisms are created for each cycle by means of multichannel analysis. In one case, constant, laminar wear over 2*a is assumed (U-shape). In the other limit case, only a boundary load is assumed (W-shape). The constant change of a results in different geometries (Figure 2). Quantification: The models are proportionally calculated with each other, and the resulting geometries are compared in the integral with the isolated wear track. The ratio whose integral shows the greatest correlation with the measured area provides information about the proportions of the respective wear mechanisms. Figure 2: Top left: unedited Profile, top right: isolated profile, bottom: W-shaped and U-shape models with the same width an area. 4. Results and Conclusions It was possible to develop a tool that can perform an analysis of the underlying wear mechanisms in particle-delivering, unlubricated friction contacts by means of a small amount of data. The finding of a percentage ratio and the minimum in height, allows conclusions to be drawn about the influence of a wear mechanism on the lifetime. In the case of the specimens studied, it can be obtained that the W-ratio is three times higher for smooth specimens than for unstructured specimens. A possible explanation for this is that emerging particles are prevented from moving outward by the microstructure. References [1] K. Kato in Wear - Materials, Mechanisms and Practice (Hrsg.: G. W. Stachowiak), Wiley, 2005, S. 9-20. [2] M. Godet, Wear 1984, 100, 437. [3] R. Gopi, I. Saravanan, A. Devaraju in Lecture Notes in Mechanical Engineering (Hrsg.: I. A. Palani, P. Sathiya, D. Palanisamy), Springer Nature Singapore, Singapore, 2022, S. 803-811. [4] N. Singh, A. Mehta, H. Vasudev, P. S. Samra, Int J Interact Des Manuf 2023, 31, 598. [5] C. Gachot, A. Rosenkranz, S. M. Hsu, H. L. Costa, Wear 2017, 372-373, 21. [6] T. Sube, M. Kommer, M. Fenker, B. Hader, J. Albrecht, Tribology International 2017, 106, 41. [7] F. Köhn, M. Sedlmajer, J. Albrecht, M. Merkel, Coatings 2021, 11, 1240. 24th International Colloquium Tribology - January 2024 149 EHL Simulation for the Design Workflow of Contacts with Limited Lubricant Availability Pastor Cesar 1* , Solovyev Sergey 2 1 Robert Bosch GmbH Corporate Research, Fluid Dynamics and Reliability, Renningen, Germany 2 Robert Bosch GmbH Corporate Research, Applied Mathematics, Renningen, Germany * Corresponding author: cesar.pastor@de.bosch.com 1. Introduction The importance of a reliable component design process for the industry is undeniable and at the same time a great challenge. For decades, limited computational resources forced engineers to develop costly experimental infrastructure to evaluate and design contacts. In the last years, the increasing computational capabilities opened new scenarios in which costly experiments can not only be simulated but also a great number of operating parameters, manufacturing tolerances, materials etc. can be numerically replicated before even having physical samples or prototypes. This creates great possibilities for the optimization and predictability of contacts in all kinds of situations, which offers a great potential for faster development cycles but also requires time-efficient and robust simulation software. 2. Grease lubrication Grease lubricated contacts are very well established in a wide range of industrial applications. The end-user benefits from one-in-a-lifetime greased systems because of their needless maintenance, making them suitable to be positioned in the most inaccessible machine parts. However, even considering the highest quality standards, a failure cannot always be avoided. The contact can run into starved to boundary lubricating conditions leading to a system failure. Moreover, the lifetime of line and elliptical contact surfaces is reduced due to the wiping effect on the lubricant displacing it away from the inlet region (figure 1). Therefore, a macroscopic system consideration is needed since (1) the lubricant film thickness depends on the inlet conditions and (2) the lubricant presence close to the inlet area depends on its availability. Figure 1. Initial grease distribution in gearbox (left). Grease distribution (wiping effect) after short operation without replenishment measures (right). 3. Component design 3.1 Methodological approach In this study a multiscale approach for systems with risk of limited lubricant availability, specifically greased contacts, is proposed. Lubricant availability affects the lubricant presence, but both phenomena have different causes and effects. Furthermore, starved conditions don’t necessarily mean a risk of failure but just a reduction of the inlet and central film thickness. Therefore, a consideration of starvation and its relevance at different operation points is very valuable in early design phases. In [1] a gear-specific simulation toolchain was presented in which an in-house developed EHL software is used. Insights into the software can be found in [2], [3] and [4]. The differences between full flow and starved conditions have been numerically in CFD simulations [5], [6], experimentally demonstrated [7], [8] and integrated in the EHL simulations at Bosch Corporate Research. 3.2 Simulation challenges: convergence methods By these means, the effect of multiple contact design variables and operation conditions may be evaluated simulatively. The main challenge for numerically solving the Reynolds equation when coupled with a fluid model and the contact deformation is its convergence. The solution is achieved by coupling several non-linear equations, the resulting system being highly non-linear. These equations are individually iteratively solved through a “weak”-coupling. Since the equation system is non-linear, damping coefficients (relaxation factors) need to be considered in the solution strategy. For the selection of these coefficients a method based on Bayesian Optimization is proposed and successfully implemented. The Gaussian process associated to the BO algorithm minimizes a predefined objective function in a minimum number of iterations. For that purpose a suitable acquisition function considering exploitation and exploration margins is needed (figure 2). 150 24th International Colloquium Tribology - January 2024 EHL Simulation for the Design Workflow of Contacts with Limited Lubricant Availability Figure 2. An example of Bayesian optimization on a 1D design problem from [9]. 4. Results Using the described methodological approach and convergence methods, a tribological system can be designed attending to certain requirements as minimum permissible film gap or maximum permissible solid contact pressure or surface stress. Figure 3. Fluid level and saturation from the starvation calculation of a 100Cr6 ball vs. sapphire disc with a (a) 0.05mm³/ s, (b) 0.108 mm³/ s and (c) 0.208mm³/ s meniscus volume. Table 1: Evaluation of simulation results for a point contact with different starvation levels. Meniscus volume [mm³/ s] h min [nm] h 0 [μm] Max elastic deformation [μm] 0.05 82.97 86.58 1.93 0.108 86.53 86.86 1.84 0.208 270.9 355.0 1.89 The results show that, due to the different contact deformation at different starvation levels, there is not always a direct relationship between meniscus volume and deformation (see Table 1 and figures 3 and 4) Figure 4. Contact deformation of a 100Cr6 ball vs. sapphire disc for a 0.208mm³/ s meniscus volume. 5. Conclusion The presented results using the proposed multiscale design workflow, EHL simulations and Bayesian Optimization methods demonstrate the value of the approach and the significant advantages of numerical simulation in the design process. References [1] Uhlig M., Pastor C., Simulation Tool-Chain For Plastic Gear Design. In: International Conference on Gears 2019, VDI-Society for Product and Process Design, 2019, 1453-1459 [2] Solovyev S., Reibungs- und Temperaturberechnungen an Festkörper- und Mischreibungskontakten, PhD- Thesis, University of Magdeburg, 2006 [3] Redlich A.C. et al., A Deterministic EHL Model for Point Contacts in Mixed Lubrication Regime. In: 26th Leeds-Lyon Symposium on Tribology, Tribology Series 38. Amsterdam : Elsevier, 2000, 85-93 [4] Solovyev S. et al., Temperaturberechnung in konzentrierten Kontakten. In: Tribologie-Fachtagung GfT. Göttingen, 2004 [5] Cen H., Lugt P.M., Film thickness in a grease lubricated ball bearing. Tribology International, 2019 [6] Zhang S. et al., Prediction of film thickness in starved EHL point contacts using two-phase flow CFD model, Tribology International, 2023, 178 [7] Nogi T., Film thickness and rolling resistance in starved elastohydrodynamic lubrication of point contacts with reflow, Journal of Tribology, 2015, 137 [8] Kochi T. et al., Experimental Study on the Physics of Thick EHL Film Formation with Grease at Low Speeds, Tribology Letters, 2019, 67 [9] Brochu E. et al., A Tutorial on Bayesian Optimization of Expensive Cost Functions, with Application to Active User Modeling and Hierarchical Reinforcement Learning, arXiv: 1012.2599, 2010 24th International Colloquium Tribology - January 2024 151 A Novel Mortar Multiphysics Computational Method for Thermal Elastohydrodynamic Lubrication Volker Gravemeier 1* 1 AdCo Engineering GW GmbH, Unterföhring, Germany * Corresponding author: E-mail gravemeier@adco-engineering-gw.com 1. Introduction There are numerous applications of thermo-fluid-structure interaction (TFSI) in engineering and nature, such as airbags, supersonic re-entry from space, hypersonic flight, gas turbines, rocket nozzles, heat exchangers, and quenching, just to name a few. Thermal elastohydrodynamic lubrication (TEHL) represents a specific subfield of TFSI, where the involved fluid domain typically features a drastically reduced thickness in at least one spatial direction. The interaction of contacting structure surfaces separated by a thin fluid film - with or without thermal interaction - is generally of great importance in various engineering as well as biomechanical applications. Due to this importance, improved designs, for which particularly valuable support might be provided by advanced computational methods, among other things, will be instrumental in both enabling substantial energy savings and reducing CO 2 emissions in the future. A comprehensive multiphysics computational method for TEHL and results obtained from applying it to tribosystems will be presented. Taking the multiphysical nature of tribosystems into account when simulating such systems is inevitable in most of the cases for truly reflecting their real-world features. For this purpose, it is typically both mandatory and challenging to consider all (nonlinear) effects of the individual physical fields as well as their mutual interactions. Only this way, though, it is ensured that one obtains reliable simulation results eventually. This is particularly true as soon as one approaches, for instance, the threshold range for dimensioning technical systems such as those prevalent in tribology. Among other things, the proposed new method overcomes typical limitations frequently reported in the literature for existing simulation methods for TEHL, such as (i) restrictions to reduced-dimensional or static/ steady-state problems, respectively, (ii) the availability of merely rather simple material laws (e.g., linear elasticity), (iii) the inevitable avoidance of contact scenarios within boundary and mixed lubrication regimes at all or the use of simplified modeling assumptions (e.g., elastic half-spaces), respectively, (iv) limitations on “code couplings” using commercial or opensource CAE software packages as “black-box” components, and (v) restrictions to node matching or accuracy-reducing interpolation procedures, respectively, at domain interfaces, to name a few. In contrast, the presented advanced computational method, available within a singular CAE software enables predictive, fully-coupled and detailed 3-D resolved simulations along the complete spectrum of the Stribeck curve, which displays the regimes of lubrication, and beyond. 2. Computational Method and Results Our computational method AVM 7 is embedded in our inhouse finite-element-based CAE software AMSE (“AdCo Multiphysics Software Environment”). In the following paragraphs of this section as well as in the first core part of the presentation, it will be introduced, among others, by five of its main (or “m-”) features: multiphysics, mortar, multilevel, multiscale, and multigrid. After the method will have been introduced, computational results obtained with it for various tribosystems featuring TEHL problems, such as bearings and seals, as well as a challenging TFSI application will be shown as the second core part of this presentation. From a computational point of view, TFSI and TEHL are particularly complex coupled multiphysics problems, involving in general four fields to be adequately considered numerically, as depicted in Figure 1: a fluid/ lubrication field, a structural (or solid) field, and two temperature fields, one within the fluid/ lubrication domain and one within the solid domain. Accordingly, there are four couplings or interactions, respectively: on the one hand, the fluid/ lubrication-structure interaction (FSI) and the thermo-thermo interaction (TTI), which occur at the interface between fluid and solid domain as a surface coupling, and on the other hand, the thermo-fluid interaction (TFI) and the thermo-structure interaction (TSI), which occur within the respective domain as a volume coupling. For all of these couplings, both monolithic and partitioned computational coupling approaches as described in [3] are available and may be chosen depending on the respective problem. Figure 1: Fields/ domains and their couplings for TFSI and TEHL Particularly important for successfully simulating coupled multiphysics applications is an adequate numerical consideration of the involved interfaces, both with respect to sur- 152 24th International Colloquium Tribology - January 2024 A Novel Mortar Multiphysics Computational Method for Thermal Elastohydrodynamic Lubrication face and volume couplings. Mortar methods were initially proposed for non-overlapping domain decomposition and later applied to various problem types such as contact (see, e.g., [3]), FSI and fluid flow (see, e.g., [1]). Mortar methods were proven to ensure consistent load and motion transfers at non-conforming interfaces, where collocation methods typically fail. Thus, they represent a key component of our computational method (integrated in three variants, enabling contact, meshtying and meshsliding at various coupling interfaces) for ensuring overall solution quality while enabling discretization flexibility. Among others, its geometric flexibility avoids any of the frequently observed specifications of existing numerical approaches to conformal vs. counterformal contact or point vs. line vs. area contact regions, respectively, in favor of a general approach. In particular, we make use of mortar methods in so-called dual formulation, which allows for condensing the related Lagrange multiplier degrees of freedom in the system of linear equations obtained eventually. Simulations along the complete spectrum of the Stribeck curve and beyond are ensured by a multilevel approach to solving the lubrication/ fluid field, ranging the mathematical formulations with which those levels are governed from (i) the standard Reynolds equation via (ii) the generalized Reynolds equation to (iii) the complete Navier-Stokes equations. Thus, with the third level, a seamless transition to simulating even more general thermo-fluid-structure interaction (TFSI) configurations is enabled. The most adequate resolution level of the fluid field may thus be chosen depending on the problem at hand, particularly the fluid-film thickness to be expected within the respective configuration. Aside from the fact that variational multiscale formulations are used for both fields in the fluid domain, which apply to all lubrication regimes, another multiscale feature of the method enables to address the specific challenge in the context of TEHL posed by boundary and mixed lubrication regimes. First of all, it is of paramount importance to integrate an adequate computational approach to contact mechanics, if it is aimed at a detailed resolution of the contact phenomena occurring within boundary and mixed lubrication processes. Dual mortar methods, as already addressed above for the interfaces in general, in fully linearized formulation and combined with geometrically nonlinear structural mechanics including various nonlinear material laws serve this purpose for our computational method, representing one of the currently most promising computational approaches to “dry” contact mechanics (i.e., irrespective of any additional lubrication effect). Furthermore, the dual mortar method is embedded in a multiscale approach to lubricated rough thermal contact, that is, the physical problem of boundary and mixed lubrication within rough surfaces. In this context, resolved-scale and subgrid-scale surface roughness are distinguished, where the former is resolved, while the effect of the latter is taken into account. A basis of the multiscale approach is a dynamically adaptive computational lubrication/ fluid domain, using an active-set strategy in combination with a semi-smooth Newton solution procedure for this highly nonlinear problem, among others. Multigrid methods are among the most efficient iterative algorithms for solving systems of linear equations associated with partial differential equations, which are typically obtained at the end of the discretization process. Two types of multigrid approaches may be distinguished: geometric multigrid (GMG) and algebraic multigrid (AMG). So-called aggregation-based AMG methods for solving systems of linear equations, originally proposed in [4], are another key component of our computational method, particularly regarding computing efficiency. Typically, they are integrated into our overall procedure as (block) preconditioners and combined with a subsequent Krylov-subspace iterative solver. For enabling their use in combination with the aforementioned (dual) mortar methods at interfaces, be it in condensed or non-condensed form, we developed specific adaptations of the basic procedures underlying aggregation-based AMG methods. 3. Conclusion Benefitting from various features briefly addressed in this abstract, our proposed novel simulation method AVM 7 enables detailed insights into tribosystems beyond the ones having been made possible by existing methods to date, thus contributing to an advancement of the digital transformation in tribology. In this presentation, among others, we will describe the main features of the method in more detail and show computational results obtained with it for various tribosystems. References [1] A. Ehrl, A. Popp, V. Gravemeier, W. A. Wall, A dual mortar approach for mesh tying within a variational multiscale method for incompressible flow, Internat. J. Numer. Methods Fluids 76 (2014) 1-27. [2] V. Gravemeier, S. M. Civaner, W. A. Wall, “A partitioned-monolithic finite element method for thermo-fluid-structure interaction,” Comput. Methods Appl. Mech. Engrg. 401 (2022) 115596. [3] A. Popp, M. W. Gee, and W. A. Wall, “A finite deformation mortar contact formulation using a primal-dual active set strategy,” Internat. J. Numer. Methods Engrg. 79 (2009) 1354-1391. [4] P. Vanek, J. Mandel, M. Brezina, Algebraic multigrid based on smoothed aggregation for second and fourth order problems, Computing 56 (1996) 179-196. 24th International Colloquium Tribology - January 2024 153 Development of a Digital Twin through Simulation of PVD/ PACVD Coatings Analysis of Both Dry and Lubricated Conditions Vincent Hoffmann 1* , Emanuel Tack 2 , Nick Bierwisch 3 1 Tribo Technologies GmbH, Barleben, Germany 2 Oerlikon Surface Solutions AG, Balzers, Liechtenstein 3 SIO - Saxonian Institute of Surface Mechanics, Ummanz, Germany 1. Introduction A better understanding of coatings and, in particular, their behavior in tribological systems is crucial for the optimization of coated components with regard to their area of application. The development of a digital twin in combination with laboratory and functional tests helps to accelerate the selection of a suitable coating for an application more quickly. In addition, such a model, which includes the coating, the substrate and all existing interfaces, helps to better understand the mechanical-technological behavior of coating systems. Simulations can help to uncover optimization potential within the substrate and layer architecture and to derive a targeted optimization of the coating architecture. 2. Methodology First the material parameter including the Young’s modulus, yield strength and tensile strength of the materials are determined to gain necessary input data for the simulation models. As a first simulation a dry scratch test shall be modelled with the help of analytical models. The work is illustrated using the example of a DLC coating produced by PVD/ PACVD processes. The method is described in detail in [1] and has already been successfully applied in [2] for PVD coatings and in [3] just as successfully for DLC coatings. As a second simulation a lubricated SRV test of the same coating system is analyzed with the help of thermo-elastohydrodynamic (TEHD) simulations. The results are validated by SRV model tests. Both the hydrodynamic and the solid-state load component within the contact, their prevailing temperatures and the lubrication film thickness can be calculated. The generated frictional losses can be evaluated in combination with the stress distribution within the layer architecture and in the substrate, allowing the interaction with the opposing body to be visualized.This approach paves the way for the prediction of the wear mechanism and rate, as well as the locally prevailing friction coefficient for the selected load collectives in the lubrication area of mixed friction. The introduction of additional features of the coating system, such as the surface finish or the properties of the oil in the lubricated system, can help to predict possible failure mechanisms before selecting a coating for application. The underlying equations and implementation has been validated successfully eg. in [4, 5 and 6]. 3. Determination of Material Parameters In order for the layer model to be analyzed from bottom to top, i.e. to be built up in the digital twin, a calotte is ground into the coated sample. The structure of the model starts with the substrate, continues with the adhesive layer, the intermediate layers and ends with the top layer. To measure Young’s modulus and hardness, a classical nanoindentation with a load of 2 mN is performed. Together with the Poisson’s ratio and the extended standard method, the individual Young’s moduli and yield limits of the individual layers are determined. The influence of the underlying layer structure is taken into account. Table 1: Determination of material parameters Calotte Grinding Nanoindentation Create access to individual layers Determine hardness, yield and tensile strength 4. Simulation of Dry Contact In order to determine the mechanical-technological limits of the coated substrate, critical load cases are modeled by increasing the normal force in a scratch test (preliminary test). As the load increases, the substrate deforms plastically and microcracks appear on the surface in the a-C: H top layer. For the simulation of the dry contact two different surfaces are analyzed. Whereas the coating is identical for the two brushed specimens, only the substrate of one specimen is polished for this analysis. Therefore, the rough specimen has a roughness depth of 0.3 µm and the smooth specimen of 0.05 µm. Both specimens will be tested in a scratch test with 4.16-N. In addition, the material stress at defined points of time and positions are calculated. Due to the differences in the surface structure, the stress in the material varies. 154 24th International Colloquium Tribology - January 2024 Development of a Digital Twin through Simulation of PVD/ PACVD Coatings Table 2: Results for Scratch Test Rough Specimen Smooth Specimen Microscope Images of Scratch Calculated von Mises Stress A critical value of 6 GPa was found within the surface layer for the von Mises stress, where local layer spalling occurs. For values smaller than 6 GPa micro cracks are created in the top layer. When the stress exceeds 6 GPa, chipping cracks are created behind the indentor. In this analysis the critical value is not exceeded for the smooth specimen but for the rough specimen. 5. Simulation of Lubricated Contact To investigate wear behavior and rate under lubricated conditions, one can carry out SRV tests. If hydrodynamic effects in combination with a contact of rough surfaces shall be analyzed, one can apply TEHD simulation in combination with a mixed friction model. Bases on the resulting pressure distribution it is possible to evaluate resulting material stress. To investigate this method two different SRV tests are carried out, which are summarized in table 3. Table 3: Test Parameters of the SRV Test Parameter Decreased Hydrodynamics Test Increased Hydrodynamics Test Ball Diameter 10 mm 10 mm Stroke 4.6 mm 4.6 mm Frequency 20 Hz 30 Hz Load 200 N 5 N Temperature 150 °C 35 °C Whereas one test is a typical SRV test where the effect of hydrodynamics is mostly avoided. The second has increased frequency and dynamic viscosity due to lower temperature of the lubricant. In addition to this the load is reduced significantly to provoke a visible effect of the hydrodynamics in the test. Table 4: Results for SRV Test Decreased Hydrodynamics Test Increased Hydrodynamics Test Microscope Images of Wear Mark Stress Distribution in Coating Layers The simulation could show that for the test with increased hydrodynamics up to 33% of the load is carried by hydrodynamics, hence reducing the degree of the roughness contact which in turn leads to a significantly reduced wear in this area. This could be proofed by the respective wear mark which shows a significant width reduction in the center of the sliding length. In comparison the “decreased hydrodynamics test” shows a uniform wear progression along the complete wear mark. Simulation could show that only a maximum of 0.5% of the load was carried by hydrodynamics. Based on the resulting contact pressure distribution resulting from hydrodynamic and solid contact pressure the stress distribution for both tests could be evaluated as well. Here one can clearly identify the formation of maximum stress values in the intermediate layers. 6. Conclusion With the help of simulation a coating system under dry and lubricated conditions could be simulated successfully. The derived distribution of the material stress correlates with the test data and hence paving the way to establish simulation as a tool to improve coating layers or apply as a digital twin for lubricated applications. References [1] N. Schwarzer, “Scale invariant mechanical surface optimization applying analytical time dependent contact mechanics for layered structures”, 2017. [2] N. Schwarzer, N. Bierwisch et al.: ”Optimization of the Scratch Test for Specific Coating Designs”, Surface and Coatings Technology, volume 206, 2011. [3] M. Zawischa, S. Makowski et al.: “Scratch resistance of superhard carbon coatings”, Surface and Coatings Technology, Volume 308, 2016. [4] M. Kroneis, L. Bobach et al.: „Calculation of the actuator system in swash plate axial piston machines by a coupled multibody and TEHL simulation”, J Engineering Tribology, volume 236(5), 2021. [5] L. Bobach, J. Mayer et al.: „Reduction in EHL Friction by a DLC Coating”, Tribology Letters, volume 60, 2015. [6] L. Bobach, R. Beilicke, D. Bartel: „Transient thermal elastohydrodynamic simulation of a spiral bevel gear pair with an octoidal tooth profile under mixed friction conditions”, Tribology International, volume 143, 2020. 24th International Colloquium Tribology - January 2024 155 Lubrication Mechanism Analysis of Textures in Journal Bearings Using CFD Simulations Yujun Wang 1* , Georg Jacobs 1 , Florian König 1 , Weiyin Zou 1 , Benjamin Klinghart 1 1 Institute for Machine Elements and Systems Engineering, RWTH Aachen University, Aachen, Germany * Corresponding author: Yujun Wang, E-mail: yujun.wang@imse.rwth-aachen.de 1. Introduction Journal bearings are known for their good NVH behaviour and high power density in drivetrains. The growing demand for energy efficiency and sustainability requires lower friction and higher load-carrying capacity of journal bearings [1]. Surface textures have been demonstrated to be a promising way for improving tribological behaviours of journal bearings [2]. Several beneficial effects of surface textures have been identified experimentally and numerically. On the one hand, textures can function as lubricant reservoirs and entrap wear debris to prevent the three-body abrasion. On the other hand, the most significant advantage of surface textures is the generation of an additional micro-hydrodynamic pressure due to the microflow in each texture [3]. This micro-hydrodynamic pressure contributes to an increase in load-carrying capacity in cases of mixed and hydrodynamic lubrication conditions. To determine suitable geometries and positions for textures, the lubrication mechanism of textures in a whole journal bearing, e.g. loading enhancement mechanisms, needs to be clarified. The microflow effects in textures and the flow in the bearing lubrication film were reported to be influenced by each other. Therefore, both of them should be considered in one model. The classic numerical models of textured journal bearings were mostly based on the Reynolds equation, which is simplified from the Navier-Stokes equations by neglecting the inertia effect and the flow across the film gap. Consequently, it cannot simulate the microflow accurately [4]. However, the vortex, one type of microflow, was found inside textures and was indicated to be related to textured bearing performance from our previous study [5]. Hence, an accurate numerical model should be utilized for the lubrication mechanism analysis of textured journal bearings and the microflow analysis should be involved in the lubrication mechanism investigation. Therefore, the objective of this study is to analyse the lubrication mechanisms of the textures in the journal bearings from the micro-flow perspective while considering the interactions between textures and the film formation in the whole bearing. In the current study, Computational Fluid Dynamics (CFD) models of textured journal bearings with different textured geometries will be built up to investigate the macro-effect of textures on the bearing performance. Furthermore, the microflow of textures is captured to clarify the micro lubrication mechanisms. 2. Numerical model The investigated journal bearing in this study consists of a rotating smooth wall and a stationary textured wall, as depicted schematically in Figure 1. The lubricant flows into the bearing from the upper inlet hole and flows out from both sides. Figure 1: Schematic representation of hydrodynamic journal bearing 3. Lubrication mechanism 3.1 Macro-effect of textures The macro-effect of textures on the loading performance of journal bearing is investigated firstly. The pressure contours of the smooth and textured bearings are shown in Figure 2 and the pressure profiles for the bearing centrelines is compared in Figure 3. When the cavitation occurs, the negative pressure is limited by the saturation pressure which caused an asymmetric but overall lifting pressure. This indicates only positive load-carrying capacity can be generated when textures are distributed in the cavitation region. In contrast, the high pressure of textured journal bearing is lower than that of the smooth bearing. Therefore, in order to improve the load performance of textured journal bearing, the micro-hydrodynamic pressure should be enhanced. Figure 2: Comparison of pressure contour 156 24th International Colloquium Tribology - January 2024 Lubrication Mechanism Analysis of Textures in Journal Bearings Using CFD Simulations Figure 3: Comparison of pressure profile 3.2 Micro-effect of textures To enhance the micro-hydrodynamic pressure, the micro-lubrication mechanism needs to be clear. Firstly, the micro-hydrodynamic pressure under different texture depth ratios is compared in Figure 4. It can be observed that, with the increase of texture depth ratio, the micro-hydrodynamic pressure shows a tendency to increase first and then decrease. When the texture depth ratio is 0.5, the micro-hydrodynamic pressure is maximum. Figure 4: Micro-hydrodynamic pressure under different texture depth ratio To clarify the mechanisms for the build-up of micro-hydrodynamic pressure, the microflow inside the textures at different texture depth ratio are captured as shown in Figure 5. The texture depth ratio is defined the ratio of texture depth d and bearing clearance C. It shows that no recirculation flow occurs when the depth ratio is small, while vortex formation can be observed at the depth ratio of 0.5 and gradually increases with higher depth ratios. Combining the information from Figure 4, the micro-hydrodynamic pressure reaches the maximum at the texture depth that makes the vortices appear. When the vortices become too large, the micro-hydrodynamic pressure is decreased. Figure 5: Microflow under different depth ratio Conclusion In this work, the hydrodynamic lubrication models of textured journal bearings are built to investigate the macro-effect of textures on the lubrication performance of journal bearings. The lubrication mechanisms of textures in journal bearings are analysed from microflow perspective inside textures. Based on the results obtained, the conclusions are summarized in the following: 1. The load-carrying capacity can be lifted by the micro-hydrodynamic pressure of textures generated by cavitation effect in the divergent gap. However, the high pressure of textured journal bearings is reduced compared to that of smooth bearings. 2. The micro-hydrodynamic pressure is influenced by the microflow inside textures. The maximum micro-hydrodynamic pressure occurs when the vortices appear. When the vortices are strong, the micro-hydrodynamic pressure is weakened. Acknowledgements This work was supported by China Scholarship Council (No. CSC202106450023). Simulations were performed with computing resources granted by RWTH Aachen University under project ID rwth1399 and rwth 1311. References [1] G. Xiang, T. Yang, J. Guo, J. Wang, B. Liu, and S. Chen, “Optimization transient wear and contact performances of water-lubricated bearings under fluid-solid-thermal coupling condition using profile modification,” Wear, vol. 502, p. 204379, 2022. [2] V. Brizmer and Y. Kligerman, “A Laser Surface Textured Journal Bearing,” Journal of Tribology, vol. 134, no. 3, 2012. [3] D. Gropper, L. Wang, and T. J. Harvey, “Hydrodynamic lubrication of textured surfaces: A review of modeling techniques and key findings,” Tribology International, vol. 94, pp. 509-529, 2016. [4] S. Cupillard, S. Glavatskih, and M. J. Cervantes, “Inertia effects in textured hydrodynamic contacts,” Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, vol. 224, no.-8, pp. 751-756, 2010. [5] Y. Wang, G. Jacobs, F. König, S. Zhang, and S. von Goeldel, “Investigation of Microflow Effects in Textures on Hydrodynamic Performance of Journal Bearings Using CFD Simulations”. Lubricants, vol. 11, no.-1, p. 20, 2023. 24th International Colloquium Tribology - January 2024 157 Investigation of Wear Protection and Friction Losses in Ultralow Viscosity Lubricant Formulations: A Combined FEM-CFD Simulation- Javier Blanco-Rodríguez 1 *, Jacobo Porteiro 1 and Marti Cortada-Garcia 2 , Silvia Fernández 2 1 CINTECX, Universidade de Vigo, GTE, Lagoas-Marcosende s/ n, Vigo, 36310, Spain 2 Repsol SA, Madrid, Spain * Corresponding author: javier.blanco.rodriguez@uvigo.gal 1. Introduction For decades, researchers have delved into the realms of durability and reliability, relying heavily on trial-and-error experiments. Nonetheless, in certain application fields, the costs tied to this method have become prohibitive. In instances involving lubricated machines enduring strenuous dynamic loads, such as high-power-density engines, simulation tools bring to the forefront significant advantages over resource-intensive testing. They enable the economical simulation of prototypes and various scenarios for the evaluation of different lubricants and engine configurations. This work, as presented here, provides an in-depth examination of wear protection and friction losses, with its foundations resting on a well-established elastohydrodynamic (EHD) simulation model concerning the connecting rod journal bearing. This model thoughtfully considers elastic deformation by utilizing a finite element model (FEM) representing the connecting rod. In addition, it incorporates diverse properties of lubricants, as determined through specific experimental tests, which are subsequently integrated into the simulation software. Accordingly, a novel wear algorithm is introduced to forecast the progression of wear depth and roughness parameters over time within a typical wear cycle, aligning with the standard operating conditions of internal combustion engines (ICE). 2. Modelling A novel wear algorithm is developed based on experimental rig and data available in (1,2). The simulation model used for this study has been validated with experimental data in previous works (3). The following sections presents the case study selected and the proposed wear algorithm developed. 2.1 Case study In Figure 1, it is depicted the case study selected for this work, where: a) Describes the experimental rig selected to represent the engine load at the cone bearing. b) Shows a CAD model of the test machine where the lubricants are tested. c) Shows the specific simulation model validated for this machine. d) FE model used for the EHD simulation, where elastic deformation takes significant importance. e) 3D surface data from bearing and shaft, which are in contact when specific oil film thickness (SOFT) is below 3. f) Schematic representation of the real scenario where the oil film is crucial to avoid contact between surfaces. g) 2D scheme of the system to solve in the simulation model. Figure 1. Case study 2.2 Proposed algorithm To rectify the inaccuracies in predicting contact conditions within a bearing system, the authors devised and implemented a novel algorithm with the primary objective of calculating wear and interface evolution. The algorithm leverages GT- Suite V2023, a software developed by Gamma Technologies (Westmont, Illinois, USA), to conduct Elastohydrodynamic (EHD) calculations. Additionally, MATLAB R2023a, developed by MathWorks (Natick, Massachusetts, USA), is employed for managing shape evolution, executing wear computations, and postprocessing the resultant data. Figure 2 illustrates the underlying logic of the proposed model, where: a) Describes the lubricant data needed to carry out a detailed characterization. b) Represents the EHD simulation model, which has multiple input data from the lubricant and geometrical model. c) Shows a representation of the MATLAB code developed to predict wear, where the nonlocal averaging algorithm is applied. d) Presents a scheme of the wear evolution through the cycle. e) Updates the wear cycle boundary conditions before the simulation launch. Figure 2. Wear algorithm 158 24th International Colloquium Tribology - January 2024 Investigation of Wear Protection and Friction Losses in Ultralow Viscosity Lubricant Formulations: A Combined FEM-CFD Simulation This wear algorithm is also accounting surface topography variation within wear, though an in-house developed routine. Where the Greenwood-Tripp roughness parameters are recalculated each time that the surface vary with wear, as shown in Figure 3. Figure 3. Dynamic surface roughness routine 3. Results In this work, two different assessments will be carried out to compare the performance of 4 different lubricants in friction losses and wear protection. 3.1 Friction assessment A detailed analysis of friction losses will be undertaken to determine which lubricant candidate performance better in the selected case study. This analysis will go through oil film formation, contact incidence, and lubricants overall comparison. An example of this is depicted in Figure 4. Figure 4. Lubricants friction comparison 3.2 Wear assessment Following with the lubricants analysis, the wear protection capabilities of these candidates will be evaluated through the wear depth prediction carried out in the proposed wear algorithm. Several result will be used in this section to assess the performance, as it can be wear load, wear depth, roughness parameters variation, contours. Some of these results are shown in Figure 5 and Figure 6. Figure 5. Wear depth evolution Figure 6. Final wear depth contours 4. Conclusion In this work, an assessment procedure is presented to select the best lubricant candidate for sliding bearing applications in relation with its friction and wear protection performance. This procedure is a valid approach for OEMs and oil companies interested in carry out virtual lubricant testing as previous step to final validations, and as a great cost cut in experimental campaigns. References [1] Sander DE, Allmaier H, Priebsch HH, Witt M, Skiadas A. Simulation of journal bearing friction in severe mixed lubrication - Validation and effect of surface smoothing due to running-in. Tribol Int. 2016 Apr 1; 96: 173-83. [2] Sander DE, Allmaier H, Priebsch HH, Reich FM, Witt M, Skiadas A, et al. Edge loading and running-in wear in dynamically loaded journal bearings. Tribol Int. 2015 Dec 1; 92: 395-403. [3] Blanco-Rodríguez J, Porteiro J, López-Campos JA, Domínguez B, Cortada-Garcia M. Friction assessment of ultralow viscosity lubricant formulations based on a validated elastohydrodynamic simulation. International Journal of Engine Research. 2023 Mar 16; 146808742311621. 24th International Colloquium Tribology - January 2024 159 Towards the Prediction of Lubricated Contacts by Machine Learning Max Marian 1* 1 Department of Mechanical and Metallurgical Engineering, School of Engineering, Pontificia Universidad Católica de Chile, Macul, Chile * Corresponding author: max.marian@uc.cl 1. Introduction The prediction of lubricated tribo-contacts plays a critical role in optimizing mechanical system performance and durability. However, accurately forecasting their behaviour is a complex and numerically costly task due to the intricate interplay between contacting surfaces, lubricants, and the environment. Artificial Intelligence (AI) Machine learning (ML) techniques have emerged as powerful tools to enhance prediction accuracy and efficiency [1]. This contribution explores the utilization of ML algorithms, such as artificial neural networks, to model and predict the behaviour of lubricated tribo-contacts. 2. Prediction of hydrodynamic contacts using Physics-Informed Machine Learning Utilizing physics-informed machine learning (PIML), one can seamlessly integrate foundational physics knowledge into ML models, particularly for hydrodynamically lubricated (HL) contacts. Methods like physics-informed neural networks (PINN) leverage established physical principles and equations specific to lubricated contacts, such as the Reynolds equation, see Fig. below. This integration serves to guide the learning process, resulting in precise and interpretable models. During the training phase, the relevant equations become an integral part of the neural network‘s loss functions, introducing a dual aspect that blends data-driven and physics-driven elements into the loss function. This is achieved by sampling input training data, encompassing spatial coordinates and/ or time stamps, and processing them through the neural network. Subsequently, the network‘s output gradients are computed with respect to these inputs at designated locations. These gradients can often be efficiently calculated using auto differentiation (AD) and are then employed to determine the residual of the underlying differential equation. This residual is incorporated as an additional term in the loss function. The inclusion of this „physics loss“ in the overall loss function serves to ensure that the solution acquired by the network adheres to the established laws of physics. This approach has rapidly evolved in just two years, progressing from the 1D Reynolds equation for converging sliders to more intricate scenarios, including the 2D Reynolds equation, addressing journal bearings with load balance, variable eccentricity, and accounting for cavitation effects. [2] 3. Data-driven prediction of elastohydrodynamic contacts using Machine Learning Apart from PIML, ML algorithms can be trained to learn patterns in data sets. Thus, approaches, such as support vector machines, Gaussian process regressions and artificial neural networks can predict relevant film parameters or even film thickness distributions more efficiently and with higher accuracy and flexibility compared to sophisticated elastohydrodynamic lubrication (EHL) simulations and analytically solvable proximity equations, respectively. By optimizing the ML algorithm hyperparameters, it is possible to achieve high prediction accuracies. However, the data set to train the ML models is of crucial importance. Graphical representation of a PINN approach. Reprinted from [2] with permission by CC BY 4.0. 160 24th International Colloquium Tribology - January 2024 Towards the Prediction of Lubricated Contacts by Machine Learning Data-based predicting EHL contact parameters using artificial neural networks. Reprinted from [3] with permis-sion by CC BY 4.0. Acknowledgement The support from the Vicerrectoría Académica (VRA) of the Pontificia Universidad Católica de Chile within the Programa de Inserción Académica (PIA) is greatly acknowledged. References [1] M. Marian, S. Tremmel: Current Trends and Applications of Machine Learning in Tribology—A Review, Lubricants, 9, 2021, 86, DOI: 10.3390/ lubricants9090086. [2] M. Marian, S. Tremmel: Physics-Informed Machine Learning—An Emerging Trend in Tribology, Lubricants, 11, 2023, 463, DOI: 10.3390/ lubricants11110463. [3] M. Marian, J. Mursak, M. Bartz, F. J. Profito, A. Rosenkranz, S. Wartzack: Predicting EHL Film Thickness Parameters by Machine Learning Approaches, Friction, 11, 2023, 6, DOI: 10.1007/ s40544-022-0641-6. 24th International Colloquium Tribology - January 2024 161 Detection of Critical Operation in Porous Journal Bearings Using Machine Learning Josef Prost, Guido Boidi, Georg Vorlaufer, Markus Varga * AC2T research GmbH, Wiener Neustadt, Austria * Corresponding author: markus.varga@ac2t.at 1. Introduction Many tribosystems are designed for long-lasting operation. Therefore, a condition monitoring strategy is necessary to timely identify critical operation and to avoid catastrophic failure. This work introduces a semi-supervised Machine Learning (ML) approach for the classification of operational states on the example of porous journal bearings [1]. Time and frequency feature sets allow to combine data from multiple sensors in order to gain new insights from tribological data by training more robust and accurate models. With this approach, ML algorithms can be trained to assess the operational state, indicating the current health status, as well as to predict the remaining useful lifetime of machine elements. 2. Experimental The presented model was developed using data generated during accelerated lifetime tests of porous journal bearings. The test rig allows to run up to five tests simultaneously and was equipped with sensors measuring, e.g., friction torque, sample holder displacement, and acceleration, see Fig. 1. Ironand bronze-based bearing materials were investigated at room temperature, 100°C, and 160°C. The bearings run on a ø8 mm X90CrMoV18 shaft and were impregnated with apolyglycol-based lubricant prior to the tests. A specific bearing load of 3 MPa was applied at all experiments, and they were run with a triangular profile from +20 to -20 rpm with 0.1-Hz frequency. Especially the constantly changing rotation direction leads to mixed lubrication conditions and accelerated wear, which allowed for lifetime tests of the bearings. Tests were run until a preset critical temperature was exceeded after several 10 to 300 hours. Tests running longer than 300 hours were stopped manually. 3. Machine Learning To achieve higher robustness and classification accuracy, data from these sensors were combined by calculating time and frequency feature sets from 60 second time windows of hourly recorded high-speed data. Each of these time windows was assigned to one of four operational regimes: ‘Run-in’, ‘Steady’, ‘Vibration’, and ‘Critical’. The labelling procedure was assisted by multivariate statistics, i.e., principal component analysis (PCA) and clustering. The most relevant features were selected by an automated iterative feature selection algorithm to further enhance the performance of the model. In a comparative study, six different “classical” ML classifiers were investigated. Hereby, analgorithm based on aggregated decision trees (“Extra-Trees”) proved best performing and was selected for training the model. Figure 1: Detailed view of one sample stage of the accelerated lifetime test rig with instrumentation. 4. Results and Discussion Exemplarily, acceleration data of two running conditions are presented in Fig. 2. The top image depicts the stable running condition, with low vibrations even at the change points of the running direction. In the bottom picture, the acceleration signal of critical running behavior shortly before the failure is shown, where at low velocities very high accelerations can be measured in both running directions. Figure 2: Accelerometer signal from different running conditions during one test run. 162 24th International Colloquium Tribology - January 2024 Detection of Critical Operation in Porous Journal Bearings Using Machine Learning Figure 3: Classification of operational states of an iron-based porous journal bearing operated at 160°C. Each colored dot represents a 60 second time window. The grey line indicates the mean coefficient of friction for each time window. An example of the trained model applied to a test run is shown in Fig. 3. The trained model was able to identify the correct operational state with an overall accuracy of 0.98. Thereby, critical operation was detected up to 50 hours before the stopping criterion of the test rig was reached. A similar model may be integrated into a predictive maintenance strategy, as it can be implemented into an online monitoring tool for identifying critical operation in real time. Impending failures can be detected hours before an alarm raised by a conventional threshold based system. This allows the operator to undertake timely countermeasures before the machinery would suffer catastrophic damage. 5. Acknowledgments This work was funded by the Austrian COMET-Program (Project K2 In Tribology1, no. 872176) and carried out at the Austrian Excellence Center for Tribology (AC2T research GmbH). Part of the work was also funded by the Interreg AT- CZ 279 project ‘‘AI-based Predictive Maintenance’’. References [1] J. Prost, G. Boidi, A.M. Puhwein, M. Varga, G. Vorlaufer. Classification of operational states in porous journal bearings using a semi-supervised multi-sensor Machine Learning approach. Trib. Int. 184 (2023) 108464. 24th International Colloquium Tribology - January 2024 163 A Machine Learning approach to Tribological Performance Prediction of New Lubricant Formulations Wahyu Wijanarko 1* , Nuria Espallargas 1* 1 Norwegian University of Science and Technology * Corresponding author: wahyu.wijanarko@ntnu.no, nuria.espallargas@ntnu.no 1. Introduction Research on finding new Environmentally Acceptable Lubricants (EALs) has become more critical due to new regulations requiring lubricants to be non-toxic, readily biodegradable, and non-bioaccumulative [1]. New regulations put limit on the usage of petroleum-based oils, especially for their use in the maritime industry, where there is a potential continuous leakage from different parts of the vessel to the sea. A fully formulated lubricant consists of 70% to 99% base lubricant and 30% to 1% lubricant additives. To be considered as an EAL, both base lubricant and additives need to be environmentally acceptable. Examples of environmentally acceptable base lubricants are vegetable oils, synthetic esters, glycols, low viscosity polyalphaolefins, and water [2]. If those are used as base lubricants to formulate an EAL, the toxicity of the fully formulated lubricants will be controlled by the additives used in the formulation. Finding the right additives for environmentally acceptable base lubricants is challenging due to the limited types of chemicals that are both, compatible with the environmentally acceptable base lubricants, and can perform as lubricant additives. Our research focuses mostly on water-based lubricants as a family of EALs. Water-based lubricants are interesting lubricants due to their readily environmentally acceptability [1,2]; however, they have poor performance compared to mineral or synthetic oils. Therefore, finding new additives for water-based lubricants becomes our main goal. One family of chemicals that is attracting attention due to their potential benefits as lubricants is ionic liquids. Ionic liquids are organic salt consisting of cation and anion moieties [3]. Ionic liquids are environmentally acceptable chemicals due to their low volatility, non-flammability, and high thermal stability [4]. In addition, ionic liquids are highly polar substances, meaning they are highly surface active with a tendency to adsorb on the metallic surfaces, making them suitable as additives [5]. There are large numbers of cations and anions precursors available for ionic liquids synthesis, therefore over 1 million possible combinations of ionic liquids can be produced. Nowadays, around 300 to 400 ionic liquids are commercially available. Such large availability of ionic liquids will take huge number of experiments to study their tribological performance, therefore machine learning can be a great tool to predict the tribological performance of water-based lubricants formulated with ionic liquids using small experimental datasets saving time in formulating new lubricants. In the future and with enough results, machine learning can also be used to predict the relationship between structure and performance to propose new ionic liquid chemistries. 2. Methodology In this work, the coefficient of friction of water-based lubricants formulated with ionic liquids was predicted using machine learning. The dataset consisted of water (W), four glycols, four water-glycols, and two polyalphaolefins as the base lubricants. Glycols that were used are monoethylene glycol (MEG), diethylene glycol (DEG), monopropylene glycol (MPG), and dipropylene glycol (DPG). Polyalphaolefins that were used are polyalphaolefin with 2 cSt viscosity (PAO2) and polyalphaolefin with 8 cSt viscosity (PAO8). Water-glycols were formulated by mixing the water with each glycol in a ratio of 1: 1, namely, water-monoethylene glycol (WMEG), water-diethylene glycol (WDEG), watermonopropylene glycol (WMPG), and water-dipropylene glycol (WDPG). Each base lubricant was formulated with six ionic liquids, namely, 1,3-dimethylimidazolium dimethylphosphate (IM), (2-hydroxyethyl)trimethylammonium dimethylphosphate (AM), tributylmethylphosphonium dimethylphosphate (PP), 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate (BMP), trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate (PB), and trihexyltetradecylphosphonium decanoate (PC). One weight percent (1 wt.%) of additives is utilized for the lubricant formulation. In addition, the base lubricant alone and the individual ionic liquids were also tested. Each test was performed twice to check the repeatability. In total, 83 lubricants were tested to generate 166 samples for building up the experimental dataset. Each sample in the dataset needs to be transformed to a machine-readable format to be able to be used in the machine learning programming. The transformation required Simplified Molecular Input Line Entry System (SMILES) to represent each chemical structure in the lubricant. As explained in the previous paragraph, seven compounds were used to formulate the base lubricants and six ionic liquids were used as lubricant additives, so in total 13 chemicals were involved in the lubricant formulation. The SMILES for each chemical can be obtained from the database webpage, for example PubChem from National Library of Medicine - National Institutes of Health (NIH) or CompTox from United States Environmental Protection Agency (EPA) [6,7]. Once the SMILES is obtained, the molecular descriptors were calculated using AlvaDesc software [8]. The software calculates 4179 molecular descriptors for each chemical based on its physical and chemical properties. Cleaning (curing) the molecular descriptors was done by removing the molecular descriptors that had constant and missing values throughout the 13 chemicals, resulting in 912 molecular descriptors that could be used for machine learning. In addition to the molecular descriptors generated A Machine Learning approach to Tribological Performance Prediction of New Lubricant Formulations 164 24th International Colloquium Tribology - January 2024 by AlvaDesc, nine experimental molecular descriptors were also added to the dataset, namely, pH, electrical conductivity, molecular weight, number of C, H, O, N, P, and F atoms. So, in total 921 molecular descriptors for the 13 chemicals were generated. As explained before, 83 lubricants were formulated using 13 chemicals, thus the molecular descriptors for each lubricant were calculated based on the volumetric ratio of the chemicals in the lubricant. For the machine learning part of the work, Python version 3.9.13 combined with Jupyter Notebook version 6.4.12 were used as the program language. Scikit-learn free online package tool version 1.2.2 together with Random Forest Regressor model were utilized to perform the coefficient of friction prediction [9]. The number of estimators used for Random Forest Regressor were 1000 decision tress. 3. Results and discussion The coefficient of friction of the water-glycols and polyalphaolefins based lubricants formulated with IM were predicted. This prediction was made by splitting the dataset into a training and a testing sub-dataset. The training dataset contains samples or lubricants with no IM in the lubricant (142 samples). The testing samples were PAO2-IM, PAO8-IM, WMEG-1IM, WDEG- 1IM, WMPG-1IM, and WDPG-1IM (6 samples). The results of the coefficient of friction predictions are shown in Figure 1. For each prediction, twenty-five iterations were performed, and the maximum, median, and minimum values were reported in the graph along with root mean square error (RMSE) and accuracy calculations. The true value (from experimental result) was also reported. The prediction results showed that Random Forest Regressor performed very well in prediction of water-glycols and polyalphaolefins formulated with IM ionic liquids with accuracy in the range of 83.26% to 99.47%. Figure 1. Prediction of coefficient of friction for water-glycols and polyalphaolefins formulated with IM ionic liquid. 4. Conclusion The application of machine learning for tribological performance prediction of new lubricant formulation was investigated in water-based and synthetic oil-based lubricants. From this study Random Forest Regressor could predict the coefficient of friction of lubricants formulated with ionic liquids with high accuracy. This high accuracy could be due to similar ionic liquids present in the training dataset, hence, predicting completely a new structure of chemical could be challenging. Therefore, for future work, the dataset will be expanded using more base lubricants and additives to increase the variation and the learning input. In addition, important molecular descriptors that influence the prediction accuracy will be analyzed. References [1] Agency, U.S.E.P., Environmentally Acceptable Lubricants. 2011, United States Environmental Protection Agency [2] Das R. Eco-friendly Lubricants for Tribological Application. Handbook of Ecomaterials. Cham (Switzerland): Springer, 2017 [3] Welton T. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chemical Reviews 99: 2071-2084 (1999) [4] Earle M J, Seddon K R. Ionic liquids. Green solvents for the future. Pure and Applied Chemistry 72: 1391- 1398 (2000) [5] Zhao H. Innovative applications of ionic liquids as ‘green’ engineering liquids. Chemical Engineering Communications 193: 1660-1677 (2006) [6] Explore Chemistry. Quickly find chemical information from authoritative sources. https: / / pubchem.ncbi.nlm. nih.gov [7] Computational Toxicology Chemicals Dashboard (CompTox Dashboard). https: / / comptox.epa.gov/ dashboard/ advanced-search [8] Mauri A. AlvaDesc: A Tool to Calculate and Analyze Molecular Descriptors and Fingerprints. Ecotoxicological QSARs. Methods in Pharmacology and Toxicology. New York (US): Humana, 2020 [9] Pedregosa et al. Scikit-learn: Machine Learning in Python. The Journal of Machine Learning Research 12: 2825-2830 (2011) 24th International Colloquium Tribology - January 2024 165 Per Aspera ad Astra Design of Friction Reducing Star Polymers from Computer Simulation to Lubricant Application. L. B. Kruse 1* , K. Falk 1 , M. Moseler 1,2 , D. Markert 3 , R. Klein 3 , J. Rausch 4 , R. Luther 4 1 MikroTribologie Cetrum µTC - Fraunhofer IWM, Freiburg, Germany 2 University of Freiburg, Freiburg, Germany 3 Fraunhofer Institutefor Structual Durability and System Reliability LBF, Darmstadt, Germany 4 Fuchs Lubricants GmbH, Mannheim, Germany * Corresponding author: Lars.Kruse@iwm.fraunhofer.de 1. Introduction The Presentation covers results of a novel star polymer (Fig.1b), which enables low viscous base oils (Fig. 1a, blue arrow), by delaying boundary lubrication through surface adsorption (Fig.1a, green arrow). The topic was addressed by three project partners, Fraunhofer LBF, Fraunhofer IWM and Fuchs Lubricants GmbH, which worked on the respective topics of synthesis, simulation and tribological experiments. Figure 1: Schematic Stribeck curve (a), divided into boundary, mixed and fluid lubrication and a MD-snapshot of a four-arm star polymer (b), with a carbon backbone (orange) and functional groups (blue). 2. Synthesis A synthesis via the controlled polymerization was found by the Fraunhofer LBF to be a successful choice for the novel star polymers. In contrast to the radical polymerization, a defined star shaped structure is obtained by linking of multiple polymer chains. In this way the molar mass of the star polymer can be easily adjusted by the initiator/ monomer-ratio with a very narrow chain length distribution. Figure 2: Schematic representation of the synthesis of a 4-arm star polymer via controlled polymerization (a) and the laboratory setup (b). 3. Simulation Accompanying atomistic simulations are carried out by the Fraunhofer IWM to calculate structure-property relationships, in order to support the synthesis with design rules. To analyze the surface attachment of the star polymers, the adsorption process is simulated via steered molecular dynamics. A linear trend in the free energy of adsorption with respect to the pulling distance allows an adsorption energy scale up (or scale down) with the number of functional groups. The tribological behavior of the star polymers was analyzed in squeezeout- (Fig.2a) and shear- (Fig.2b) simulations. In both simulations, the star polymers showed a great surface attachment, preventing solid contact, while non-thickening the low viscous base oil. Finally microscopic friction results could be combined with contact theory (Persson), to calculate macroscopic friction coefficients for the boundary lubrication regime. 166 24th International Colloquium Tribology - January 2024 Per Aspera ad Astra Figure 2: Snapshots of a squeezeout- (a) and a shear- (b) molecular dynamics simulation, for conditions of T-=-100-°C and P load = 200MPa. 4. Experiment Tribological experiments by Fuchs Lubricants GmbH, allowed investigations on the lubricating film structure, thickening effect, surface adhesion, friction and shear strength. Results of Ball-on-disc experiments (Fig.3a) show the superiority of the novel star polymers, compared to commercial additives mixed in a model base oil (Fig.3b), with an agreement of the experimental and simulation results. Further experiments proved the star polymers mode of action also for a fully additivated engine oil (Fig.3c). Figure 3: Schematic tribological ball-on-disc setup (a), friction coefficient results for various rolling speeds and additives mixed in a model base oil (b) and mixed in a fully additivated engine oil (c). 24th International Colloquium Tribology - January 2024 167 Computational Modeling of Tribological Systems: Insights into Grinding Processes, Materials Tribology and Tribofilm Formation through Molecular Dynamics Stefan J. Eder 1,2,* , Philipp G. Grützmacher 2 , Manel Rodríguez Ripoll 1 , Andreas Nevosad 1 , Karen Mohammadtabar 3 , Ashlie Martini 3 , Nicole Dörr 1 , Daniele Dini 4 , Carsten Gachot 2 1 AC2T research GmbH, Viktor-Kaplan-Straße 2/ C, 2700 Wiener Neustadt, Austria 2 Institute of Engineering Design and Product Development, TU Wien, Lehárgasse 6 - Objekt 7, 1060 Vienna, Austria 3 Department of Mechanical Engineering, University of California Merced, 5200 N. Lake Road Merced, CA 95343, USA 4 Department of Mechanical Engineering, Imperial College London, South Kensington Campus, Exhibition Road, London SW7 2AZ, UK * Corresponding author: stefan.j.eder@tuwien.ac.at 1. Introduction Molecular dynamics (MD) simulation has long been a fancy academic toy for solid-state physicists that suffered from limited credibility due to the system size restrictions and its inability to capture chemical bond breaking and formation. The past 10 years have seen an enormous increase in manageable system sizes and the ability to treat chemical reactions without the necessity to consider electrons explicitly. In this contribution, we will present three examples from different corners of tribology that showcase how this simulation method has matured to a level that makes it interesting to surface processing and lubrication experts, materials tribologists, and mechanical engineers. 2. Surface finishing and abrasive processes The first example introduces an atomistic approach for modeling and analyzing a finishing process, offering a glimpse into the microscopic aspects of workpiece development that are often elusive in experiments, see Figure 1. The method involves four key system parameters: workpiece material, abrasive shape, temperature, and infeed depth [1]. The model’s validation demonstrates its potential, accurately reproducing critical process parameters and even predicting surface quality deterioration due to tool wear, despite considerable differences in grain sizes between simulation and macroscopic experiments. Utilizing the validated model, we delved into time-resolved stress profiles within the ferrite/ steel workpiece and mapped microstructural changes near the surface [2]. This investigation highlighted the approach’s capability to unveil the intricate and dynamic effects of blunt abrasives at elevated temperatures on near-surface microstructure and stresses, where multiple processes vie for dominance. Figure 1: Overview of a nanoscopic model designed to study surface finishing and abrasion processes. 3. Materials tribology of high-velocity sliding Numerous contemporary applications rely on the reliable operation of high-speed sliding components, such as those in e-mobility, high-speed manufacturing, or impact-resistant materials. While extensive research has explored the impact of mechanical energy and heat on the friction and wear behavior of dry metallic interfaces [3], the effect of extreme speeds on near-surface deformation mechanisms in polycrystalline metals has remained elusive [4]. This example addresses this crucial issue by employing large-scale molecular dynamics simulations, investigating sliding velocities 168 24th International Colloquium Tribology - January 2024 Computational Modeling of Tribological Systems: Insights into Grinding Processes, Materials Tribology and Tribofilm Formation through Molecular Dynamics ranging from 10 to 2560 m/ s in CuNi alloys, see Figure 2. Four distinct deformation regimes emerge, with microstructural responses remaining consistent up to 40 m/ s but undergoing significant changes between 150 and 500 m/ s, depending on composition. The decrease in plastic deformation at the highest speeds is attributed to a substantial rise in contact temperature, approaching or surpassing the bulk melting point. This phenomenon results in a reduced contact area and diminished sliding resistance. Understanding this nonlinear and non-monotonic material response to increasing sliding velocities offers valuable insights for selecting materials and designing durable components for high-speed sliding and wear applications. Figure 2: Dependence of several tribological key indicators on the sliding speed, with results obtained from large-scale molecular dynamics modeling. 4. Reactive molecular modeling Molecular modeling also comes in a “reactive flavor” that allows the elucidation of tribochemical processes [5]. For example, lubricated mechanical components subjected to extreme conditions rely on protective films that develop through the presence of additives in lubricants. The formation of these films is influenced by heat, load, and shear forces within the sliding interface, yet the specific impact of each factor remains poorly understood. Here, reactive molecular dynamics simulations were applied to untangle the effects of heat, load, and shear force on chemical reactions between di-tertbutyl disulfide, an extreme-pressure lubricant additive, and Fe(100), a model representing the ferrous surfaces of mechanical components [6]. The study characterizes the reaction pathway in terms of the number of chemisorbed sulfur atoms and the release of tert-butyl radicals during various heat, load, and shear stages in the simulation, see Figure 3. It was observed that chemisorption is constrained by the accessibility of reaction sites, with shear promoting the reaction by facilitating the movement of radicals to available sites. By plotting the reaction yield of the rate-limiting reaction for tribofilm formation, we provide a tool for surface engineers to optimize the growth of protective sulfurous films. Figure 3: Representative example of the power of reactive modeling. The reaction pathway for tribofilm formation and its bottleneck-reaction are identified, then the reaction yield of that reaction is evaluated as a function of shear stress and temperature. Acknowledgements This work was supported by the Austrian COMET program (project K2 InTribology, no. 872176). D.D. acknowledges the support of the Engineering and Physical Sciences Research Council (EPSRC) via his Established Career Fellowship EP/ N025954/ 1. References [1] Eder, S. J., Grützmacher, P. G., Spenger, T., Heckes, H., Rojacz, H., Nevosad, A., & Haas, F. (2022). Friction, 10(4), 608-629. [2] Grützmacher, P., Gachot, C., & Eder, S. J. (2020). Materials & Design, 195, 109053. [3] Eder, S. J., Grützmacher, P. G., Rodríguez Ripoll, M., Dini, D., & Gachot, C. (2021). Materials, 14(1), 60. [4] Eder, S. J., Grützmacher, P. G., Ripoll, M. R., Gachot, C., & Dini, D. (2022). Applied Materials Today 29, 101588. [5] Martini, A., Eder, S. J., & Dörr, N. (2020). Lubricants, 8(4), 44. [6] Mohammadtabar, K., Eder, S. J., Dörr, N., & Martini, A. (2022). Tribology International, 176, 107922. 24th International Colloquium Tribology - January 2024 169 Tribochemical Reactions in the Degradation Process of Iron Nitride with Reactive Molecular Dynamics Simulation Development of Reactive Force Field Parameters for Elucidation of Tribochemical Reactions of Iron Nitride Mizuho Yokoi 1 , Masayuki Kawaura 1 , Shogo Fukushima 1 , Yuta Asano 1 , Yusuke Ootani 1 , Nobuki Ozawa 2, 1 , Momoji Kubo 1, 2* 1 Institute for Materials Research, Tohoku University, Sendai, Japan 2 New Industry Creation Hatchery Center, Tohoku University, Sendai, Japan * Corresponding author: E-mail: momoji@tohoku.ac.jp 1. Introduction Modern structural materials must exhibit long service lifespans and withstand harsh environmental conditions. Especially, components subjected to friction, such as gears, require exceptional wear resistance. Nitriding is a surface treatment technique that enhances wear resistance by forming a compound layer that consists of Fe 3 N, Fe 4 N, or a combination of both crystals. Previous studies have suggested that the compound layer is harder than steel, and thus more resistant to wear [1]. In addition, it has been reported that the Fe 4 N layer exhibited less wear than Fe 3 N [2]. As these previous studies show, the amount of wear has been thought to correlate with the hardness and crystal structure. However, the recent studies have shown that metal wear phenomena also depend on the chemical reactions between the sliding material and substance in diverse atmospheres. This dependence is evident from the observation of the reduced wear of steel in high-humidity environments and the increased wear of stainless steel in oxygenated environments [3, 4]. Therefore, to enhance the wear resistance of nitrided steels, understanding the chemical reaction dynamics between the surface compound layer and substances in diverse atmospheres is required. Especially, chemical reactions with water are important because they can strongly affect wear behaviors; water causes corrosive wear of steel in aqueous environments [5], whereas water reduces wear of steel in high humidity conditions [3]. However, the observation of chemical reactions at friction interface in an atomic scale is still challenging. Molecular dynamics (MD) simulations using the reactive force field (ReaxFF) play a crucial role in analyzing such chemical reaction dynamics [6]. ReaxFF determines the potential energy of a system based on variable bond-order and atomic charge depending on the atomic configuration. Therefore, ReaxFF can handle chemical reaction dynamics with less computational costs than first-principles MD calculations. ReaxFF has been widely used for studying the chemical reactions of various structural and catalytic materials [7, 8] as well as tribological materials [9]. Therefore, we aimed at the analysis of tribochemical reactions between iron nitride and water with ReaxFF for revealing the effects of tribochemical reactions on the wear. The simulation necessitates ReaxFF parameters for Fe, N, H, and O. While the parameters dealing with the chemical reactions between H/ O/ Fe, as well as those of H/ O/ N, have been previously published [10, 11], there exists no published parameter set of Fe and N suitable for iron nitride. This proceeding described the development of the ReaxFF parameters of the Fe/ N system to perform these tribochemical reaction simulations. 2. Parameterization Method The ReaxFF parameters of Fe/ N were developed so as to reproduce of the structure and energy obtained from the density functional theory (DFT) calculations. For this development, we performed DFT calculations for several iron nitride models. Bulk iron nitride and iron nitride crystal surfaces were used to develop the ReaxFF parameters. First, we need to develop ReaxFF parameters that can reproduce the stability of bulk iron nitride. Thus, volume energy curves were calculated for Fe 3 N and Fe 4 N crystals by changing the lattice constants a, b, and c uniformly using DFT for the reference to develop the ReaxFF parameter. Second, we need to develop ReaxFF parameters that can reproduce the stability of the iron nitride surfaces; Thus, the surface energy was calculated for Fe 3 N (001), (100), (110), and (111), as well as for Fe 4 N (100), (110) and (111) using DFT for the reference to develop the ReaxFF parameters. The surface energy E surface , was calculated from the bulk energies E bulk , slab model energies E slab , and the surface area of each orientation A using the following equation: In the DFT calculations, generalized gradient approximation type Perdew-Burke-Ernzerho (GGA-PBE) exchange-correlation functional and DNP basis set were used with effective core potentials. We employed DMol 3 code for DFT calculations [12, 13]. 3. Results and Discussion First, we performed DFT and ReaxFF calculations for volume-energy curves of Fe 3 N and Fe 4 N. Figure 1 illustrates the volume-energy curves obtained by DFT as well as ReaxFF 170 24th International Colloquium Tribology - January 2024 Tribochemical Reactions in the Degradation Process of Iron Nitride with Reactive Molecular Dynamics Simulation with the developed parameters. The crystal structures of Fe 3 N and Fe 4 N are shown in the inset. The origin of each volume-energy curve is the minimum energy value for each curve. As shown in Figure 1, the lattice volume that gives the minimum energy obtained from the DFT calculations and those obtained using ReaxFF are almost identical for both crystals. This indicates that the developed ReaxFF parameters reproduce the lattice constants of both crystals. Moreover, since the curve shapes obtained from the DFT and ReaxFF are in good agreement, the developed ReaxFF parameters are expected to reproduce the expansion and compressibility coefficients of the Fe 3 N and Fe 4 N. Figure 1 Volume-energy curves of (a) Fe 3 N and (b) Fe 4 N crystal obtained by DFT as well as ReaxFF with the developed parameters. Second, to reproduce the energetic stability of Fe 3 N and Fe 4 N surfaces, which is important for describing tribochemical reactions, we performed the DFT and ReaxFF calculations for slab models of Fe 3 N and Fe 4 N. Figure 2 shows slab models of Fe 3 N (001), (100), (110) and (111), as well as for Fe 4 N (100), (110) and (111) planes. Figure 2 Slab models of (a) (001), (b) (100), (c) (110), and (d) (111) planes of Fe 3 N and (e) (100), (f) (110), and (g) (111) of Fe 4 N. Figure 3 presents the surface energy values obtained by DFT as well as ReaxFF with developed parameters. The DFT calculation results indicate that the surface energy of Fe 4 N is generally lower than that of Fe 3 N, except for the Fe 3 N (111) surface. Furthermore, the surface energy of Fe 3 N exhibited greater variation across surface orientations compared to the surface energy of Fe 4 N. These surface energy trends are also well reproduced in the developed ReaxFF parameters. The errors in the surface energy obtained by ReaxFF are within 10% for all planes. Therefore, ReaxFF with the developed parameters adequately reproduces the surface stability, although the ReaxFF with the developed parameters slightly overestimates the surface energies. Figure 3 Surface energies of Fe 3 N and Fe 4 N crystal obtained by DFT as well as ReaxFF with the developed parameters. From these comparisons, it is evident that the newly developed ReaxFF parameters for Fe/ N effectively represent the bulk and surface states of iron nitride. The developed ReaxFF parameters are listed in the Appendix. 4. Conclusion To investigate the wear and degradation mechanisms of iron nitride, we developed the ReaxFF parameters of the Fe/ N system. The ReaxFF parameters can reproduce volume-energy curves and surface energies of iron nitrides. Therefore, we can utilize these ReaxFF parameters for sliding simulations to elucidate the tribochemical reactions of iron nitrides. References [1] S. Karaoğlu, Mat. Charact., 49, 349 (2002). [2] T. Peng, et al., Surf. Coat. Technol., 403, 126403 (2020). [3] G. Bregliozzi, et al., Tribol. Lett., 17, 697 (2004). [4] M. Esteves, et al., Tribol. Int., 88, 56 (2015). [5] J. Jiang, et al., Wear, 261, 964 (2006). [6] A. C. T. van Duin, et al., J. Phys. Chem. A, 105, 9396 (2001). [7] Y. Ootani, et al., J. Phys. Chem. C, 126, 2728 (2022). [8] L. Chen, et al., Appl. Surf. Sci., 563, 150097 (2021). [9] Y. Wang, et al., Sci. Adv., 5, eaax9301 (2019). [10] M. Aryanpour, et al., J. Phys. Chem. A, 114, 6298 (2019). [11] Y. Wang, et al., J. Phys. Chem. C, 124, 10007 (2020). [12] B. Delley, J. Chem. Phys., 92, 508 (1990). [13] B. Delley, J. Chem. Phys., 113, 7756 (2000). 24th International Colloquium Tribology - January 2024 171 Towards a Continuum Description of Lubrication in Highly Pressurized Nanometer-wide Constrictions: the Importance of Accurate Slip Laws Andrea Codrignani 1,2 , Stefan Peeters 1 , Hannes Holey 1,2 , Franziska Stief 1,3 , Daniele Savio 1,4 , Lars Pastewka 5 , Gianpietro Moras 1 , Kerstin Falk 1 and Michael Moseler 1,2,3,* 1 Microtribology Center μTC, Fraunhofer Institute for Mechanics of Materials IWM, Wöhlerstraße 11, 79108 Freiburg, Germany 2 Freiburg Materials Research Center, University of Freiburg, Stefan-Meier-Straße 21, 79104 Freiburg, Germany. 3 Institute of Physics, University of Freiburg, Hermann-Herder-Straße 3a, 79104 Freiburg, Germany. 4 Freudenberg Technology Innovation SE & Co.KG, Höhnerweg 2-4, 69469 Weinheim, Germany 5 Department of Microsystems Engineering, University of Freiburg, Georges-Köhler-Allee 103, 79110 Freiburg, Germany * michael.moseler@iwm.fraunhofer.de 1. Introduction Modern, compact and efficient tribological systems are often operated in mixed lubrication or even boundary lubrication, meaning small gaps, frequently high pressures and occasional solid-solid contacts between the lubricated sliding surfaces (1). In particular, the need for climate-friendly lubricants with low viscosities (2), an increase in assembly precision of lubricated contacts for electric vehicles (3) and new high-performance coating techniques (4) that allow smaller assembly tolerances have driven the shift to operating devices in the mixed lubrication regime. Downsizing dramatically increases loads in tribological components (often in the GPa range) resulting in an additional driving force towards boundary lubricated contacts (5). Under such severe conditions, the film thickness in typical applications can reach a few nanometers (6), becoming comparable to the size of the lubricant molecules themselves. At this scale, a current state-of-the-art continuum description of lubricant flow is expected to lose its validity due to density layering (7), solvation forces (8), the emergence of solid-like states (9, 10), increased viscosities or wall slip (11-16) - see also (17) for a comprehensive review of atomistic simulations of confined lubricant films. The Reynolds Lubrication Equation (RLE) is the most commonly used continuum equation for flow calculations in lubricated systems (18). Although the RLE was proposed at the end of the 19 th century for incompressible laminar Newtonian flows (19), the past decades have seen research in extending its applicability to lubricants exhibiting compressibility, piezoviscosity, shear thinning and cavitation (1, 11, 20). These extensions have rendered the RLE a predictive description for elastohydrodynamic lubrication (EHL) of technically relevant tribocontacts (20), provided quantitative constitutive laws for compressibility and viscosity are employed (21). In combination with empirical friction coefficients for boundary lubrication regions, the RLE is also used for mixed lubrication problems (22, 23). Technically, the RLE is employed for local gap heights exceeding a certain threshold (of the order of 0.1 - 1 µm) to obtain the hydrodynamic contribution to friction while smaller gaps are assumed to be in solid-solid contact and modeled via a Coulomb-Amontons friction law or a Bowden-Tabor (24) constant interfacial shear stress. The choice of this threshold is more a matter of convenience than of a physical reasoning. It would be very useful to explore the lower gap size limits for a continuum description of the lubricant flow - especially for high local pressures characteristic for EHL contacts. By extending the RLE realm to smaller scales the importance of empirical solid-solid contact friction laws could be reduced and therefore the predictive power of mixed lubrication calculations would improve substantially. In the present work, isothermal non-equilibrium molecular dynamics (MD) simulations of hexadecane in a gold converging-diverging channel (depicted in Fig. 1A and B) are performed to generate realistic benchmark data representative of mineral oils lubricating an asperity contact between two metal surfaces. In a previous related study, some of the authors have used this alkane/ gold model to study the onset of cavitation and its continuum description in a parallel channel with heterogeneous slip conditions at moderate pressures (25), while the current work addresses much higher pressures and a variation in the channel height (26). A profound MD characterization of our atomistic hexadecane lubricant model provides an equation of state ρ( p) as well as a pressureand shear-rate-dependent constitutive law for the viscosity h( p, γ ̇). With these data the validity limit of the RLE description for gaps h 0 in the single-digit nanometer range and pressures p approaching the GPa regime is explored. A failure of such a traditional RLE treatment for high pressures can be traced back to the violation of the no-slip boundary condition on the gold (111) facets in the converging-diverging channel. By a separate parametric MD study of hexadecane in parallel gold channels (Fig. 1C and D) the pressure-dependence of wall slip is quantified and the existence of a constitutive law v s- =-v s (τ, p) that relates local shear stress τ at the wall with slip velocity v s is demonstrated. A v s (τ, p) law is also found for technically relevant systems such as diamond-like carbon (DLC) channels filled with poly-α-olefin (PAO) lubricant (Fig. 1E). Finally, we show that an extension of the RLE by v s- =-v s (τ, p) results in a model that allows for a quantitative description of pressure and velocity profiles in our MD simulations of converging-diverging channels for minimum gap sizes and local pressure that are of the order of 1 nm and 1 GPa, respectively. 172 24th International Colloquium Tribology - January 2024 Towards a Continuum Description of Lubrication in Highly Pressurized Nanometer-wide Constrictions: the Importance of Accurate Slip Laws Fig. 1: Atomistic models used in the molecular dynamics simulations. References [1] Y. Meng, J. Xu, Z. Jin, B. Prakash, Y. Hu, A review of recent advances in tribology. Friction 8, 221-300 (2020). [2] S.-W. Zhang, Recent developments of green tribology. Surface Topography: Metrology and Properties 4, 23004 (2016). [3] L. I. Farfan-Cabrera, Tribology of electric vehicles: A review of critical components, current state and future improvement trends. Tribology International 138, 473-486 (2019). [4] B. Fotovvati, N. Namdari, A. Dehghanghadikolaei, On coating techniques for surface protection: A review. Journal of Manufacturing and Materials processing 3, 28 (2019). [5] J. F. Archard, M. T. Kirk, Lubrication at point contacts. Proceedings of the Royal Society A 261, 532-550 (1961). [6] R. P. Glovnea, A. K. Forrest, A. V. Olver, H. A. Spikes, Measurement of Sub-Nanometer Lubricant Films Using Ultra-Thin Film Interferometry. Tribology Letters 15, 217-230 (2003). [7] J. Gao, W. D. Luedtke, U. Landman, Layering transitions and dynamics of confined liquid films. Physical Review Letters 79, 705 (1997). [8] R. G. Horn, J. N. Israelachvili, Direct measurement of structural forces between two surfaces in a nonpolar liquid. The Journal of Chemical Physics 75, 1400- 1411 (1981). [9] S. T. Cui, P. T. Cummings, H. D. Cochran, Structural transition and solid-like behavior of alkane films confined in nano-spacing. Fluid Phase Equilibria 183, 381-387 (2001). [10] M. L. Gee, P. M. McGuiggan, J. N. Israelachvili, A. M. Homola, Liquid to solidlike transitions of molecularly thin films under shear. The Journal of Chemical Physics 93, 1895-1906 (1990). [11] B. J. Hamrock, S. R. Schmid, B. O. Jacobson, Fundamentals of Fluid Film Lubrication (CRC press, 2004). [12] W. Habchi, D. Eyheramendy, S. Bair, P. Vergne, G. Morales-Espejel, Thermal elastohydrodynamic lubrication of point contacts using a Newtonian/ generalized Newtonian lubricant. Tribology Letters 30, 41-52 (2008). [13] D. Savio, K. Falk, M. Moseler, Slipping domains in water-lubricated microsystems for improved load support. Tribology International 120, 269-279 (2018). [14] R. Pit, H. Hervet, L. Leger, Direct experimental evidence of slip in hexadecane: solid interfaces. Physical Review Letters 85, 980 (2000). [15] J. P. Ewen, S. K. Kannam, B. D. Todd, D. Dini, Slip of Alkanes Confined between Surfactant Monolayers Adsorbed on Solid Surfaces. Langmuir 34, 3864-3873 (2018). [16] Y. Zhu, S. Granick, No-Slip Boundary Condition Switches to Partial Slip When Fluid Contains Surfactant. Langmuir 18, 10058-10063 (2002). [17] J. P. Ewen, D. M. Heyes, D. Dini, Advances in nonequilibrium molecular dynamics simulations of lubricants and additives. Friction 6, 349-386 (2018). [18] D. Gropper, L. Wang, T. J. Harvey, Hydrodynamic lubrication of textured surfaces: a review of modeling techniques and key findings. Tribology International 94, 509-529 (2016). [19] O. Reynolds, IV. On the theory of lubrication and its application to Mr. Beauchamp tower’s experiments, including an experimental determination of the viscosity of olive oil. Philosophical Transactions of the Royal Society of London, 157-234 (1886). [20] P. M. Lugt, G. E. Morales-Espejel, A review of elasto-hydrodynamic lubrication theory. Tribology Transactions 54, 470-496 (2011). [21] P. Vergne, S. Bair, Classical EHL Versus Quantitative EHL: A Perspective Part I—Real Viscosity-Pressure Dependence and the Viscosity-Pressure Coefficient for Predicting Film Thickness. Tribology Letters 54, 1-12 (2014). [22] H. Christensen, BSc, PhD, CEng, MIMechE, A Theory of Mixed Lubrication. [23] A. Martini, D. Zhu, Q. Wang, Friction Reduction in Mixed Lubrication. Tribology Letters 28, 139-147 (2007). [24] F. P. Bowden, D. Tabor, The friction and lubrication of solids (Oxford University Press, 2001). [25] D. Savio, L. Pastewka, P. Gumbsch, Boundary lubrication of heterogeneous surfaces and the onset of cavitation in frictional contacts. Science Advances 2, e1501585 (2016). [26] A. Codrignani, S. Peeters, H. Holey, F. Stief, D. Savio, L. Pastewka, G. Moras, K. Falk and M. Moseler, Towards a continuum description of lubrication in highly pressurized nanometer-wide constrictions: the importance of accurate slip laws, Science Advances (2023). 24th International Colloquium Tribology - January 2024 173 Tribochemical Properties of Glycerol as a Green Lubricant on Ferrous Substrates: Atomic-scale Study by Reactive Molecular Dynamics Simulation Vahid Fadaei Naeini 1,2* , J. Andreas Larsson 1 , Roland Larsson 2 1 Applied Physics, Division of Materials Science, Department of Engineering Sciences and Mathematics, Luleå University of Technology, Sweden 2 Machine Elements, Division of Machine Elements, Department of Engineering Sciences and Mathematics, Luleå University of Technology, Sweden * Corresponding author: vahid.fadaei.naeini@ltu.se 1. Introduction Annually, the leakage of oil-based lubricants into the environment presents significant ecological hazards [1] . When industrial lubricants spill, they can contaminate extensive areas, especially affecting water bodies, leading to widespread environmental damage [2] . The increasing demand for eco-friendly lubricants has made vegetable oils popular, despite their limitations in heat resistance and viscosity range. [3] . Glycerol, as a green and biocompatible base fluid, demonstrate better physical properties at cooler temperatures positioning it as a promising green substitute for vegetable oils. Recent research has delved into the complexities of tribofilm formation and its role in elastohydrodynamic lubrication. Zhang et al. examined the mechanochemical behaviors of zinc dialkyldithiophosphate (ZDDP) in full-film EHL, focusing on the critical influence of shear stress [4]. In this study, we present a computational framework based on reactive NEMD simulations, designed to enhance understanding of the tribochemical interactions of glycerol. 2. Computational Methods and System Setup 2.1 Stress-Augmented Thermally Activated Process Recent advances in nanoscale experiments, especially those involving asperities, have offered new perspectives for studying the single lubricant molecules [5]. Gosvami et al. employed atomic force microscopy (AFM) to investigate the growth rate of ZDDP tribofilms on aluminum-based interfaces, discovering that the formation rate is concurrently affected by temperature (T) and normal stress (σ). [6]. The tribofilm growth observed in these experiments follows a stress-augmented thermally activated (SATA) process described by a modified Arrhenius equation that is widely known as the Bell model: (1) In this equation, A represents the pre-exponential factor, E a the activation energy of the reaction, ∆V ∗ the activation volume, and k B the Boltzmann constant. This study aims to determine the mechanochemical factors that control the growth of glycerol lubricant films, using data from glycerol dissociation and the Bell model. 2.2 Simulation Method and System Setup Molecular dynamics (MD) simulations using the ReaxFF force field provide a reliable computational approach for studying physical and mechanochemical properties of lubricants, particularly excelling in simulating chemical reactions like bond breaking and formation. Its functional form, expanded by Aktulga et al. [7] , is expressed as: E ReaxFF = E bond + E angle + E tors. + E vdW + E Coulomb + E specific + E 0 (2) where E bond denotes the energy of bond formation, E angle and E tors. correspond to energies of three-body angles and four-body dihedrals. E Coulomb and E vdW represent electrostatic and van der Waals energies. E Specific covers system-specific properties like hydrogen bonding, while E over. prevents atom over-coordination. The dynamic atomic partial charges in the simulation are calculated using the charge equilibration method [7] . This study employs the ReaxFF parameters by Khajeh et al. for systems containing C/ H/ O/ Fe [8] . To evaluate the surface impact on tribochemical properties of glycerol, a layer containing 333 intact glycerol molecules was placed between α-Fe(110) and amorphous functionalized magnetite substrates (a-Fe 3 O 4 -OH), individually. The magnetite substrate was functionalized with hydroxyl groups. The simulation box measured 54 Å × 54 Å × 89 Å along the X, Y, and Z axes. After setting up the box, the system was equilibrated at T=300 K for 0.1 nanoseconds (ns), using the Nosé- Hoover thermostat [9] to be published. For the production run, the temperature was varied between 300-500 K and the normal pressure was increased from 1 to 4 GPa, as indicated in Figure 1. Figure 1. The initial configuration of the system To achieve the desired normal stress, a constant force was applied to the outermost atomic layers of each slab using steered MD simulation with a constant force. The sliding velocity was achieved by applying equal and opposite velocities (±5 m/ s) to the outermost layers of atoms along the x-axis. The heating, compression, and shear starin were all carried out simultaneously during the 1 ns production run using the LAM- MPS package [10], with a simulation timestep of 0.25 fs. 3. Results and Discussion Figure 2 displays the time evolutions of the number of intact glycerol molecules confined between a-Fe 3 O 4 -OH and 174 24th International Colloquium Tribology - January 2024 Tribochemical Properties of Glycerol as a Green Lubricant on Ferrous Substrates: Atomic-scale Study by Reactive Molecular Dynamics Simulation α-Fe(110) surfaces at T=300 K under various loading conditions. Numerical assessments show that glycerol dissociation between magnetite substrates adheres to a second-order reaction, while the decline in glycerol molecules between α-Fe slabs indicates a first-order reaction. Figure 2. The time evolution of the number of intact glycerol molecules for pure glycerol confined between: a. a-Fe 3 O 4 -OH surfaces and b. α-Fe(110) at T=300 K under various loading s. As depicted in Figure 3, we applied 3D surface fitting to the dissociation rate data of glycerol between a-Fe 3 O 4 -OH and α-Fe slabs to analyze the temperature and stress effects on reactivity. Figure 3. Panels a and b illustrate the dissociation rate alter-ations by temperature and shear stress for glycerol confined between a. a-Fe 3 O 4 -OH and b.α-Fe slabs, respectively. The approach depicted in Figure 3 paved the way to determine the key parameters of the Bell model like ΔV * , ln(A), and E a , as listed in Table 1. Table 1.The values ΔV*, ln(A), and Ea for glycerol confined between a-Fe3O4-OH and α-Fe(110) slabs Ea (kJ/ mol.) ln A (s-1) ΔV* (Å3) Gly._a-Fe3O4-OH 18.02±4.21 -3.75±0.40 8.71±0.49 Gly.-α-Fe(110) 8.27±1.67 3.78±0.75 12.21±2.33 4. Concluding Remarks In this study, molecular dynamics (MD) simulations with the ReaxFF reactive force field were used to study the tribochemical properties of glycerol between typical ferrous surfaces. The research focused on determining the decomposition rate of glycerol molecules and the mechanochemical factors affecting tribofilm growth. This computational method allows for validating results and assessing the performance of conventional lubricants under operational conditions. References [1] P. Nowak, K. Kucharska, and M. Kamiński, “Ecological and health effects of lubricant oils emitted into the environment,” Int. J. Environ. Res. Public Health, vol. 16, no. 16, p. 3002, 2019. [2] L. Spilsbury and R. Spilsbury, The oil industry. The Rosen Publishing Group, Inc, 2011. [3] M. A. Dandan, S. Samion, N. F. Azman, F. Mohd Zawawi, M. K. Abdul Hamid, and M. N. Musa, “Performance of polymeric viscosity improver as bio-lubricant additives,” Int. J. Struct. Integr., vol. 10, no. 5, pp. 634-643, 2019. [4] J. Zhang, J. P. Ewen, M. Ueda, J. S. S. Wong, and H. A. Spikes, “Mechanochemistry of zinc dialkyldithiophosphate on steel surfaces under elastohydrodynamic lubrication conditions,” ACS Appl. Mater. Interfaces, vol. 12, no. 5, pp. 6662-6676, 2020. [5] L. O. Laier et al., “Surface-dependent properties of α-Ag 2 WO 4: a joint experimental and theoretical investigation,” Theor. Chem. Acc., vol. 139, pp. 1-11, 2020. [6] N. N. Gosvami, J. A. Bares, F. Mangolini, A. R. Konicek, D. G. Yablon, and R. W. Carpick, “Mechanisms of antiwear tribofilm growth revealed in situ by single-asperity sliding contacts,” Science (80-. )., vol. 348, no. 6230, pp. 102-106, 2015. [7] H. M. Aktulga, J. C. Fogarty, S. A. Pandit, and A. Y. Grama, “Parallel reactive molecular dynamics: Numerical methods and algorithmic techniques,” Parallel Comput., vol. 38, no. 4-5, pp. 245-259, 2012. [8] A. Khajeh et al., “Statistical analysis of tri-cresyl phosphate conversion on an iron oxide surface using reactive molecular dynamics simulations,” J. Phys. Chem. C, vol. 123, no. 20, pp. 12886-12893, 2019. [9] S. Nosé, “A unified formulation of the constant temperature molecular dynamics methods,” J. Chem. Phys., vol. 81, no. 1, pp. 511-519, 1984, doi: 10.1063/ 1.447334. [10] S. Plimpton and L. S. National, “Fast Parallel Algorithms for Short-Range Molecular Dynamics,” J. Comput. Phys., vol. 117, no. June 1994, pp. 1-42, 1995. 24th International Colloquium Tribology - January 2024 175 Effect of Polar Additives on the Slip and Bulk Shear of Hydrocarbon Oils Seyedmajid Mehrnia 1* , Maximilian Kuhr 1 , Peter F. Pelz 1 1 Chair of Fluid Systems, Technical University of Darmstadt, Darmstadt, Germany * Corresponding author: seyedmajid.mehrnia@tu-darmstadt.de 1. Introduction Different liquid lubricants are utilized on sliding surfaces and rotating parts to reduce the unfavorable consequences of friction. Despite this, most researchers are moving to decrease the usage of lubricant fluids because of their undesirable influence on the environment. Polar additives are interested in developing research considerations. This is because of their specifics, such as improving viscosity and reducing friction. In this paper, the wall slip and bulk shear of a polar lubricant additive blended with the non-polar hydrocarbon oil, i.e., Polyalphaolephin (PAO) base oil, at various temperatures in interaction with metal surfaces will be investigated. The polar molecule used here is Poly Alkyl Methacrylate (PAMA), a Viscosity Index (VI) improver lubricant additive. Molecular Dynamics (MD) simulation is employed as a tool for capturing the liquid-metal interaction. Figure 1 shows calculated dynamic viscosity in different apparent shear rates for three forms of 1-Decane molecule of PAO oil by MD simulation. In this simulation, the lubricant is confined to a stationary and moving wall. There is a Couette flow between the walls due to wall movement. The of 1-Decane tetramer molecule with various branches are used in this simulation. A hybrid force field consisting of different potential energy functions was employed in this simulation. The potential energy functions calculate the force for each atom, depending on the position of the other atoms. Newton’s laws define how those forces will influence the atoms‘ movements. The potential energy depends on the angle, bond, dihedral, and improper of the atoms. Force field application is representing the time evolution of angle, stretching, and torsion rotation of the bonds, besides the non-bonded interactions within atoms [1-3]. 1.1 Simulation method All molecules models are designed using Avogadro software, then assembled and optimized by the Packmol software. The model utilized for the liquid molecules is a full atom-style model that includes bond stretching, bending, and torsion. The MD equations of motion were integrated using the velocity-Verlet algorithm with an integration time-step of 1.0-fs for the selected force field. The MD modeling in this research was performed by the LAMMPS molecular dynamics simulator, an open-source code [4]. Updating velocity and positions of atoms in the group each time step was performed by a fixed NVE integration. This generates a system trajectory compatible with the microcanonical ensemble. At the start of the simulation, the system was relaxed to equilibrium in 0.05-ns. Hence, the system achieved a thermodynamic stability state. Through all simulations, the temperature is uniformly controlled by a Langevin thermostat. The main outcome of this research is calculation of wall slip and the effect of polar additives on viscosity and friction in different temperatures. Figure 1: Schematic structure of PAMA-L C 2 O2 (CH2)7 (CH3)4 1.2 Slip and bulk shear The concept of apparent and real measures has proven to be practical in rheology and is hence used here also in the context of MD tribology. If we assume a fluid film between two parallel walls having the distance h sliding relative to each other with velocity U the apparent shear rate is γ ̇ app -=-U⁄ h, the apparent shear stress is τ app = m γ ̇ app . For any stationary Couette flow, the real shear stress τ(z) = τ w is constant across the channel height 0 ≤ z ≤ h and equal to the wall shear stress τ w . For small Reynolds number, the shear stress is dominated by molecular, i.e. viscous forces and the result of the constant shear stress is a constant field of shear rate τ =-τ w -=-m γ ̇ =-cons. [1]. In the context of polar additive molecules, their polar functional groups exhibit an affinity for the fluid molecules. This interaction can give rise to a thin layer of fluid molecules in proximity to the surface, commonly referred to as the boundary layer or adsorbed layer. The boundary layer possesses distinct characteristics when compared to the bulk fluid, leading to alterations in viscosity and flow behavior. In this specific scenario, slip values were exceedingly small, approaching zero. This occurs when the fluid molecules strongly bind with the polar molecules, causing them to adhere closely to the surface. Such minimal slip results in heightened friction and increased resistance to flow, leading to reduced flow rates and elevated pressure drop. Figures 2 and 3 depict the impact of polarity on the black-marked PAO 6 near the metal wall. 176 24th International Colloquium Tribology - January 2024 Effect of Polar Additives on the Slip and Bulk Shear of Hydrocarbon Oils Figure 2: Side view of a 3D simulation box depicting a black marked PAO 6 molecule along a metal wall. Figure 3: Side view of a 3D simulation box depicting a black marked PAO 6 molecule along a metal wall in interaction with PAMA molecules. The viscosity of fluids typically decreases significantly as temperature rises, which can be a significant issue for lubricants that often operate across a range of temperatures. The role of a Viscosity Index (VI) improver is to enhance the viscosity of a low-viscosity base fluid to achieve a desirable viscosity level at high temperatures while avoiding excessive viscosity increase at low temperatures. VI improvers are employed to modify the relationship between viscosity and temperature in lubricants. Figure 4 illustrates the impact of polar additives on viscosity as temperature increases. Figure 4: Calculated apparent viscosity in different temperature. 1.3 Conclusion In simulations involving confined lubricants, various methods can be employed to evaluate slip. One such approach involves determining slip velocity by analyzing the velocity profile of lubricant molecules near the solid surface. The findings from this analysis suggest that slip values are minimal, mainly due to the presence of polar atoms at the wall surfaces. Calculating the viscosity of confined polar lubricant additives mixed with PAO oil between metal surfaces in MD simulations necessitates considering the interactions among the lubricant additives, PAO oil, and the metal surfaces. To achieve this, a shear gradient is applied, the stress tensor is computed, and stress autocorrelation is analyzed to estimate viscosity. The resulting viscosity values demonstrate that polar molecules exert a significant influence on viscosity, primarily as a result of coil expansion. The concept of coil expansion proposes that a polymer initially maintains a coiled conformation at lower temperatures and then expands as solubility increases at higher temperatures, leading to elevated viscosity. Furthermore, the interaction between polar molecules and carbon groups contributes to the heightened viscosity. References [1] Pelz P. F., Corneli T., Mehrnia S., Kuhr M. (2022): Temperature-dependent wall slip of Newtonian lubricants. Journal of Fluid Mechanics, 948, A8. [2] Mehrnia S., Pelz P. F. (2021): Slip length of branched hydrocarbon oils confined between iron surfaces. Journal of Molecular Liquids, 336, 116589. [3] Mehrnia S., Pelz P. F. (2022): Tribological design by Molecular Dynamics simulation - The influence of molecular structure on wall slip and bulk shear. Chemical Engineering & Technology, 202200448. [4] Plimpton S. (1995): Fast parallel algorithms for shortrange molecular dynamics. Journal of Computational Physics, 117, 1-19. 24th International Colloquium Tribology - January 2024 177 Numerical and Experimental Analyses of the Multiscale Effects in the Tribological System Rotary Shaft Seals * Jeremias Grün 1* , Marco Gohs 1 , Simon Feldmeth 1 , Frank Bauer 1 1 University of Stuttgart, Institute of Machine Components, Stuttgart, Germany * Corresponding author: jeremias.gruen@ima.uni-stuttgart.de 1. Introduction In many applications, the tribological system [1] rotary shaft seal [2] is subjected to a variety of dynamic loads on multiple scales. Especially considering the trends in electromobility, rotary shaft seals are exposed to increasingly extreme operating conditions and loads. To address this, it is essential to provide suitable models for numerical computation, as well as conduct appropriate experiments for verification and validation. The following contribution focuses on both numerical and experimental analyses of the function of rotary shaft seals. The numerical methods encompass nonlinear finite element analyses (FEA) of the multiscale structural mechanics [3] and transient multi-phase computational fluid dynamics (CFD) of the flow processes in the sealing gap on the microscale [4]. Test results on endurance test rigs and visual examinations using specialized devices are employed to validate and verify the numerical methods. 2. Numerical simulations and experimental analyses Figure 1 illustrates the general process of multiscale modeling and simulation of rotary shaft seals. The initial step involves utilizing FEA to compute the structural mechanics of the rotary shaft seal. This encompasses evaluating the deformation of the sealing ring during mounting on the macroscale and analyzing the microscopic distortions in the sealing contact between the shaft counter face and the sealing edge [3], [5]. The post-processing of the FEA results establishes the computational domain for the subsequent CFD modeling [4]. The minimum sealing gap height serves as a crucial input parameter for defining the computational domain. Lubricant film thickness equation according to [6], [7] serve to determine the sealing gap height under consideration of surface and material parameters as well as operating conditions. The CFD simulations allow to evaluate the sealing performance on the basis of parameters such as the pumping rate. Moreover, they provide a more in-depth insight into the complex transient flow processes in the lubricant film in the seal gap. Figure 1: Multiscale modeling and simulation of rotary shaft seals 178 24th International Colloquium Tribology - January 2024 Numerical and Experimental Analyses of the Multiscale Effects in the Tribological System Rotary Shaft Seals* Figure-2: Structural mechanical results 3. Results Figure-2 illustrates the results of the structural mechanics analysis, while Figure-3 showcases the findings from the fluid dynamics analysis. Figure-2 shows above the numerical results for the microscopic mechanics in the sealing contact, showcasing the deformed sealing edge surface (Figure-2 ① ) and the contact pressure distribution (Figure-2 ② ). The black dashed line represents the mean displacement of the sealing edge. At the bottom (Figure-2 ③ ), a comparison is made between the numerical results (m = 0.25 … 0.35) and the experimental results (RSS 1 - RSS 4) for the tangential displacement of the sealing edge surface. In Figure-3 the fluid dynamics results are depicted. The upper part illustrates the phase transitions between air and oil (Figure-3 ① ) and the hydrodynamic pressure, along with the formation of cavitation areas (Figure-3 ② ). The bottom section (Figure-3 ③ ) presents a comparison between the pumping rates obtained from endurance tests ṁ exp and those computed numerically ṁ sim . 4. Conclusions The results obtained from both the numerical simulations and experimental analyses demonstrate a substantial level of agreement. This suggests that the employed numerical methods are well-suited for studying the multiscale tribological effects. In summary, the attained numerical and experimental findings offer a comprehensive understanding of the tribological behavior exhibited by rotary shaft seals, serving as a foundation for future advancements and developments in this field. Figure-3: Fluid dynamics results References [1] F. Bauer, Tribologie. Wiesbaden (DE): Springer Fachmedien Wiesbaden, 2021. doi: 10.1007/ 978-3-658- 32920-4. [2] F. Bauer, Federvorgespannte-Elastomer-Radial-Wellendichtungen. Wiesbaden (DE): Springer Fachmedien Wiesbaden, 2021. doi: 10.1007/ 978-3-658-32922-8. [3] J. Grün, M. Gohs, und F. Bauer, „Multiscale Structural Mechanics of Rotary Shaft Seals: Numerical Studies and Visual Experiments“, Lubricants, Bd. 11, Nr. 6, S.-234, Mai 2023, doi: 10.3390/ lubricants11060234. [4] J. Grün, S. Feldmeth, und F. Bauer, „Multiphase Computational Fluid Dynamics of Rotary Shaft Seals“, Lubricants, Bd. 10, Nr. 12, S. 347, Dez. 2022, doi: 10.3390/ lubricants10120347. [5] J. Grün, S. Feldmeth, und F. Bauer, „The sealing mechanism of radial lip seals: A numerical study of the tangential distortion of the sealing edge“, Tribology and Materials, Bd. 1, Nr. 1, S. 1-10, 2022, doi: 10.46793/ tribomat.2022.001. [6] N. Marx, J. Guegan, und H. A. Spikes, „Elastohydrodynamic film thickness of soft EHL contacts using optical interferometry“, Tribology International, Bd. 99, S. 267-277, Juli 2016, doi: 10.1016/ j. triboint.2016.03.020. [7] P. Sperka, I. Krupka, und M. Hartl, „Analytical Formula for the Ratio of Central to Minimum Film Thickness in a Circular EHL Contact“, Lubricants, Bd. 6, Nr. 3, S. 80, Sep. 2018, doi: 10.3390/ lubricants6030080. 24th International Colloquium Tribology - January 2024 179 Simulative and Experimental Characterization of the Tribo-Electrical Contact of Roller Bearings Stefan Paulus 1* , Simon Graf 1 , Oliver Koch 1 , Stefan Götz 2 1 RPTU Kaiserslautern-Landau, Chair of Machine Elements, Gears and Tribology (MEGT), 67663 Kaiserslautern, Germany 2 RPTU Kaiserslautern-Landau, Division of Mechatronics and Electrical Drives (MEAS), 67663 Kaiserslautern, Germany * Corresponding author: stefan.paulus@rptu.de 1. Introduction The usage of fast switching frequency inverters provides high performance operation of electric drive trains. Therefore, they are commonly used in modern drive systems, especially in electromobility. Beside their advantages, these inverters also induce parasitic voltages in the drive system that also affect the bearings. When the breakdown voltage of a lubricated contact is exceeded, discharge currents occur with a high energy density [1]. These currents can cause different kind of damages such as grey frosting [2], flutings [3] and white etching cracks [4]. All these can lead to premature failure of the bearing [5]. To predict the behavior of a bearing under electrical load, it has to be described as an electrical component. This is usually done by regarding the rolling contact between a rolling element and the raceway as a parallel connection of three capacitances, the capacitance of the Hertzian contact area C Hertz , the capacitance of the inlet zone of the contact C Inlet and the capacitance of the outlet zone C Outlet . The Hertzian contact area is thereby regarded as a parallel plate capacitor, due to the deformation of rolling element and the raceway und mechanical load. The capacitance of a parallel plate capacitor can be calculated by equation 1 (1) where 𝜖 0 is the permittivity of vacuum and 𝜖 r is the relative permittivity of the dielectric between the plates, here the lubricant. Inserting the Hertzian contact area h 0 , and the central lubrication gap height the Hertzian capacitance can be calculated. The relation between the total contact capacitance C Contact and C Hertz is commonly estimated with help of correction factors. In this work an electrically extended EHL-simulation model was used to investigate the contact capacitance, including the inlet and the outlet zone. The results are compared with experimental data. 2. Methods and Material In this work the capacitance of a thrust bearing of type-51208 is investigated using two different oils, mineral oil and polyglycol. The axial load was varied in the steps 700-N, 800-N, 1000-N, 1100-N, 1300-N, 2000-N, the rotational speed in the steps 1500-rpm, 2750-rpm, 4000-rpm and the temperature in the steps 40-°C and-60-°C. 2.1 Simulation Model To calculate the deformation of the lubrication gap the Reynolds-equation is solved iteratively. The dimensions of the calculation area in rolling direction x and perpendicular to the rolling direction y are defined by -4a < x < 4a and -3b < y < 3b where a and b are the half axes of the Hertzian contact ellipse. In this way the inlet zone and the outlet zone are part of the calculation area. The finite volume method is used for discretization. The Fischer-Burmeister-Newton-Schur-(FBNS) algorithm is used to solve the Reynolds-equation. This algorithm provides the lubrication gap height and the distribution of pressure and cavitation degree in the calculation area after converging. The oil density and viscosity are calculated in dependency of the pressure distribution and the inlet temperature with help of a set of Bode-equations. When the calculation of the lubrication gap has finished, the contact capacitance is calculated. For this purpose, every single control volume in the calculation area is assumed to be a parallel plate capacitor and the capacitance of each control volume is calculated with help of equation 2. The permittivity of oil is calculated in dependency of pressure and temperature with help of another Bode-equation. In the outlet region, cavitation occurs. Therefore, the capacitance of each control volume in the outlet region is calculated as a series connection of the permittivity of oil and air. The ratio between both phases is given by the cavity fraction, that was calculated in the EHL-simulation. The overall contact capacitance is calculated by summation of all control volume capacitances. 2.2 Experimental setup The experimental investigation of the capacitance of a thrust bearing type 51208 is carried out at the test bench called GESA (ger.: „Gerät zur erweiterten Schmierstoffanalyse“, eng.: „device for extended lubricant analysis“). At this test bench, thrust bearings can be loaded both, mechanically and electrically. 3. Results The simulative and experimental results for mineral oil are given in figure-1, the results for polyglycol are given in figure-2. 180 24th International Colloquium Tribology - January 2024 Simulative and Experimental Characterization of the Tribo-Electrical Contact of Roller Bearings Figure 1: Capacitance of bearing type 51208 with mineral oil determined by simulation (left) and testing (right) Figure 2: Capacitance of bearing type 51208 with polyglycol determined by simulation (left) and testing (right) The capacitance of the polyglycol is basically higher compared to the capacitance using mineral oil. This is due to the different permittivities of the oils. The permittivity of the mineral under normal conditions is about 2.2, the permittivity of the polyglycol is about 5.5. Both oils show a similar dependency of the bearing capacitance on the operating conditions. The capacitance increases with higher axial loads, due to the stronger deformation of the rolling elements which results in a larger Hertzian contact area, i.e. larger capacitor plates. At the same time, decreasing rotational speed and increasing temperature leads to an increase of the capacitance. Both parameters affect the gap height and thereby the distance between the capacitor plates. Lower gap heights lead to higher capacitances. This effect can be seen in both, the simulative and the experimental results but is slightly stronger developed in the simulative results. Furthermore, the experimental determined capacitance is overall a bit higher compared to the simulation results. Both observations can be attributed to the fact, that a bearing capacitance does not only consists of the capacitance values of the rolling contacts. Moreover, the bearing components outside the contact zone also contribute to the total capacitance of the bearing. These influences are not covered by the EHL contact simulation. Nevertheless, the main capacitance source is the contact capacitance and therefore the simulative results in general show good agreement to the experimental determined values. 4. Conclusions In this work an electrically extended EHL simulation model was used to determine the contact capacitance of a thrust bearing of type 51208. Overall, the results of the electrically extended simulation show good agreement with experimental determined values. The influence of varying axial load, speed and oil temperature on the capacitance can be captured, although the total bearing capacitance determined by simulation is in general lower than the experimental determined capacitance. For a more accurate determination of the bearing capacitance, the simulation is being expanded to include the capacitance sources outside the roller contacts. Furthermore, the calculation of the temperature inside the roller contact is to be implemented in the EHL model. 5. Acknowledgment The authors thank the Deutsche Forschungsgemeinschaft (DFG) for funding „Determination of ball bearing impedances under steady-state operating conditions by means of a further developed rolling contact model at full film lubrication“ (SA898/ 32-1/ 470273159). References [1] A. Jagenbrein: Investigations of bearing failures due to electric current passage, Technische Universität Wien Dissertation. Wien 2005. [2] S. Graf, B. Sauer: Surface mutation of the bearing raceway during electrical current passage in mixed friction operation. Bearing World Journal 2020 (2020) 5, S.-137-147. [3] T. Zika: Electric discharge damaging in lubricated rolling contacts, Technische Universität Wien Dissertation. Wien 2010. [4] J. Loos, I. Bergmann, M. Goss: Influence of High Electrical Currents on WEC Formation in Rolling Bearings. Tribology Transactions 64 (2021) 4, S.-708-720. [5] V. Schneider, J. O. Stockbrügger, G. Poll, B. Ponick: Stromdurchgang am Wälzlager - Verhalten stromführender Wälzlager. Abschlussbericht FVA 863 I Nr.-1501 (2022). Test and Measurement Methodologies 24th International Colloquium Tribology - January 2024 183 Comparison of Different Standard Test Methods for Evaluating Greases for Rolling Bearings under Vibration Load or at Small Oscillation Angles Markus Grebe 1* , Henrik Buse 2 , Alexander Widmann 3 1 Competence Center for Tribology, UAS Mannheim 2 Tribologie Engineering Mannheim GmbH 3 Steinbeis Transferzentrum at UAS Mannheim, Department Tribology * Corresponding author: m.grebe@hs-mannheim.de 1. Introduction Rolling bearings that are often only operated at small oscillation angles or that are exposed to vibrations when stationary show typical damage after only a short period of operation. This can be classic false brinelling damage, so-called standstill marks or fretting damage. It is important to differentiate here according to the so-called amplitude ratio (x/ 2b), which indicates the ratio between the movement of the rolling element (x) and the Hertzian contact half-axis (b). Depending on this ratio, suitable laboratory test methods must be used to test the lubricating grease in a practical manner for the respective application. In the context of the lecture, the scientific fundamentals of these special operating and test conditions are explained and test results of model lubricants in these three standard rolling bearing tests as well as in a classical Fretting test under oscillating sliding friction are compared with each other. 1.1 Aim of the Test Series The aim of this in-house series of tests was to compare various known laboratory test methods for evaluating lubricating greases for rolling bearings that only perform small oscillating movements or are only subjected to vibration loads (e.g. blade bearings in wind turbines). The Fafnir wear test according to ASTM D4170 [1] and the SNR-FEB2 test [2], which is frequently required in Europe, can be mentioned here as standard procedures. In addition, an in-house test was carried out that simulates bearings at very small angles of oscillation and vibration (KTM QSST) [3], [4], [5]. The new NLGI grease specification for high performance multi-purpose (HPM) greases [6] also requires the SRV test according to ASTM D7594 [7] for greases with higher loads, so this was also included in the test series. For the series of tests, four mineral oil-based grease samples were prepared with two types of soap thickeners (lithium and calcium sulfonate), which are known to give different results in the Fafnir test. However, the aim of the series of experiments was not to find a particularly suitable grease for the application, but to show that the greases give different results depending on the tribological stress collective. In the first part of the presentation, the scientific principles of rolling bearings that are only operated under small angles of oscillation or are subjected to vibrations are explained. In these studies, it is important to consider the amplitude ratio x/ 2b, which is the ratio between the rolling element’s rolling path (x) and twice the Hertzian contact width (2b). At a ratio smaller than 1, parts of the contact are never opened, which makes the re-entry of lubricant much more difficult or even impossible. At an amplitude ratio greater than 1, reflow is possible in principle and depends strongly on the rheology of the grease [8], [9]. In the second part of the presentation, the operating and test conditions of common and partly standardized rolling bearing test methods as well as the test results are presented and discussed. 2. Results All conducted tests show that the performance of the lubricants strongly depends on the test conditions and that until today there seems to be no universal lubricant for these different operating conditions. This also confirms the results of previous tests at KTM [5]. A high oil release is advantageous for rolling bearings operating with relatively small vibration angles. However, changing the thickener can also be promising (Figure 1). On the whole, the ranking in the Fafnir test and the SNR-FEB2 test does not differ. In the Fafnir test, however, slightly different values are obtained at the industry partner and at the KTM. The results show, however, that no major differences in the rankings are to be expected for amplitude ratios significantly above 1, although the C/ P ratio in the SNR-FEB2 test, for example, is significantly smaller (Fafnir approx. 7.6; SNR-FEB2 approx. 3.1). Figure 1: Results of the bearing tests with x/ 2b > 1 The result is completely different for the tests with a small amplitude ratio (Figure 2) (here 0.55). The evaluation on the basis of the school grades shows that grease sample No. 1 with the highest oil release also shows certain advantages in 184 24th International Colloquium Tribology - January 2024 Comparison of Different Standard Test Methods for Evaluating Greases for Rolling Bearings under Vibration Load or at Small Oscillation Angles this test. Calcium sulfonate grease No. 15, which performed very well under wider angles, is hardly better than the other two samples under these conditions. The lithium soap grease with the lower oil release (sample 2) shows the greatest damage. It is noticeable here that the damage is already pronounced after just one minute. Figure 2: Results of the standstill tests; Evaluation by means of (subjective) school grades A comparison with a „classic“ Fretting test under oscillating sliding friction in the SRV test rig shows the problem of the very high contact pressure at the beginning of the test. Two of the four grease samples could not be tested under these standard test conditions. For both lithium soap greases, adhesive failure already occurred during the run-in phase with reduced normal force. Both calcium sulfano greases, on the other hand, ran through without any major differences in friction and wear behavior. The high contact pressure in point contact requires a special lubricant or additive chemistry to prevent seizure, which is ultimately of secondary importance in rolling bearings. The test should therefore only be used if high local contact pressures and pure sliding friction are to be expected in practice. 3. Conclusion It can be shown that the performance of the lubricants strongly depends on the test conditions and that so far there does not seem to be one universal lubricant for these different operating conditions. This series of tests was intended to show that, for meaningful screening, a laboratory test must be selected that reflects practical conditions as closely as possible. This was clearly demonstrated using the specially formulated model greases. In particular, small x/ 2b ratios of less than 1 represent a major challenge for the lubricants used. Greases that still perform very well at x/ 2b ratios greater than 1 can be completely unsuitable here. The SRV fretting test (ASTM D7594) is not suitable for reliably predicting the lubrication behavior of a grease in a rolling bearing. If necessary, results from several tests must also be taken into account for a final decision, if different conditions may prevail in practice. For example, the conditions in the blade bearings of wind turbines are very different. Large x/ 2b ratios occur during the adjusting motion of the blades, while small x/ 2b ratios must be considered when the blades are stationary under vibration [10]. Therefore, the lubricant selected must always represent a compromise. References [1] American Society for Testing and Materials. ASTM D4170: 2016 - Standard Test Method for Fretting Wear Protection by Lubricating Greases, 2016. [2] Normalisation Francaise. Nft 60-199 - aptitude à résister au faux effet brinell, 1995. [3] M. Grebe, P. Feinle. Brinelling, False-Brinelling, „false“ False-Brinelling? - Ursachen von Stillstandsmarkierungen und geeignete Laborprüfungen; Jahrestagung der Gesellschaft für Tribologie, (GfT) Tagungsband, 2006. [4] M. Grebe. False Brinelling - Standstill marks at roller bearings. PhD thesis, Slovak University of Technology, Bratislava, 2012. [5] M. Grebe. False-Brinelling und Stillstandsmarkierungen bei Wälzlagern - Schäden bei Vibrationsbelastung oder kleinen Schwenkwinkeln. Expert-Verlag, Renningen-Malmsheim, ISBN 978-3-8169-3351-9; 2017. [6] R. Shah, J. Jinag, and J. Kapernik. Next-generation NLGI grease specifications. NLGI Spokesman, 83(4): 63-73, 2020. [7] American Society for Testing and Materials. ASTM D7594: 2019 - Standard Test Method for Determining Fretting Wear Resistance of Lubricating Greases Under High Hertzian Contact Pressures Using a High-Frequency, Linear-Oscillation (SRV) Test Machine, 2019. [8] C. Schadow and L. Deters. Abschlussbericht Forschungsvorhaben Nr. 540 I: False Brinelling - Stillstehende fettgeschmierte Wälzlager unter dynamischer Belastung. FVA-Forschungsheft 951, 2010, Arbeitskreis Schmierstoffe und Tribologie, 951: 127, 2010. [9] S. Tetora, C. Schadow, and D. Bartel. Abschlussbericht Forschungsvorhaben Nr. 540 III: Stillstehende fettgeschmierte Wälzlager unter dynamischer Belastung. FVA-Informationsblatt, 2022, Arbeitskreis Schmierstoffe und Tribologie, 1500: 4, 2022. [10] M. Stammler. Endurance Test Strategies for Pitch Bearings of Wind Turbines. Ph.D. Thesis, Fraunhofer Verlag, Stuttgart, Germany, 2020. 24th International Colloquium Tribology - January 2024 185 Panta Rei: Everything Flows But not Everything Flows the Same René Westbroek 1* , Ben Habgood 2 , Daniel Williams 2 1 Axel Christiernsson International AB, Nol, Sweden 2 Centre for Industrial Rheology, Hampshire, UK * Corresponding author: rene.westbroek@axelch.com 1. Introduction Around 500 BC the famous Greek philosopher Heraclitus allegedly said that everything flows. His term Panta Rhei has generally been adopted by the rheology world, but although studies on the flow behaviour of lubricating grease date back to the mid 20 th Century [1] there still is a lot of confusion on what flow of grease actually means! Experience from the field has taught us that two greases with the same NLGI grade can behave very differently in for example central lubrication systems [2]. Previous studies have shown that not only the NLGI grade of the grease affects the flow behaviour of the grease [3], but also the grease thickener type can play a significant role in the pumpability of the grease [4]. Besides pumpability in for example central lubrication systems, the flow behaviour of the grease will also play a role inside the bearing, during for example the churning phase [5]. This manuscript aims to study the underlying role the thickener plays in the flow behaviour of lubricating greases. 2. Material For this study several greases were manufactured on the lab aiming to keep the variation between the greases at a minimum. All greases are adjusted to an NLGI 2 consistency with PAO-6 as the main oil and neither of the greases contain any additives. For the grease thickened with polypropylene (PP) a small amount of oil soluble polyalkylene glycol (OSP) is added, while the other greases contain a similar amount of an alkylated naphthalene (AN). As the polar oil only makes up a tiny amount of the total oil composition, the effects that this small deviation in the composition could bring are judged to be negligible. Table 1 presents the details of the greases. 3. Method Description The rheological properties of the greases were tested in three different experiments on an Anton Paar MCR 301 at 0-°C, 20- °C and 40- °C. All experiments were performed with a 25 mm plate-plate geometry and the temperature was controlled with a Peltier hood. The gap size for all experiments was set to 1000 µm. Table 1: Grease Properties Grease Thickener Oil Composition Worked Penetration 60-strokes LiX Lithium Complex 95% PAO6 5% AN 276 PP Polypropylene 95% PAO6 5% OSP 281 PU MDI-Polyurea 95% PAO6 5% AN 284 In the first experiment the greases were exposed to a strain sweep, increasing the strain from 0,01 to 1000%. From these experiments both the yield stress (σ y ) and the stress in the crossover point (σ f ), where G’=G’’, were determined. To determine the thixotropic behaviour of the greases, a socalled hysteresis test was performed. In this experiment first the shear rate is increased from 2 to 100 s -1 . The shear rate was then maintained at 100 s -1 for 50 seconds, after which it was decreased back to 2 s -1 . A larger difference of the apparent viscosity for the increaseand decrease step indicates a larger thixotropic behaviour of the grease. Lastly, flow curves were determined at the three test temperatures by increasing the shear stress from 100 to 3000 Pa. The obtained results from these experiments were fitted with the Herchel-Bulkely model: As well as the Casseau-Yasuda model: From these models the flow index (n) and the zero shear viscosity (h 0 ) can be determined for each grease at the three test temperatures. To get more insight in how the different thickener structures behave, rheo-microscopy analysis of the greases was performed using a TA Instruments Discovery HR10 fitted with a TA Instruments Modular Microscope Accessory capturing images in cross polarisation mode with a Nikon MRH08430 40X (NA 0.60) objective and a THORLABS M470L5 470 nm 186 24th International Colloquium Tribology - January 2024 Panta Rei: Everything Flows light source. Temperature was controlled using a TA Instruments Upper Peltier Plate and was set to 25-°C throughout testing. The rheometer was equipped with a 40 mm diameter flat plate geometry with a glass lower plate. The gap size was 200 µm. Each grease was subjected to subsequent shear rate peak holds (0.1/ 1/ 10/ 100/ 500/ 1000 s -1 ) for 30 s with data sampled at an interval of 1 s pt -1 and a 30 s equilibration period (0 s -1 ) allowed between each peak hold. 4. Results and Discussion Combining all the results in a spiderweb diagram makes it easy to visualize the difference between the different greases. Figure 1 shows the comparison for the experiments at 20-°C. The results show clear differences between the different greases, indicating that the thickener plays a major role in the flow behavior of the greases as both the NLGI grade and oil composition are almost identical. Figure 1: Comparison of rheological test results at 20-°C. Experiments with the rheo-micrsocope have shed some light on the different nature of the thickeners as can be seen in the example in figure 2, which compares a lithium complex greases (left) with a polypropylene grease (right). Figure 2: Comparison of thickener structures from lithium complex (left) and polypropylene (right) greases. 5. Conclusion This study has presented controlled laboratory experiments which show that the thickener in a lubricating grease can play a significant role in the flow behavior of the grease. These results support the experience from the field which has shown large differences in the pumpability of NLGI 2 greases with different thickener types. Acknowledgement The authors would like to thank Anurag Singh at Axel Christiernsson International for performing the rheological testing on the greases. References [1] Sisko, A.W. The Flow of Lubricating Greases. Ind. Eng. Chem., 1958, 50, 1789-1792 (https: / / doi.org/ 10.1021/ ie50588a042). [2] Personal communication, Roger Persson, Volvo Group. [3] Westerberg, L.G.; Lundström, T.S.; Höglund, E.; Lugt, P.M. Investigation of Grease Flow in a Rectangular Channel Including Wall Slip Effects Using Microparticle Image Velocimetry. Tribology Transactions, 2010, 53: 4, 600-609. (https: / / doi.org/ 10.1080/ 10402001003 605566). [4] Farré-Lladós, J.; Westerberg, L.G.; Casals-Terré, J.; Leckner, J.; Westbroek, R. On the Flow Dynamics of Polymer Greases. Lubricants, 2022, 10, 66 (https: / / doi. org/ 10.3390/ lubricants10040066). [5] Lugt, P. M., Velickov, S., Tripp, J. H. (2009), On the Chaotic Behaviour of Grease Lubrication in Rolling Bearings, Tribology Transactions, 2009, 52, 581-590. (https: / / doi.org/ 10.1080/ 10402000902825713). 24th International Colloquium Tribology - January 2024 187 Enhancing Understanding of Grease-Retention and Lubrication-Mechanisms of Oscillating Sliding Contacts with Long Stroke Lengths Andreas Keller 1* , Markus Grebe 1* , 1 Hochschule Mannheim/ Kompetenzzentrum Tribologie, Mannheim, Deutschland * Corresponding author: andreas.keller@hs-mannheim.de 1. Introduction Modern automotive tribosystems face strict demands for performance, cost-effectiveness, and energy efficiency. Many of these systems, like small worm gear drives, linear guides and actuators use lifetime lubrication with grease to enable compact, low-maintenance designs. However, as friction partners displace grease from the contact area, particularly in systems with long displacement distances, the risk of dry running and subsequent system failure is a constant problem. The lubrication’s success hinges on the ability of displaced grease to return to the contact region, a complex process influenced by grease’s rheological properties and chemical composition. Practice shows that current standardized tribological and rheological testing methods do not permit a sufficient prediction of the suitability of lubricants for lifetime-lubricated linear plain bearings and lead screw drives. 2. Current State of Tribological Testing of Greases There is already a great amount of knowledge and experience available to evaluate the suitability of greases for lifelong lubrication in rolling bearings. Common testing methods are described in DIN51819 ( FE8) and DIN51821 ( FE9 ) . Most literature search results show a focus on rolling bearing lubrication and related grease research areas, including investigations into thickener structural degradation, chemical advancements in additives and thickeners. These include, for example, the structural degradation of the thickener [1] [2], the thickener influence on lubrication state [3] and the impact of thickener type and base oil viscosity on lubricant film height [4]. However, research on tribosystems involving grease movement over larger distances under oscillating sliding friction is still limited. For instance, when grease adheres to a lead screw nut and is displaced to the ends of the spindle, relying solely on the flow of released oil for lubrication upon the nut’s return is inadequate (see Figure 1). In the absence of mechanisms that facilitate the “return transport” of the grease, the stickiness of the lubricating grease becomes crucial. It helps retain lubricant at the friction point or minimizes the amount of displaced lubricant. Currently, there is no standardized test-method for these kinds of load-collectives and lubrication conditions. The standard test for grease classification in tribology is the 4 ball method, described in ASTM D2596 and ASTM D2266 and their counterpart DIN variants (DIN 51 350/ 4 and / 5). Obviously, these test-methods are very far away from any real-life application and cannot sufficiently recreate those conditions, as contact pressures and speeds are too high and grease/ oil can easily reflow from the sides. Figure 1 Schematic depiction of grease displacement in lead screw nut 3. Current State of Rheological Testing of Greases Lubricating greases are primarily characterized by their NL- GI-Class, which describes their consistency properties. The NLGI class is determined using the ASTM D217 “cone penetration of lubricating grease” test at 25°C. Other common rheological characterization methods use: • Rotational viscometry/ rheometry with a cone-and-plate or parallel-plate setup, which measures viscosity as a function of shear rate, offering insights into shear resistance, flow properties, and linear viscoelastic properties like storage and loss moduli as well as other factors like thixotropy and creep behavior • Thermal Analysis (Differential Scanning Calorimetry - DSC), identifying phase transitions like melting or softening across temperature ranges. These methods provide crucial data on how greases behave under various conditions and might offer a better understanding of grease mobility compared to standard NLGI classification alone. Additional major grease characteristics might be oil release and stickiness/ tackiness. If grease is pushed out of a friction point by the movement of the friction partners, then the oil release capacity is one of the factors that determine how much lubricant remains in the tribological zone. The oil release capacity is usually determined according to DIN 51817. As mentioned before, part of the grease might return due to tackiness when the friction partner resumes the original position. Achante et al [5] were the first to investigate the tackiness of greases in a systematic manner following the tackiness tests for pressure sensitive adhesives in ASTM D2979. In the case of tackifier-free greases, they were able to demonstrate that the thickener itself has a decisive influence on the 188 24th International Colloquium Tribology - January 2024 Enhancing Understanding of Grease-Retention and Lubrication-Mechanisms of Oscillating Sliding Contacts with Long Stroke Lengths stickiness of greases. A correlation of stickiness and friction coefficient showed a weak correlation for sliding friction (plate-cylinder geometry) and a strong correlation for rolling friction (plate-ball geometry) was also shown by E.P. Georgiou et. al. [6]. 4. Working hypothesis, research approach and conclusion Our working hypothesis is that the limited service life of linear plain bearings and spindle drives can be explained by the depletion of grease from the contact zone. When lifetime-lubricated systems are assembled, sufficient grease is available in the contact zone at the beginning. Due to the component-specific movement of the friction partners, the grease is displaced over long distances from the actual tribological zone. If it does not return to the loaded contact after stressing, it can no longer provide its function as a lubricant reservoir. This is presumably the case if a lubricating grease has a low stickiness and therefore does not adhere to the moving mating body, or if a lubricating grease “stiffens” to quickly and therefore remains in the edge zone after displacement. Without additional supply of grease, then the contact point will become depleted of lubricant over time. From this working hypothesis, it can be concluded that, on the one hand, the stickiness, viscoelasticity and oil release properties of lubricating greases must be characterized in a suitable manner and, on the other hand, both the time dependence of the lubricating grease distribution and friction and wear in test configurations with translational sliding at low frequencies with long strokes and must be determined. The correlation of all results should therfore allow the formulation of a robust and meaningful screening test strategy to derive the suitability of greases for lifetime lubricated linear sliding systems. We propose the following composition of rheology and tribometry: Regarding rheology, greases should be characterized by determining viscosity curves following DIN 51810-1 and viscoelasticity (yield point, shear modulus, loss modulus) according to DIN 51810-2. In addition, stickiness and oil release capacity are to be determined. There is currently no standardized test method for determining the tackiness of lubricating greases. Here, it makes sense to develop and evaluate a rheological measurement method based on the known publications as described by [5] [6]. There is a standard for the evaluation of the oil release capacity; however, strictly speaking, DIN 51817 only applies to oil separation under static conditions (storage conditions) and not tribological induced stress. Therefore, more practical approaches might be necessary. Regarding tribometry the distribution of lubricating greases is to be recorded optically in oscillating sliding friction tribometers capable of long stroke oscillation to provoke “pushout” of grease and subsequent starvation. In a first step this can be done with a simple category 5 model test setup which would allow easy correlation of qualitative and quantitative grease distribution and friction and wear measurements (see figure 2). Figure 2 Depiction of pin on disc testing with continuous optical documentation of grease distribution In a second step, results from the tribometric model testing and advanced rheology have to be confirmed by Cat. IV component testing for example by testing application relevant trapezoidal screw drive assemblys or linear plain bearings in a suitable test setup. Using this testing methodology together with a specifically defined and produced set of model greases should yield further understanding on the behavior of greases in long stroke sliding contacts and their dependency on rheological characteristics. Ultimately this should improve the ability of lubricant producers to tailor their grease specifically for these kinds of applications and industrial end users to design their systems towards longer lifetime and/ or higher load capacity. References [1] M. A. Delgado, C. Valencia, M. C. Sanchez, J. M. Franco,* and C. Gallegos: Influence of Soap Concentration and Oil Viscosity on the Rheology and Microstructure of Lubricating Greases; Ind. Eng. Chem. Res. 2006, 45, 1902-1910. [2] Grebe, M; Ruland, M: .Influence of mechanical, thermal, oxidative and catalytic processes on the thickener structure and thus on the service life of rolling bearings; Lubricants 2022, 10(5), 77; https: / / doi.org/ 10.3390/ lubricants10050077; [3] P. M. Cann; Grease Degradation in a Bearing Simulation Device; Tribol. Int., 39, 1698 - 1706 (2006) [4] B. Vengudusamy; C. Enekes; R. Spallek: On the film forming and friction behaviour of greases in rolling/ sliding contacts; Tribology International, Volume 129, pages 323-337, 2019 [5] S. Achanta; M. Jungk; D. Drees: Characterization of cohesion, adhesion, and tackiness of lubricating greases using approach-retraction experiments; Tribology International; 44 (10), 1127 - 1133, 2011. [6] E.P. Georgiou, D. Drees, M. De Bilde, M. Feltman, M. Anderson; Grease Adhesion and Tackiness: Do They Influence friction? ; NLGI Spokesman, 2020, 84(4), 20 - 25. 24th International Colloquium Tribology - January 2024 189 Correlation of MTM Striebeck Curves with Efficiency Data for Predictive Analysis of Coaxial EV Gearbox Performance An Experimental Study Dmitriy Shakhvorostov 1* , Mirjam Bäse 2 1 Oil Additives, Evonik Operations GmbH, Kirschenallee 64293 Darmstadt, Germany 2 Magna Powertrain GmbH & Co KG, Plant Lannach, Lannach, Austria * Corresponding author: dmitriy.shakhvorostov@evonik.com 1. Introduction The transition towards fully electric or hybrid powertrains in passenger vehicles has become the preferred solution to meet greenhouse gas emission reduction targets [1]. Ongoing optimization efforts are focused on achieving maximum efficiency and durability for all the components, including the mechanical transmission, electric motor, and power electronics. Improving efficiency not only reduces CO 2 emissions during the vehicle‘s use phase, but also extends the range of electric vehicles. Additionally, it can help reduce costs and CO 2 emissions associated with production, such as by reducing the size of the battery. This research study investigates the correlation between Mini Traction Machine (MTM) Stribeck curves and efficiency data obtained from a coaxial EV gearbox, aiming to predict and enhance the overall efficiency of the transmission system. By employing both ball-on-disk and low-pressure barrel-on-disk setups, the Stribeck curves were measured for eight different lubricating fluids. Notably, the study revealed that the low-pressure barrel-ondisk contact configuration provided a better correlation between the MTM Stribeck curves and efficiency data compared to the ball-on-disk contact configuration. The obtained data was then compared to efficiency measurements from the Worldwide Harmonized Light Duty Testing Procedure (WLTP) to establish correlation using the Pearson coefficient. 2. Results The results showed a significant correlation, enabling the utilization of the MTM methodology for predicting the efficiency of the transmission in a WLTP driving cycle. Additionally, specific operating points in terms of torque and speed demonstrated satisfactory correlation, although a few operating points did not exhibit the same level of correlation. The temperatures utilized in the MTM tests were precisely aligned with the operating temperatures within the transmission. The precision of the transmission tests and the MTM predictions, calibrated using five fluids with established correlation, were within the range of +- 0.02%. Notably, the correlation performed well at 40-°C operating temperature (see Fig 1), while the correlation at 80-°C operating temperature was relatively less robust. Fig. 1 Result of correlation of efficiency improvement in electric vehicle transmission in WLTP simulated cycle to a relevant domain averaged friction coefficient in a MTM with low pressure barrel-on-disk contact. 3. Conclusion While our study shows that it is useful to apply MTM (Fig.-1) for additives and base oils selection for transmission efficiency optimization, we could identify limits of the methodology at conditions resembling an intensive mixed lubrication regime, where MTM specimen surfaces do not replicate the surfaces in the real application (transmission). Special care needs to be taken for proper conditions selection (unlike conditions selection approach seen in the literature [2-3]) for a meaningful screening. References [1] UNECE Vehicle Regulations. [Online] https: / / unece. org/ transport/ vehicle-regulations. [2] Cañellas, G.; Emeric, A.; Combarros, M.; Navarro, A.; Beltran, L.; Vilaseca, M.; Vives, J. Tribological Performance of Esters, Friction Modifier and Antiwear Additives for Electric Vehicle Applications. Lubricants 2023, 11, 109. [3] Costello, Michael T. “Effects of basestock and additive chemistry on traction testing.” Tribology Letters 18 (2005): 91-97. 24th International Colloquium Tribology - January 2024 191 LIF Signal Calibration for Bench Simulating Experiments and Engine Oil Film Thickness Investigations Polychronis S. Dellis 1* 1 National Technical University of Athens, School of Mechanical Engineering, Mechanical Design and Automatic Control Section, Athens, Greece * Corresponding author: pdellis@mail.ntua.gr 1. Introduction The Laser Induced Fluorescence (LIF) measurement technique when carefully applied to engine cylinder liners or flat reciprocating simulation mechanisms is able to produce oil film measurements data that can, either be compared with direct simultaneous capacitance oil film thickness (OFT) measurements or when calibrated appropriately, provide a picture of the oil film in between and under the piston-rings. For the purpose of calibration, insitu dynamic measurements or static calibration techniques have been applied in the past [1, 2]. Bench calibration being less prone to thickness errors as its concept relies on a micrometer, meticulously modified to be used as the measurement reference, whereas dynamic calibration offers significant advantages, as it takes into account local temperatures at the measurement point, lubricant degradation, i.e. issues that affect fluorescence [3]. This study is focused on presenting the two calibration methods, propose a new dynamic method and how is temperature affecting the calibration coefficient. Eventually, light is shed on the inconsistencies found between the proposed methods. 2. Background Engine oil consumption reduction is an ever-increasing trend that requires controlling of the OFT in between and under the piston rings. The combination of fibre optics and LIF for measuring OFT is a non-intrusive method based on the emission of photons during the relaxation process following the initial excitation of the substance molecules with a light source,that is the blue laser light. Spectral discrimination, achieved by appropriate optical filtering, is giving the intensity of the fluorescence that is related to the OFT [3]. 2.1 Experimental Techniques - Previous Applications of Static and Dynamic Calibration Ring profile fitting with known film thickness taken from the capacitance measurements was used to calibrate the LIF signal. Temperature can bring about significant changes in the viscosity of engine oils and their fluorescence spectrum. Previous attempts were performed to achieve static calibration with a high-resolution micrometer [2, 4]. The optical fibre was installed to the modified micrometer anvil that accommodates the fibre and supplies a continuous oil flow between the anvil and the micrometer spindle. Thus, the OFT flowing through the interrogation zone could be read from the micrometer scale respectively. The area of interest was fully flooded with oil as it is pumped from a small reservoir, a condition which is important for very thin oil films ranging from 0 to 5 μm, where the photobleaching effects might have a more pronounced effect in the quantum yield [2]. Brown et al [5], proposed an in-situ calibration method with grooves etched on the piston-ring surface. The static calibration method showed very low repeatability for the data points corresponding to an OFT of 10 μm. Obtaining calibration coefficients of high accuracy in the region 0 to 10 μm is vital for improving the accuracy of the LIF system [2]. Experiments with an immersed silica block incorporating grooves of known depth overcame this deficit [2]. Nakayama et al [6], measured OFT on engine bearings using LIF and presented a static calibration apparatus. The researchers also presented a dynamic calibration technique. Figure 1 shows the device that used a shaft with two thickness gauges pasted on its surface having a known thickness of 48 μm and Figure 2 shows the OFT tests. Figure 1: Dynamic calibration using thickness gauges [6] Figure 2: OFT by dynamic calibration method [6] 2.2 Dynamic Calibration with Grooves on Piston-Ring For the purpose of dynamic calibration, a piston-ring specimen was modified to be used for the experimental set-up of the simulating single-ring test rig according to the specifications of these type of measurements (microns range, flat reciprocating liner and steady piston-ring). The dynamic calibration is based on etching a groove or series of grooves of known depth on the surface where the optical fibre travels above. The best surface finish possible was desired to be achieved with the spark eroding machine (Electro-Discharge Machining, EDM) with the groove depth ideally being between 15 and 25 μm and the surface roughness R a <1 μm [7]. In Figure 3, the first attempt shows the groove depth from 0 to 65 μm. 192 24th International Colloquium Tribology - January 2024 LIF Signal Calibration for Bench Simulating Experiments and Engine Oil Film Thickness Investigations Figure 3: First unsuccessful attempt to etch groove on the surface of the piston-ring [7] The electrode used for spark eroding was specially manufactured Cu-Cr-Zr and to enhance the accuracy of the manufacturing process the technique was applied for longer periods of time to minimize the risk of failed attempts. Figure 4 shows the measured 3-D surface profile of the groove. Figure 4: 3-D meshed axonometric of groove [7] Results were derived from tests at an average oil temperature of 40-°C. Test oil coded 3B was supplied by CASTROL, the properties of which are shown in Table 1. Laser power was set at 50 mW, 0.39 kV photomultiplier (PMT) voltage. Table 1: Tested lubricant properties [7] CASTROL Code 003B SAE Grade 0W-30 Viscosity Index 182 V 100 (cSt) 12.16 V 40 (cSt) 68.93 HTHS (mPa s) 3.30 Polymer Castrol Code A Base Fluid Poly alpha olefin V100, V40: Kinematic Viscosity at 100- °C, 40- °C HTHS: High Temp High Shear viscosity The dynamic coefficient is derived when LIF and groove data are directly superimposed. Figure 5 shows a schematic of the groove area over which the optical fibre travels. Five surface roughness profiles were averaged so that a mean derived value would correspond to the areas over which the fibre passes. In Figure 6 the LIF points vs the averaged groove data are superimposed. After this representation, the dynamic calibration coefficients were derived and a temperature parametric study followed. Figure 5: The groove with the blue - highlighted line shows the area over which the optical fibre travels [7] Figure 6: Matching of groove and averaged LIF data [7] The shortest stroke available was set (5 mm) for the experiments so that as many data as possible could be acquired. When the optical fibre travels on top of the specially machined groove, the highest number of data points will be acquired from the data acquisition system over the groove width measured at 1,525 mm. The averaged data will represent the highest accuracy possible. 3. Conclusions - Matching LIF and surface roughness data is the key point of the dynamic calibration technique. - A higher calibration coefficient is derived for high temperature parametric testing. - Considerations such as background noise, groove flooding, PMT voltage, absolute OFT (less than 5 μm) and OFT to voltage ratio affect the measurements’ accuracy. References [1] Arcoumanis C., Duszynski M., Lindenkamp H. and Preston H., “Measurements of oil film thickness in the cylinder of a firing diesel engine using LIF”, SAE 982435, 1998. [2] Duszynski M., “Measurement of Lubricant Film Thickness in Reciprocating Engines”, PhD thesis, Imperial College of Science, Technology and Medicine, 1999. [3] Dellis, P. “Aspects of lubrication in piston cylinder assemblies”, PhD Thesis, Mechanical Engineering Department, Imperial College London, April 2005. [4] Pyke E. A., “Investigation of Piston Ring Lubrication Using Laser Induced Fluorescence”, PhD thesis, Imperial College of Science, Technology and Medicine, March 2000. [5] Brown M. A., McCann H. and Thompson D. M., “Characterisation of the Oil Film Behaviour Between the Liner and Piston of a Heavy-Duty Diesel Engine”, SAE Paper 932784, 1993. [6] Nakayama K., Morio I., Katagiri T. and Okamoto Y., “A Study for Measurement of Oil Film Thickness on Engine Bearing by using Laser Induced Fluorescence (LIF) Method”, Central R&D, Daido Metal Co., Ltd, SAE 2003-01-0243, 2003. [7] Dellis P., “An Attempt to Calibrate the Laser Induced Fluorescence Signal used for Oil Film Thickness Measurements in Simulating Test Rigs”, Tribology in Industry, 37 No. 4, 525-538, 2015. 24th International Colloquium Tribology - January 2024 193 Digital Twin Parametrization of a Roller Bearing based on Ultrasonic Film Thickness Measurement Fabio Tatzgern, Boris Gigov, Michal Kracalik, Georg Vorlaufer, Markus Varga * AC2T research GmbH, Wiener Neustadt, Austria * Corresponding author: E-mail (optional) 1. Introduction Lubricant film thickness is a crucial parameter in machine elements to ensure optimal and safe operation [1]. Measuring the film thickness in-situ allows the development of an accurate Digital Twin predicting the film thickness under real operational conditions and opens up new possibilities for efficiency optimization and extension of lifetime. In this work, the Digital Twin is based on the empirical Hamrock & Dowson (H&D) model [2], which has long been used to estimate the film thickness to a reasonable degree of accuracy, but relies on a complex set of empirical parameters, which cannot be determined by direct measurements. Therefore, in this work we base a Digital Twin on the H&D model and setup a test rig with novel ultrasonic sensors to measure the film thickness. With this input we can parametrize the used lubricant for further use in Digital Twins. 2. Experimental The work was carried out on a test rig applying industrial rolling element bearings following the DIN51350-6 standard. It comprises a pair of SKF32008X rolling element bearings from SKF that are loaded axially (Fig. 1). The bearings were lubricated with two greases, namely one with mineral base (NLGI 2) and one with synthetic base (NLGI 2.5). Lithium soap as main additive was used in both. Figure 1: Test setup and control units. A parameter range from 2.3-6.9-kN load and 1000-2000-rpm rotational frequency were investigated in a full matrix approach. Additional to load and torque and temperature sensors, the test rig is equipped with a sensor based on ultrasound reflectometry for in-situ lubricant film thickness measurement. A customized Lead Metanobiate transducer with 10 MHz central frequency bonded on the outside of the bearing outer raceway was used. 3. Model The goal of the Digital Twin is to predict the film thickness based on the dynamical loading of the bearings with the applied grease. In our test study the system dynamics is fully characterized by the rotational speed w 21 of a roller, F raceway - the force between outer raceway and the roller and the temperature T (Fig. 2). Figure 2: Loading of a single roller. In this work, only the basic form of the H&D model is retained (Eq. 1) and the model parameters are based on real-time data collected at the system level, such as load and speed. This allows the adaption of the H&D model to a wide range of operating conditions and types of lubricants for improved accuracy in prediction. (1) Equation 1: Hamrock & Dowson formula for film thickness [2]. In the next step, the exponential values in the model Eq. 1 are chosen to be variables and then fit to the film thickness measurements as: (2) Equation 2: where a, b and c are the parameters of interest and r serves as a parameter for the offset. 194 24th International Colloquium Tribology - January 2024 Digital Twin Parametrization of a Roller Bearing based on Ultrasonic Film Thickness Measurement 4. Results The data from this sensor are used to characterize a given lubricant by determining its specific H&D parameters, which can then be used to calculate film thickness under real load conditions in machinery. This also allows the prediction and simulation of other parameters, such as slip, torque, viscosity of the lubricant, and many more, to further improve the tribological efficiency of the machine element. Fig. 3 shows exemplarily the measured values during the run parameter set of the mineral base grease. The film thickness results show increasing film thickness with increasing velocity, and contrary, decreasing film thickness with increasing load level. Naturally, the measured torque increases with increasing load, while the temperature overall increases with increasing test time. Figure 3: Results of the mineral based grease. Several regression models were applied to the obtained measurements, to identify the parameters a, b, c of Eq. 2 and so parametrize the Digital Twin for the film thickness of the rolling element bearing with the given lubricant. Fig. 4 shows the results of linear regression of the parameters for the mineral base grease. It is clearly visible, the mean film thickness can be predicted accurately based on the loading conditions applied to the bearing. Hence, the H&D Digital Twin model for the film thickness is fully defined by the parameters a, b, c and those parameters are potentially unique and fully define the grease under different operating conditions. Figure 4: Linear regression (red) of the measured film thickness (black) of the mineral base grease. 5. Conclusions Digital Twins of tribosystems are a useful tool to understand the influence of load and system parameters onto critical tribological conditions, such as the minimum film thickness of a lubricated contact. Therefore, in this work, we have established a Digital Twin of the film thickness of a rolling element bearing with grease lubrication. Ultrasonic film thickness measurements based on reflectometry allowed us for the determination of the real film thickness in-situ. These results were applied to a Hamrock & Dowson prediction of the film thickness. Regression models allowed us to describe the system with three parameters, which abled us to uniquely and fully define the grease behaviour under the different operating conditions. References [1] M. Schirru, M. Varga: A Review of Ultrasonic Reflectometry for the Physical Characterization of Lubricated Tribological Contacts: History, Methods, Devices, and Technological Trends, Tribology Letters 70 (2022), 129. [2] Engineering Tribology. In Engineering Tribology (Fourth Edition); Stachowiak, G.W., Batchelor, A.W., Eds.; Butterworth-Heinemann: Boston, 2013; ISBN 978-0-12-397047-3. 24th International Colloquium Tribology - January 2024 195 Oil Aging on a Test Rig to Introduce Sustainable Lubricants in Electric Vehicle Transmissions Timo Koenig 1* , Marco Kohnle 1 , Luca Cadau 1 , Lukas Steidle 1 , Didem Cansu Gueney 2 , Katharina Weber 2 , Joachim Albrecht 2 , Markus Kley 1 1 Aalen University, Institute of Drive Technology Aalen IAA, Aalen, Germany 2 Aalen University, Research Institute for Innovative Surfaces FINO, Aalen, Germany * Corresponding author: timo.koenig@hs-aalen.de 1. Introduction There is currently a growing demand for electric vehicles. In order to make them even more environmentally friendly, there is the need to replace the conventional transmission oil with sustainable alternatives that must have at least similar properties. The demand for sustainable lubricants is increasing due to the finite fossil oil resources and the release of conventional lubricants into the environment. The sustainable lubricants, which are biodegradable, could reduce future damage when used in transmissions. This paper compares the properties of sustainable oils at different stages of aging. The properties of the oils are measured using rheological investigations. An innovation of this publication is that the aging of the sustainable oil is achieved by an standardized oxidation test, the so-called TOST test, according to DIN EN ISO 4263-1 [1] and real test rig experiments with an electric vehicle transmission. The aged oils are compared with the new oil in terms of sustainability to allow a better understanding of the aging behaviour. 2. State of the Art This publication shows a comparison between new, oxidized and aged oil in a transmission. As a reference for aging of lubricants, a publication is used which shows a concept for comparison of new and aged lubricants in transmissions and a method for oil aging [2]. The focus here is to highlight the differences between the aged oils in comparison to the new oil. High torques and speeds as well as a lot of other factors are among the biggest stress factors that can affect the performance of lubricants in electric vehicles [3]. For this reason, it is necessary to develop and apply special lubricants that are adapted to the conditions and requirements of electric vehicles. For most applications, conventional, mineral oil-based lubricants are still used. In order to meet the sustainability goals, also in the area of lubricants, it is essential to use sustainable oils. [2] These oils generally must have a suitable viscosity, be oxidation-resistant and thermally stable. High load capacity and corrosion protection are also requirements as well as the friction coefficient. These are crucial points for all lubricants, regardless of the sustainability. [2] It is a well-known point that the properties of lubricants deteriorate with increasing operating time. The aging behavior of a biodegradable lubricant, in this application a polyalkylene glycol-containing lubricant with an amine phosphate as an additive to reduce friction, is therefore to be investigated. The requirements of the sustainable lubricant are adapted to those of the conventional oil used so far. It can be emphasized that in the literature a direct comparison between new, oxidized oil and oil aged in a transmission has been made only to a limited extent. 3. Oil aging process and oil analysis Two different approaches are used for the aging of the sustainable oil in this paper. The aged lubricants are compared with each other as well as with the new oil. The first test is a standardized oxidation test, the so-called TOST test with a total running time of 312 h, and the second test is represented by realistic aging on a test rig in a transmission. For the tests on the drive test rig, a load collective is defined to reflect the real operating conditions of the transmission, so that the oils can be aged on the test rig under conditions that are as real as possible (Cf. with figure 1). The load points are randomized but with a repeating sequence so that the lubricant can be aged for 250 h in total. [2] Figure 1: Defined load collective for oil aging [2] The aged lubricants, each with an almost identical aging time, and the new lubricant are analyzed with different rheological tests at different temperatures. The conducted rheological experiments are described in more detail in a previous paper [2]. In the following diagrams, new oil is denoted in blue (- -), oxidized oil in red (- -), and real aged oil in green (- -). The curves shown are each calculated from three individually performed tests. The flow behavior of the oils is visualized in figure 2. The viscosity at constant temperature T is determined as a function of shear rate γ. There is a clear contrast between oxidized and real aged oil in a transmission. The analyzed oils show a shear-thinning behavior at 20-°C and an ideal viscous behavior at 60-°C. The viscosity of real aged oil remains significantly lower than that of oxidized oil throughout. 196 24th International Colloquium Tribology - January 2024 Oil Aging on a Test Rig to Introduce Sustainable Lubricantsin Electric Vehicle Transmissions Figure 2: Flow Behavior Figure 3 shows the temperature behavior of the analyzed sustainable oil. Here, the temperature is increased at a constant shear rate (100 1/ s) and the viscosity h is measured at different temperatures T. The curves of the oxidized and the new oil are almost identical, whereas the curve of the real aged oil shows lower viscosities over the temperatures. However, the temperature dependent behavior is identical in all tests. Figure 3: Temperature Behavior Figure 4 illustrates the jump test. The study investigates the regeneration and recovery of the structure after a sudden, strong shear loading of the specimen. The findings are presented as a function of the viscosity h over the time t. Here, it is also shown that the real aged oil has a lower viscosity, although the regeneration can be evaluated equally in comparison to all samples. Figure 4: Jump Test Figure 5 presents the frequency test. The time-dependent behavior of the oil in the non-destructive range is analyzed by changing the oscillation frequency while keeping the amplitude constant. The storage moduli G 9 and the loss moduli G 0 are depicted in relation to the angular frequency w. All samples exhibit nearly the same intersection point. The intersection points of G 9 and G 0 shift to the right at higher temperatures. Figure 5: Frequency Tests at 20-°C and 60-°C As can be seen in the figures, there are notable differences between the real aging and the standardized TOST test. The results of the laboratory oxidation tests do not fully match the results of the rig tests for the sustainable oil. In the real aging curves, the viscosity is significantly lower than in the calculated curves in new condition and after the oxidation test. This decrease in viscosity can possibly be attributed to moisture absorption during rig testing. 4. Conclusion In summary, differences can be observed in rheological tests according to different aging methods. Therefore, the TOST test may not adequately represent the real aging of the sustainable oils as reflected in the differences in viscosity, and new laboratory test procedures could be adjusted for the new generation of sustainable gear oils. References [1] DIN EN ISO 4263-1: 2004. Mineralölerzeugnisse und verwandte Produkte - Bestimmung des Alterungsverhaltens von inhibierten Ölen und Flüssigkeiten - TOST Verfahren - Teil-1: Verfahren für Mineralöle [2] König, T., Cadau, L., Steidle, L., Güney, D. C., Albrecht, J., Weber, K. and Kley, M.: A concept for comparison of new and aged lubricants in transmissions of electric vehicles and a method of oil aging on a test rig. Forschung im Ingenieurwesen (2023), doi: 10.1007/ s10010-023-00705-3. [3] Aguilar-Rosas, O. A., Farfan-Cabrera, L. I., Erdemir, A. and Cao-Romero-Gallegos, J. A.: Electrified fourball testing - A potential alternative for assessing lubricants (E-fluids) for electric vehicles. Wear 522 (2023), pp.-204676, doi: 10.1016/ j.wear.2023.204676. 24th International Colloquium Tribology - January 2024 197 Copper Wire Resistance Corrosion Test for Assessing Potential Fluids as E-Thermal Fluids in BEVs Immersion Cooling Applications Bernardo Tormos 1* , Vicente Bermúdez 1 , Jorge Alvis-Sanchez 1 , Leonardo Farfan-Cabrera 2 1 Universitat Politècnica de València - CMT - Clean Mobility & Thermofluids, Valencia, Spain 2 Tecnológico de Monterrey - Escuela de Ingeniería y Ciencias, Puebla, Mexico. * Corresponding author: E-mail (betormos@mot.upv.es) 1. Introduction The electrification of the transport sector worldwide is paving the way for Battery Electric Vehicles (BEVs) to become a prominent part of the modern automotive landscape. To enhance BEVs’ efficiency, safety, and lifespan, immersion cooling is emerging as a promising solution to manage the thermal loads of high-capacity batteries (enabling fast charging), ensuring safe and optimal performance [1] . For such cooling systems, selecting an appropriate dielectric fluid is critical to prevent adverse effects on the immersed components, particularly copper components, commonly found in electric applications. While assessing various aspects of potential E-Thermal fluids, material compatibility holds significant importance, primarily owing to the anticipated extended lifespan of these fluids (fill for life). Consequently, conducting compatibility assessment tests becomes imperative, and copper being the primary conductor of electricity in this application, both the fluid and the copper materials must not negatively affect each other. The Copper Wire Resistance Corrosion Test (CWRCT) method has emerged as an important tool for assessing the compatibility of fluids with copper materials [2] . This study presents a CWRCT designed to assess different fluids as potential E-thermal fluids for BEV: a base stock Polyalphaolefin (PAO), a base stock API GIII, a synthetic base stock Diester, an electrical insulating oil (TRANSF), and a fully formulated dielectric coolant (AMPC). The experiment was conducted in collaboration with the Tecnológico de Monterrey University (TEC), where the PAO, the insulating oil, and the fully formulated oil were tested at their facilities in Puebla, Mexico, and the Diester, the API GIII, and another batch of PAO were also tested at CMT - Clean Mobility & Thermofluids research center at Universitat Politècnica de València (UPV), Spain. 2. Materials and methods The experimental setup consisted of placing 500 mL of each test fluid in a 1 L beaker. A 1 m length of 64 microns diameter (42 American Wire Gauge [AWG] caliber) copper wire was submerged in each fluid, while another wire was placed above the fluid to evaluate both oil and vapor phases. The test was conducted at a working temperature of 130°C (± 2ºC) for 336 hours. A DDM (Digital Data Multimeter) was employed to measure the resistance of the copper wires with a 1mA direct current. Figure 1. CWRCT in-house TEC setup The change (increase) in the resistance is an indicator of the corrosiveness of the fluid to the copper due to removal of material, since the resistance of a wire is provided by the formula: (1) Where R is the resistance of the wire, ρ is the resistivity of copper, L is the length of the wire, and d is the diameter of the wire. If the fluid is corrosive to the copper, the wire’s cross-sectional area will decrease, increasing the overall resistance. 3. Results and discussion Qualitative (SEM) and quantitative (resistance measurements and ICP measurements) aspects are considered in the results. 3.1 Resistance measurements The measurements obtained at Tecnológico de Monterrey showed that the wire in the PAO vapour phase failed after 140 hours, while the rest increased their resistance by less than 1,5%. Figure 2. Resistance measurements at Tecnológico de Monterrey (TEC). 198 24th International Colloquium Tribology - January 2024 Copper Wire Resistance Corrosion Test for Assessing Potential Fluids as E-Thermal Fluids in BEVs Immersion Cooling Applications Oscillations observed in the resistance measurements appear as a consequence of the high fluctuations in ambient temperature between day and night; taking into account that heating plates were used to control temperature; therefore, there is a higher difference in resistance between the vapor and oil liquid phases. Measurements obtained at Universitat Politècnica de Valencia (Figure 3), where a thermal bath was used in the setup, show more stable conditions (no oscillations) and smaller differences between vapor and oil phases. Figure 3. Resistance measurements. CMT With the exception of the wires exposed to Diester and PAO (TEC) in the vapor phase, most wires did not exhibit a substantial increase in resistance over the course of the experiment. When wire failure occurred, the presence of green droplets around the wire was observed (Figure 4). This phenomenon is often indicative of the generation of Copper (II) hydroxide (Cu(OH)2) as a corrosion byproduct, typically due to the presence of moisture. Figure 4. Green droplets were found in the PAO (TEC) and Diester wires in vapor phase. Table 1. SEM Element analysis FLUID Cu C O Si S PAO OIL TEC 65,45 27,69 6,71 - 0,15 PAO VAP TEC 30,65 43,19 25,84 0,18 0,07 AMPC OIL 67,49 25,95 6,38 0,08 - AMPC VAP 55,62 36,38 7,61 - - TRANSF OIL 61,96 26,54 10,76 0,13 0,55 TRANS VAP 66,98 28,84 4,18 - - PAO OIL CMT 72,91 22,83 4,09 0,1 - PAO VAP CMT 65,56 30,86 3,49 0,09 - DIEST OIL 73,32 21,7 4,74 - 0,05 DIEST VAP 34,86 44,68 20,16 0,06 0,17 G_III_OIL 72,66 23,07 4,27 - - G_III_VAP 60,13 33,39 6,4 - 0,07 3.2 SEM Portions of the wires were observed under a Scanning Electron Microscope (SEM) to analyze the effects of corrosion and quantify the elements found on the surface. Table 1 shows the material analysis on the surface of the wire and the percentage of elements found. The wires most affected by corrosion were the vapor phase of the PAO (TEC) and the vapor phase of the Diester; both showed a significant decrease in copper found on the surface and high amounts of oxygen. Figure 5 also shows the images of these two wires at 800x magnification and 20kV of power at the SEM. Figure 5. SEM images: a) PAO_VAP_TEC (Left) and b) DIEST_VAP (Right) 4. Conclusions Overall, resistance measurements, green droplets, SEM images, and the surface analysis of the failed copper wires are complementary evidence of corrosion. Ambient conditions might play an important role in the process of corrosion since the PAO only failed in one location. The methods of corrosion might differ, given the exponential failure of the PAO_VAP_TEC and the linear increase in resistance of DIEST_VAP. Further research must be conducted to determine viability of fluids as a E-thermal fluids. Other factors such as thermal and electrical performance, costs, biodegradability, and sustainability have yet to be considered, and they could greatly influence its suitability. 5. Acknowledgements This research was partly funded by the project CIAI- CO/ 2021/ 013 from GVA, Generalitat Valenciana. The authors also want to acknowledge “Programa de Apoyo para la Investigación y Desarrollo” (PAID-01-22) for financing the PhD. Studies of J. Alvis-Sanchez at Universitat Politècnica de València. References [1] Pambudi, N.A.; Sarifudin, A.; Firdaus, R.A.; Ulfa, D.K.; Gandidi, I.M.; Romadhon, R. The immersion cooling technology: Current and future development in energy saving. Alexandria Engineering Journal 2022, 61, 9509-9527. [2] Hunt, Gregory J., Michael P. Gahagan, and Mitchell A. Peplow. “Wire resistance method for measuring the corrosion of copper by lubricating fluids. Lubrication Science 29.4 (2017): 279-290. 24th International Colloquium Tribology - January 2024 199 Shear Stability and Thermal Performance Analysis of Engine Oils for Electric Vehicles Victor Nino, Fabio Alemanno, Deepak Halenahally Veeregowda 1 Ducom Instruments, Global Applications Team, Groningen, Netherlands * Corresponding author: fabio.a@ducom.com 1. Introduction The soaring adoption of electric vehicles (EVs) is steering the automotive industry toward a greener horizon, with profound implications for the lubricants sector. This manuscript explores the potential values of modifying lubricant testing methods to evaluate the behaviour of lubricants in the presence of changing temperature. 2. Materials and Methods The lubricant used in this study are newly formulated EV fluids given by a major lubricant manufacturer. The main properties of the fluids are listed in Table 1. Table 1. Fluids physical properties. Unit A1 B1 C1 Kinematic Viscosity (@ 40 °C) mm²/ s 40 55 18.5 Density kg/ m³ 950 950 950 Flash point °C 200 200 200 The lubricants were tested in a Four Ball Tester according to the ASTM D4172-B test method, and in a KRL Shear Stability Tester according to the CEC L-45-99 test protocol. Both the test method require a precise control of the test temperature during their execution. A modified version of both procedures was also followed. The ASTM D4172-B test method was modified by starting the test at 30 °C (instead of 75 °C as prescribed) and letting the temperature rise independently as a consequence of the friction generated at the sliding contact. The CEC L-45-99 test method was shortened, and divided into segment during which the temperature control was activated to keep the temperature to the prescribed value of 60 °C, to segments in which the temperature control system was deactivated to allow the temperature to rise spontaneously as a result of the friction generated. An example of such a cycle is reported in Figure 1. Figure 1. Temperature profile during the modified KRL test protocol. 3. Results Four Ball Tester Results The Wear Preventive (WP) tests run according to ASTM D 4172-B were analyzed to highlight differences in the lubricants in terms of friction and mean wear scar diameter on the test balls. As shown in Figure 2, the friction coefficient showed by the fluid C1 is slightly higher than the values showed by both A1 and B1, which resulted in similar friction values. Figure 2. Evolution of friction coefficient over time during ASTM D4172-B tests. The mean wear scar diameter values obtained with the three fluids showed a difference comparable to the precision of the measurement system, and were therefore considered not significantly different. Shear Stability and Thermal Performance Analysis of Engine Oils for Electric Vehicles 200 24th International Colloquium Tribology - January 2024 The average friction and wear values obtained with both the standard and the modified ASTM D4172-B test protocol are shown in Figure 3 and Figure 4, respectively. Figure 3. Average friction coefficient measured during both standard and modified ASTM d4172-B tests. Both friction and wear values measured after the modified test protocol were lower than the ones measured after the standard procedure. In particular, fluids A1 and B1 resulted in a wear reduction of about 4%, whereas C1 resulted in a 10% reduction. Figure 4. Mean Wear Scar Diameter (MWSD) measured during both standard and modified ASTM d4172-B tests. Since the temperature of the modified protocol was not controlled, the three fluids resulted in a different final temperature. Figure 5 shows the temperature change from the initial temperature of 30 °C. The temperature change for fluids A1, B1 and C1 were 31, 38 and 30 °C, respectively. Figure 5. Temperature changes with respect to the initial temperature during modified ASTM D4172-B tests. KRL Shear Stability Tester Results The conventional KRL Shear Stability tests according to CEC L-45-99 resulted in a viscosity loss of around 4% for both fluid A1 and B1, while C1 resulted in a 1.7% viscosity loss, as shown in Figure 6. Figure 6. Viscosity measurement before and after the CEC L-45-99 tests. The thermal cycling resulted in 5 cycles of spontaneous heating for each test. Figure 7 compares the average of each of the 5 cycles for each test. The temperature change for fluids A1, B1 and C1 were 18, 22 and 8 °C, respectively. Figure 7. Temperature changes with respect to the initial temperature during modified CEC L-45-99 tests. 4. Conclusions The results shows that fluids with negligible differences in their ASTM D4172-B friction and wear properties, can still be differentiated with respect to their response when one additional degree of freedom (i.e. temperature) is added to the Four Ball test setup. In the KRL test setup, differentiating the lubricants with respect to their response to a thermal stimulus helped in increasing the small differences observed with the conventional measurand (i.e. viscosity loss). The temperature increase in both the Four Ball and KRL Shear Stability tester seems to correlate with the viscosity value, with a direct proportionality between viscosity and temperature increase, whereas an inverse proportionality between viscosity and Four Ball Tester friction coefficient could be observed. 24th International Colloquium Tribology - January 2024 201 Go Greener by In-situ Characterization of Lubricants for Cold Rolling - Droplet Size Distribution and Physical Separation/ Emulsion stability Arnold Uhl 1* , Stefan Küchler 1 , Sylvain Gressier 2 , Titus Sobisch 1 1 LUM GmbH, Berlin, Germany 2 LUM France, Plaisir, France * Corresponding author: info@lum-gmbh.de 1. Introduction A lubricant is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, transporting foreign particles, or heating or cooling the surfaces. Typically, lubricants contain 90% base oil (most often petroleum fractions, so-called mineral oils) and less than 10% additives. Vegetable oils or synthetic liquids such as hydrogenated polyolefins, esters, silicones, fluorocarbons and many others are sometimes used as base oils. Additives deliver reduced friction and wear, increased viscosity, improved viscosity index, resistance to corrosion and oxidation, aging or contamination, etc. [1] There are non-liquid lubricants and dry lubricants available, but they are not subject to this talk. A good lubricant generally possesses the following characteristics: • A high boiling point and low freezing point (in order to stay liquid within a wide range of temperature) • A high viscosity index • Thermal stability • Hydraulic stability • Demulsibility • Corrosion prevention • A high resistance to oxidation. [1] Based on the above, lubricants for cold-rolling are formulated e.g., as oil-in-water (o/ w) emulsions. Instrumental methods for the analytical characterization of emulsion stability/ demulsibility and for particle/ droplet characterization become more and more popular offering a greener approach for the analysis compared to conventional methods based on chemical analysis. This talk presents three case studies, where the patented STEP-Technology ® ( S pace and Time resolved measurement of Extinction Profiles) was applied in combination with accelerated stability/ separation testing. It allows for the quick and reliable characterization of lubricants requiring much less sample volume and avoiding expensive and environmentally unfriendly cleaning of tools and apparatus. 2. STEP-Technology ® STEP-Technology has been developed and patented by LUM GmbH and allows for the in-situ visualization of nanoand microparticle movement using different optical wavelengths, e.g., invisible near infrared (NIR), visible red and blue wavelengths as well as the use of invisible X-radiation. The sample is filled into a closed sample cell of appropriate cell geometry based on sample properties. Then the sample cell is positioned between the light or radiation source and the corresponding detector. Light or X-radiation is sent to the sample simultaneously from top to bottom and the transmitted light or radiation is recorded, again simultaneously from top to bottom at preset time intervals. This can be done either at earth gravity to investigate the real-time separation or at higher gravity for accelerated testing, using relative centrifugal acceleration (RCA) to physically accelerate the particle movement with a predefined multiplicator of earth gravity. The experiments are done either at a constant preset temperature in the range from 4 to 80°C or even in some instruments with a temperature ramp. For further technology details and applications see [2] and the references therein. The three case studies were performed at higher gravity and at a constant temperature, being different for each case. The NIR wavelength 870 nm was used. 3. Case studies 3.1 Accelerated testing vs. conventional The accelerated stability testing of o/ w emulsions within few minutes by using a single analytical instrument, based on ISO/ TR 13097 [3], is compared with a rather complex emulsion stability index (ESI) determination lasting for several hours and having a high negative economic and environmental impact other than the presented method. Three lubricants for cold-rolling - o/ w emulsions - were analysed by accelerated stability testing at higher gravity (11 times physically accelerated). Go Greener by In-situ Characterization of Lubricants for Cold Rolling - Droplet Size Distribution and Physical Separation/ Emulsion stability 202 24th International Colloquium Tribology - January 2024 Clarification speed by LUMiSizer/ LUMiFuge within 17 min, RCA 11, 45°C, high homogenizer speed. ESI values obtained after several hours, provided by customer [4]. Customer ‘s temperature of 45°C for comparative chemical analysis was applied in the analytical instrument, too. During the separation the change of near infrared transmission was recorded and quantified by Integral transmission (area below curve) as function of time. 3.2 Droplet size distributions Two lubricants for cold rolling o/ w emulsions were compared, first the droplet size distribution was determined based on ISO 13318-2 [5]. Second, the combination of different analytical methods (instrumental optical technique and naked eye evaluation) for the reliable separation stability characterization of a challenging sample was applied. Samples in duplicate. For the white lubricant the mean value of the distribution was also determined by another method (impedance based) as x50 = 3.5 μm, which is in good agreement with the LUMiSizer ® result. The much broader droplet size distribution and the large deviations in the duplicates of the brown lubricant are confirmed by having a look at the samples by naked eye. The brown lubricant is a very inhomogeneous sample. 3.3 Instability index of lubricants 6 different lubricants were qualitatively and quantitatively compared. For qualitative comparison the clarification profiles were used, for quantitative comparison the instability index, both as defined in [6]. The instability index after 10 min discriminates all 6 samples. Samples 3, 5, 6 require 40 s only. 4. Conclusions Accelerated stability testing based on STEP-Technology ® is enabled for up to 12 samples simultaneously, requiring only a small sample volume in disposable sample cells and providing analytical results in short time. This is regarded much greener compared to conventional large volume single sample testing, in glassware to be extensively cleaned afterwards, lasting for several hours, based on chemical analysis. Costs and environmental impact are significantly reduced, when applying STEP-Technology. The stability ranking of the lubricants for cold rolling is identical for both methods. The reliable characterization of lubricants including challenging samples consists of two independent results when applying STEP-Technology in combination with physically accelerated testing, the droplet size distribution, and the instability index for the sample in its original concentration. The combination of the instrumental optical approach with the always available visual inspection and possibly further characterization methods is the way of choice. Independent results show the identical ranking. The near infrared clarification profiles recorded under accelerated conditions as well as the derived instability index allow for a quick qualitative and quantitative discrimination of lubricants for cold rolling. To be applied either in formulation or in quality assurance or in application development. References [1] https: / / en.wikipedia.org/ wiki/ Lubricant Available on 12.4.2021 at 13: 51 [2] Comprehensive Characterization of Nanoand Microparticles by In-Situ Visualization of Particle Movement Using Advanced Sedimentation Techniques, Dietmar Lerche, KONA Powder and Particle Journal 2019, 36, 156-186 [3] ISO/ TR 13097: 2013 Guidelines for the characterization of dispersion stability, https: / / www.iso.org/ standard/ 52802.html, 31.10.2023 1945 [4] Private communication from the customer (producer of lubricants, not to be disclosed) to LUM GmbH [5] ISO 13318-2: 2007, Determination of particle size distribution by centrifugal liquid sedimentation methods Part 2: Photocentrifuge method, https: / / www.iso.org/ standard/ 45771.html, 31.10.2023 19: 45 [6] Instability Index, T. Detloff, T. Sobisch, D. Lerche, Dispersion Letters Technical, T4 (2013) 1-4, Update 2014, https: / / www.dispersion-letters.com/ technical-notes/ instability-index, 31.10.2023 19: 45 24th International Colloquium Tribology - January 2024 203 Investigtion of Functional Lubricity of Water-Based MWFs by an Innovative Tool Ameneh Schneider, Felix Zak Optimol Instruments Prueftechnik GmbH, Munich/ Germany Motivation: The coolant system for metal working is a typical example of a tribomechanical system, where several wear mechanisms are present simultaneously. Additionally formulation of MWFs must meet many challenges such as: Environment, Health and Safety, Corrosion, Staining, Formulation Stability, Foaming, Fluid Longevity. Particulate composition varies significantly depending on the cutting process and operating conditions, as well as coolant characteristics and coolant systems. 2002 the autors mentioned that the workpiece (and its relative machinability) and tool materials can vary greatly, and there are a multitude of different machining processes (milling, drilling, grinding, broaching, etc.), each with their own fluid needs [1]. 20 years later in 2022 the authors [2] mentioned also that the greatest challenge is the machining of ferrous and non-ferrous alloys with a single MWF. The complex fluids can be mineral oil based, synthetic based, semisynthetic or water based (emulsion). Big challenge will be to find the right type of AWand EP-additive to work on ferrous and non-ferrous alloy. For example, a positive additive effective on a ferrous alloy may be antagonistic to a non-ferrous alloy and visa versus. For these reasons measuring the tribological performances of different formulated will give more insides about the lubricity and protection performances of MWFs. Optimol Instruments developed a new setup in cooperation with his industry partners for meeting these challenges and help this MFWs manufacturers to develop high performance coolants and MWFS for future, Design of set up for fluids with high flow rates and high rotary velocities for SRV ® rotation modus: Figure 1 shows a cross section of new set up. The fluid will be supplied directly into the contact area by using a nozzle element. Secondly it is possible to add an addition temperature sensor in fluid drain. Through using this setup in the rotation modus of SRV ® high rotation speeds up to 2000 rpm are achievable. Figure 1: Gross section of construction design The designed is very user friendly as can be seen in Figure 2. Lower disk specimens, fluid bath, different holders for variable contact geometry (point, line, area) and fluid pump are included in this setup. Figure 2: New MWF-Set up in the test chamber of SRV ® rotation modus. Experimental part: The functionality of new device was proved by 42 tests. Various specimens’ combination for three different fluid composition (Coolant A. B, C) were chosen. All fluids were prepared as 6% water emulsion. 4 different materials as a mushroom-shaped specimen - as tool piece were available for this study: • 20MnCr5 (Gear steel) • 16MnCr5 (Higher wear protection) • 115CrV3 (Tool steel) • X8CrNiS18-9 (Stainless steel) Following two materials as work pieces were selected for this investigation: • 16MnCr5 untampered, E-Module: 208 [kN/ mm²] • Aluminum alloy AlMg3, E-Module: 70 [kN/ mm²] 204 24th International Colloquium Tribology - January 2024 Investigtion of Functional Lubricity of Water-Based MWFs by an Innovative Tool The temperature changes for the disk as well as for the fluids were registered for all test combinations. After some pretests and discussions with industry partner the following test parameters were chosen: • Normal load: 150 N • Rotation speed: ramp 0 to 3.14 m/ s in 10 min. • Temperature: RT • Test time: 30 min Repeatability of results concerning friction and wear value were evaluated. Example of results Figure 3 shows the values of coefficient of friction (CoF) during the test time for one fluid (Coolant C) in material combination steel and 15MnCr5. Each test combination was repeated once. The repeatability of results was very good. The online measurement of wear was also possible however the wear values at the tool material at the end of the test is the most import one and the values war repeatable as well. Figure 3: repeatability of CoF value for coolant C As the repeatability for all tests was very good the ranking of three fluids, regarding their tribological performances was easily possible. The CoF curves for each coolant for material combination steel and 20MnCr5 is compered in Fig. 4. Coolant C delivered the lowest and most stable values during the entire test time. Figure 4: Comparison of CoF for three fluids Mean CoF and wear (last 15 min + standard deviation) for steel with all other 4 tool materials are presented as a balk diagram in Figure 5: Figure 5: Mean CoFand wear values (last 15 min + standard deviation) for Steel For all material pairing the coolant C have shown the best tribological performances followed by coolant B and A. Optimizing of speed and other test parameters such as normal load for aluminum is carried on by Optimol Instruments for these three and many other fluids. The results are ongoing and will be presented in future publications. Summary: For Steel as workpiece • A good differentiation in performances can be shown for all selected material pairings with this new setup. • Repeatability of results is very good. • Temperature developments during the tests for disk and coolants are stable: • + 1 to 2-°C due to good cooling effects • Online wear measurements work precisely. This new MWF setup for SRV ® rotation modus enables the formulators to identify, try and confirm and finally find the best MWF for multi-metal application. Additionally, it helps to solve the upcoming challenges such as improved economic efficiency of manufacturing processes by using more efficient formulations for extending the tool life. References: [1] Radoslav Rakic et al, The influence of the metal working fluids on machine tool failures; Wear, Vol. 252 (2002), Iss. 5-6, Pages 438-444. [2] Neil Canter, Metalworking fluids: Current options for machining multi-metal alloy, TLT magazine March 2022, Pages 44-54. Corresponding author: ameneh.schneider@optimol-instruments.de www.optimol-instruments.de 24th International Colloquium Tribology - January 2024 205 Tribological Testing for the Assessment of Friction and Metal Transfer in Sliding Contacts between Cemented Carbide and Aluminum during Metal Forming N. Cinca 1* , M. Olsson 2 , M. G. Gee 3 1 Hyperion Materials & Technologies, Polígono Industrial Roca Calle Verneda 12-24, Martorelles 08107, Barcelona, Spain 2 Dalarna University, SE-791 88, Falun, Sweden 3 Department of Materials and Mechanical Metrology, National Physical Laboratory, Hampton Road, Teddington, Middlesex TW110LW, UK * Corresponding author: nuria.cinca@hyperionmt.com 1. Introduction Cemented carbides are well-known heterogeneous materials used to manufacture tools are employed in many industries where wear resistance is required together with a proper balance of hardness and fracture toughness. Diverse wear mechanisms can occur depending on the operating conditions for the components. In the metal forming industry, a sliding friction wear mode takes place with transfer of the metal work material to the tool [1]. Different types of tribological testing can be used to characterize the friction and wear phenomena during sliding, but it is always difficult to extrapolate the behavior from lab testing to field performance. Decreasing the length scale down to the microlevel can provide some insights of initial metal transfer due to surface asperities [2, 3]. In the present study, tribological characterization of Al metal transfer in sliding motion onto two cemented carbides was performed two ways. Firstly, to evaluate the initial sticking on a polished surface, an Al tip was made to slide onto different cemented carbide grades under well-defined contact conditions. In the second test, a pin on disc system was used with the capability for continuous imaging and capture of changes to the wear surface. 2. Methodology The two commercial cemented carbide grades tested in the present work are presented in Table 1. Grade A is a plane WC-Co grade, while grade B has cubic carbides. Both tribological tests were performed under dry conditions. Table 1. Chemical Composition and Hardness Grade WC-phase [vol.%] γ-phase [vol.%] Binder [vol.%] Hardness, HV 30 [kg/ mm 2 ] A 79.6 - 20.4 Co 1294 ± 9 B 63.3 19.8 16.9 Co 1517 ± 5 In the first test, an aluminum stylus was slid over a scratch that had been made intentionally onto a well-polished surface and the coefficient of friction (COF) was recorded during the test. The testing conditions were normal load 10 N, sliding speed 10 mm/ min, sliding distance 10 mm, 1&5 passes. For the second test, a pin on disc tribometer fitted with linescan camera to obtain continuous images of wear track on disc was used. The COF was continuously measured. Lapped cemented carbide plates (Ra 1.03 mm and 0.684 mm for A and B respectively measured with Alicona InfiniteFocus) of 75 mm diameter were tested against the domed end (10 mm radius) of an Al pin of 20mm diameter sliding at 200N with a 59mm wear track. The test duration was 1 hr at a speed of 2 rev/ min. 3. Results & Discussion 3.1 Multipass Testing with Aluminum Stylus The initial metal transfer represented in this test illustrates that both cemented carbide grades present similar friction characteristics, with the friction coefficient increasing after reaching the intentionally introduced scratch and decreasing afterwards. This behaviour can be explained by the repassivation of the fresh aluminum surface that was scrapped off when passing over the scratch. However, the friction peaks after first and fifth passes are higher for grade A than grade B. Since grade A is softer, its scratch ridges of are higher, which induces that more aluminum is scraped off in connection to the scratch. However, after passing the scratch, the COF remains higher for grade B, resulting in higher transfer tendency. Figure 1. COF of aluminium pin sliding against the tested cemented cabide grades. Grey line is 1 pass and black line is 5 passes. 3.2 Pin on Disc in Situ Testing In the pin on disc tests (Figure 2), the COF continuously increased along the test time for the softest grade A, starting Tribological Testing for the Assessment of Friction and Metal Transfer in Sliding Contacts between Cemented Carbide and Aluminum during Metal Forming 206 24th International Colloquium Tribology - January 2024 at values around 0.4 and reaching 0.7 at the end of the test. By contrast, the COF for grade B was relatively constant for the whole test at about 0.6-0.7. With the linescan camera, the sticking progression of aluminum was recorded. Small areas of transferred aluminium were seen almost immediately in both tests. These were often associated with scratches of the lapped surfaces. The higher transfer tendency in grade B discussed in Figure 1 can be the reason for the higher values of friction here recorded here. Using real time in situ measurement techniques provide information on wear mechanisms related to microstructural features of tested surfaces. Figure 2. Visual and graphical representation of the change of COF with the time along the pin on disc test for both cemented carbide grades. The brighter, the higher the COF value. 4. Conclusions Two cemented carbide compositions have been tribologically tested for aluminum metal forming applications. With the multipass testing to evaluate the initial metal transfer and the ball on disc test, it has been found that the surface topography plays a key role in the aluminum sticking, which results in an increase in the friction coefficient. Both methodologies provide insights on the wear mechanisms. References [1] V. Westlund, J. Heinrichs, S. Jacobson, On the role of material transfer in friction between metals: Initial phenomena and effects of roughness and boundary lubrication in sliding between aluminum and tool steels, Tribol.Lett. 66 (2018) 66-97. [2] J. Heinrichs, M. Olsson, S. Jacobson, New understanding of the initiation of material transfer and transfer layer build-up in metal forming—In situ studies in the SEM, Wear 292-293 (2012) 61-73. [3] V. Westlund, J. Heinrichs, M. Olsson, S. Jacobson, Investigation of material transfer in sliding friction-topography or surface chemistry, Tribol.Int. 100 (2016) 213-223. [4] M. Gee, T. Kamps, P. Woolliams, J. Nunn, K. Mingard, In situ real time observation of tribological behaviour of coatings, Surf Coat Technol 442 (2022) 128233. 24th International Colloquium Tribology - January 2024 207 Analysis of Tribo-Films in Industrial Applications Joerg W. H. Franke 1* , Janine Fritz 1 , Daniel Merk 2 1 Schaeffler Technologies AG & Co. KG, Herzogenaurach, Germany 2 Schaeffler Technologies AG & Co. KG, Schweinfurt, Germany * Corresponding author: frankjer@schaeffler.com 1. Introduction The performance of rolling bearings in industrial applications is strongly dependent on lubrication, including formation of tribo-films in the mechanical contact. As a result of the research a detailed description of these tribo-films is available. These analysis methods are rarely available in the industry, but mainly at research institutes and universities. In industrial practice it is necessary to use less time consuming, flexible, widely available, and in the best case, non-destructive techniques. In [1] a selection of specimens from FE8 WEC tests were investigated by “micro-X-ray Fluorescence Spectroscopy” (m-XRF) and “Attenuated Total Reflection Fourier Transform Infrared Spectroscopy” (ATR-FTIR). The presentation at 24 th International Colloquium on Tribology is focused on following questions: a. How repeatable are tribo-films? b. Are the methods of analysis sufficiently discriminating? c. How robust are tribo-films? 2. Methods 2.1 Rolling bearing testing The FE8 test rig [2] is usually used to test standardized test setups according to the DIN standards DIN 51819-02 [3] or DIN 51819-03 [4]. The aim is to test lubricants (oils and greases) according to their general behavior, i.e., the anti-wear behavior of specific additives. In this context, a test procedure was established to investigate the mixed friction behavior of point and line contacts. The FE8-25 tests with the cylindrical roller thrust bearing 81212 exhibit different failure modes depending on the lubricant chemistry. In addition to the harsh test conditions of mixed friction at high speeds and slippage, this bearing type exhibits special kinematic conditions that lead to different values of frictional energy across the raceway. The main failure mechanisms of this test are sub-surface fatigue damage due to White Etching Cracks (WECs) and/ or surface-initiated fatigue damage (SIF). The various stress zones with different energy inputs perpendicular to the raceway of a washer after a FE8-25 test run can often be easily recognized visually. Figure-1: Different stress zones and the friction energy density along the raceway of an FE8 axial washer 2.2 Surface analysis The selected methods, m-XRF and ATR-FTIR microscopy, allow the characterization of areas of interest via mapping in their entirety. Both methods are non-destructive, so it was possible to analyze the specimen generated on FE8 test without any mechanical sample preparation. Before the analysis, it was only necessary to clean the sample by rinsing off lubricant residues on the surface with a suitable organic solvent, e.g., n-heptane (CAS No. 142-82-5). In µ-XRF spectroscopy a high energetic radiation is used to excite atoms of the specimen. In these atoms electrons from inner shells are removed from the atom. In a very short time these vacancies are filled with electrons from outer shells. The free energy can be emitted as an AUGER-electron or as an X-ray photon. The energy of the emitted X-ray photon depends on the difference of binding energies of both involved electron levels - the vacancy and the level from which the electron jumps into the vacancy. Because this difference is characteristic for every element, the excited specimen emits a characteristic fluorescence radiation. This can be used to analyze both qualitative and quantitative composition of specimens. The m-XRF offers the possibility of a position sensitive elemental analysis of non-homogeneous material. ATR-FTIR microscopy was used to determine the chemical structure of the generated tribo-films on the washer surface. FTIR spectroscopy exploits the fact that most molecules absorb light in the infrared region of the electromagnetic spectrum. The absorbed energy leads to a change in the molecular dipole moment. These molecular vibrations generate an IR spectrum that serves as a characteristic “molecular fingerprint” from which the determination of the specimen’s chemical structure can be made. In addition, the ATR technique uses the effect that optical absorption spectra can be easily obtained by observing the interaction of the totally reflected light emerging from the optically dense medium with the optically thin medium. The ATR-FTIR microscopy on the raceway side of the washer was analyzed by mapping a grid of 13 × 13 measurement points both in the circumferential and transverse to the direction of the raceway of the thrust bearing surface. The 13 measurement points of each row (identical tribological conditions) were used to calculate a sum spectrum. The detailed equipment parameters were identical as described in [1]. 3. Results 3.1 Similar oil compositions More than 10 years of FE8-25 testing with a variety of different types of lubricants allows the comparison of tribo-film 208 24th International Colloquium Tribology - January 2024 Analysis of Tribo-Films in Industrial Applications formation with several similar but not identical oil formulations. As an example, the resulting tribo-film of five oil formulations with at least ZDDP and calcium sulfonate was compared. In 3 out of 5 specimens the typical amorphous calcium carbonate bands are observed in the infrared spectrum. In addition, an absorption band at 1006 cm -1 (i.e. P-O typical) is visible. In the other two specimens, however, this band is shifted to 1110-1130-cm -1 and no carbonate typical bands are detectable. This also correlates with the results of the element analysis. The phosphorous signal is strongly dominant when no carbonate peaks are detectable. In the other three cases the sulfur signal is on a higher level. So similar formulations usually result in similar tribo-films, but the exact oil formulation determines the true result. 3.2 Different additive formulations A less additivated ester oil was used to produce a different tribo-film. As expected, the specimen failed early after 21 hours with surface-initiated failure. The element analysis shows only a low occupancy of the surface. Traces of sulfur and nearly no identification of other elements. The IR-spectrum is dominated by an iron oxide typical band. Unfortunately, oxygen is not detectable by the selected surface analysis method. Therefore, only the infrared information is available. Nevertheless, the result is a clear differentiation compared to the phosphorus or calcium based tribo-films on most other specimens. Figure-2: Surface analysis - samples run with different chemistries 3.3 Stability of tribo-film A common doubt regarding tribo-films is their stability after preparation procedures, e.g., cutting of the specimen by using cooling fluids etc. Thus, two specimens from one test were used. The first specimen was analyzed directly after the test run. The second one was first cut to prepare a sample for structure analysis. The remaining piece was analyzed exactly as the first part. The analysis shows a similar if not identical tribo-film. The films seem robust enough and an analysis after the cutting procedure is also promising. Figure-3: Surface analysis of cut and uncut part from same test run. 4. Conclusion By combining the two surface analysis methods developed (m-XRF and ATR-FTIR microscopy), it was possible-based on the samples available-to differentiate tribo-films on used parts. These correlative spectroscopy techniques are suitable to describe the tribo-film in an adequate precision for industrial application. In addition, compared to externally available complex methods, significantly larger areas can be characterized in a spatially resolved manner via mapping. This is a smart way to determine the actual respective chemical composition of the generated tribo-films in a sufficient resolution. It supports the objective to characterize tribo-films by using comparatively inexpensive, fast, and non-destructive analysis methods, which are more commonly widespread in the industry. References [1] Franke, J.W.H.; Fritz, J.; Koenig, T.; Merk, D. Influence of Tribolayer on Rolling Bearing Fatigue Performed on an FE8 Test Rig - A Follow-up. Lubricants 2023, 11, 123. https: / / doi.org/ 10.3390/ lubricants11030123 [2] DIN 51819-1; Testing of Lubricants-Mechanical-Dynamic Testing in the Roller Bearing Test Apparatus FE8-Part 1: General Working Principles 2016-12. Beuth Verlag: Berlin, Germany, 2016. [3] DIN 51819-2; Testing of Lubricants-Mechanical-Dynamic Testing in the Roller Bearing Test Apparatus FE8-Part 2: Test Method for Lubricating Greases-Applied Test Bearing: Oblique Ball Bearing or Tapered Roller Bearing. Beuth Verlag: Berlin, Germany, 2016. [4] DIN 51819-3; Testing of Lubricants-Mechanical-Dynamic Testing in the Roller Bearing Test Apparatus FE8-Part 3: Test Method for Lubricating Oils-Applied Test Bearing: Axial Cylindrical Roller Bearing, 2016- 12. Beuth Verlag: Berlin, Germany, 2016. 24th International Colloquium Tribology - January 2024 209 Detection of Wear in Modern Naval Engines Theodora Tyrovola 1* , Fanourios Zannikos 2 . 1 Laboratory of Fuels and Lubricants - National Technical University, Athens, Greece (Technical Academy Esslingen eV) 2 Laboratory of Fuels and Lubricants - National Technical University, Athens, Greece (Technical Academy Esslingen eV) * Theodora Tyrovola: theodoratirovola@gmail.com 1. Introduction: Cutting Emissions from Shipping Industry. 1.1 Establishing Stricter Regulations for Emissions Cut. Maritime transport is the linchpin of the global economy, acting as the physical support for its flows of freight. It remains dominated by longitudinal interactions which are considerable, having some great advantages apart from speed, like the continuity and the capacity to handle large amounts of cargo. Nevertheless, it is a growing source of greenhouse gas (GHG) emissions and a major source of air pollution, underwater noise and oil pollution. At EU level, maritime transport represents 3 to 4% of the EU’s total CO2-emissions and in terms of equivalent tonnes of CO2, over-124 million in 2021. The initial GHG strategy of the International Maritime Organization (IMO) sets a fundamental target to reduce CO2 emissions per transport work, as an average across international shipping, by-at least 40% by 2030 and 70% by 2050. IMO implemented strict regulations in 2020 (IMO2020 Rule), forcing ships to use low sulphur fuels for public health reasons. The new rules lowered the maximum percentage of sulphur from 3.5% to 0.5% for all ships operating worldwide and the highest permissible sulphur content in fuels of ships sailing in the SECAs (Sulphur Emission Control Areas) is 0.1% m/ m. The immediate shrinking of sulphur emissions is mentioned in the sixth (IV) Annex of the Marine Pollution (MAR- POL) Convention of the International Maritime Organisation (IMO) [1]. The switch from residual fuels to low sulphur distillate ones is the most viable way for shipowners and operators to comply with the emission limitation requirements. 1.2 Notorious Emitted Pollutants. Air pollution-from ships is generated mostly by-diesel engines-that burn high-sulphur-content-fuel oil, also known as bunker oil. The complete and incomplete combustion of conventional fuels inside the naval engine along with the high temperature of the intake or scavenger air inside the cylinder, result in the formation of a complex mixture of exhaust gases and particles. Shipping emissions are constituted by primary and secondary particulate matter, mainly in the fine size fraction (PM 2.5 ) and including black carbon (BC), and in addition by sulphur dioxide (SO 2 ), nitrogen oxides (NOx), non-methane volatile organic compounds (NMVOC) and carbon dioxide (CO 2 ) [2]. Nearly 70% of these emissions occur near coastlines, posing immediate environmental and human health risks to the coastal populations. Port cities especially, with high population densities, are more susceptible to ship emissions. Global shipping emissions could grow by up to 50% by 2050, depending on future economic and energy developments. The most challenging emissions are currently sulphur oxides - SO x , and nitrogen oxides - NO x. The main fuel type used today is the low-cost, highly viscous residual or else heavy fuel oil with high sulphur content and potentially carcinogenic substances. These combustible gases, emitted into the environment in the form of smoke, can have adverse effects on the ozone layer in the troposphere, which results in the “greenhouse effect” and may contribute to the global warming phenomenon. 2. Sustainable Low Sulphur Marine Gasoils. 2.1 Properties of Low Sulphur Marine Gasoils. With a far lower carbon footprint than petroleum-based fuels, environmentally friendly marine fuels are the most sustainable solution at the moment, for lowering SOx and NOx emissions, achieving ecological justice and promoting energy saving in the maritime sector. Low Sulphur Marine Gasoil (LS-MGO) is a distillate fuel with maximum sulphur content of 0,1% m/ m. Distillate marine fuel is mostly used in ships sailing in EU ports or in the Emission Control Areas (ECAs). Distillates can be unstable since they undergo chemical changes in the short term that can cause severe operational problems, they often experience low lubricity, they might contain contaminants and finally they can be incompatible. Due to fuel’s limited lubricity, pumps can suffer from adhesive wear where pump internal parts stick accelerating wear. Existing seals may need to be changed in order to deal with the challenges of the new fluid. 2.2 Weak Tribological Properties of Marine Gasoils. A diesel fuel’s lubricity is a measure of its ability to prevent or minimize wear in the components that utilize the fuel as a lubricant. Obviously, components with the greatest dependence on the fuel for lubrication demand a higher lubricity fuel. Fuel’s lubricity is related to the chemical composition of the fuel. The sliding surfaces in fuel injection system are protected from wear by hydrodynamic and boundary lubrication mechanisms. The lubricant’s ability to keep the surfaces separated is governed by its viscosity. Sulphur is one of the compounds in the fuel that imparts lubricity characteristics. The currently established refining processes for the production of low sulphur marine gasoils, remove not only the sulphur and nitrogen compounds but also a significant proportion of oxygenated and polyaromatic (polar) compounds. Loss of the polar compounds is considered to be responsible for the limited tribological abilities of low sulphur marine gasoils, making them insufficient and eventually leading to leading to excessive wear and scarring on the engine’s components [3]. Detection of Wear in Modern Naval Engines 210 24th International Colloquium Tribology - January 2024 3. Detection of Wear. 3.1 High Frequency Reciprocating Rig (HFRR) Test Method. The lubricating capacity of marine distillates is determined by the High Frequency Reciprocating Rig (HFRR) test which constitutes a microprocessor-controlled reciprocating friction and wear test system which provides a fast, repeatable assessment of the performance of fuels and lubricants. It is considered the industry’s standard test for all diesel fuels lubricity. The parameters of the HFRR test method simulate boundary lubrication conditions. The result given is the corrected with respect to the standard water vapor pressure at 1.4 kPa wear diameter (WS1.4) expressed in micrometers (μm) and constitutes the lubricating capacity of the fuel [4] . Lubricity has been recognized as a potential quality issue with low-sulphur marine fuel distillates, and therefore a specific acceptable limit for lubricity is set in the recently revised version of the ISO 8217: 2017 standard for the classification and specification of marine fuels -. The maximum specified limit is 520 µm WSD (wear scar diameter) and is determined by the HFRR method according to ISO12156- 1. This limit only applies to fuels with less than 0.1% (1000 ppm) sulphur content. 3.2 Modifications in the Original Parameters of HFRR Test Method. The implementation of IMO2020 Rule, establishes more assumptions and inaccuracies related to marine distillates’ lubricity, making it not easily perceived in the marine industry. The use of a fuel with poor lubricity may result in fuel pump seizure, but it is not the only factor that provokes a failure. Marine distillates are proven to have positive impact both on the marine environment and coastal health, but they are accompanied by a huge range of side effects related to their storage, combustion, ignition and lubricity. In order to increase the sensitivity and accurateness of HFRR over marine distillates, we rely both on the basic parameters of ISO 12156-1 standard and on the modifications of them. Altering the basic parameters of ISO 12156-1 is performed in order to identify the possible poor lubricating capabilities of low-sulphur marine gasoils and to track wear on the metallic parts of the engine’s equipment that cannot be detected by the original method. In this scientific research both temperature and the imposed load are factors that can challenge the efficacy of marine distillates. By keeping temperature stable and changing the load there is a significant limitation in fuel’s lubricity, which in fact degrades considerably. The load factor affects several aspects of the engine’s operation and can impact its efficiency in various ways by provoking lower efficacy and increased wear on the mechanical components. When the temperature progressively rises, as certain parts of the naval equipment experience regionally higher temperatures due to adhesive wear, the wear scar diameter is remarkably rising. When both the imposed load and heat are rising, WSD increases rapidly, escaping from the imposed lubricating limit and therefore the fuel’s lubricating capacity is remarkably reduced. Determining lubricity by using more vulnerable HFRR ball specimens, excess wear is observed on their surfaces leading to diminishing of the fuel’s tribological properties. At higher temperatures and loads from those that are normally applied (200g, 60 o C), wear is measured to be significantly higher when using the original hardness specimens rather than when using more susceptible to abrasion and scuffing ones. Table 1: MWSD for Marine Diesel Fuel (S content less than 0.1%) with original Rockwell hardness. Table 2: MWSD for Marine Diesel Fuel (S content less than 0.1%) with modified Rockwell hardness. 4. Conclusion Fluctuations of temperature and load in a naval diesel engine are factors that can challenge the efficacy of marine distillates. The application of more vulnerable ball specimens proves that they suffer extreme wear on their surfaces leading to excessive friction. It is necessary and imperative to conduct targeted and thorough research so as to establish an exclusive control standard for the ship’s fuel pumps and be able to avoid future breakdowns. While worldwide maritime transport sector plays a vital role for the economic well-being it has an urgent responsibility to step up its efforts to reduce the sector’s environmental footprint. While steps have been taken already based on European and international policies, much more is needed for a fundamental shift towards a sustainable maritime transport sector that contributes to secure the future well-being and survival of our most sensitive ecosystems and coastal areas, and the well-being of citizens. References: [1] MARPOL 73/ 78. 2015. “International Convention for the Prevention of Pollution from Ships” Practical Guide 38: 1-57. [2] Balcombe, P., Brierley, J., Lewis, C., Skatvedt, L., Speirs, J., Hawkes, A., Staffel, I. (2019). How to decarbonize international shipping: Options for fuels, technologies and policies. Energy Convers. Manage. 182, 72-88.- [3] Wie D., The lubricity of Fuels II, Wear Studies using model compounds, J. of Petrol. (Pet. Processing.) 1988, Vol 4, No.1, p90. [4] Diesel Fuel - Assessment of lubricity using the high frequency reciprocating rig (HFRR), Draft International Standard ISO/ DIS 12156-1. 24th International Colloquium Tribology - January 2024 211 Unveiling the Butterfly Effect in Tribology: The Impact of Surface Profile Yulong Li 1,2 , Nikolay Garabedian 1,2 , Johannes Schneider 1,2 , Christian Greiner 1,2* 1 Institute for Applied Materials (IAM), Karlsruhe Institute of Technology (KIT), Kaiserstr. 12, 76131 Karlsruhe, Germany 2 KIT IAM-ZM MicroTribology Center (µTC), Str. am Forum 5, 76131 Karlsruhe, Germany * Corresponding author: christian.greiner@kit.edu 1. Introduction In many tribological studies, a surface that seems homogenous is, in reality, a planar anomaly riddled with inconsistencies. Even surfaces refined to a mirror-like polish manitain microscale undulations and deviations, characterized by pronounced, coarse, or jagged protrusions termed “asperities” (derived from the Latin word “asper”, meaning “rough”). These asperities have various scales, displaying self-affine or fractal patterns. The intricate configuration of the interface between a substance and its external environment critically influences the tribological characteristics of that surface [1-3]. Usually, surface deviations are quantified using mean roughness and waviness metrics, typically represented as simple scalar values (for example, roughness R a and waviness W t as per ISO-4287 standards). These metrics for roughness and waviness are not directly derived from the primary surface profile. Instead, they are from their respective roughness and waviness profiles after the primary profile undergoes filtering in line with prevailing standards (e.g., ISO- 116610 or ASME-B46.1), as illustrated in Figure 1. Roughness pertains to the minor, closely spaced deviations observable at the microscopic scale on a surface. These deviations commonly arise from the manufacturing process and can include characteristics such as scratches, indents, and others. Conversely, waviness denotes the more pronounced, broadly spaced deviations visible at the macroscopic scale on a surface. Such irregularities frequently arise from processes like machining or assembly, encompassing features such as undulations, bumps, and other substantial surface anomalies. [4] The impact and control of roughness parameters on friction and wear have been thoroughly explored in the literature. However, achieving a perfectly flat surface or replicating the same surface consistently remains an unachievable goal. As a result, each tribological experiment is conducted on a distinct surface, even if certain parameters (such as R a ) deem them similar. On the one hand, the subtle surface deviations in experimental conditions may exert a surprisingly significant and yet unknown influence on tribological behavior. On the other hand, the effects of these minor surface deviations on tribo-logical behavior could potentially shed light on unre-solved questions that continue to perplex the tribological community. Figure 1: (a) Roughness and waviness on a surface [5]; (b) Schematic of arithmetical mean deviation of roughness profile R a [6]. 2. Results and discussion The tribological experiments in this contribution were carried out using a pin-on-disk configuration with pins and disks made from bearing steel (100Cr6, AISI 5210). Maximum efforts were made to control the surface topography of the disk, maintaining its roughness within a range from R a = 0.08 to 0.11 µm. The radial height discrepancy along the frictional track was kept under 2-µm, as depicted in Figure 2. Ensuring that the height variation of the 132-mm disk’s sliding track stays within 2-µm, representing the minimum achievable value in our laboratory. Figure 2: Extracting the surface profile from chromatic profilometry data. The height difference of the 132-mm sliding track is below 2-µm [7]. 212 24th International Colloquium Tribology - January 2024 Unveiling the Butterfly Effect in Tribology: The Impact of Surface Profile However, when comparing the average friction coefficient along the entire disk’s sliding track with the roughness distribution (as shown in Figure 3a) and the waviness profile (presented in Figure 3b), we observe no discernible correlation between the friction coefficient and roughness. In contrast, a partial correlation exists between the friction coefficient and the disk’s waviness profile, as illustrated in Figure-3b. The friction coefficient peaks where the waviness profile reaches its maximum height. A “hill” with an approximate height of 2-µm elevates the friction coefficient by 91%. The influence of this mere “2-µm” is remarkable, especially considering our rigorous efforts to regulate waviness, ensuring it remains below 2 µm over such an extended sliding track (132-mm). Even in precision semiconductor manufacturing, exemplified by the 7-nm node lithography process, the permissible height variance on a 300 mm wafer is just under 5-µm [8]. Thus, a 132-mm sliding track with a height differential of only 2 µm is conventionally deemed “flat” within tribological studies. However, this research has unveiled that even such a tiny difference can significantly dictate tribological behavior, which is a point often overlooked by tribologists and deserves broader attention in the future. Figure 3: Comparing friction coefficient with roughness distribution(a) and waviness profile(b) [7]. 3. Conclusion This research unveils the “butterfly effect” in tribology, where even minute surface topographical variations can exert a decisive influence on frictional performance. The insights presented in this thesis offer a lucid explanation for why frictional behavior is impossible to repeat. Even when surface topography is minimized and controlled, there exists a significant and hitherto unrecognized impact on tribological behavior. Given that the surface topography is distinct in each experiment, achieving absolute consistency in frictional behavior is inherently unattainable! References [1] Jacobs T. D. B., Pastewka L. Surface topography as a material parameter. MRS bulletin 12 (2022) 1205-10. [2] Hanaor D. A., Gan Y., Einav I. Contact mechanics of fractal surfaces by spline assisted discretisation. International Journal of Solids and Structures (2015) 121-31. [3] Aghababaei R., Brink T., Molinari J.-F. Asperity-Level Origins of Transition from Mild to Severe Wear. Physical review letters 18 (2018) 186105. [4] Aghababaei R., Brodsky E. E., Molinari J.-F., Chandrasekar S. How roughness emerges on natural and engineered surfaces. MRS bulletin (2023). [5] American Society of Mechanical Engineers. Surface texture: Surface roughness, waviness, and lay. New York: American Society of mechanical engineers; 2020. [6] DIN EN ISO 4287: 2010-07, Geometrische Produktspezifikation (GPS)_- Oberflächenbeschaffenheit: Tastschnittverfahren_- Benennungen, Definitionen und Kenngrößen der Oberflächenbeschaffenheit. Berlin: Beuth Verlag GmbH. [7] Li Y., Garabedian N., Schneider J., Greiner C. Waviness affects friction and abrasive wear (2022). [8] Iida S., Nagai T., Uchiyama T. Standard wafer with programed defects to evaluate the pattern inspection tools for 300-mm wafer fabrication for 7-nm node and beyond. Journal of Micro/ Nanolithography, MEMS, and MOEMS 02 (2019) 1. 24th International Colloquium Tribology - January 2024 213 Soft and Highly Sensitive Contact Pressure Sensors Based on Randomly Rough Surfaces Luciana Algieri 1,2 , Luigi Portaluri 1,2 , Marco Bruno 1,2 , Massimo De Vittorio 1,2 , Michele Scaraggi 1,2* 1 Center for Biomolecular Nanotechnologies, Istituto Italiano di Tecnologia, Arnesano (LE), 73010, Italy 2 Department of Innovation Engineering, University of Salento, Lecce, 73100, Italy * e-mail: michele.scaraggi@unisalento.it 1. Introduction (TAE_Heading) Soft contacts are widely used in macro, micro, and nano-electromechanical systems, including flexible electronics for applications in translational medicine, energy, and the automotive sector [1]. Contact mechanics properties such as friction, adhesion, thermal and electrical contact resistance, etc., do strictly depend on the roughness and rheological properties of the mating surfaces. Nowadays, they can be predicted by recurring to mean-field formulations of the contact mechanics [2]. The availability of such mean field theories makes it possible to construct bio-interfaces and -sensors based on arbitrarily rough surfaces, which are often characterized by a facile microfabrication process. In this work, we adopted randomly rough surfaces to develop capacitance-based soft contact pressure sensors. Due to its inherent characteristics, which include high sensitivity, temperature stability, low consumption, and simplicity, the capacitive transductive approach was chosen. Capacitive sensing can make use of the dielectric permittivity change, the overlapping area, or the distance between the plates. Changing the separation and the corresponding capacitance variation is the most widely used method of pressure sensing. Indeed, due to the applied pressure, the dielectric material between the parallel plate electrodes is compressed, resulting in a change in capacity. [3]. For this reason, an ideal dielectric layer should be as soft as possible to be highly sensitive to several pressure ranges. An effective approach toward realizing high sensitivity is to employ dielectric layer micropatternig. In the following, we established that the sensitivity of capacitive soft contact pressure sensors based on dielectric having random roughness is higher compared with the capacitive soft contact pressure sensors based on dielectric with deterministic topography. 2. Experimental Details Chemicals and materials PDMS used in the work was prepared by thermal curing a silicone elastomer (Sylgard ® 184) with a curing agent in a 10: 1 ratio. Nanoscribe IP-S photoresist was used in DLW configuration to obtain deterministic mold. The conductive glass slides used have a dimension of 25 x 75 mm with 25 nm of conductive Indium Tin Oxide (ITO) (Xin Yan Technology LTD). Poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrene sulfonate) (PEDOT: PSS, Sigma-Aldrich, 3-4 wt%) was used to cover the surface of PDMS domes. Soft Surface Fabrication Random deterministically-patterned roughness was fabricated in PDMS with dimensions of 8 mm × 8 mm by soft lithography and subsequently PDMS double casting method. The mold used in soft lithography is made to employ two different fabrication strategies. A pressurized water steam system was optimized to obtain the random roughness while a two-photon polymerization by Nanoscribe Photonic Professional GT system (Nanoscribe, Karlsruhe, Germany) was used to obtain a deterministic array. Surface Characterization The surface topographies were characterized with AFM (AFM Nano-Observer CSI ) and profilometer (Dektak XT Bruker) to extract the surface power spectral densities - needed in the contact mechanics theory. 3. Results and Discussion Experimentally, we have unraveled the role of roughness randomicity on the generation of capacitance thanks to the development of a home-made opto-electro-mechanical triboscope (Fig.1a). The contact interface is made by an ITO-coated, optically smooth microscope slide in contact with either a i) randomly rough self-affine or ii) deterministically-patterned PDMS layer with back electrode, the latter in adhesive contact on the top of a PDMS dome (Figure 1b). Both dome and layer share the same rheological properties. The surfaces have a root mean square roughness of ≈15 µm for both random and deterministic patterns. The power spectral density of the random roughness is reported in the inset of Figure 1b, whereas the deterministic pattern is constituted by a square array, with a lattice distance of 400 µm, of hemispheres with a radius of 100 µm. The domes are approached with a constant speed to the conductive glass with a load range of 0.1N to 1.6N. Changing the separation gap induced by the different loads applied allows for recording a variation in capacitance and therefore capacitive pressure sensing. The capacitive measurements were performed using the Digilent Analog Discovery 2, connected to the sample and the conductive glass while the triboscope allowed to record the image during the true contact. The contact area and the capacitance are reported as a function of the applied normal load (Fig.1c), where the black (blue) line is for the deterministic (random) topography. We observe that the capacitance range (sensitivity) is superior for the random roughness compared to the deterministic pattern, due to the multiscale nature of the random topography. In Figure 1 e) and f) we show the optically acquired true contact domains for the random and deterministic roughness cases, respectively. Different colors identify different simply connected contact patches. We note that a larger contact area is coupled with a smaller average 214 24th International Colloquium Tribology - January 2024 Soft and Highly Sensitive Contact Pressure Sensors Based on Randomly Rough Surfaces interface separation [2], thus an enhanced capacitance range, in agreement with our experimental findings. 4. Conclusion In summary, we built a highly sensitive soft capacitive pressure sensor based on random roughness using a novel and low-cost fabrication method. These remarkable characteristics show that the developed sensor can be implemented as a fully flexible sensing device in different applications, such as for robotics hands, artificial skin, and human health monitoring. References [1] R. B. Mishra et al., Advanced Materials Technologies 6, (2021),2001023. [2] M. Scaraggi et al., Journal of Chemical Physics 143 (2015), 224111. [3] Mishra, Rishabh B., et al. Advanced materials technologies-6.4 (2021): 2001023. Figure 1: (a) Schematic of the opto-electro-mechanical tribometer adopted in this study. (b) Contact interface, as made by an ITO-coated, optically smooth microscope slide in contact with a randomly (left) or deterministically (right) rough PDMS layer with back electrode. (c) Contact area (right) and the capacitance (left) as a function of the applied normal load. (e) and f) optically acquired contact domains for the random and deterministic case, respectively (at different loads). 24th International Colloquium Tribology - January 2024 215 The Importance of Inocula for Biodegradation Testing of Lubricants Dr. Peter Lohmann 1* 1 Hermann Bantleon GmbH, Ulm, Germany * Corresponding author: plohmann@bantleon.de 1. Introduction In nature, degradation processes of organic materials such as hydrocarbons are strongly influenced by environmental conditions. Here, for example, the temperature, the circulation in waters, but also the presence of bacteria capable of degradation play a major role. Higher temperatures accelerate the degradation process, while in colder regions, biodegradation processes take place only very slowly or will completely stop. Similarly, a sea rich in oxygen due to large tidal range, such as in Brittany, accelerates biodegradation processes [1]. There are several hundred species of petroleum-degrading microorganisms, on land, in freshwater and in saltwater [2, 3, 4, 5]. These microorganisms require special enzymes to convert the hydrocarbons into digestible fatty acids [6]. Short hydrocarbons degrade quickly, longer chains take more time, and complex molecules can take months or years [7]. In addition, most microorganisms cannot metabolize all hydrocarbons in the same way. Rather, they have a „main degradation profile“ (Table 1, [8]). Table 1. Petroleum hydrocarbon-degrading bacteria and their preferred degradation substrates. 2. Relevance for degradation tests Well, what does it matter whether a lubricant has been degraded by 50, 60 or 70% after 28 days in a laboratory degradation test? Even if a lubricant has jumped the 60% hurdle of a test in the laboratory, the speed of real degradation in nature remains completely unclear. For this reason, the focus of the laboratory test is only on the comparison of different lubricants. To determine this comparison with possible advantages in terms of degradation rate for a product, the test parameters must be precisely defined and as comparable as possible. In addition to the general laboratory parameters (GLP), the inoculum with its microorganisms plays the most important role in the degradation process. But can an inoculum, which consequently comes from different sources depending on the location, do justice to this comparability? Even the composition of the microorganisms of an activated sludge from one and the same wastewater treatment plant shows great temporary differences (Table 2, [9]). Table 2. Variability of the „same“ activated sludge. Date Number of diff. genera March 2022 215 June 2022 323 July 2022 235 August 2022 309 216 24th International Colloquium Tribology - January 2024 The Importance of Inocula for Biodegradation Testing of Lubricants The idea of a precise inoculum with a defined set of oil-degrading specialists has already been extensively discussed and rejected. Since then, laboratories mostly used activated sludge from the local sewage treatment plant as an inoculum, as required by various degradation standards. As a criterion for the suitability of the inoculum, only the number of colony-forming units (CFU) is determined. The number of CFU should be in the range of 10 3 - 10 6 / ml colony-forming units in the test vessel. The species or genera of microorganisms are not further questioned, although a close correlation between diversity of degrading microorganisms and biodegradability is evident (Figure 1). Figure 1.: Biodegradation of mixed bacterial strains. 3. Conclusion & Outlook As the „main player“, the inoculum is of enormous importance for testing the biodegradability of lubricants. Due to the large variability in diversity of inocula from wastewater treatment plants, the inoculum should be characterized and go through a process of suitability-check before use. The method of next-generation sequencing (NGS), which can provide information about the individual composition and diversity of the inoculum, seems to be suitable for this purpose. References [1] www.wissenschaft.de/ allgemein/ wie-die-natur-dasoelverdaut (2012). [2] Koska et al. (2011) Hydrocarbon-degrading bacteria and the bacterial community response in gulf of Mexico beach sands impacted by the deepwater horizon oil spill. Appl. Environ. Microbiol., 77(22), 7962-74. [3] Liu, Z. and Liu, J. (2019) Evaluating bacterial community structures in oil collected from the sea surface and sediment in the northern Gulf of Mexico after the Deepwater Horizon oil spill. MicrobiologyOpen, 2. [4] Looper, J. K. et al. (2013) Microbial comunity analysis of Deepwater Horizon oil-spill impacted sites along the Gulf coast using functional and phylogenetic markers. Environ. Sci: Processes Impacts, 15 (11), 2068−2079. [5] Leahy, J. G., and R. R. Colwell. (1990) Microbial degradation of hydrocarbons in the environment. Microbiol. Rev. 54: 305-315. [6] Maeng, J. H. et al. (1996) Diversity of dioxygenases that catalyze the first step of oxidation of long-chain n-alkanes in Acinetobacter sp. M-1. FEMS Microbiol. Lett. 141: 177-182. [7] Rehm, H. J., and I. Reiff. (1981) Mechanisms and occurrence of microbial oxidation of long-chain alkanes, p. 175-215. In A. Fiechter (ed.), Advances in biochemical engineering, vol. 19. Springer-Verlag, Berlin, Germany. [8] Xu, X. et al. (2018) Petroleum Hydrocarbon-Degrading Bacteria for the Remediation of Oil Pollution Under Aerobic Conditions: A Perspective Analysis. Front. Microbiol. 9: 2885. [9] Weyandt, R. SGS Fresenius, UNITI Workshop Berlin, March 28 th 2023. 24th International Colloquium Tribology - January 2024 217 Active, Real-Time Friction Control with ElectroAdhesion: Application to Soft Contacts for Augmented Tactile Perception Luigi Portaluri 1* , Luciana Algieri 2 , Massimo De Vittorio 1,2 and Michele Scaraggi 1,2,3** 1 Department of Engineering for Innovation, University of Salento, Lecce, IT 2 Center for Biomolecular Nanotechnologies, Istituto Italiano di Tecnologia, Arnesano (LE), IT 3 Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, UK * luigi.portaluri@iit.it ** michele.scaraggi@unisalento.it 1. Introduction Virtual reality technologies make it possible to interact in an environment recreated entirely by a computer. The sense of sight and hearing are reproduced through visors and headsets respectively, which have developed exponentially in recent years, reproducing reality almost faithfully. Haptic technologies, on the other hand, enable/ modulate the sense of touch experienced by humans when touching an object, adapting in real time the mechanical interaction that occurs between the object itself and the skin tissue. The human body recognizes stimuli from the external world through the presence of receptors in the skin. When pressure is applied to the skin, the deformation stimulates these receptors, generating the sense of touch [1]. One way to achieve the sense of touch is to exploit the phenomenon of electro-adhesion (EA), where an electrical potential applied between two nominally flat conductive surfaces in contact, with an interposed dielectric gap, causes charges of opposite sign to accumulate on the contacting surfaces. This generates an electrostatic attraction (known as Maxwell stress) which, when added to the externally applied normal load, increases the actual contact area (maintaining the external normal load) [2,3,4]. This modulation of contact area determines the change in friction and adhesion (and, to name a few, electrical contact resistance, thermal contact resistance, etc.). In order to provide an augmented sense of touch, most haptic feedback studies have focused on recreating a surface through a rising/ lowering pin array. This movement of the pins was enabled by piezoelectric actuators [5], linear electromagnetic actuators [6], shape memory alloy (SMA) actuators [7], electroactive polymers, or EAPs [8]. But these displays can only be improved by making the actuator system smaller, to increase the pin density of the surface. These displays can only be improved by making the actuation system smaller, to increase the pin density of the surface. By exploiting the phenomenon of electro-adhesion, friction modulation can be obtained by simply varying the voltage applied to the display, without using an actuation system. The objective of this work was to set the basis for the control of the sliding friction occurring in the finger-touchscreen contact, with the purpose of developing a tactile feedback (augmented sense of touch) screen prototype. 2. Materials and method The phenomenon of electro-adhesion (EA) for soft contacts was investigated to mimicking the human finger vs touchscreen interaction. A soft rubber spherical sample was fabricated for mimicking the effective elastic properties of the human finger, whereas electrical conductivite is added by coating the prototyped finger with a compliant conductive layer. The prototyped finger is thus put in contact with a home-made EA-active transparent display, and the contacting pair dynamically actuated in pure squeeze motion thanks to a novel home-made thermally-controlled opto-electromechanical tribometer. The latter allows to quantify the extent to which the applied voltage, thus the EA, is effective into increasing, reversibly, the true contact area between the prototyped finger and the EA-display, thus to provide a modulated active interaction and, finally, sense of touch. The prototyped finger (Figure 1.a). was developed using soft Polydimethylsiloxane (PDMS), hemisphere shaped, covered with a rough layer (mimicking the finger small scale roughness) of a stretchable and conductive polymer Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS). EA-active screen (Figure 1.b) was built through a microscope slide covered with a transparent and conductive patterned thin layer of Indium Tin Oxide (ITO), on the top of which a few µm Parylene C optically-transparent dielectric layer was deposited. The prototyped finger is attached at a load cell to evaluate the load present in the contact. The Johnson-Kendall-Roberts (JKR) experimental technique was adopted to quantify the EA-tailored adhesion and contact formation in the contact pair described above. In the typical experiment: 1. the PEDOT covered PDMS dome is moved in contact at constant approach speed of 0.05mm/ s until a squeezing load of 0.1 N is reached; 2. the contact is left under constant penetration for 5 s; 3. the dome is moved out of contact at constant speed of 0.05 mm/ s until the original (starting) position is reached. Cycle 1) to 3) is repeated N times at the same voltage, then a new voltage is set and the cycle 1) to 3) repeated again N times until the total number M of voltage is tested. The adopted voltages are 0, 10, 20, 40 and 80 V. Active, Real-Time Friction Control with ElectroAdhesion: Application to Soft Contacts for Augmented Tactile Perception 218 24th International Colloquium Tribology - January 2024 Figure 1. (a) PDMS hemispherical dome coated with a thin layer of PEDOT: PSS to replicate the surface and geometrical finger properties. (b) Microscope slide coated with 35 nm layer of ITO and a thin film of ParyleneC to replicate a generic transparent touchscreen. (c) Schematic of the prototypes contact stack. (d) Contact area at different voltage with 4x magnification. (e) Schematic of the opto-electro-mechanical tribometer adopted in this study. (f) Pull-off force measured at different voltage. 3. Conclusion Figure 1.d shows how the contact area remarkably increases with increasing applied voltage, and this finally qualitatively validate the adoption of EA as a physical approach to augment the interaction between two solids in a controllable way, i.e., it is expected a higher pull-off force and contact area as the applied voltage increases. Also the pull-off (Figure 1.f) was successfully measured to quantify the strength of the EA process through the rough interaction, mimicking the finger vs touchscreen contact. The greatest effect of electroadhesion occurs at 80V. Applying the other voltages, the work remains constant or increases slowly. References [1] Johnson KO, Yoshioka T, Vega-Bermudez F. Tactile functions of mechanoreceptive afferents innervating the hand. J Clin Neurophysiol. 2000 Nov; 17(6): 539- 58. doi: 10.1097/ 00004691-200011000-00002. PMID: 11151974. [2] Ayyildiz M, Scaraggi M, Sirin O, Basdogan C, Persson BNJ. Contact mechanics between the human finger and a touchscreen under electroadhesion. Proc Natl Acad Sci U S A. 2018 Dec 11; 115(50): 12668-12673. doi: 10.1073/ pnas.1811750115. Epub 2018 Nov 27. PMID: 30482858; PMCID: PMC6294909. [3] Omer Sirin. Mehmet Ayyildiz. Bo Persson. Cagatay Basdogan. Electroadhesion with application to touchscreens. Soft Matter. 10.1039/ C8SM02420K. [4] Persson BNJ. The dependency of adhesion and friction on electrostatic attraction. J Chem Phys. 2018 Apr 14; 148(14): 144701. doi: 10.1063/ 1.5024038. PMID: 29655360. [5] Wang, Qing-Ming & Du, Xiao Hong & Xu, Baomin & Cross, L.. (1999). Theoretical analysis of the sensor effect of cantilever piezoelectric benders. Journal of Applied Physics. 85. 1702-1712. 10.1063/ 1.369314. [6] Juan José Zárate, Giordano Tosolini, Simona Petroni, Massimo De Vittorio, Herbert Shea. Optimization of the force and power consumption of a microfabricated magnetic actuator.2015. [7] Robert D Howe, Dimitrios A Kontarinis, and William J Peine. Shape memory alloy actuator controller design for tactile displays. Proceedings of 1995 34th IEEE Conference on Decision and Control, New Orleans, LA, USA, 1995, pp. 3540-3544 vol.4, doi: 10.1109/ CDC. 1995. 479133. [8] Hidenori Okuzaki, Satoshi Takagi, Fumiya Hishiki, Ryo Tanigawa. Ionic liquid/ polyurethane/ PEDOT: PSS composites for electro-active polymer actuators. 2014. 24th International Colloquium Tribology - January 2024 219 Limit Values for the Evaluation of Lubricant Analyses Stefan Mitterer OELCHECK GmbH, Brannenburg, Germany stm@oelcheck.de 1. Introduction The analysis of lubricants, but also coolants or fuels, is an established field for the condition monitoring of machines. This results in recommendations for action that are included in the maintenance of the machines. The analyses are used in the areas of R&D, maintenance or during operation at the end customer. Important requirements for monitoring are an analysis scope suitable for the application and the use of standardized test methods. An analysis in the laboratory can include 40-50 measured values per sample, which provide information on the condition of the lubricant and the machine itself. 2. Limit values and their benefit In order to derive recommendations for action from measured parameters, appropriate limit values are required as orientation in addition to the knowledge and experience of an evaluating tribologist. The limit values can be defined for the various categories on a laboratory report. These include: - Wear, e.g. iron, copper - Impurities, e.g. water, sodium - Oil condition, e.g. viscosity, oxidation - Additives, e.g. calcium, phosphorus - Various additional tests, e.g. cleanliness class, MPC, i-pH value Lubricant and fuel analysis are used in a wide range of industries. Roughly, this can be shown in the following overview. These are the branches in which OELCHECK is active with analytics: Diagram 1: Main branches for lubricant analyses This very large scope already suggests that the performance and evaluations of analyses must be adapted to the respective concerns. Obviously, there is no fixed evaluation scheme that can be used for all analyses in the same way. More precisely, the following aspects play an important role in the diversity of evaluation criteria that must be used in the evaluation of analyses: 3 Different applications require individual limits  no general scheme available 3 Stress level is very different (hydraulics, engines, gears, transformers…) 3 Within one application: “hydraulic system is not just a hydraulic system” 3 Wide range of surrounding conditions 3 Different requirements for the evaluation of the analyses 3 Optimizing the availability of machines 3 Avoidance of unplannend downtimes The question arises, which possibilities are available to define corresponding limit values and to ensure their relevance. For the development and implementation of such values, various procedures are presented in the presentation: 1. Limit values according to existing specifications 2. Limit values from the laboratory because of the data basis and the experience from many analyses 3. Limit values in agreement with customers and as a result of feedback from the field It must be ensured that the stored limits are automatically made available during the evaluation of samples in order to be able to provide the customer with a targeted recommendation. With the help of a sophisticated system, these parameters can be used for general applications (e.g. gearboxes of wind turbines) as well as for a single machine. The system for processing must be easy and intelligent to ensure adjustments at any time. However, the requirement for many lubricant analyses is not only to evaluate according to fixed limit values. Information such as oil operating time, oil volume or the trendline of samples should also be included in the evaluation to avoid misinterpretations. 3. Complex analysis options With the help of limit values not only individual samples from individual machines can be analyzed and evaluated. Rather, evaluations of large amounts of data can then also be carried out. However, an important prerequisite for this is to have a stable data structure available. This includes correct information about the machines, the lubricants used, running times, etc.. If you want to evaluate not only individual laboratory reports, but also a large number of samples and associated machines as part of a data analysis, it is important to have knowledge of the existing data structure. When important information about the application or the lubricants is missing, a Big Data analysis becomes more difficult. If information on the oil types used is missing, a more in-depth evaluation with regard to changes in fresh oil-specific val- 220 24th International Colloquium Tribology - January 2024 Limit Values for the Evaluation of Lubricant Analyses ues (additives, viscosity-index, changes in the infrared spectrum...) can only be partially possible. If the above-mentioned prerequisites are met, extensive evaluations can be implemented on the basis of the analysis data. Various concerns and questions can now be looked at more closely for customers. Some of these questions are, for example: Which machine types often show conspicuous analysis values? Which analysis values are mostly critical? What correlations can be derived from this? These are just a few concerns that may be of interest to a maintenance or service department. As shown in diagram 2, it is possible to evaluate which values were mostly critical in the analyzes and which conclusions can be drawn from them. In the case in question, the customer’s machines frequently have high water values, which result in reactions and thus problems with the additive package (phosphorus). The problem here is therefore not primarily due to overheating of the oil or mixing of lubricants or mechanical wear, but was primarily to be found in defective seals in moisture entering. In a further evaluation, individual machine types of the customer could be identified which were particularly affected here. This makes it possible to identify which adjustment screws on machines need to be addressed and optimized in order to plan service activities better or to avoid unexpected problems. Diagram 2: Critical values in a machine fleet 4. Meaningfulness The definition of limit values is an important tool for ensuring that lubricants remain in use for as long as possible. Ultimately, this also contributes to the conservation and sustainable use of resources. Using practical examples, the presentation will show procedures for creating and optimizing threshold values, as well as how to deal with these values during evaluation. Furthermore, the digital possibilities for the evaluation of a large amount of analytical data will be shown. 24th International Colloquium Tribology - January 2024 221 The European Tribology Centre Tribology as a Service towards a Sustainable World Franz Pirker 1 , Alberto Alberdi 1 , Xavier Borras 1* 1 i-TRIBOMAT: The European Tribology Centre, Wiener Neustadt, Austria * Corresponding author: xavier.borras@i-tribomat.eu 1. Introduction Tribology is relevant to everyone because it impacts various aspects of daily life. It affects the performance and efficiency of machines such as vehicles, appliances, and equipment, leading to cost savings and reduced environmental impact. Improved tribology results in a longer lifespan of products and a reduced maintenance costs. Additionally, tribology plays a crucial role in many industries, including transportation, manufacturing, and healthcare, directly affecting the economy and quality of life. A better understanding of tribology leads to advancements in technology, improved product design, lower environmental impact, and increased safety. The potential for saving energy and reducing costs was the key reason to define tribology as its own scientific discipline. Nowadays, digitalisation and the Green Deal are the drivers and challenges at the forefront of industrial development and take first place on the agenda of the European Union. The European Green Deal is a set of initiatives from policy side by the European Commission with the central aim to make Europe climate neutral in 2050. An impact assessed plan will also be implemented to increase the EU’s greenhouse gas emission reductions for 2030 to at least 50% and towards 55% compared with 1990 levels. Lowering the friction and wear by developing new material solutions for tribological applications has substantial environmental effects and directs towards a low carbon footprint future. The study carried out on the influence of tribology on global energy consumption, costs, and emissions, by Holmberg and Erdemir in 2017 [1], concluded that in total, about 23% (119 EJ) of the world’s total energy consumption and 8120 Mt/ year of CO 2 emissions originates from tribological contacts. Digitalization has been accurately defined as the use of digital technologies to change a business model and provide new revenue and value-producing opportunities; it is the process of moving to a digital business. Very illustrative examples of the implementation of digital business models are the so called GAFAs. These companies are the biggest tech giants in the world, and they all have in common a platform where the customer can use and buy services easily and quickly on a transparent cost basis. In the technological field, there are the first attempts at digital business models such as SaaS (Software as a Service). The advantage of SaaS is that the client does not require to install any software or using a computer with high computing power to run simulations. 2. i-TRIBOMAT: The European Tribology Centre With the European H2020 research project i-TRIBOMAT („Intelligent Open Test Bed for Tribological Materials Characterisation “), the path towards TaaS - Tribology as a Service - is presented. i-TRIBOMAT: The European Tribology Centre was funded in February 2023 to support industry to contribute to the European Green Deal initiatives through a new digital business model. i-TRIBOMAT developed new digital services, which facilitate the rapid and cost-efficient selection of materials, as well as the prediction of the tribological performance of products regarding efficiency and lifetime. i-TRIBOMAT connects the entire tribological characterisation infrastructure of five leading European research centres in tribology and links it to an IT-platform using IoT technology. The clients can choose between over 100 different material and tribology characterisation tools. The data is centrally stored and further processed in a newly developed cloudbased material database. The clients can access their data any time and can easily request an advanced analysis or create their own reports. Without needing a particular expertise, clients can carry out simulations in virtual workrooms, allowing them to use their material data to predict operational characteristics rapidly and cost-efficiently, without constructing a prototype. All digital services can be customised and booked by the client on the web-based platform. The connection of infrastructures and the new digital services result in the emergence of Europe’s largest tribology centre, which offers and markets all services on a web-based platform. i-TRIBOMAT: The European Tribology Centre ’s core is a platform on which various services can be booked - from standardised tribometer tests and characterisation services to data driven services and simulations based on a SaaS concept. All these three kinds of services combined are representing the integrated workflow to upscale materials performance in a tribological component. If a customer wants to know how a new material performs in his component under operation, the material will be tribologically tested, the test results and test data will be stored in a secure manner, and ultimately integrated into simulation models seamlessly. These models upscale the performance of the component with the input of real test data. These leads to an accelerated component development process saving time and costs for the customer. 3. Success stories The following are examples of how the ETC infrastructure has been used to deliver relevant results to European companies in different sectors and fields of application. 222 24th International Colloquium Tribology - January 2024 The European Tribology Centre 3.1 Friction and wear performance of bronze-based materials in air and vacuum. At the request of the company Federal-Mogul DEVA GmbH, the friction and wear behaviour of different low friction copper-tin alloys with low-friction additives, such as graphite, were tested at room temperature in air and vacuum environments using the pin-on-disc testing technique, using Cu-Sn pins against X5CrNi18-10 (1.4301) steel discs. Typical uses of these materials are sliding or cylindrical bushing, joint bearings, and wind- & gas-turbines. Figure 1 shows the time evolution of the friction coefficient in three of these tests (test V4 in air, and tests V8 and V11 under vacuum, at 4 x 10-6 mbar). Figure 1: CoF time evolution 3.2 Friction and wear performance of PVD TiHfCN and TiN coatings. The main applications of these coatings are bearing support and guide rotating or oscillating machine elements and transfer loads between machine components. They must provide high precision and low friction to achieve high speeds while reducing heat generation, energy consumption and wear. In a study carried out for CITRA, the friction coefficient of TiN and TiHfCN PVD coatings applied on discs were tested against Al2O3 balls at 200 °C and 400 °C. Figure 2 shows how TiHfCN coatings improve friction behaviour in comparation with conventional TiN coatings. Figure 2: The friction evolution of (a) TiN coated discs and (b) TiHfCN coated discs sliding against Al2O3 balls in pin-on-disc tests at 200 °C temperature. 3.3 Performance of hydraulic, pneumatic, and rotary seals The objective of this study, carried out for the company EPI- DOR Seals and Rubber Technology was to see the effect of friction and wear in seals that are made in HPU, NBR, and FPM to avoid contamination (oil-ambient interface), and maximize the lifespan of their components. A Finite Elements (FE) model of the seal was developed using the seal profile and operating conditions given by the customer (Figure 3). Once a representative seal-shaft contact pressure was deduced, tribological experiments were carried out on pins of the three different sealing materials (Figure 4). Therefore. this service request counted with two stages: a) Downscaling (modelling). b) Experimental Testing. Figure 3: Finite Elements model of the reciprocating seal Figure 4: Frictional hysteresis loop resulting from the three sealing materials tested (HPU, NBR, FPM). References [1] K. Holmberg, A. Erdemir, Friction 5(3) 263 (2017) 24th International Colloquium Tribology - January 2024 223 Tribological Investigations under Varying Pressure Atmosphere Introduction of a novel Tribotest Option Felix S. M. Zak 1 , Ameneh Schneider 1 , Gregor Patzer 1 1 Optimol Instruments Prüftechnik GmbH, Munich, Germany * felix.zak@optimol-instruments.de 1. Introduction Pressurized gas atmospheres are commonly encountered in aerospace, oil and gas, energy generation and various manufacturing processes. The behavior of materials and lubricants under elevated pressure conditions can significantly impact the efficiency, safety, and reliability of these systems. However, traditional tribology test methods often fall short in accurately simulating the complex conditions found within pressurized gas environments. A novel option for tribotesting is presented, specifically designed to operate under different pressurized gas atmospheres, especially hydrogen. The development of this test option addresses the need to simulate real-world conditions encountered in many industrial applications where components and materials are subject to mechanical friction and wear in specific gas environments. By exposing test samples to controlled gas atmospheres at different pressure levels, the Tribotest option provides a valuable tool for evaluating the tribosystem in terms of performance and durability of materials under different operating conditions. 2. State of the art Velkavrh et al. demonstrated the significant influence of gaseous atmospheres in their research [1]. They expanded the SRV ® 4 tribometer by incorporating a rubber gas chamber and filled it with various gases. Their study unveiled substantial impacts on wear behavior. Due to the elastic pressure chamber, the experiments were only feasible at a low overpressure of 0.02 bar. Mishina in 1992 already emphasized the influence of gas pressure, manifested through gas molecule movement, on the tribological behavior of the system. [2] Moreover, NASA research groups in the 1960s delved into atmospheric influences, categorizing them into inert, reducing, and oxidizing atmospheres, although the influence under pressure was not the primary focus [3]. Subsequent investigations mostly centered on vacuum or low-pressure experiments. [4] Balasooriya et al. outlined current limitations and forthcoming challenges associated with pressurized hydrogen atmospheres concerning the mechanical requirements of materials utilized [5] This study can be extended to encompass tribologically relevant aspects. Gachot et al. successfully modified a friction and wear testing setup to conduct experiments under a pressure gas atmosphere of up to 10 bar, yielding distinguishable results under CO 2 atmosphere [6]. Noteworthy institutions have conducted studies under high-pressure hydrogen atmospheres, identifying effects related to wear behavior [7,8,9]. However, the research landscape remains insufficient given the paramount importance of this theme. 3. Design and Features The new Tribotest option presents an extension module of the SRV ® 5, an established testing machine for tribological investigations. It retains the standard specifications of the tribometer and additional adjustable parameters, like the pressure up to 100 bar The technical dependencies of set values, in the form of the tribological loading spectrum, give rise to several limitations. These dependencies require additional corrections within the regulation, such as compensating for force due to pressure differentials between the pressure chamber and the environment, necessary for normal force control. The Tribotest option offers variable adjustment of the surrounding atmospheric pressure, capable of encompassing various gases, with a primary focus on hydrogen. Due to hydrogen’s unique properties, predominantly stemming from its small molecular size and associated hazards concerning its high energy density, specific design considerations are imperative. The small molecule size, governed by Graham’s law, results in a high diffusion rate. To counter this, aside from hermetic sealing, an additional protective gas layer is employed. This protective gas (nitrogen) operates at the same pressure as the test gas, conferring advantages in terms of the system’s dynamic sealing. The pressure is measured in both circuits and controlled separately and also recorded for examination purposes. Given the hazards posed by different test gases, particularly hydrogen, an elaborate safety concept is pursued. This includes the utilization of external sensors for detection purposes and consideration of safety parameters, such as the maximum allowable temperature for sealing materials, through continuous monitoring and shutdown mechanisms. In addition to diffusion, other effects emerge when dealing with hydrogen, such as increased permeation and hydrogen embrittlement. These are mitigated through suitable material selection. For permeation, a specific polymer blend, verified through permeation testing, has been employed. The amalgamation of pressure tightness and the dynamic load from tribological investigations presents diverse challenges to individual components within the sealing concept. To counter these challenges, appropriate kinematics and well-chosen sealing methods have been implemented. No detectable leakage of the test gas is tolerated, while minimal leakage of the protective gas is accepted. Parasitic forces, like spring forces or damping forces resulting from the sealing components, are minimized through suitable calibration processes, corrections, and a deliberate distribution of stiffness and mass within the testing system. Despite the necessity for a substantially robust construction due to pressure-induced requirements, the Tribotest option 224 24th International Colloquium Tribology - January 2024 Tribological Investigations under Varying Pressure Atmosphere offers temperature measurement close to the friction point for regulating the test body contact or measuring friction-induced temperature rise. 4. Experimental Validation Initial attempts to demonstrate the specified kinematic characteristics of the construction under normal load were successfully conducted in sub-trials. Likewise, evaluations regarding pressure stability within the test gas region were accomplished. However, due to numerous optimizations of the Tribotest option throughout the development process, the completion of a full test series was not achieved by the time of submission. These pending experiments will be presented and elucidated in subsequent works. Various standard test procedures are being employed under additional pressurized test gas atmospheres (constant or variable) in forthcoming trials. Owing to the existing dataset, well-established test materials under dry and lubricated conditions are utilized for comparative purposes with normal environmental conditions. 5. Conclusion In conclusion, the exploration of pressurized gas atmospheres within tribological investigations holds significant implications across diverse industrial sectors. Traditional tribology test methods often lack precision in replicating the intricate conditions encountered in pressurized gas environments. The development of the novel Tribotest option addresses this gap, offering a specialized testing apparatus designed explicitly to operate under various pressurized gas atmospheres. This advancement aligns with the critical need to simulate real-world scenarios where materials and components undergo mechanical friction and wear within specific gas environments. By subjecting test samples to controlled gas atmospheres at different pressure levels, the Tribotest option emerges as a crucial tool for assessing material performance and durability under varied operating conditions. The collaborative efforts of researchers and institutions pave the way for comprehensive studies, promising advancements in material engineering and industrial applications within diverse pressure-based operational contexts. Figure 1: Design of Tribotest option References [1] I. Velkavrh, F. Ausserer, S. Klien, J. Brenner, P. Forêt, A. Diemeferences: The effect of gaseous atmospheres on friction and wear of steel-steel contacts. Tribology International 79, 99-110, Elsevier Ltd, 2014. doi.org/ 10.1016/ j.triboint.2014.05.027 [2] H. Mishina: Atmospheric characteristics in friction and wear of metals. Wear 152, 99-110, Elsevier Ltd, 1992. doi.10.1016/ 0043-1648(92)90207-O [3] D.H. Buckley, R.L. Johnson: EFFECT OF INERT, RE- DUCING, AND OXIDIZING ATMOSPHERES ON FRICTION AND WEAR OF METALS TO 1000°F. NASA, D-1103, 1961. [4] D.H. Buckley: FRICTION, WEAR, AND LUBRICA- TION IN VACUUM. NASA SP-277, 1971. [5] W. Balasooriya, C. Clute, B. Schrittesser, G. Pinter A Review on Applicability, Limitations, and Improvements of Polymeric Materials in High-Pressure Hydrogen Gas Atmospheres, Polymer Reviews, 62: 1, 175- 209, 2022 DOI: 10.1080/ 15583724.2021.1897997 [6] F. Ausserer, I. Velkavrh, F. Kafexhiu, C. Gachot: Experimentelle Methodik für die Prüfung tribologischer Systeme unter einer Druckgasatmosphäre. GfT Tagungsband 2023, 77-81. [7] K. Nakashima, A. Yamaguchi, Y. Kurono, Y. Sawae, T. Murakami, J. Sugimura. Effect of high-pressure hydrogen exposure on wear of polytetrafluoroethylene sliding against stainless steel. Journal of Engineering Tribology. 2010; 224(3): 285-292. doi: 10.1243/ 13506501JET642 [8] E. R. Duranty, T.J. Roosendaal, S. G. Pitman, J. C. Tucker, S. L. Owsley Jr., J. D. Suter, K. J. Alvine: In Situ High Pressure Hydrogen Tribological Testing of Common Polymer Materials Used in the Hydrogen Delivery Infrastructure. Vis. Exp. (133), e56884, doi: 10.3791/ 56884 (2018) [9] Y. Sawae, K. Fukuda, E. Miyakoshi, S. Doi, H. Watanabe, K. Nakashima, J. Sugimura: Tribological Characterization of Polymeric Sealing Materials in High Pressure Hydrogen Gas. STLE/ ASME 2010 International Joint Tribology Conference. San Francisco, California, USA. October 17-20, 2010. pp. 251-253. ASME. https: / / doi.org/ 10.1115/ IJTC2010-412 24th International Colloquium Tribology - January 2024 225 Efficiency Improvements of In-Situ Hydrogen Permeation Measurements in Lubricated Bearing Steel Contacts Using the Modified Devanathan-Stachurski Cell (MDSC) Method Edward Vernon-Stroud 1 , Ajay Pratap Singh Lodhi 1* , Frederick Pessu 1 , Ivan Delic 2 , Nicole Dörr 2 , Markus Varga 2 , Josef Brenner 2 , Ardian Morina 1* 1 Institute of Functional Surfaces, University of Leeds, Leeds, United Kingdom 2 AC2T research GmbH, Wiener Neustadt, Austria * Corresponding author: A.P.S.Lodhi@leeds.ac.uk; a.morina@leeds.ac.uk 1. Introduction The ever-increasing need for products towards sustainability, combined with the increasing demands for product reliability and ease of maintenance, presents a unique challenge to all forms of engineering, especially in the field of tribology. In wind turbine gearbox, one of the most common causes of failure is thought to be due to hydrogen embrittlement during operation of the steel bodies used in the tribosystems, leading to downtime and costly repairs. Hydrogen is produced in the system through lubricant decomposition and water contamination, which then permeates the steel structure and becomes irreversibly trapped. This impairs the mechanical properties of the steel and creates micro-fractures such as those seen in White Etching Cracks (WECs), which then propagate and lead to total bearing or gear failure if not detected in time. These failures severely impact the sustainability of these technologies, to the detriment of both the public and businesses alike [1-4] which can lead to the failure of wind turbines. The mechanisms of hydrogen diffusion in bearings are not yet fully understood, but it is assumed under tribological loading that the lubricant degradation releases hydrogen which diffuses into the contacting surfaces (i.e. steel). Real time study of hydrogen generation/ permeation in a tribological contact is a challenging task to perform accurately. In the recent five years, a method was developed by the researchers using a Modified Devanthan-Stachurski Cell (MDSC). This method was then utilized to study and measure the hydrogen permeation through a thin steel membrane. In the current study, this method was re-evaluated, and several potential improvements were identified and addressed with the aim to reduce the time and resources required to complete each test. 2. Methodology 2.1 Cell design A new electrochemical cell or hydrogen detection cell (MDSC) was designed (see Fig. 1) and manufactured to work with a Cameron-Plint TE77 reciprocating pin-on-plate tribometer and validated by completing tests using different base oils, base oil-water mixture and deionized (DI) water. Water is well known to increase hydrogen permeation in bearing steels due to its low lubricity (high friction) and presence of hydrogen. The 3-electrode (counter, working, and reference) cell used to oxidise the permeated hydrogen consisted of an EN31 bearing steel membrane as working electrode, Ag/ AgCl electrode as reference electrode and a platinum wire as counter electrode. The 0.1M deaerated sodium hydroxide solution was used as an electrolyte. Fig. 1 (a) Modified Devanathan-Stachurski Cell (MDSC) design and (b) MDSC cross-section 2.2 Material Poly-alpha-olefin (PAO4), perfluorinated polyether (PFPE), DI water and a mixture of PAO4 and DI water were selected as lubricant to measure the hydrogen permeation current density. EN31 steel was selected as both disc (0.8 mm thick and 40 mm diameter) and pin (10 mm diameter) material. Steel samples were polished up to 600 grit paper on the sliding side. The other side (detection side) of the steel sample was polished using up to 1200 grit paper and diamond paste (6 mm, 3 mm, and 0.5 mm). After polishing, polished samples were cleaned using ultrasonic cleaner for 3 minutes in various solvents (acetone, propanol, ultra-pure DI water) and dried using a N 2 gun. After cleaning, the detection side of the polished steel sample was coated by a 100 nm thick palladium (Pd) layer for permeation current measurement. 2.3 Experimental parameters Table 1 show the experimental parameters selected to conduct the hydrogen permeation measurements. 226 24th International Colloquium Tribology - January 2024 Efficiency Improvements of In-Situ Hydrogen Permeation Measurements in Lubricated Bearing Steel Contacts Using the MDSC Method Table 1 Tribomaterial and electrochemical parameters Process parameters Values Load (N) and contact pressure (GPa) 50 Stroke length (mm) 7 Frequency (Hz) and sliding speed (m/ s) 10 and 0.14 Sliding and stabilization time (hr) 2.5 Polarization current (V) +0.115 V Temperature (°C) Ambient 3. Results During sliding between steel disc and pin, the hydrogen atoms generated from the surrounding environment (lubricant decomposition, presence of water, etc.) permeated through the steel membrane and reached the opposite face of the steel membrane for detection. The Pd coating on the detection side oxidized these hydrogen atoms. The amount of hydrogen permeated through the steel membrane was measured by the oxidizing current density (hydrogen oxidation current). The increment in current density confirms the hydrogen generation and detection. Fig. 2 shows the variation in current density with sliding time for the selected lubricants. The increment in current density was observed highest for pure water. While for base oils (PAO and PFPE) the permeation current was observed lowest, as expected. The current for the oil-water mixture was observed between the pure water and the base oils. For PAO, the permeation current is higher compared to PFPE. It may be due to the presence of hydrogen in PAO, which is a hydrocarbon-based oil, whereas PFPE as perfluorinated oil does not contain hydrogen in its chemical structure. Fig. 2 Variation in hydrogen permeation current with sliding time 4. Conclusion The results from the validation tests showed that the newly designed Modified Devanthan-Stachurski Cell (MDSC) is able to accurately detect varying levels of hydrogen permeation for different base oils or base oil-water mixtures. The newly designed cell significantly reduced the time required for testing from 8-12 hrs to 2-3- hrs. After these validation tests, base oils with potential future gearbox applications have been selected and are being tested. The hydrogen permeation characteristics obtained from these tests shall be compared with commonly used PAO base oil. The validation of this experimental setup in combination with the testing completed opens the door to many more opportunities regarding efficient testing of a wide variety of lubricants, additives, or materials. It is hoped that the findings from this work can be used to informed decisions for the next generation of sustainable gearbox oils. References [1] Oberle N, Amann T, Kürten D, et al (2020) In-situ-determination of tribologically induced hydrogen permeation using electrochemical methods. Proc Inst Mech Eng Part J J Eng Tribol 234: 1027-1034. https: / / doi. org/ 10.1177/ 1350650119889196 [2] Evans MH (2016) An updated review: white etching cracks (WECs) and axial cracks in wind turbine gearbox bearings. Mater Sci Technol (United Kingdom) 32: 1133-1169. https: / / doi.org/ 10.1080/ 02670836.201 5.1133022 [3] Wranik J, Holweger W, Lutz T, et al (2022) A Study on Decisive Early Stages in White Etching Crack Formation Induced by Lubrication. Lubricants 10: 1-17. https: / / doi.org/ 10.3390/ lubricants10050096 [4] Esfahani EA, Soltanahmadi S, Morina A, et al (2020) The multiple roles of a chemical tribofilm in hydrogen uptake from lubricated rubbing contacts. Tribol Int 146: 106023. https: / / doi.org/ 10.1016/ j.triboint.2019.106023 24th International Colloquium Tribology - January 2024 227 Parallel Wear Testing - an Update Can We Produce Enough Data to Enable AI in Tribology? Lais Lopes 1 , Dirk Drees 1 , Pedro Bai-o 1 , Emmanouil Georgiou 2 1 Falex Tribology NV, Rotselaar, Belgium 2 Hellenic Air-Force Academy, Faculty of Aerospace Studies, Dekelia Air Force Base, Athens, Greece * Corresponding author: ddrees@falex.eu 1. Introduction The durability of materials (wear resistance) or anti-wear efficiency of lubricants can be measured in many different ways, both non-standardized and standardized. Non-standard methods are used to be close to an application. However, such methods cannot be used to compare materials or lubricants systematically from one another. In the case of polymers, for instance, available standard methods are very limited, the major ones being ASTM D3702 and G137. Each method has their advantage, but both have one important drawback : there is a large variation in test results. This large variation is mostly the result of inhomogeneity in materials and wear mechanisms, and can not be reduced by better test methods. Dry wear of polymers is inherently a very stochastic property. Since it is not feasible, and even not useful to improve on the repeatability of the test methods - due to the lack of repeatability in the materials themselves, it is a better approach to consider multiple data points per materal, and to use trends and outlier statistics to evaluate wear resistance. Multiple data points can be easily produced by reversing the mind-set of the tribological experiment : instead of producing one very expensive test result with an equally expensive single station tribometer with many sensors, we aim to generate only wear data with a multi-station ‘wear generator’ with a single measurand, namely material wear loss. This enables the plotting of wear trends, rather than a single point with a very wide variation. The risk of misinterpreting outliers reduces, and an interpretation of wear evolution becomes more feasible. The ability to produce larger datasets economically, may also open possibilities for AI based evaluation of materials. 2. Experimental approach The wear generator used is a 10-station, constant load, constant speed system, comprised of a rotating shaft where 10 individual loading stations apply the same load to 10 test samples. The rotating shafts can be acquired in different materials, or with different coatings, but the base shafts are available at very moderate costs. This reduces the test cost considerably. The materials under study can be machined in the shapes of blocks or cylinders can be used, they are held in sample holders. 10-station wear generator setup 10-station wear generator sample holders (10) Once the machine has been set up with 10 sample holders running against the rotating shaft, the machine does not require supervision or any online monitoring devices, although it may be beneficial to add some sensors to this system. This Parallel Wear Testing - an Update 228 24th International Colloquium Tribology - January 2024 is the subject of further research and development. To date, we focus on running standard sets of conditions to compare many different materials. One such case is the wear resistance of ‘lubricant containing’ polymers. We tested the wear resistance of 16 different polymers against a standard steel shaft. Test conditions were optimized for time: test duration of no longer than 1 week, or 200.000 test cycles. The wear evaluation method is optimised, rejecting a weight loss measurement (effect of water absorption, and too light wear losses), comparing 2Dand 3D-wear scar measurements. It is found that the 3D measurement is over-complicated in this case, and gives no more information than the simpler 2D-method. The wear volume on the blocks (see picture below) can be easily estimated geometrically by measuring just the wear scar width. So a more time-efficient method has proven to be adequate for this characterisation. 3D image of a typical wear scar of plastic material test against rotating steel shaft. Typical wear scars, easily measureable with 2D (optical) technique. Illustrating typical scar width variation for a single material in the same test. 3. Results and conclusions The previous image shows the typical variation that is measured on nominally same materials. All 5 samples in this test have undergone the exact same, and parallel wear test (same test conditions), and samples were taken from a single production batch. Nevertheless, there is some notable variation in wear scars. Only by collecting enough repeats, will it be possible to conclude trends and general test results for different materials. Within a few days, a total of 16 materials, each tested 5 times (80 test results) can be produced easily. Comparison of 16 different polymer grades, in terms of wear (scar) resistance. Higher is poorer wear resistance. Note the variation per material, as indicated. Some materials are clearly more repeatable than others. Wear evolution: In addition to the total wear of a material after given duration, it is also efficient to plot a wear evolution : 10 stations are available, and 10 samples of the same material can be loaded at the same setup. Then, pairs of loading stations can be retracted after given intervals during a long test, and the wear of each can be measured after the final test duration. This quickly gives an indication of the wear evolution of a material. The following graph shows the evolution for two materials, showing in both cases a rapid run-in wear, followed by a more steady low wear rate. The question becomes : how to define a wear rate ? Total wear divided by total duration ? Or slope of the wear curve, ignoring run-in wear ? Wear evolution measured in one run : subsequent removal of wear stations. CONCLUSION: parallel wear testing opens the possibility to produce a lot of wear data efficiently, both in personnel time, materials, and characterisation methods. It allows for the first time to study the variability of materials durability, the wear evolution of materials, and the confidence level of any wear metric in an economically feasible way. 24th International Colloquium Tribology - January 2024 229 Building Tribology Application Testing to Determine Wear and Characterization of Polymer-based Composites Abstract from Versiv™ Composites Michael Katzer 1* , David Rich 2 , Diarmaid Williams 1 1 Versiv™, Kilrush, Ireland 2 Saint-Gobain, Merrimack, USA * Corresponding author: michael.katzer@versivcomposites.com 1. Introduction Tribology testing is vital for understanding, optimising, and predicting the frictional behaviour, wear resistance, and overall performance of polymer-based composites. It aids in material selection, performance optimisation, design, failure analysis, quality control, and cost reduction, ultimately leading to enhanced product performance, durability, and sustainability. It finally is the base for the correct material selection for the individual use case. 1.1 Methodology Objectives At Versiv™ Composites, our approach is comprehensive in establishing a reliable testing process close to the application, to demonstrate how to improve coating formulations, determine carrier materials and develop polymer-based composite products with optimised wear and friction performance tailored to the individual use case. Developing testing methods using customer insights Versiv™ Composites has a long-standing relationship with our customer. We were approached to develop an improved product for friction applications in a solenoid valve and a printer. The goal for the solenoid application was to produce a version with similar durability (number of cycles) to the existing model but thinner in profile to increase magnetic force. The customer requested a specific thickness level, and the challenge centred on creating the right material to ensure friction performance remained consistent while meeting the specified lifetime expectation. For the low friction lining assigned to the printer, the aim was to create a material capable of a longer lifetime, while ensuring the lowest possible friction. This project enabled the customer to access a premium market segment by introducing a durable product designed to enhance energy efficiency through its low friction value. Developing an application test Following initial sampling we received negative feedback, due largely to a long feedback loop as we relied on customers to test in the applications. To address this, we opted to create an application test capable of mimicking various scenarios related to friction usage. We acquired a tribometer after which we had to figure out a) what the right physical counterpart is to test against our polymer composite material and b) what the right parameters (speed, pressure) are to get an accelerated test and c) how to quantify that for our materials. The initial phase focused on formulating an experimental procedure that considered the aforementioned key points. Picture 1: Scheme of the experimental procedure to develop the tribology application test Co-development process to quantify wear and identify of optimal mix for real-world application Partnering with the customer, several samples, that had previously seen the application, were used to understand the impact of different parameters on the material and ensure that it correlated with the outcome in real-world use. Through benchmarking different quantification methods, e.g., wear scares, cross-section images, positive and negative value results were established and assigned. These value qualities were analysed through optical profilometry, and microscope and SEM were used to quantify the different results with wear depth and impact on the material surface and structure. Using methods and tribometer to differentiate wear performance Using techniques to quantify wear, including measuring the depth and width of the wear pattern, helped gauge the impact, aligning it with the effects of the use cases on the composite polymer materials. Profilometry was then applied to ensure changes were noted and understood due to the use of glass fabric or rigid polymers as reinforcement. Cross-section pictures helped with the observation of the impact on the different components of the composite material. 230 24th International Colloquium Tribology - January 2024 Building Tribology Application Testing to Determine Wear and Characterization of Polymer-based Composites Picture 2: Wear pattern of different polymer composite materials Tribology development (tool) / understand setting impact on materials The tool is based on a stainless-steel ball with a diameter of 6,35mm based on standards for plastic polymeric materials. Different parameters, such as the modification of pressure (single digit Newton) and speed to mimic the different load scenarios in the application, were used to understand the effect of the various settings generated by the tribology machine. Relaying back the results of testing to build the better performance product The different sample materials used, such as Fluoropolymer-coated Polyimide or Glass fabrics provided several different outcomes depending on their composition. More detail was discovered about how each component, carrier material and coating of Versiv™ Composites can influence wear and friction performance. The team’s findings also included that incorporating reinforcement materials, such as texture variations and diverse compressibility methods, can greatly enhance friction performance. Learnings from the evolution of the products The learnings garnered from this customer co-development activity initiated an internal study on how to optimise wear and friction performance by modifying the polymeric composition of dispersions that are coated on our reinforcement materials. The knowledge available in the group around filler materials was used to determine our path forward. By using different thermoplastic materials in different ratios, an optimisation could be achieved. 2. Conclusion Through demonstrating how we developed and then processed materials through a specific test matrix, conclusive evidence is provided on how to improve coating formulations and the choice of base carrier materials. These formulations, optimised by fillers and coated on the application-specific carrier material, are fit for use in the development of products where optimised wear and friction performance is essential. Sustainability and Resource Efficiency 24th International Colloquium Tribology - January 2024 233 How Oil Care Can Reduce Oil and Maintenance Costs Steffen D. Nyman 1 1 C.C.JENSEN & Noria Partner, Svendborg, Denmark 1. Why discuss reduction of oil & maintenance costs? We have known for decades that clean oil will improve system reliability and uptime, as well as prolonging component and oil life in service [1]. This is especially top of mind for owners and operators of critical machinery within power generation, marine, mining and many other industries e.g. automotive, steel and paper etc. With increasing oil and energy prices, component delivery times and pressure from governments, the reduction in maintenance cost as well as CO 2 has never been more important. Alternative ways to create electricity and heating is on everyone’s lips, but very few can grasp how much oil care and a reliable asset can contribute - the potential saving in your company is enormous! 1.1 How the session will address the problem? C.C.JENSEN has been on the journey to become CO 2 neutral since 2008, but since 1953 we have been using oil care to extend in-service-life of oils and machine components inhouse and at our customers [2]. This presentation will inspire the attendees to implement similar best practices to yield huge savings. 1.2 Implementing the best practice in this presentation including oil care will result in: • Lower power and electricity consumption • Reduced oil & maintenance cost • Reduced carbon footprint • Extended oil and machines life • Increased operational reliability and machine uptime • Less production and QSHE issues • Less handling, transportation and storage of oil and machine wear/ spare parts If you want to improve the uptime of machines, it is best done by maintaining the oil clean using a combination of good air breathers, in-line oil filters and offline depth filters, while monitoring the oil properties and cleanliness e.g. particle count acc. to ISO 4406. Particle counters are typically using optical light extinction sensors [3] and detect particles bigger or equal to 4-micron, 6-micron and 14-micron. Testing of particle distribution in used gear oil (illustration below) [4] show that normal abrasive wear results in exponential distributed particles with close to no particles of size 14 mm or above - meaning the number of tiny particles in oil multiplies and cause most harm. Figure 1: Particle distribution after abrasion test. OCM sensor. 1.3 Why is this important for the attendees? The constant pressure to reduce cost of operation, oil & maintenance as well as CO 2 emissions can be quite frustrating if you don’t know where to start or which assets you can address as the “low hanging fruits”, giving the largest savings for each Euro spend. This presentation will inspire and help you understand where the biggest savings can be found. 1.4 What will the attendees learn: - Ideas to reduce CO 2 emissions in your company and production within e.g. electricity, heating, asset management etc. - How oil care will reduce oil & maintenance costs including CO 2 emissions by means of longer oil life, less waste, fewer machine components being replaced due to wear etc. - Cases from Manufacturing Industries, Marine, Mining, Power Generation and Wind Segments will be presented. References [1] R.S. Sayles, P.B. Macpherson, Influence of wear debris on rolling contact fatigue, Rolling contact fatigue testing of bearing steels. A symposium sponsored by ASTM committee A-1 on steel, stainless steel, and related alloys, ASTM STP 771 (1982) 255-274. [2] C.C.JENSEN’s Energy and CO 2 Account, 2008-2021. CO 2 Group and Lars Questgaard. [3] Krogsøe, K.; Henneberg, M.; Eriksen, R.L. Model of a Light Extinction Sensor for Assessing Wear Particle Distribution in a Lubricated Oil System. Sensors 2018, 18, 4091. [4] Henneberg, M. Eriksen, R.L., Tribological test and optical measurements of particles and their distribution as function of wear mode; Oildoc, 2017, January 24-26 th , (Proceedings), Rosenheim Germany. 234 24th International Colloquium Tribology - January 2024 How Oil Care Can Reduce Oil and Maintenance Costs Speaker biography Steffen Nyman earned his B.Sc. degree in Mechanical Engineering in 1996 and joined technical sales for three and a half years. Since February 2000 he has been responsible for developing and conducting technical training and documentation for sales, service and technical staff. He is certified ICML Machinery Lubrication Analyst II and Technician II as well as 4-MAT trainer in adult teaching skills. He has worked as Corporate Trainer for C.C.JENSEN since 2004, conducting hundreds of customized seminars in understanding oil maintenance including oil filtration technologies for different industries like manufacturing, steel and paper, as well as marine, mining, power generation and wind. He has been representing Noria Corporation as a partner in Denmark since Dec. 2012. Speaker experience Steffen Nyman has presented in front of more than 100 people several times and has conducted hundreds of training courses/ seminars/ presentations for 10-30 people e.g. Reliable Plant Conference, Machinery Lubrication conference and OilDoc conference in 2011, 2012, 2014, 2015, 2017, 2018, 2019 and 2022 (online) and latest in May 2023. The organizations C.C.JENSEN develops and manufactures high quality oil maintenance systems with specialty in offline filters for wind turbines, steam and gas turbines, marine engines, mining equipment, quenching oil systems and numerous other lubrication and hydraulic applications. C.C.JENSEN has 70-years of experience and is represented in over 40 countries worldwide. CJC filters are well reputed for efficient removal of fine particles, water, varnish and acidity from oil and other special lubricants. Noria Corporation is a training & consulting company with partners worldwide, who conduct certified machinery lubrication maintenance training and consultant work to help companies enable reliability through better lubrication processes and applying best practice. 24th International Colloquium Tribology - January 2024 235 Using Molecular Modelling to Anticipate Future Toxicity Classifications of Anti-oxidants and Identify Safer Structures Siegfried Lucazeau 1 , Grégoire Hervé 2 , Florence Séverac 3 1 NYCO, Paris, France 2 NYCO, Paris, France 3 NYCO, Paris, France 1. Regulatory background 1.1 Toxicity of classical chemistries As more toxicity studies are carried out, new information on the toxicity of classicial chemistries used in lubrication, once considered as safe, have caused them to now be classified as toxic and possibly make the formulated product itself hazardous. Such changes have required reformulation work, and changes in additive purchasing strategies. 1.2 Lack of confidence in the future Outside of the immediate impact of additive classification changes, a number of questions arise as more chemistries might be found toxic in the future. In order to regain confidence about the long term safety and availability of commonly used additives, we need to be able to identify truly safe compounds and anticipate the toxicity profile of existing or new molecules. This work covers the area of anti-oxidant additives. 2. Computational chemistry to assess toxicity 2.1 Similar molecules have similar activities Computational chemistry uses computer simulation to solve chemical problems; it is widely used in drug design. In particular, QSAR (Quantitiave Structure Activity Relationship) modelling, also called 2D modelling, may be used as a predicting tool. QSAR models are complex mathematical equations that associate the biological activity of a known molecule with its structural features (descriptors or predictors), in order to anticipate the activity of non evaluated compounds. 2.2 Robustness of QSAR models QSAR models have to be educated and evaluated using sets of compounds with known, measured toxicity features. A part of a set is used to build the model, while the other part is used to test the model. The quality of the model may be described by metrics such as sensitivity, specifity, accuracy, or AUC (Area Under the Curve). 3. Additional in-vivo testing Freshwater planarians are flatworms that possess a cholinergic system. They may be used as living models to assess neurodevelopmental toxicology and deliver additional information on potential toxicity of tested compounds. Cholinesterase activity on planarians 4. Results on antioxidants: aminics, phenolics and polyaminic oligomers 4.1 Scope Various classes of anti-oxidants were evaluated using 4 dedicated QSAR models: • Carcinogenicity • Mutagenicity • Reprotoxicity • Neurotoxicity Assessed compounds include alkylated diphenylamines with various alkyl chains, various phenolic anti-oxidants, as well as oligomeric anti-oxidants. 4.2 Results 4.2.1 General comments A few examples of well-known chemical structures used as anti-oxidants in lubricants do confirm the robustness of the QSAR models that have been designed for the purpose. Results on planarians are in good agreement with those of QSAR models, thus giving us more confidence in the overall evaluation system. 4.2.2 Takeaway • Toxicity of phenylalphanaphthylamine is known and confirmed by QSAR modelling on the 4 categories; alkylation of this molecule seems to reduce mutagenicity and neurotoxicty, however carcinogenicity and reprotoxicity may still be an issue. Using Molecular Modelling to Anticipate Future Toxicity Classifications of Anti-oxidants and Identify Safer Structures 236 24th International Colloquium Tribology - January 2024 Alkylated phenyalphanaphthylamine • Diphenylamines carrying iso C4 alkyl groups do show indications of high carcinogenicity and reprotoxicity potential; unfortunately, it appears like none of the evaluated alkyl groups on diphenylamines are able to exclude any risk of toxicity. Alkylated diphenylamines • The evaluation of several phenolic anti-oxidant structures leads us to believe that phenolics do not represent a satisfactory, long term safer alternative. Methylene bis(diterbutyl paraphenol) One specific polyaminic oligomeric antioxidant, however, shows an excellent toxicity profile according to our models, on all 4 toxicity categories; this compound has no effect on planarians either, confirming the credibility of this concept. More structures will be investigated. Oligomer of aminic anti-oxidants 5. Anti-oxidancy performance of oligomers Oligomeric anti-oxidants have long been used in the aviation industry to maximize resistance to thermo-oxidation and cleanliness of jet engine oils. This concept has also been introduced in high temperature industrial applications such as high temperature chain oils, greases, and compressor oils, with proven performance. The question remains as to how the oligomeric anti-oxidant that was identified as safe will perform in the variety of standard, less stringent conditions usually met with classical anti-oxidants; this is an ongoing project. 6. Conclusion Formulators, more than ever, are in need of future-proof, safe technologies to design their lubricants. Carefully selected polyaminic oligomeric anti-oxidants may represent such a beneficial technology, as an alternative to current anti-oxidants. The level of confidence in the toxicity profile of this technology is high, thanks to the metrics of the QSAR models that were used, as well as supporting data from additional in-vivo testing. 24th International Colloquium Tribology - January 2024 237 Oxidation Effects on the Rheology and Tribology of Sustainable Lubricants for Electromechanical Drive Systems Didem Cansu Güney 1* , Joachim Albrecht 1 , Katharina Weber 1 1 Research Institute for Innovative Surfaces FINO, Aalen University, Beethovenstr. 1, D-73430 Aalen, Germany * Corresponding author: didem_cansu.gueney@hs-aalen.de 1. Introduction Lubricants are essential for improving the performance of vehicles and construction equipment, particularly in the automotive industry, to optimise efficiency [1]. Due to leakage and disposal, sustainable and eco-friendly alternatives are essential [2]. The rise of electric vehicles poses new challenges, including high speed operation in excess of 20,000-rpm at elevated temperatures [3]. A current challenge is scarcity of commercially available lubricants that are sustainable, biodegradable, non-toxic, and performance-enhancing for electromechanical drives [4]. Gear oils are vital for reducing friction, wear, noise and corrosion [5]. Sustainable gear oils aim to extend change intervals and achieve a 0.5-% powertrain efficiency increase while reducing environmental impact. This aligns with the standard requiring 25-% renewable content and significant biodegradability for biolubricants [6, 7, 8]. This study compares sustainable lubricants for electromechanical drives derived from native oil (“Native”), synthetic ester (“Synth”) and polyalkylene glycol (“PAG”) with a conventional mineral oil-based lubricant (“Mineral”), all with the same 40-°C kinematic viscosity. The effect of ageing on these lubricants is investigated by assessing their rheological, tribological and electrochemical properties in aged and non-aged conditions, with the aim of improving sustainability without sacrificing technical suitability. 2. Experimental details and results The ageing of the lubricants was carried out using the Turbin Oxidation Stability Test method [9]. 100 ml of oil was heated at 95-°C for 13 days in a dark environment and subjected to of compressed air. 2.1 Rheological Investigation 2.1.1 Temperature-dependent behaviour Dynamic viscosity is determined using a rotational rheometer in a cone-plate system. Samples are sheared at a constant shear rate of and at temperatures ranging from 25-°C to 100-°C. To calculate kinematic viscosity, the obtained dynamic viscosity is divided by the previously determined density. The kinematic viscosity is then used to create Ubbelohde-Walther diagrams, as shown in Figure 1a and 1b for non-oxidised and oxidised lubricants [10]. The linear slope represents the temperature gradient m, which is related to the viscosity index. Low m values indicate a higher viscosity index. A high viscosity index with low viscosity values is desirable. Fig. 1: Ubbelohde-Walther diagrams of the a) non-oxidised; b) oxidised lubricants with the respective gradients. In black: mineral oil, in red: native oil, in green: synthetic ester, in blue: polyalkylene glycol. “Native” shows dramatically increased viscosity values after oxidation and thus poor oxidation stability. The viscosity values of “PAG” and “Synth” are in the range of the values of “Mineral”. After ageing, the viscosity index of all samples, except “PAG”, increases. However, “Mineral” has the highest m-value, which means that “Synth” and “PAG” show a lower sensitivity to temperature fluctuations in contrast to “Mineral”. The results show that “PAG” and “Synth” can be considered as gear oils, while “Native” appears unsuitable due to its strong viscosity change after oxidation. 2.1.2 Jump test The jump test is used to investigate the reaction of the lubricant to a sudden change in load or deformation. The lubricant is first subjected to a low shear rate of , which is then abruptly changed to and finally returned to the initial value. The relaxation time is found by measuring how long it takes for the lubricant´s dynamic viscosity to return to its original state after a sudden change. Figure 2 shows the jump tests of the lubricants in the non-oxidised and oxidised states. The jump test results of the oxidized “Native” sample are not shown due to its strong increase in viscosity upon oxidation. “Mineral” and “Synth” do not regenerate during the entire test period. “Native” also does not regenerate in the non-oxidised state. In the oxidised state, however, regeneration occurs after 122.4-s (not shown in Figure 2). “PAG” regenerates after 32.4-s in the non-oxidised state and after 14.4 s in the oxidised state. The relaxation time shortens as the oil ages. “Native” is no longer considered in the following measurements due to the very high viscosity after oxidation. 238 24th International Colloquium Tribology - January 2024 Oxidation Effects on the Rheology and Tribology of Sustainable Lubricants for Electromechanical Drive Systems Fig. 2: Jump test of the non-oxidised and oxidised lubricants. In black: mineral oil, in red: synthetic ester, in green: native oil, in blue: polyalkylene glycol 2.2 Tribological Investigation An oscillating tribometer was used to determine the coefficient of friction according to DIN 51834-2 [11]. A spherical counter body, here 100Cr6, acts with a constant force of F-=-50-N perpendicularly on the surface (100Cr6) lubricated with 0.3 ml oil. The lubricated surface moves simultaneously with a certain path length of s-=-1 mm and a frequency of f-=-50-Hz in a linear oscillating motion for a duration of t-=-120 min. Figure 4 shows the coefficients of friction of the non-oxidised and oxidised lubricants. Fig. 4: Coefficient of friction of the lubricants over time. In black: mineral oil, in red: synthetic ester, in blue: polyalkylene glycol. The coefficients of friction of the lubricants are all between 0.08 and 0.12. Since the tribological contact is in the partially lubricated state, there is always solid contact between the interacting components, which explains the observed coefficients of friction. Differences can be seen between the different oils and between the non-oxidised and oxidised states. “PAG” shows the lowest coefficients of friction. “Synth” shows the highest friction values. It is noticeable that the coefficients of friction of the lubricants decrease upon oxidation. This is possibly due to the increasing viscosity after ageing which increases the film thickness and thus reduces the solid body interaction in the partially lubricated state. 2.3 Electrochemical Investigation The electrical properties of “Synth” and “PAG” were characterised by electrochemical impedance spectroscopy in the frequency range of 10 5 - 0.1 Hz. The results of the non-oxidised and oxidised samples are shown in Nyquist plots, Figure 5 and Figure 6. Fig. 5: Nyquist plot of the non-oxidised and oxidised synthetic ester. Fig. 6: Nyquist plot of the non-oxidised and oxidised po-lyalkyleneglycol-based oil. The real part of the impedance is plotted on the x-axis of the Nyquist diagram and the imaginary part on the y-axis. The real part corresponds to the ohmic resistance. For both samples, the oxidised lubricant has a lower resistance than the non-oxidised lubricant. The oxidised samples therefore have a higher electrical conductivity than the non-oxidised samples. When comparing the two lubricants, it is noticeable that “Synth” has a significantly higher resistance (GΩ range) than “PAG” (MΩ range). 3. Conclusion In this study, sustainable lubricants were compared with a mineral oil-based lubricant in terms of various performance parameters. The results highlight the potential advantages of “PAG” and “Synth” in certain applications, whereas “Native” is severely impacted by ageing. References [1] M. Woydt, Wear, 2022, 488-489 (1), 204134. [2] N. Salih et al., Biointerface Res. Appl. Chem., 2021, 11-(5), 13303. [3] L. I. Farfan-Cabrera, Tribol. Int., 2019, 138 (40), 473. [4] M. Beyer et al., Tribol. Online, 2019, 14 (5), 428. [5] M. Arca et al., Int. J. Sustain. Eng., 2013, 6 (4), 326. [6] W. J. Bartz, Proc. Inst. Mech. Eng. D: J. Automob. Eng., 2000, 214 (2), 189. [7] DIN SPEC 51523 (2011). [8] R. Luther, Lubricants and Lubrication, Wiley-VCH Verlag, Weinheim, Germany, 2017. [9] DIN EN ISO 4263-4 (2006). [10] D. C. Güney et al., Materialwiss. Werksttech., 2023, accepted for publication. [11] DIN 51834-2 (2017). 24th International Colloquium Tribology - January 2024 239 Bio-Lubricants as Metal-Working Fluids: More than an Environmental-Friendly Choice Marco Bellini *1 , Simone Pota 1 1 Bellini SpA, Zanica, Italy * Corresponding author: mbellini@bellini-lubrificanti.it 1. Introduction Metal-Working Fluids (MWFs) are supposed to reduce Coefficient of Friction (CoF) in all lubrication regimes (boundary, mixed and hydrodynamic lubrication). To obtain so, a blend of performance additives has to be added to the base fluid. There are a plenty of additives known to be effective on performance’s enhancement for MWFs and, even if each company uses its technology, they can be grouped into four main clusters: chlorine based, sulphur based, phosphorous based additives and super-lubricity additives. However, increasing quality and quantity of additives in Metal-Working Fluids sometimes does not lead to better results. R&D departments experienced this in the past two decades, when an increase of R&D effort was not paired to better performances. This forced to find alternatives and to consider different base stocks other than the standard ones. Among the others, esters were the perfect candidate due to their chemistry and to their tribological behaviour. Nowadays bio-lubricants are a trend topic in lubricants market. The raising focus on green alternatives and HSE advantages of bio-lubricants in respect of mineral oil are the main drivers of this big change in lubricants market. However, bio-lubricants are more than an environmental-friendly choice: they offer a complete range of advantages. 1.1 Bio-lubricants technology Bio-lubricants have many advantages compared to mineral oil lubricants. From molecular point of view, esters are more polar than mineral oil. Esters create a layer of organized molecules on the workpiece surface due to their polarity. This layer is responsible to CoF drop observed in tribology measurement. Each type of ester, depending on many factors such as carbon chain length and viscosity, has its typical tribological behavior, extensively studied in the first screening. The selection of low CoF ester base stock to be used has been characterized and deeply studied. The main advantages of ester compared to mineral oil are related to its chemistry: biodegradability, lower CoF, vapor tension, higher flash and fire point and so on. How much is biodegradable lubricants technology ready to replace mineral oil technology? Bellini SpA tried to reply to this question by testing extensively biodegradable formulations on the field in the past years in many different applications. Replacement of mineral oil based product with ester based product often leaded to improvement of performances in terms of lubrication, reduction of oil consumption and reduction of tool wear. 1.2 Health, Safety & Environment Some types of esters are biodegradable according to OECD 301 B method and they have a lower carbon footprint compared to mineral oil. Moreover, some classes of synthetic esters come from renewable sources while natural esters has vegetable origin. Bio-lubricants are the right choice even for Health and Safety reasons. It is well known that some mineral oil based products contains polycyclic aromatic hydrocarbon (PAH) compounds and PAH content increases because of cutting fluids use [1]. We tested concentration of PAH in new MWF, and in the same fluid after 3, 6 and 9 months of intensive use. PAH concentration has been studied in new and after 6 months MWF (ester based and mineral oil based both). Analysis in workplace atmosphere and on the clothes and skin of the operators show high concentrations of PAH when mineral oil is used. References [1] Apostoli P. et al, Int Arch Occup Environ Health (1993) 64, 473-477 24th International Colloquium Tribology - January 2024 241 Potential and Performance of Pure Water Lubrication in Gearboxes Model-tribometer and Prototype Testing Andreas Nevosad 1* , Stefan Krenn 1 , Michael Adler 1 , Dominik Cofalka 2 , Siegfried Lais 2 , Uwe Gaiser 2 1 AC2T research GmbH, Wiener Neustadt, Austria 2 Reintrieb GmbH, Wien, Austria 3 Gleason Corporation, Rochester NY, USA * Corresponding author: E-mail (andreas.nevosad@a2t.at) 1. Introduction Gearboxes are widely used for multiple applications in mechanical drive systems. Sizes may vary from small handheld tools to large ships, wind turbines or lightweight high revolution and high-power applications like in e-mobility drivetrains. Conventionally, these gearboxes are lubricated using mineral oils, where the oils provide cooling and friction reduction for the rolling and sliding contacts in the gears as well as in the bearings. These oils are particularly problematic in areas where they can enter the environment, like in maritime applications, or are otherwise harmful, such as in food production. This study presents a radically new approach in sustainable systems and demonstrates the feasibility and performance of a gearbox that is lubricated with pure water. The here presented prototype with bevel gears was developed with maritime applications in mind, but results can be transferred to any other gearbox [1]. Corresponding FZG tests were performed by Raddatz et al [2]. 2. Experimental We present model tribometer experiments and the first long run prototype of a real water lubricated gearbox 2.1 Topography and surface mapping The wear volume was measured with a Leica DCM 8 and calculated in Leica Map Premium software. 2.2 Tribotesting Tribocorrosion experiments were performed in an Optimol SRV4® tribometer, equipped with a sample holder for corrosive liquids. The room temperature tests were performed at 100 Hz with a stroke of 0.5 mm for 21600 s and a Hertzian pressure of 1.8 GPa for 10 mm ball diameter. 2.3 Materials For the tribotests, three different kinds of cemented carbides were tested against cemented carbide balls. An overview of the materials is given in Table 1. The tribocorrosion tests were performed in deionized water (DI) and tap water (TW). For the prototype test DI was used. Table 1: Investigated materials and counterbody Sample Binder phase Binder content [m%] Carbide size Ball Co 6 Fine/ medium A Co 15 Fine/ medium B CoCrNi 20 Coarse C Co 25 Fine/ medium 2.4 Gear box prototype The gearbox prototype (see Figure 1) was developed by Reintrieb GmbH. It features bevel gears and friction bearings and is designed to transmit 22 kW. The bevel gears were made from material C, due to the highest fracture toughness, whereas the bearings were made from material A. Figure 1: Gearbox prototype 3. Results and discussion 3.1 Tribocorrosion experiments The tribocorrosion tests were done in order to measure the coefficient of friction and to determine the wear resistance of the different cemented carbides in different water qualities. Figure 1 shows the variation in coefficient of friction (CoF) for all tested materials for deionized and tap water. For all tests, the curves show at the beginning of the test an increase in CoF up to 0.3 and then a reduction to around 0.25. The measured curves show scatter and irregular behavior, but no significant differences in overall CoF for the individual materials or water qualities. Potential and Performance of Pure Water Lubrication in Gearboxes 242 24th International Colloquium Tribology - January 2024 Figure 2: Coefficient of friction For the evaluation of the wear resistance, the volume of the wear tracks after the tribometer tests was calculated from topography measurements. For statistics, the measurements in DI water were performed three times and averaged values and the standard deviation as error bar is plotted with the values for wear in TW in Figure 2. Highest wear was found for material C with the highest binder content, followed by material A with the lowest binder content. Lowest wear was found for Material B, which has a binder content between the two formers, but the binder phase in B is a CoCrNi alloy with higher corrosion resistance. Also the WC grains are larger in B, which can contribute to a higher wear resistanze in this test. All materials showed higher wear in TW, indicating the tribocorrosive nature of this tribosystem. Figure 3: Wear volume for the three materials in deionizedand tap water 3.2 Prototype endurance testing Prior to the full-power endurance test, two run-in periods were done at 1.18 kW at 1000 rpm and 3.39 kW at 1500 rpm respectively. The endurance test of the prototype gearbox was performed at a torque of 65 Nm at 3000 rpm. To monitor the wear progress and rate, water samples were taken regularly during the operation (see Figure 4). After the run-in periods, at the start of the endurance test, the system was rinsed and filled with fresh water. The content in W and Co in the water samples corresponds to the wear of the cemented carbide parts. The observed steady increase of the concentrations indicates a constant wear rate during the endurance test. Figure 4: Water analyses from the prototype endurance test. After more than 500 h of testing, the experiment had to be aborted due to the breakout of one tooth on one of the gears. The fracture surface clearly showed fatigue failure and the initiation of this crack was assigned to a problem in the operation of the frequency converters at the beginning of the test run. A wear analysis of the disassembled parts after the test showed no wear for the bearings. The material loss on the gears indicated that these had reached about half of their lifetime. 4. Conclusion The experiments clearly demonstrated the feasibility of lubricating a gearbox with pure water. Model tribometer experiments showed the effect of tribo-corrosion in dependence of water quality and materials. This indicates a further reduction in wear and increase in lifetime with the use of corrosion resistant materials. Funding Presented results were realized in research projects with financial support from the participating project partners and the Austrian COMET program (Project InTribology, No. 872176). The COMET program is funded by the Austrian Federal Government and concerning InTribology by the provinces of Lower Austria and Vorarlberg References [1] S. Lais, Getriebe, EP2614000A1, 2013. https: / / patents. google.com/ patent/ EP2614000A1/ de? oq=EP2614000 (accessed October 10, 2023). [2] K. J. Raddatz, et al. Scientific Evaluation of Investigations on the Load Carrying Capacity of Carbide Cylindrical Gears Lubricated with Water. Tribologie und Schmierungstechnik, 2022, 69. Jg., Nr. 8. 24th International Colloquium Tribology - January 2024 243 Sustainability Assessment of Polyol Esters - A Comparative LCA Analysis of a Bio-Based vs. Fossil-Based Product Verena Koch 1 , Denise Haas 2 1 Peter Greven GmbH & Co. KG, Bad Muenstereifel, Germany 2 Peter Greven GmbH & Co. KG, Bad Muenstereifel, Germany 1. Introduction In the urgent context of climate change a critical question emerges: Can materials and products derived from renewable carbon reduce greenhouse gas emissions when compared to the established fossil-based counterparts? Answering such a question demands in-depth assessment and the method of choice for this kind of evaluations is a Life Cycle Assessment (LCA). We present a peer-reviewed LCA study - representing the highest possible scientific standard - that examines the carbon footprint of a product mainly made from renewable carbon vs. a fossil-based product. This kind of examination is significant as defossilisation is the right strategy to eliminate additional influx of fossil carbon into our carbon cycles and the atmosphere - but at the same time we need to ensure that the alternatives really reduce greenhouse gas emissions. The principle advantage of renewable carbon feedstock is that it originates from atmo-, bioand technosphere and therefore does not bring additional fossil carbon from the land into the carbon cycle above the ground. Instead, these feedstocks help to build and realise a truly circular economy and circular carbon loops [1]. 1.1 Life Cycle Assessment (LCA) LCAs are globally recognised as the gold standard for assessing the environmental impacts of products and services. LCA is an internationally standardized method laid out in ISO 14040: 2006 and ISO 14044: 2006. They analyse every stage of a product’s life, providing a comprehensive understanding of their environmental impacts. Peer-reviewed LCAs are particularly valuable as they undergo rigorous expert scrutiny, ensuring the reliability of their findings and enabling reliable public assertions in terms of environmental preference. The LCA procedure [2] 1.2 Carbon Footprint of Bio-based Materials It is essential to recognize that the carbon footprint of biobased materials is not automatically close to zero for two primary reasons: Fossil energy in the value chain: The growth or provision of raw materials, transportation as well as product manufacturing stages involve energy consumption and a substantial amount of grid mix energy is still derived from fossil sources. In particular the agricultural sector, as a key provider of biomass, is still strongly reliant on fossil resources e.g. for fertilisers, pesticides or simply the diesel needed to run machinery. This reliance on fossil feedstock within the value chain significantly impacts the overall carbon footprint. Land Use Change: Land use change refers to the alteration or conversion of a particular area of land from one land use type to another. It involves the transformation of land for various purposes such as agriculture, urban development, forestry, mining or infrastructure projects. Land use change has significant impacts on the environment, biodiversity, climate and socio-economic aspects. Climate change impact for biobased materials is mostly linked to the deforestation to get arable land. Understanding and managing land use change is crucial for sustainable development and environmental conservation. Effective land use policies, land zoning and conservation efforts are necessary to ensure responsible land use change and minimize negative impacts on ecosystems and communities. Sustainably certified production can significantly lower greenhouse gas emissions [3]. 2. Current Study This case study provides you with key LCA insights of polyol esters and how a bio-based product can help to mitigate climate change by reducing greenhouse gases [4]. The study was carried out following the LCA standards laid out in ISO 14040 [2] and ISO 14044 [5]. External critical review as described in the standards has been performed by a review panel consisting of three independent reviewers. This LCA covers all relevant life cycle stages from cradle-to-gate, which means from the supply of raw materials to the manufacturing of the products. For the impact assessment, the sixteen potential impacts from the EF 3.0 method were investigated. These include climate change, the depletion of resources and impact on humans and ecosystems. 244 24th International Colloquium Tribology - January 2024 Sustainability Assessment of Polyol Esters - A Comparative LCA Analysis of a Bio-Based vs. Fossil-Based Product Impact categories Impact category Indicator Climate change Radiative forcing as Global Warming Potential with a time horizon of 100 years (GWP100) Resource use, fossils Abiotic resource depletion ultimate reserves Resource use, minerals and metals Abiotic resource depletion - fossil fuels Particulate matter Disease incidence due to exposure to PM2.5 Ozone depletion Ozone Depletion Potential Photochemical ozone formation Tropospheric ozone concentration increase Ionising radiation Human exposure efficiency relative to U235 Acidification Accumulated Exceedance Eutrophication, freshwater Fraction of nutrients reaching freshwater end compartment (P) Eutrophication, marine Fraction of nutrients reaching marine end compartment (N) Eutrophication, terrestrial Accumulated Exceedance Land use Soil quality index Water use User deprivation potential Human toxicity, non-cancer Comparative Toxic Unit for humans Human toxicity, cancer Comparative Toxic Unit for humans Ecotoxicity, freshwater Comparative Toxic Unit for ecosystems A comprehensive sensitivity analysis on different explorative scenarios and allocation scenarios was carried out in order to determine how value and methodological choices related to this issue affect the results and conclusions of this LCA. 2.1 Assessed Products LIGALUB 19 TMP is a polyol ester with a bio-based carbon content of 81% (measurement based on ASTM D 6866: 2008) used for lubricant applications and produced by Peter Greven, a leading manufacturer of oleochemical products based on renewable raw materials. It is produced by the esterification of a fatty acid made from palm kernel oil and an alcohol, trimethylolpropane (TMP). Conventional lubricant esters based on isotridecanol and adipic acid, such as diisotridecyladipate (DITA) can be considered as direct counterpart for LIGALUB 19 TMP. Both reactants for the production of DITA are commonly derived from petrochemical feedstocks. DITA was used as fossil-based reference system because it is a lubricant ester with similar product properties compared to LIGALUB 19 TMP. Both are equally suitable as lubricant component e.g. for engines, gearboxes and hydraulic oils. The results of an investigation of the physical properties support the selection of DITA as suitable reference [4]. 2.2 Results The environmental impacts were found to be lower for LIGA- LUB 19 TMP in comparison to DITA in the following evaluated categories: climate change, use of fossil resources and photochemical ozone formation. In summary, the bio-based alternative to DITA can help to reduce carbon emissions and support the defossilisation. References [1] Plum, M. et al. 2023: Case Studies Based on Peer-reviewed Life Cycle Assessments - Carbon Footprints of Different Renewable Carbon-based Chemicals and Materials [2] ISO 2006: Environmental management - Life cycle assessment - Principles and framework (ISO 14040: 2006). [3] Schmidt, J. and De Rosa, M. 2020: Certified palm oil reduces greenhouse gas emissions compared to non-certified. Journal of Cleaner Production, Vol. 277 10.1016/ j.jclepro.2020.124045 [4] nova-Institut für politische und ökologische Innovation GmbH 2023: Life Cycle Assessment -Ligalub 19 TMP [5] ISO 2006: Environmental management - Life cycle assessment - Requirements and guidelines (ISO 14044: 2006); German and English version EN ISO 14044: 2006. 24th International Colloquium Tribology - January 2024 245 How can Esters Improve the Sustainability of Both Intrinsic and Extrinsic Factors? Gareth Moody 1 , Gemma Stephenson 2* 1 Cargill, York, United Kingdom 2 Cargill, York, United Kingdom * Corresponding author: Gemma_Stephenson@Cargill.com 1. Introduction Sustainable development is happening on a global basis [1]. At the core is a call for concerted efforts towards building an inclusive, sustainable and resilient future for people and planet. For this to happen, it is crucial to harmonize the three elements of social development, economic growth and environmental protection. Central to environmental protection is the recognition that greenhouse gas levels, particularly CO 2 , in the atmosphere need to be reduced to slow down the global temperature rise and avoid catastrophic events [2]. It has been realized that within the lubricants industry, decarbonization is required across the whole value chain and collaboration is essential in making this happen. Initiatives are underway to reduce carbon embedded in raw materials (scope 3 emissions), as well as reduce reliance on fossil-based processes within manufacturing (scope 1 and 2). Within the scope 3 emissions umbrella, the “use of sold products” [3] allows for lubricant additive manufacturers to design technologies that allow for carbon reductions and savings. As an industry, product performance is the primary consideration when designing additives, base oils and finished lubricants. A product MUST deliver performance benefits to the customer that are second to none. These are known as the extrinsic benefits. It is also desirable to maximize intrinsic benefits at the same time, e.g., biodegradability or bioaccumulation potential. However, we understand that with product performance will also come a carbon footprint, or an environmental footprint of some kind. Reports have indicated that up to 80% of a materials carbon footprint can be attributed to carbon embedded in raw materials [4]. It is therefore a balancing act, but also vital to create products which are safe and sustainable by design, and to use innovation as a tool to create ingredients that deliver maximum performance benefits, but which also have a minimal carbon/ environmental footprint. Biobased materials are derived from biomass and are defined as renewable resources, which can be replenished over time. Biobased materials allow for the consideration of CO 2 sequestration as the raw materials are grown and can lead to reductions in the CO 2 footprint of a product. Care must be exercised when calculating product carbon footprints (PCFs) and appropriate methodologies should A simple way to evaluate base fluid traction coefficient is by measuring them using a Mini Traction Machine (MTM), the test conditions for which are shown in Table 2.be followed. Similarly, it is important to consider other complex factors when assessing sustainability of such biobased materials (e.g., land use change) to ensure that a positive effect in one area is not being cancelled out by a negative effect elsewhere. Continuing efforts to minimize product carbon footprints can help reduce CO 2 levels, however extrinsic factors can save even more over the lifetime of a vehicle if a fluid can be more efficient or have a longer lifespan. Choosing a lubricant which has a low product carbon footprint, but which is inefficient in use may well have a negative effect overall. In order to evaluate which of the lubricants is the best choice overall both the intrinsic and the extrinsic benefits must be evaluated. In this paper, the intrinsic and extrinsic properties of both petrochemical and highly biobased formulations have been tested. Base fluids for EV transmission oils have been evaluated using a suite of tribological tests. Fully formulated EV transmission oils have also been considered. Additionally, an ester whose structure is optimally designed for film formation within EV transmission oils has also been evaluated. 2. Maximizing Efficiency and Biobased Content in Esters The properties of biobased esters having a viscosity similar to typical base fluids (group III, PAO 4) are shown in Table 1. Table 1: Neat base oils for evaluation Product kV at 40-°C (mm 2 / s) kV at 40-°C (mm 2 / s) Biobased content (%) Group III 20 4.2 0 PAO 4 19 4.1 0 Ester 1 19 4.5 31 Ester 2 25.3 5.5 47 Ester 3 19.6 4.4 82 Ester 4 9.6 2.9 0 Ester 5 6.1 2.0 85 A simple way to evaluate base fluid traction coefficient is by measuring them using a Mini Traction Machine (MTM), the test conditions for which are shown in Table 2. 246 24th International Colloquium Tribology - January 2024 How can Esters Improve the Sustainability of Both Intrinsic and Extrinsic Factors? Table 2: MTM Test Conditions Parameter High speed, low severity Samples Base oils Test type Traction curves Load/ N 16 SRR 0-100 Temperature/ °C 40, 100 Figure 1 shows the traction coefficient curves for these tests. The solid line shows the traction coefficient curve at 100-°C, and the dotted line shows the traction coefficient curve at 40-°C. Figure 1: Traction coefficients by MTM at 40 and 100-°C The first thing to notice is that the traction values at 40-°C (dotted line) are higher than their corresponding lines at 100-°C (Solid lines), this is because of viscosity and the general trend that the lower the viscosity, the lower the traction. At 40-°C there is a large difference between the base oil types. Group III has the highest traction, next are Ester 1 and PAO. Ester 1 is a predominantly petrochemical based ester and has some branching which can give it higher traction. Esters 2 and 3 have a higher biobased content and were designed to have lower traction. This occurs at both 40-°C and 100-°C. Esters 4 and 5 are designed to have maximum efficiency and lowest possible traction as shown in Figure 1. There are two approaches to achieving this. The first is to use petrochemical derived raw materials which have a higher PCF value but have the lowest possible traction and Ester 5 which uses biobased raw materials but still has very low traction. 3. Conclusion Using esters can greatly benefit the intrinsic and extrinsic properties of a lubricant. For the intrinsic properties the choice of raw materials (petrochemical vs biobased), the way they are processed and the way they are transported will influence the product carbon footprint of a molecule. Even if a material is 100% biobased it does not mean that it will be the most sustainable option over the lifetime of an oil as it must also perform in use. When evaluating a lubricant for an application, the intrinsic and extrinsic properties must both be considered to make the most sustainable choice. Esters shown in this paper demonstrate that it is possible to improve EV transmission fluid efficiency through designing molecules that deliver significant reduction of traction. References [1] United Nations. The Sustainable Development Goals Report, 2022. [2] United Nations Climate Change UNFCCC. The Paris Agreement - Publication 2018 [3] Carbon Chain, 2023. Scope 1, 2 and 3 emissions. Available at https: / / www.carbonchain.com/ carbon-accounting/ scope-1-2-3-emissions/ [4] C. Cherel-Bonnemaison, G. Erlandsson, B. Ibach & P. Spiller. Buying into a more sustainable value chain. Mckinsey & Company, 2021. Available online at https: / / www.mckinsey.com/ capabilities/ operations/ our-insights/ buying-into-a-more-sustainable-value-chain 24th International Colloquium Tribology - January 2024 247 Moving towards Sustainable Lubrication - Challenges and Findings for Lube Components from Biobased Sources Marcella Frauscher 1 *, Jessica Pichler 1 , Rosa-Maria Nothnagel 1 , Adam Slabon 2 1) AC2T research GmbH, Wiener Neustadt, Austria 2) University of Wuppertal, Wuppertal, Germany Corresponding author: marcella.frauscher@ac2t.at 1. Introduction With the mandatory change towards a sustainable economy, renewable, waste-derived or plant-based materials are investigated as possible replacements for lubricant components with fossil origin. The studied materials include various forms of lignin, food waste such as spent coffee grounds, further valorised after coffee brewing or fish oil waste. One main challenge of bio-based lubricants is the determination and improvement of their stability to fulfil the performance characteristics in industrial applications [1]. For this purpose, a comprehensive approach combining stability assessment, characterization of component degradation and evaluation of triboperformance is necessary [2]. 2. Sustainable sources for lubricant components Within this presentation, three different bio-based materials as potential lubricant components are presented: Lignin, coffee ground oil, and bio-based friction modifiers. 2.1 Green depolymerization of lignin as base for additives Lignin as source for lubricant components is characterized by a strong structural variation depending on the origin. The challenges for lignin-derived components are separation and purification of compounds of interest to create materials and chemicals with added value. A green depolymerization route is presented, resulting in building bricks for synthesis of potential lubricant additives [3]. The effectiveness of the electrochemical setup for cleavage of the main linkage present in the lignin macromolecule and the mechanism and selectivity of lignin depolymerization was evaluated using the cleavage of β-O-4 linkage of 2-phenoxyacetophenone (2-PAP) as a simple model compound [4]. First triboresults for dispersions of nanoscopic flower-like lignin particles (NP) doped with a minimal amount of MoS 2 in polyalphaolefin (PAO) base oil showed significant improvement of the lubricating properties (figure 1) [5]. Figure 1: Tribological properties of lignin/ MoS 2 NPs hybrid additive in mineral oil (0.05 wt.%) [6] 2.2 Spent coffee ground oil as lubricant component source Oil extracted from spent coffee grounds (SCGO) was investigated for friction and wear properties used as both base oil and additive by an oscillating tribometer and rheometer. Furthermore, properties such as viscosity, acid value, water content, thermogravimetric analysis and differential scanning calorimetry were determined. The composition of oils was studied by ATR-FTIR, elemental analysis (CHNSO) and gas chromatography coupled with mass spectrometry (GC-MS). The tribological properties (figure 2) of SCGO were investigated as a base oil (100-%) and as a 5 % additive in polyalphaolefin (PAO 8). Figure 2: Coefficient of friction (COF) of synthetic base oil (PAO 8), spent coffee grounds oil (100-% SCGO), spent coffee grounds oil 5-wt% in PAO 8 (5-% SCGO). Moving towards Sustainable Lubrication - Challenges and Findings for Lube Components from Biobased Sources 248 24th International Colloquium Tribology - January 2024 This revealed that 100 % SCGO as well as 5 % SCGO led to an improvement in the coefficient of friction (COF) compared to neat PAO 8. Microscopically determined wear traces confirmed this improvement (figure 3). Figure 3: Wear scar PAO 8 (A), 100 % SCGO (B), 5 % SCGO (C). 2.3 Friction modifiers from bio-sources Bio-based friction modifiers (FM) were compared with conventional ones by using a developed rheometer method (see figure 4), designed under variation of normal force, temperature, and sliding speed [8]. Bio-based friction modifiers, such as rapeseed and salmon oil, were selected based on a toxicological assessment of literature data. Comparison of performance revealed the advantages and disadvantages of bio-based and conventional friction modifiers, respectively. Measurements of both types of FM in PAO at 80 °C and in distilled water at 30 °C showed that bio-based friction modifiers can perform at least as good as the best performing conventional FM [9]. Figure 4: Test setup - Ball-on-three-plates [8] 3. Conclusion To show the possibilities for sustainable lubricant components and selected results, 3 different materials and approaches were presented. A straightforward approach to produce value-added biobased building blocks from lignin for additives was demonstrated. This was realized by a simple depolymerization process of biomass waste in a biomass-based solvent and a cheap transition metal as electrocatalyst. Spent coffee grounds was used as a valuable waste resource, serving as high-quality feedstock for biodiesel or bio-lubricants. SCGO showed superior friction reduction behavior compared to a synthetic lubricant. Based on a toxicological assessment and the availability on the European market, bio-based FM were selected. Two different applications were considered: bio-based FM mixed in water and FM with petrochemical origin mixed in a conventionally used PAO. The performed tests indicated the advantages and disadvantages of both FM in the respective system. Comparing both it was revealed that bio-based FM could be a promising alternative for conventionally used fossil-based FM in lubricants. Acknowledgements The work presented was funded by the Austrian COMET program (Project InTribology, Nr. 872176) and carried out at the “Excellence Centre of Tribology” (AC2T research GmbH). References [1] A comprehensive review of sustainable approaches for synthetic lubricant components. Pichler J., Eder R.M., Besser C., Pisarova L., Dörr N., Frauscher M., Marchetti-Deschmann M. Green Chemistry Letters and Reviews, Vol 16 No 1 2023 [2] Assessment and design of modern lubricants supported by mass spectrometry. Frauscher M. European Conference on Tribology 2023 Bari (IT) [3] Electrochemical depolymerization of lignin in a biomass based solvent. da Cruz M.G.A., Gueret R., Chen J., Piatek J., Beele B., Sipponen M. H., Frauscher M., Budnyk S., Rodrigues B., Slabon A. ChemSusChem - Chemistry - Sustainability - Energy - Materials, Vol 15 2022 [4] On the product selectivity in the electrochemical reductive cleavage of lignin model compounds. da Cruz M.G.A., Rodrigues B., Ristic A., Budnyk S., Das S., Slabon A. Green Chemistry Letters and Reviews, Vol 15 Is 1 2022 [5] MoS2 nanoflower-decorated lignin nanoparticles for superior lubricant properties. Lindenbeck L.M., Beele B., Morsali M., Budnyk S., Frauscher M., Chen J., Rodrigues B., Sipponen M. H. Nanoscale, Vol 15, 2023 [6] Secret Wood - source of molecules for sustainable lubrication? Frauscher M., Budnyk S., da Cruz M.G.A., Rodrigues B., Slabon A. International Conference on Tribology and Systainable Lubrication 2023 Düsseldorf (DE) [7] Moving towards green lubrication: tribological behaviour and chemical characterization of spent coffee grounds oil. Pichler J., Eder R.M., Widder L., Varga M., Marchetti-Deschmann M., Frauscher M. Green Chemistry Letters and Reviews, Vol 16 Is 1 2023 [8] Anton Paar.: Ball-on-three-Plates/ Pins (BTP) Calculation 2020 [9] Testing sustainable and safe lubricants from plant-feedstocks. Eder R.M. Master Thesis 2021 24th International Colloquium Tribology - January 2024 249 New Technologies of Antiwear and Antioxidant Additives Used for Designing Nonhazardous Turbine Oils and Sustainable High-Performance Lubricants Including Greases Grégoire Hervé 1 , Florence Severac 1 1 NYCO, Paris, France 1. Introduction Sustainability is increasingly vital in technological developments, with final users desiring non-hazardous materials that pose minimal environmental and safety risks for their operations. Evolving regulations are unveiling the true toxicity of various chemicals, particularly affecting performance additives, transcending industries. Thus, the quest for effective and safer additive chemicals remains a big challenge for the global industry. 2. Methods Toxicity assessment is a complex and multifaceted domain, characterized by numerous uncertainties. It depends on various factors, including the type and level of exposure, sensitivity of the organisms involved, individual differences such as gender, and more. Usually, a thorough evaluation spanning several years and incurring high costs is required to gain a comprehensive understanding of chemical toxicity. In some cases, conflicting results from toxicity studies have led to prolonged debates within the scientific community. In our research, we employ a holistic approach that integrates both modeling and practical experiments to assess the toxicity of anti-wear and antioxidant additives effectively. 2.1 Modeling Approach [1] For the modeling aspect of our assessment, we have established two types of models: - Quantitative Structure-Activity Relationship (QSAR) Models - 3D Models based on the Harmonic Spheric methodology as previously reported in the literature. [2] 2.2 Biotesting Experiments - In Vitro Experiments - In Vivo Experiments using the freshwater planarian model [3] The combination of these modeling and biotesting approaches forms the foundation of our holistic toxicity assessment, which aims at comprehensively evaluating the toxicity level of anti-wear and antioxidant additives used in lubricants. 3. Main results 3.1 Antiwear toxicity assessment In our assessment of antiwear and organophosphate substances (OPs), we modeled several classes of OPs. Our 3D and QSAR models revealed that most conventional antiwears fall into a group of molecules with an undesirable toxicity profile. Figure 1 illustrates the distinct 3D model patterns, emphasizing variations in shape and chemical functionalities. Additionally, our modeling tool quantifies the accessibility of the potent phosphate functions directly linked to toxicity. The alignment between our in vitro and in vivo experimental results corroborates our modeling findings. For instance, Figure 2 illustrates the disparate neurotoxicity patterns observed in vitro aligning with the 3D models. 3.2 Antioxidant toxicity assessment In the assessment of antioxidants (AOs), we primarily employed QSAR models, encompassing four types: C, M, R, and N models (N for neurotoxicity). Reprotoxicity (R) emerged as the most critical endpoint among these. Our in vivo experiments validated the potential toxicity of most commercial aminic and phenolic AOs. Encouragingly, our in-house developed polyaminic antioxidant technology exhibits a significantly safer profile based on our both predictive and in vivo data. 3.3 Performance evaluation Subsequently, we formulated our safer and nonhazardous antiwear and polyaminic antioxidant additives, resulting in various finished lubricants, including turbine oils, gear oils, and greases. Thermal and tribological evaluations underscore their superiority over existing market lubricants. Our bearing rig, as depicted in Figure 3, recorded outstanding thermal properties when subjecting turbine oils to high temperatures and rigorous conditions (260-°C, 200-h, 10,000-rpm). Furthermore, we observed reduced coke formation compared to commercial references. Notably, our new antiwear molecules demonstrate excellent frictional antiwear performance, with some exhibiting remarkable extreme pressure properties, as illustrated in Figure 4. This latter achievement is exceptional as it relies solely based on phosphorus, oxygen, carbon, without the need for other heteroatoms like N, F, Cl, or S. Turbine oils and gear oils derived from these innovative chemistries find applications with excellent seal and material compatibilities, thanks to the low additive aggressiveness. Additionally, greases stand to benefit from these technologies as hazardous phosphorus additives can be replaced, allowing for the formulation of pictogram and risk-free materials. It is noteworthy that biodegradability can be further enhanced, particularly in ester/ PAO-based grease formulations) using such carefully selected additives. 250 24th International Colloquium Tribology - January 2024 New Technologies of Antiwear and Antioxidant Additives Used for Designing Nonhazardous Turbine Oils and Sustainable High-Performance Lubricants 4. Conclusion Our research addresses the complex toxicity issue through an innovative, holistic approach that integrates modeling and practical experiments on anti-wear and antioxidant additives. Both computational and experimental approaches align harmoniously, providing robust consistency and clear trends. This enables us to judiciously select the safest chemistry of additives for antioxidant and anti-wear applications. Our lubricant formulations, including greases, exhibit exceptional thermal and tribological performance. This research demonstrates the harmonious coexistence of safety, sustainability and performance, in accordance with industry and environmental needs. Figure 1: 3D models predict different bioactivity-toxicity patterns between the class of standard organophosphate antiwears (cluster A) and the new class of AW molecules (cluster B). Calculated phosphate access is considerably reduced in cluster B versus cluster A. [1] Figure 2: In vitro results specific to neurotoxicity assessment based on the Elman’s method (cholinesterase (ChE) inhibition). [4] IC50: OP concentration (in mg/ L) for 50% of inhibition. Figure 3: Bearing rig used for assessing the level of cokefaction of turbine oils. Figure 4: AW Standard 1 is the commercial benchmark, AW1, AW2, AW3 the new safer antiwears. Load resistance performance using MTM2 tribometer. Conditions: 0,2 m/ s speed, 50 N load, slide-to-Roll Ratio: 30 s step from 300% and up to failure, 100-°C. References [1] Modeling work on OP toxicity assessment is under publication. [2] Karaboga et al. Journal of Molecular Graphics and Modelling 2013, 41, 20-30. [3] Hagstrom, D., Hirokawa, H., Zhang, L., Radic, Z., Taylor, P., & Collins, E. S. Archives of toxicology 2017, 91(8), 2837-2847. [4] Ellman, G. L., et al. Biochem. Pharmacol. 1961, 7, 88-95. 24th International Colloquium Tribology - January 2024 251 The Effects of Applying the Tribological Compound TZ NIOD Philipp Harrer 1 , Dmitrii Svetov 2 , Patrick Eisner 3 , Maximilian Lackner 4 , Erich Markl 5 1 UAS Technikum Wien, Industrial Engineering, Vienna, Austria 2 Dmitrii Svetov www.tribo.at, Vienna, Austria 3 UAS Technikum Wien, Industrial Engineering, Vienna, Austria 4 UAS Technikum Wien, Industrial Engineering, Vienna, Austria 5 UAS Technikum Wien, Industrial Engineering, Vienna, Austria 1. Introduction Tribology’s economic and technical relevance in terms of energy loss, material deterioration and waste has long been accepted and has recently been augmented with sustainable viewpoints such as environmental awareness, longer life of device, reducing waste and enhancing the quality of life. Due to the vast potential and relevance in various sectors, improvements in the field of tribology remain of high importance. [1] One potential contributor could be TZ NIOD [2], which is a tribological compound which shall be applied to moving parts with the goal of reducing friction, energy consumption, renewing worn out surfaces, increasing the service life of the entire device, reducing the temperature, reducing the coefficient of friction as well as reducing the rate of wear. It consists of a complex mixture of silicate material powder, specifically serpentinite, which uses oil or grease as a transport medium to reach worn surface and highly loaded friction points. These particles allegedly react with the material under the influence of temperature and pressure creating a modified surface layer. The aim is to analyse the effects of TZ NIOD on tribo-systems and devices it is applied to. In order to determine the effect of TZ NIOD a literature review covering tribology, tribometry, wear, common methods of reducing wear as well as nanoparticles was conducted. Additionally, empirical analyses were performed in alignment with the tribological test chain consisting of model tests aligned on the test procedure of the commonly applied pin-on-disc tribometer with the goal of analysing the effect of TZ NIOD on a material level of simplified tribo-systems. Followed by an empirical analysis consisting of machinery tests analysing the effects of an application of TZ NIOD on piston compressors. These analyses consisted of temperature and power consumption measurements during various modes of operation of the compressor and different modes of applying TZ NIOD to the compressor. The projects were sponsored by Dmitrii Svetov (www.tribo.at) who is the appointed European general representative for TZ NIOD. Dmitrii Svetov is the responsible and qualified representative to distribute TZ NIOD for all of Europe. 2. Tribological Compound - TZ NIOD TZ NIOD [2] is a novel agent to improve properties of friction partners. TZ NIOD is a complex mixture of silicate material powder with particle sizes ranging from 5 to 50 micrometers. The basis of TZ NIOD is made up of finely distributed and divided particles of Serpentinite. It consists of nanoparticles which must be dispersed in oil or grease and the intensity of its penetration into the material surface is proportional to the pressure and temperature of contact zones. 2.1 Claimed Benefits of TZ NIOD The sponsor [2] claims that the nanoparticles of TZ NIOD accumulate in worn areas of contact zones of tribo-systems due to the higher surfaces roughness. Thus, the oil and grease act as a transporting agent of the TZ NIOD particles to the areas of highest wear. The increased friction, temperature and higher pressure, due to wear, stimulate the penetration of TZ NIOD particles into the contact surface resulting in a mending and self-healing effect. This self-healing effect results in a restoration of the friction partners of the tribo-system and shall have the following positive effects. • reduce the coefficient of friction • reduce energy losses due to friction • lower temperature increases due to friction • higher resistance against wear • ability to operate tribo-systems without lubrication for short periods of time 2.2 Working Principles of TZ NIOD An application of TZ NIOD is performed directly on the device during its operation and consist of three phases [3]. In phase one, finely dispersed TZ NIOD particles are transported to the areas of wear via the oil and grease it is dispersed in. The particles abrasive effect polishes the contact areas due to the hardness of the silicate material, which removes oxide layers from the metal and react under the influence of temperature and pressure. In the second phase, the activated TZ NIOD particles formed in the contact zone result in a modified surface layer with increased hardness and higher resistance against wear. The process of phase 2 continues until the entire surface of the contact area is saturated with TZ NIOD particle. Since the metal structure is saturated with TZ NIOD at the end of Phase 2 the lubricant containing the remaining finely dispersed and grinded TZ NIOD can be removed from the tribological system and replaced by a fresh lubricant. The third phase, referred to as “running-in”, continues after the removal of the lubricant containing TZ NIOD. The device must continue to operate while the TZ NIOD particles embedded in the metal structure continue to positively influence the metallic structure of the device. 3. Empirical Analyses To test the effect of TZ NIOD on real equipment in a machinery test, TZ NIOD was applied to the crank case and the cylinder head of more than 40-year-old used piston air compressors. The pressure in the pressure vessel and filling time, the 252 24th International Colloquium Tribology - January 2024 The Effects of Applying the Tribological Compound TZ NIOD power consumption of the motor, as well as the temperature of the cylinder head were recorded in the initial state (before applying TZ NIOD), directly after an application of TZ NIOD as well as after a running-in phase consisting of 100 hours of standard discontinuous operation at a utilization rate of roughly 60%. The application of TZ NIOD on the piston air compressor, consisted of the following four stages: 1. Removing the initial oil from the device. 2. Filling the TZ NIOD - Oil mixture in the proper ratio into the oil pan of the device. 3. Application Phase: continuous operation for 40 minutes while exposing the cylinder head to TZ NIOD for 20 minutes, followed by the device’s standard discontinuous operation for 3 hours and 20 minutes. 4. Running-In Phase: replacing the TZ NIOD-Oil mixture with fresh lubrication followed by 100 hours of the devices standard discontinuous operating mode. 3.1 Results of TZ NIOD applied to the Piston Air Compressor During the empirical analysis TZ NIOD was applied to a piston air compressor in operation without the need of disassembling the compressor or modifying components. The application resulted in a down time of only 60 minutes. It was observed that the pressure over time characteristic of the compressor improved noticeably, see Figure 1. A clear difference between the characteristic after running-in (blue line in Figure 1) can be noticed compared to the initial state (black line in Figure 1) and the state directly after applying TZ NIOD (green line in Figure 1) Figure 1: Pressure-Time Diagram of initial, after application and after running-in Further, the application of TZ NIOD positively affected the filling time and the power consumption, as compared in Table 1. The average power consumption of each filling cycle during a typical discontinuous operating mode for filling the pressure vessel was reduced by 20.7 Wh, which represents a reduction of 7.8 percent. The average time to fill the pressure vessel was shortened by 3.9 seconds, which represents a reduction of 5.1 percent. Table 1: Result of Filling the Pressure Vessel after Running-In with TZ NIOD Unit Initial After Applying TZ NIOD Difference Avg. Time for Filling [sec] 80.30 76.37 -3.93 ( -5.1%) Avg. Power Consumption [Wh] 38.24 35.47 -2.77 (-7.8%) The course of the power consumption while filling the pressure vessel is illustrated in Figure 2. The power consumption after running-in with TZ NIOD is represented by the blue graphs in Figure 2. The blue graphs are noticeably lower than the graphs of the initial state, represented in black. Figure 2: Detail of Power Consumption filling the Pressure Vessel after running-in with TZ NIOD 4. Conclusion The results of the empirical analyses conclude that TZ NIOD is capable of unfolding its positive effects when applied to devices in operation and within the specified operating condition of the device. An application of TZ NIOD must be tailored to the specific device. The application of TZ NIOD on the piston air compressor resulted in a total down time of only 60 minutes. Overall, it can be concluded that the positive effects of TZ NIOD, on the device it is applied to, comprise of lowering the power consumption by 7.8%, increasing the efficiency by lowering the filling time of the pressure vessel by 5.1%. This indicates that the worn-out surfaces of the device were regenerated which contributed to decreasing the temperature in operation and increasing the devices service life. References [1] J. P. Davim, Progress in Green Tribology: Green and Conventional Techniques. Berlin: De Gruyter Oldenbourg, 2017. [2] D. Svetov, “Tribologie in österreich,” Tribo.at, http: / / tribo.at/ (acc. Apr. 18, 2023). [3] D. Svetov, “Die tribotechnische Zusammensetzung von Niod - Prozesse,” Tribo.at, http: / / www.tribo.at/ prozes. html (accessed Apr. 18, 2023).- 24th International Colloquium Tribology - January 2024 253 Innovative Lubricant Components with Lower Greenhouse Gas Emissions Addressing sustainability and transformation needs of the lubricants industry Dr. Sabrina Stark 1* , Edith Tuzyna 1 , Rene Koschabek 1 1 BASF S.E., Fuel&Lubricants Solutions, Ludwigshafen, Germany * Corresponding author: sabrina.stark@basf.com 1. Introduction Sustainability has always been an important topic in the lubricants industry, as lubricants manipulate tribological factors in mechanical systems to prevent wear. This enhances the systems longevity and saves resources. Additionally, energy/ fuel is also saved with reduced friction which consequently reduces Greenhouse gas (GHG) emissions in the use phase [1]. However, reporting guidelines as the GHG Protocol do not deal with energy savings due to, for example, machine efficiency increases related to the use of improved lubricants (comparative emissions impact or avoided emissions, scope 4) but only with reporting of emissions within the life cycle or value chain of a product (scope 1, 2, 3 upstream and 3 downstream) [2]. Although there is considerable interest among companies in claiming that their products can help avoid GHG emissions compared to other products in the marketplace, claims on energy conservation can only be marketed, if widely recognized procedures for determining effective CO 2 reductions through frictional reductions and extended longevities are in place [3, 4]. Studies to date have mostly focused on the efficiency of components and adjustment of operational strategies to reduce energy consumption [4]. However, with this paper we would like to continue our journey [5] on displaying the contribution of lubricant components with reduced product carbon footprint (PCF, cradle-to-gate) and focus on the emerging needs to understand and reduce climate related information of indirect emissions from upstream activities in the value chain (Scope 3 upstream). As lubricants industry associations publish industry guidelines on product carbon footprint calculation [6], and companies along the value chain start to commit to Scope 3 upstream emission reduction targets [7], the topic of regimenting claims on sustainable lubricants gains a lot of traction. Therefore, we would like to cover in this paper the contribution on the individual product level and give an insight into a new portfolio of biomass balanced lubricant components with significantly reduced carbon footprint compared to conventional products. 2. Product Carbon Footprint and the Biomass Balance Methodology- With the drive to reduce GHG emissions and dependence on fossil resources, the chemical industry is gradually starting to develop new products derived from renewable feedstocks and optimized PCF. Usage of renewable raw material is not a new concept to the lubricants industry as the terms “biolubricant” and “environmentally acceptable lubricant” (EAL) have been largely used for decades to describe lubricants containing components that are plant oil-based and/ or synthetic manufactured from modified renewable raw materials [8]. This is an established market, with companies as BASF offering a broad range of synthetic ester base stocks. However, replacing fossil feedstocks with renewable feedstocks in applications outside the biolubricant segment is a challenging task. Large investments are required in R&D and production facilities to produce bio-based chemicals. Moreover, due to technological requirements in various lubricant applications the use of renewable feedstocks is limited, and other new concepts are required. For the introduction of renewable feedstocks in existing production pathways on a broad scale and in a cost-effective way, a simplified approach based on mass balance has been proposed by BASF [9]. The main fossil feedstocks in the chemical industry are naphtha and natural gas, which are further processed either by cracking or part-oxidation at high temperatures. To produce BMB products, these feedstocks are replaced by renewable feedstocks, i.e., bio-naphtha and/ or bio-methane at certain amounts calculated through a material flow analysis [9]. BMB products are chemicals that are produced using both fossil and renewable feedstocks in an integrated chemical production facility. The output is a mix of fossil and renewable based products which are not distinguishable based on their composition or technical characteristics. As the renewable feedstock is processed together with fossil feedstock, it might not be physically traceable throughout the production processes. Therefore, there is a need to attribute the applied renewable feedstock volumes to a final product in a fully transparent and auditable way, which is achieved by a certification according to the requirements of the RedCert2 scheme (see Figure 1).- Innovative Lubricant Components with Lower Greenhouse Gas Emissions 254 24th International Colloquium Tribology - January 2024 - Fig 1. The biomass balance (BMB) approach This solution enables the use of renewable feedstocks to produce products with the same properties as those manufactured from fossil feedstock while saving fossil resources and reducing CO 2 emissions. This methodology offers hundreds of BMB products, globally, across various industries, including components for the lubricants industry.-- In absence of established standards or guidelines on how to calculate the PCF of a BMB product, a method was developed by BASF and a third-party certification confirms that the calculation procedure of BMB product carbon footprints and the associated PCF reduction follow conventional life cycle analysis (LCA) procedure as described in ISO standard 14067: 2018 and Together for Sustainability (TfS) Guideline [10]. 3. Examples of BMB products for lubricant industry Two examples relevant for the lubricants industry from the BASF portfolio on biomass balanced products, that were already topic of previous publications [5], are the high performance water-soluble polyalkylene glycol (PAG) base stocks (Breox® BMBcert™) and the polyisobutene (PIB) portfolio (Glissopal® BMBcert™). The new additions to the BMB family for lubricants are shear stable VI improvers and pour point depressants (Irgaflo® BMBcert™ series) and the Lubricity improvers (EO / PO block co-polymers) for use in metalworking fluids (Synative® RPE BMBcert™ series). Table 1 displays an overview of our BMB portfolio for the lubricants industry. Table 1: PCF (cradle-to-gate, including biogenic carbon) reduction potential of different BASF products when applying the biomass balance method. Biomass balanced product series PCF reduction potential compared to conventional product Polyisobutene (PIB) 100% Pour point depressants up to 95% PAG base stocks up to 80%* Lubricity improvers up to 65% VI improvers up to 60% *Benefits include usage of 100% green electricity in production 4. Conclusion Based on an established and independently certified method, the biomass balanced products enable transparency for sustainable purchasing decisions and a faster transition to a carbon-neutral circular economy. Providing the benefits of sustainably sourced renewable feedstock, flexible scale-up and identical product quality, the new biomass balanced product range BMBcert™ provide base stocks and lubricant oil additives that help save fossil resources and reduce overall greenhouse gas emissions, while maintaining the same performance. The biomass balance approach enables lubricant producers to differentiate their solutions from competition while achieving industry sustainability goals. References [1] M. Woydt, Material efficiency through wear protection - The contribution of tribology for reducing CO2 emissions. WEAR 488-489 (2022) 204134, https: / / doi.org/ 10.1016/ j. wear.2021.204134 [2] Scope3CalculationGuidance|GHGProtocolhttps: / / ghg protocol.org/ scope-3-calculation-guidance-2 and https: / / ghgprotocol.org/ estimating-and-reporting-avoi ded-emissions [3] M. Woydt, R. Shah and G. Thomas, Koehler, Lube Magazine, July 2023, Sustainability in lubricants: a look at how regulatory agencies can play their role. [4] M. Woydt, E. Bock, V. Bakolas, T. Hosenfeldt, R. Luther and C. Wincierz, Effects of tribology on CO 2 -emissions in the use phase of products - Contributions of tribology to defossilization, Publisher: German Society for Tribology, www. gft-ev.de, June 2023, open access [5] S. Rauch, C. Krüger, P.Saling, S.Stark, Reducing product carbon footprint of lubricants by using biomass balanced basestocks: The importance of biogenic carbon modelling in LCA, Lube Magazine Online June 2022 [6] Methodology for Product Carbon Footprint Calculations for Lubricants and other Specialties, https: / / www.ueil.org/ sustainability/ [7] Science Based Targets Initiative, https: / / sciencebasedtargets.org/ companies-taking-action [8] Biolubricants: Raw materials, chemical modifications and environmental benefits, European Journal of Lipid Science and Technology, Wiley Online Library, 2010 [9] Krüger, C., Kicherer, A., Kormann, C., Raupp, N., Biomass balance: An innovative and complementary method for using biomass as feedstock in the chemical industry, Springer International Publishing, Cham, 2018.- [10] Together for Sustainability PCF Guideline (tfs-initiative.com) 24th International Colloquium Tribology - January 2024 255 High Quality Sustainable Base Oils from Plastic Waste and Biomass Matias de Tezanos 1* , Boris Zhmud 2 1 KATA Circular Pte Ltd, Singapore, Singapore 2 Tribonex AB, Uppsala, Sweden * Corresponding author: mdt@kata.com Plastic pollution is a serious and constantly growing environmental threat. In 2022, global annual plastics production has reached 460 million tons. Estimated global leakage of plastic waste to the environment was 22 million tons in 2019, and this estimate is projected to double by 2060. The use of plastics in agri-food systems and food value chains is particularly widespread. Polyethylene and polypropylene are two common polymers used in agriculture for multiple applications. These polymers can be readily upcycled into value-added products. Unfortunately, despite the fact that plastic waste can be used as raw material in chemical industry, there is a lack of systematic collection and sustainable management. Figure 1: Estimated quantities of agricultural plastics used per hectare of land [1] Pyrolysis is one common technique used to convert a wide range of plastics into a complex mixture of alkanes, alkenes, alkynes, and aromatics that can be further used in traditional petrochemical processes. Another possibility is catalytic hydrogenolysis that uses hydrogen to cleave carbon-carbon bonds producing predominantly alkanes. Unfortunately, so far, these processes have not been economically competitive with traditional crude oil processing due to relatively low crude oil prices. However, the situation is gradually changing thanks to the introduction of carbon credits. Figure 2: Upcycling of waste plastic [2] The Fischer-Tropsch (FT) process is the basis for nearly all technological processes dealing with conversion of natural gas and coal to more valuable liquid hydrocarbons. In the past decades, lubricating oils derived from FT waxes started to gain interest. Unfortunately, the availability of FT waxes is rather limited to cover market needs. Another serious concern is the heavy carbon footprint of such processes. In such a situation, waste plastic avail itself as a perfect substitute for FT and slack waxes from petroleum refining, leading the way to a truly sustainable process with a greatly reduced carbon footprint and environmental impact: instead of dumping waste plastic to landfills, we can convert it into value-added products. KATA has developed an innovative way to chemically upcycle plastic waste originating from agriculture. In the present communication, properties and applications of novel sustainable base oils and additives produced from biomass and plastic waste are presented, with a focus on common physicochemical properties, property blending relationships and tribological performance. KATA technology allows one to convert PE and PP agricultural waste plastic into high quality sustainable fuels, solvents and base oils. Originating from a non-petroleum feedstock, KATA oils contain no aromatic and naphthenic molecules, being 100% composed of iso-paraffins. This leads to improved low temperature flow properties and oxidation stability. Compared to traditional mineral oils, KATA oils have more narrow molecular weights and boiling points distribution, as distillation curves indicate (Figure 3). Figure 3: Distillation curves for conventional mineral base oil and synthetic base oil produced from plastic. The yield of value-added products is close to 80%. The actual partitioning of individual products depends on process conditions. Thus, one can prioritize the production of fuel, the production of wax, etc. Approximate outputs of different product streams are shown in Figure 4. 256 24th International Colloquium Tribology - January 2024 High Quality Sustainable Base Oils from Plastic Waste and Biomass Figure 4: Approximate outputs of different products produced by plastic waste upcycling Thus, Table 1 presents typical properties of sustainable white oil, SBS 100, produced from waste plastic. This oil has been benchmarked against traditional API Group III/ III+ and Group IV base oils in a number of finished lubricants and demonstrated comparable, and in some cases, superior, performance. Table 1: Typical properties of SBS100 white oil Typical Properties Method Unit SBS100 Specific gravity @ 15-°C ASTM D1298 g/ cm 3 0.832 Flash Point ASTM D92 °C 200 Pour Point ASTM D97 °C -38-°C Kin. viscosity @ 40-°C ASTM D445 cSt 16.1 Kin. viscosity @ 100-°C ASTM D445 cSt 3.8 Viscosity Index ASTM D2270 - 130 Noack DIN 51581 % 15 Color ASTM D1500 - <-Lo.5 Sulfur ASTM D2622 ppm <-10 Further, Table 2 shows the properties of a different product an ester derived from castor oil. This product has been evaluated as co-emulsifier and lubricity improver in cutting oils. Table 2: Typical properties of ricinoleic ester Property Method Unit Value Kin. viscosity @ 100-°C ASTM D445 cSt 3.5 Kin. viscosity @ 40-°C ASTM D445 cSt 17.6 Viscosity index ASTM D2270 - 56 Specific gravity @ 20-°C ASTM D4052 g/ cm 3 0.92 Pour point ASTM D6892 °C -24 Flash point ASTM D92 °C 208 KATA operations rely upon a closed loop supply chain, paving the way to a truly sustainable manufacturing ecosystem. Waste plastic is collected from local farmers, converted to value-added products, such as agricultural lubricants, fuels and crop protection oils, which are sold back to the farmers. By co-processing castor oil with rapeseed oil, organic friction modifiers were produced demonstrating outstanding friction-reducing properties when used together with API Group IV base oils. Figures 5(a) and 5(b) show the MTM test data for various organic friction modifiers at 0.5% treat level in PAO4 base oil. “Base” denotes the pure base oil and “XFM” denotes the novel cross-linked polymeric friction modifier. (a) (b) Figure 5: MTM test data comparing the tribological efficiency of various friction modifiers in fresh (2 h at 100- o C) formulations and aged formulations (8 h at 100- o C + 8 h at 130- o C). The XFM polymeric organic friction modifier demonstrates top-of-the-class efficiency and superior effect retention compared to other commercial OFM systems. This makes it attractive for use in industrial and transportation lubricants as a replacement for inorganic friction modifiers [3]. References [1] FAO. 2021. Assessment of agricultural plastics and their sustainability. A call for action. Rome. https: / / doi. org/ 10.4060/ cb7856en [2] R. Hackler, K. Vyavhare, R. Kennedy, et al. Synthetic Lubricants Derived from Plastic Waste and their Tribological Performance, ChemSusChem. 10.1002/ cssc.202100912. [3] B. Zhmud, N. Stawniak, S. Ressel, Metalworking Fluids and Industrial Lubricants Based on Novel Rapeseed Oil Varieties, Lube 174 (2023) 9. 24th International Colloquium Tribology - January 2024 257 Hybrid Lubricating Grease Formulations: A Sustainable Approach for Utilizing Renewable Resources within a Circular Economy Model George S. Dodos *1 , Mehdi Fathi-Najafi 2 , Christina Dima 1 , Nora Kaframani 1 , Andreas Dodos 1 1 ELDON’S S.A., Athens, Greece 2 Nynas AB, Stockholm, Sweden * Corresponding author: g.dodos@eldons.gr 1. Introduction The UN’s Sustainable Development Goals sets 2030 as the year by which certain key targets should be achieved including the responsible consumption and production, aiming to reduce waste and protect natural resources. Waste minimization can be achieved in an efficient way by focusing primarily on the first of the 3Rs, “reduce,” followed by “reuse” and then “recycle”. “Zero Waste” is an approach that promotes the goal of reducing the amount of material we throw away and instead reincorporating by-products of one system for use for another system. The idea of the so-called Industrial Symbiosis the process by which wastes or by-products of an industry or industrial process become the raw materials for another can substantially contribute to the creation of circular development and “Zero Waste Goal”. Waste may include bio-waste, mineral-waste, and food-type waste. Particularly for the latter there is a global social and economic need to recover high added value components and to produce energy and bio-commodities. Valorizing the vast amount of food and food processing industry waste and by-products can provide economic benefits and reduce GHG emissions [1, 2]. Moreover, the industrial utilization of components with a lower nutritional value means that there is no element of competition between current food or feed streams. In the recent past, a series of successful projects to valorize waste streams, such as used cooking oil to produce sustainable greases have been presented. Also, it has been demonstrated that the reduction of the energy consumption in the grease production can contribute to an increasing sustainable profile of lubricating greases [3,4]. To move forward, the utilization of waste materials or the re-use of end-of-life components as renewable feedstock in the production of greases, can promote further the sustainable performance within a circular model. 1.1 Scope The aim of this work is to employ innovative renewable raw material streams for the formulation of lubricating greases that can show improved performance characteristics compared to a conventional base grease formulation model. Specifically, this study: • Investigate the potential of the Hybrid Concept in grease formulation. Within the hybrid approach, bio-components and waste material can be efficiently incorporated to already studied matrix and this will allow for elimination of the risk of incompatibilities and provides flexibility in the selection of efficient additive chemistries. • Assess the efficiency of incorporating crude fatty oils in the grease matrix instead of refined ones. Renewable feedstock from plant or animal sources are a good starting point as most of them do not pose classification issues. The utilization of crude (instead of refined) species of these oils is one more asset to the LCA of the final greases due to the lower GHG emissions involved in the overall processes and the potential gain in the water footprint. • Examine the incorporation of renewable particles in the grease matrix. By adding solid particles to a lubricating grease, certain properties can be enhanced. These renewable solid particles can further increase the renewable partition in the final formulation. • Create possibilities for the evolution of a new roadmap towards low carbon intensity greases. 2. Experimental 2.1 Grease formulations Lab scale grease formulations based on Ca and Li 12-HSA thickener were utilized and evaluated per the effect of a series of renewable components (liquid and solid) on certain key grease performance characteristics. A naphthenic specialty product was utilized as an effective conventional and compatible foundation to highlight the benefits of the renewable materials added. The target was to replace at least 50 percent by weight of the formulation with bio-components. 2.1.1 Natural esters Two different types of renewable straight run/ crude natural esters were incorporated in the dispersion medium that represent oils with different fatty acid composition that could be either of low nutrition value or by-products of the food sector. Cottonseed oil (CCSO) has increased levels of saturated fatty acids while olive pomace oil (COPO) is abundant in oleic acid. CCSO has certain advantages for climate change compared to other seed oils (low water footprint, potential for reduced climate change impacts). COPO is a byproduct of olive oil processing. It is intended for refining for use for human consumption, or for technical use. It is a non-GMO High Oleic Fatty acid profile which is beneficial for industrial uses. 2.1.2 Renewable solid particles Two types of renewable particles (RSP-1 & RSP-2) were added at a treating rate between 5 - 15 wt. percent. These are by-products of the food processing industry that were selected as inert components instead for chemically active compounds. 258 24th International Colloquium Tribology - January 2024 Hybrid Lubricating Grease Formulations: A Sustainable Approach for Utilizing Renewable Resources within a Circular Economy Model Li Formulations Ca Formulations Figure 1: Hybrid Grease Formulations: Effect of the various renewable bio-components on grease’s characteristics Moreover, Calcium carbonate (CaCO 3 ), a natural solid particle, was added as a controlling agent particle at 2 wt. percent. 3. Results and Discussion Figure 1 gives a summary of the effect of the various bio-components on fundamental quality parameters of the hybrid grease formulations. 3.1 Natural Esters Consistency: A penetration equal to NLGI #1 was determined in most cases with a trend of a higher consistency in Ca formulation, especially when COPO was incorporated. Dropping Point: No adverse effect on thermal properties of greases after the addition of the crude vegetable oils. Anti-wear Properties: Reasonable Four Ball wear scars were generated after the addition of the natural esters. Cold flow properties: COPO is capable of substantially upgrading the low temperature performance due to the positive effect of high monounsaturated fatty acid profile. Lower flow pressure values were measured in the Li formulations. Oxidation Stability: Results are acceptable for this type of formulations. There is a clear effect of the variant FA profile of the two natural esters. 3.2 Renewable solid particles Consistency: A thickening effect of the particles was depicted in almost all cases. Dropping Point: The addition of renewable particles shows a positive effect on Ca formulations, with no negative effect in the rest of the samples. Antiwear Properties: RSP1: at 5wt. % treating rate the wear scar was dramatically decreased from around 2.6 mm to <- 0.5- mm. RSP2: this type of particle is more efficient at higher treating rates of 15 wt.% Cold flow properties: COPO formulations show overall lower impact on low temperature flow pressure after the addition of the RSPs. Oxidation Stability: No adverse effect on oxidation stability 4. Conclusions In a holistic approach to formulate lubricating greases in a sustainable way, this study demonstrated that by-products or waste material from the food processing industry may be introduced in the grease matrix and can be compatible with conventional chemistries and base oils. The incorporation of these renewable materials produces cost-effective greases that are competitive in terms of their characteristics and performances. Hybrid formulations are advantageous in terms of the versatility they can offer in the selection of materials and additives. Finally, the authors believe that this study demonstrates as a successful benchmark of an efficient life-extension of regional by-products or/ and wastes. Moreover, the global extensions of this type of concept to other regions may contribute to a localized vertical (food) waste valorization in grease formulations that can promote the ideas of industrial symbiosis and zero-waste society. References [1] Roy, et al. (2023). ACS Environmental Au, 3(2), 58-75. [2] Tropea, A. (2022). Fermentation, 8(4), 168. [3] Dodos G. S., Eurogrease 2, 2016. [4] Fathi-Najafi M. et al. NLGI Spokesman, 84-2, 2020. Young Tribologists/ Various Tribology 24th International Colloquium Tribology - January 2024 261 Amorphous Carbon Coatings for Total Knee Arthroplasty - a Knee Simulator Evaluation Benedict Rothammer 1* , Kevin Neusser 1 , Marcel Bartz 1 , Sandro Wartzack 1 1 Engineering Design, Friedrich-Alexander-University Erlangen-Nürnberg (FAU), Erlangen, Germany * Corresponding author: rothammer@mf k.fau.de 1. Introduction The use of artificial implants is substantially improving the quality of life for millions of people worldwide. However, the success of implants, such as total hip arthroplasties (THAs) or total knee arthroplasties (TKAs), depends on their ability to integrate and function in the human body without causing undesirable reactions-[1]. One way to improve the performance of artificial implants is to use amorphous carbon/ diamond-like carbon (DLC) coatings. Due to the fact that DLC is a material with excellent mechanical, chemical, and biocompatible properties, it has promising potential for various biomedical applications [2]. Some studies have investigated DLC coatings for use in joint replacements to improve wear resistance and thus extend the life of the artificial implant [3,4]. Thus, the following research gap, investigated in this contribution, arose as to whether a double-sided DLC coating of TKAs can considerably reduce wear compared to uncoated Co28Cr6Mo (CoCr)/ conventional UHM- WPE (CPE) or Ti6Al4V (Ti64)/ CPE in a knee simulator and thus substantially increase the service life of TKAs. 2. Materials and methods Basically, the unconstrained, fixed-bearing TKA BPKS Integration Knee System (Peter Brehm, Weisendorf, Germany) was used. This TKA is commercially available as a CoCr variant. Especially for this investigation, BPKS were also made of Ti64 (not commercially available) in order to characterize its tribological behavior (uncoated and coated). The spherically shaped femoral component of the left TKA(size-4) was fabricated from a cast CoCr alloy [5] and by machining a Ti64 alloy [6], respectively. The corresponding planar tibial plateau (size-4) was forged from a forged CoCr alloy [7] or manufactured additively from Ti64 by selective laser melting. The tibial inlays (size-4) were made of UHMWPE (Granular UHMWPE Ruhrchemie, GUR 1020 [8]). Subsequent γ-irradiation (BBF Sterilisation Service, Kernen im Remstal, Germany) with 31.2-±-5.5-kGy [9] to CPE was performed for the reference inlays after their production and for the coated inlays after the deposition process. The surfaces of the uncoated (Gesellschaft für Polier- und Schleiftechnologie, Bosau, Germany) and coated TKAs (Bestenlehrer, Herzogenaurach, Germany) were polished so that the surface requirements according to ISO-7207 [10] were met for metallic (R a -≤-0.1-µm) and for polymeric (R a -≤-2.0-µm) components, respectively. Articulation partners were the femur/ inlay pairs (n-=-3 plus 1 axial reference): • CoCr/ CPE as gold standard, • Ti64/ CPE the comparative pairing, • as well as Ti64/ aC: H/ CPE/ aC: H (according to [11]) as the decisive pairing to be investigated with aC: H fully coated. Experimental in-vitro testing of the implants was performed in a modified servohydraulic knee simulator with four stations (EndoLab, Thansau, Germany) [12] according to ISO 142431 [13]. In the knee simulator, walking as an everyday activity was simulated. Prior to the actual testing, the tibial inlays were preconditioned in diluted bovine calf serum (DBCS)-[14] at body temperature until the relative weight gain of the inlays was <-10-% compared to the previous week. This preconditioning prevented soaking of the inlay during the actual test, so that falsification of the wear mass of the CPE was nearly excluded. Subsequently, the implants were tested on their articulating surface areas in the simulator using DBCS tempered to 37- °C as lubricant for a total of 3.5-×-106 cycles. At intervals of 0.5-×-106 cycles, the implants were cleaned, the test fluid was retained for future particle testing, replaced by new DBCS, and the wear mass of the tibial inlay was determined gravimetrically with an analytical balance (Sartorius BP211D, Goettingen, Germany, accuracy of 0.01-mg) according to ISO-142432 [15]. In addition, the wear masses were corrected for soaking and air buoyancy using a purely axially load. In order to minimize interstation differences, the tibial inlays were rotated between stations after 0.5- ×- 106 cycles. For the characterization of the biotribological behavior, the original system (CoCr/ CPE and Ti64/ CPE) as well as the initial condition of the coated Ti64/ CPE variant before loading served as reference. 3. Results and discussion The mean values and standard deviations of the wear mass of the polymeric tibial inlays for each test interval are shown in Figure-1. With the help of the diagram, the development of the wear over the entire test can be shown for the uncoated TKAs made of CoCr/ CPE and Ti64/ CPE as well as the fully coated TKAs made of Ti64/ aC: H/ CPE/ aC: H. After the first 0.5-×-10 6 cycles, which equate to a critical runin, a comparable wear mass of approximately 2.7-mg was determined for all TKAs tested. While the uncoated references exhibited rather higher values over the test intervals with a steady, linear upward trend in wear mass, the coated tibial inlays showed a considerably milder wear with a slight upward trend. After 3.5-×-10 6 cycles, the highest wear was determined for the Ti64 (≈- 23.7- mg), followed by the CoCr (≈-20.9-mg), and the lowest wear for the coated variant (≈-9.0-mg). This clear relation was also reflected in the wear rates, which were 6.0- ±- 0.4- mg/ 10 -6 cycles for CoCr/ CPE, 7.1-±-1.0-mg/ 10 -6 cycles for Ti64/ CPE, and 2.1-±-0.7-mg/ 10 -6 cycles for Ti64/ aC: H/ CPE/ aC: H. Amorphous Carbon Coatings for Total Knee Arthroplasty -a Knee Simulator Evaluation 262 24th International Colloquium Tribology - January 2024 Figure-1: Arithmetically averaged wear mass and corresponding standard deviation for uncoated TKAs (CoCr/ CPE and Ti64/ CPE) and fully coated TKAs (Ti64/ aC: H/ CPE/ aC: H). Generally, the test showed that the CoCr/ CPE variant achieved a reduction in wear mass of about 12-% compared with the Ti64/ CPE variant. By coating both articulation partners, a significant reduction in the wear mass of roughly 57-% compared to CoCr/ CPE and roughly 62-% compared to Ti64/ CPE could be achieved, thus the service life of TKAs could be extended by DLC coatings. However, the tested components must be examined using advanced surface analysis in order to make a full statement about the exact wear behavior and particles. The obtained tibial inlay wear rates were found to be rather low for all tested TKAs compared to wear rates already published in the literature [16-18]. However, due to the revision of ISO-142431 [13], there was an adjustment of the kinematics during wear testing, allowing a higher free tibial rotation during a gait cycle. Thus, according to the current state of knowledge, a quantitative comparison of the wear rates is not expedient yet. In summary, it can be assumed that the coating can considerably increase the service life of TKAs. In this context, the DLC coatings must be specially adapted to the biotribological system TKA in order to prevent a coating-induced, adhesion-related failure. The DLC coatings on TKAs represented a compromise between the coating combinations presented in [11] and [19] - in terms of sufficiently high wear resistance and high deformation capability to prevent near-surface fatigue. The coating itself contributes to an increase in the service life by the same amount due to its thickness, provided that the coatings do not lead to undesirable (three-body) wear. Furthermore, the coating can promote a favorable running-in of the articulating partners and thus enable slow, continuous wear - avoiding the occurrence of delamination. Even though the testing of implants in the knee simulator is an important step in the process of certification of TKAs, the limitations of testing under almost ideal in-vitro conditions at the component level must be considered as well. Besides a simplified environment, ideal gait cycles were simulated. Additionally, it was ensured that clinically relevant extreme conditions were excluded. These include, for instance, the presence of ceramic particles, which can be generated during cemented implantation under non-ideal conditions and lead to three-body wear. Basically, such extreme tests could be used to assess the wear behavior of the coatings in comparison to reference pairings under more realistic conditions. In future studies, these results must be examined using precise surface analysis methods in order to be able to provide a full explanation of the wear behaviour. 4. Conclusion The current results of experimental in-vitro testing of uncoated and coated TKAs shown in this contribution demonstrated that a significant wear reduction and thus service life extension of TKAs was possible by DLC coatings on both articulating partners. The initial wear results shown were consistent with the findings in [11,19] and confirmed the biotribological effectiveness of DLC coatings. Due to the update of ISO-142431 [13], a quantitative comparison with previously published results regarding wear rates is, at present, not expedient. Also, the general limitations of a component test rig under ideal conditions must be considered in order to be able to derive well-founded, realistic wear predictions. Nevertheless, it can be stated for the investigations carried out that the service life of TKAs could be considerably increased by biotribologically effective DLC coatings. However, the current investigations from the knee simulation must be fully continued and consolidated for a holistic prognosis of the tribological performance of coated TKAs. References [1] Marian, M., et al. Adv. Colloid Interface Sci., 2022, 307. Jg., S. 102747. [2] Malisz, K., et al. Materials, 2023, 16. Jg., Nr. 9, S. 3420. [3] Birkett, M., et al. Acta Biomaterialia, 2023. [4] Shah, R., et al. Surf. Interfaces, 2021, 27. Jg., S. 101498. [5] DIN ISO 5832-4: 2015-12. [6] DIN EN ISO 5832-3: 2022-02. [7] DIN ISO 5834-2: 2020-07. [8] DIN ISO 5832-12: 2020-07. [9] DIN EN ISO 11137-1: 2015-11. [10] ISO 7207-2: 2011-07. [11] Rothammer, B., et al. Wear, 2023, 523. Jg., S. 204728. [12] Woiczinski, M., et al. Knee Surg. Sports Traumatol. Arthrosc., 2020, 28. Jg., S. 3016-3021. [13] ISO 14234-1/ Amd 1: 2009-11/ 2020-01. [14] Rothammer, B., et al. J. Mech. Behav. Biomed. Mater., 2021, 115. Jg., S. 104278. [15] ISO 14243-2: 2016-09. [16] Ezzet, K. A., et al. Clin. Orthop. Relat. Res., 2004, 428. Jg., S. 120-124. [17] Grupp, T. M., et al. Clin. Biomech., 2009, 24. Jg., Nr. 2, S. 210-217. [18] Kretzer, J. P., et al. Orthopaedic Proceedings. Bone & Joint, 2012. S. 97-97. [19] Rothammer, B., et al. Adv. Mater. Interfaces, 2023, 10. Jg., Nr. 7, S. 2202370. 24th International Colloquium Tribology - January 2024 263 On the Relation between Friction and Surface Topography - Models and Challenges Charlotte Spies 1,2* , Arshia Fatemi 1 1 Robert Bosch GmbH, Renningen, Germany 2 University of Freiburg/ Department of Microsystems Engineering, Freiburg, Germany * Corresponding author: charlotte.spies@de.bosch.com 1. Introduction Sliding contact of different material combinations can be found in applications of various types. Naturally, the surface roughness is considered to influence friction and their relation is of interest for many researchers. With this knowledge, friction could be controlled in such a way to a.o. increase the efficiency or load carrying capacity of engineering parts. In the following, an introduction to the models and challenges of relating friction and surface topography for nominally flat surfaces is given. Therefore, in Section 2 and 3 models and parameters for surface roughness are given. Different types of friction and their relation to roughness are introduced in Section 4. Finally, a discussion on open questions and challenges, and a conclusion are given in Sections 5 and 6 respectfully. 2. Modeling roughness Surface topography and thus roughness can be considered with different approaches. For multi asperity models the roughness is assumed to consist of a multitude of asperities of the same order of magnitude. Examples of these multi asperity models are the theories by Greenwood and Williamson [1] and Bush, Gibson and Thomas-[2]. The surface roughness is assumed to consist of many asperities with a known height distribution. The asperities are spherical [1] or paraboloidal [2] and have the same radii or principal curvatures. With both theories, the real area of contact can be computed and is found to depend on different roughness parameters respectfully. Here, for the contact of rough surfaces the real area of contact is the area where asperities are in contact and differs from the larger nominal area of contact. In contrast, multi scale models take a different approach. Here, it is assumed that roughness occurs on different scales. When magnifying, roughness of a smaller scale would be found and will occur up to the atomic scale. The first to approach roughness with this multi scale theory was Archard [3]. In current research, Persson’s theory [4] and its adaptations and extensions use multi scale roughness. There, the real area of contact can also be related to the surface roughness. 3. Roughness parameters When defining roughness parameters, the multi scale nature of roughness is not necessarily considered. The amplitude and hybrid parameters given in norms and guidelines [5] can be computed for both types of models for roughness. The occurring challenges will be discussed in Section 5. Here, two roughness parameters, the root mean square (RMS) roughness Rq and the RMS slope Rdq, are highlighted. They are defined as (1) (2) where z(x) is the surface height and stands for averaging [5]. These parameters can be used to describe a rough surface and are also used with the approaches of Greenwood and Williamson [1] or Bush, Gibson and Thomas-[2] to model roughness. However, these roughness parameters are not sufficient to accurately describe multi scale roughness. For this, two different characteristics are needed [6], see Fig.-1. The probability density function gives the contribution of each surface height to the total roughness and is often assumed to be Gaussian [6,7]. The power spectral density-(PSD) describes the contribution of the respective wavelength to the total roughness [6]. Since many engineering surfaces show a self-affine fractal behavior from a specific scale on [7], the PSD shows a horizontal roll-off region and for higher wave vectors a linear decrease when plotting it on a log-log scale [7]. With these two characteristics, multi scale roughness can be described and related to a.o. real area of contact or friction. Fig. 1: Power spectral density and probability density function to describe a multi scale surface roughness. On the Relation between Friction and Surface Topography - Models and Challenges 264 24th International Colloquium Tribology - January 2024 4. Friction Friction can be distinguished based on its source. While other friction sources can occur, a.o. adhesive and deformative friction can be observed. In addition, viscoelastic friction can occur when at least one of the contacting partners is of viscoelastic material. There are different models to compute each type of friction, which also give their respective relation between friction and roughness. Bowden and Tabor use the real area of contact for the calculation of the adhesive friction [8], which itself can be determined e.g. with the Greenwood and Williamson [1] or Bush, Gibson and Thomas [2] theory. There, an increase in RMS roughness Rq or a decrease in RMS slope Rdq will result in an increase of adhesive friction. In contrast, a higher RMS slope will introduce a higher deformative friction [9]. Using Persson’s theory for viscoelastic friction, a relation between the obtained coefficient of friction and the PSD of the multi scale roughness can be shown [4]. In the same manner, the real area of contact when considering adhesion can be computed using Persson’s theory [10]. For the Persson theory, self-affine fractal surfaces with a Gaussian height distribution are assumed. 5. Discussion When relating friction and roughness a variety of challenges are arising. Especially the scale dependency of the roughness parameters must be taken into account. The different roughness parameters, e.g. the RMS roughness-Rq or RMS slope Rdq, are dominated by specific scales of roughness. While Rq is determined by long wavelengths, the short wavelengths determine Rdq [7]. This proposes multiple open questions for roughness measurements: Which cut-off wave lengths must be chosen? How can provided roughness measurements be interpreted and compared? In which way should requirements of roughness be defined? In addition, the multi scale nature of roughness is not always considered for the existing friction models. How can the models be adapted to account for multi scale roughness? Which scales are dominating for which type of friction? Can this knowledge be used for guidelines on roughness measurements of engineering parts? 6. Conclusion While there are many models and theories both for surface roughness and friction, their relation is an ongoing topic of research. Multi asperity models and roughness parameters can be used to relate adhesive or deformative friction and roughness. However, they do not consider the multi scale nature of roughness and the scale dependency of the roughness parameters. Various open questions due to these challenges were presented. References [1] Greenwood, J. A. and Williamson, J. B. P., Proceedings of the Royal Society of London Series A, vol. 295, no. 1442, pp. 300-319, 1966. [2] A. W. Bush, R. D. Gibson, and T. R. Thomas, Wear, vol. 35, no. 1, pp. 87-111, Nov. 1975. [3] Archard, J. F., Proceedings of the Royal Society of London Series A, vol. 243, no. 1233, pp. 190-205, 1957. [4] B. N. J. Persson, The Journal of Chemical Physics, vol. 115, no. 8, pp. 3840-3861, Aug. 2001. [5] “DIN EN ISO 21920-2: 2022-12”. Beuth Verlag GmbH. [6] B. Sista and K. Vemaganti, Wear, vol. 316, no. 1-2, pp. 6-18, Aug. 2014. [7] B. N. J. Persson, O. Albohr, U. Tartaglino, A. I. Volokitin, and E. Tosatti, J. Phys.: Condens. Matter, vol. 17, no. 1, pp. R1-R62, Jan. 2005. [8] F. P. Bowden, D. Tabor, Clarendon Press, 2001. [9] I. Hutchings and P. Shipway, in Tribology, Elsevier, 2017, pp. 37-77. [10] B. N. J. Persson, Eur. Phys. J. E, vol. 8, no. 4, pp. 385- 401, Jul. 2002. 24th International Colloquium Tribology - January 2024 265 Modeling of Shape Deviations for the Development of Predictive Models of TEHD Contacts Klara Feile 1* , Marcel Bartz 1 , Sandro Wartzack 1 1 Friedrich-Alexander-Universität Erlangen-Nürnberg, Faculty of Engineering, Department of Mechanical Engineering, Engineering Design, Martensstraße 9, 91058 Erlangen, Germany * Corresponding author: feile@mf k.fau.de 1. 1 Introduction The reduction of frictional losses in lubricated contacts of machine elements provides a significant contribution towards the development of energy-efficient and wear-resistant technical products and thus to the preservation of resources [1]. Manufacturing-related shape deviations, which are e.g. caused by tool vibrations [2], have a significant influence on the film formation in elastohydrodynamic (EHD) contacts, synonymous with influencing the friction and wear behavior [3,4]. A numerical consideration of shape deviations within the calculation of contact pressures and lubricant film heights in thermo-elastohydrodynamic (TEHD) contacts is complex, time-consuming and therefore not application-oriented. An approach for a comparatively simple consideration is, e.g., the extension of the widely used analytical approximation equations to determine the central and minimum lubricant film heights in EHD contacts developed by Dowson/ Higginson [5,6] and further evolved by other authors [7]. Due to additional correction factors, it is possible to consider different boundary conditions regarding thermal effects, fluid behavior and surface deviations [7]. Kumar et al. [8] developed a correction factor for 2D line contacts depending on the mean square surface roughness to extend the approximation equations of Dowson/ Higginson. However, this approach assumes a constant wavelength and neglects transient effects. The central and minimum lubricant film heights of elliptical contacts considering stochastic surface roughness can be approximated with the correction factors of Masjedi and Khonsari [9]. To date, there are no known research results regarding comprehensive, simple predictive models, such as correction factors, to complement established analytical approximation equations with respect to the consideration of manufacturing-related shape deviations of the surface topography in 2D and 3D TEHD contacts. The present work aims to provide the prerequisites to close this research gap. The development of predictive models requires the generation of comprehensive databases for 2D line and 3D point contacts, enabled by the application of simulative calculation methods. For this purpose, simulation models considering shape deviations have to be defined. Within the scope of this work, manufacturing-related shape deviations of 2 nd and 3 rd order of magnitude describing waviness and surface roughness were mathematically defined and integrated into TEHD simulation models. 2. Procedure and results In Figure-1, the conceptual approach of this work (dark blue) is shown in the context of further prospective research steps (light blue). Fig. 1. Overall conceptual approach. 2.1 Mathematical description of shape deviations Waviness and roughness were each mathematically modeled and superimposed using sinusoidal functions. Within the parametric description of the two orders of magnitude, in order to ensure a differentiated consideration of the shape deviations, the amplitudes- a 2 and a 3 as well as the wavelengths-λ 2 and λ 2 listed in Table 1 were selected as parameter limits. The limiting wavelengths were defined in accordance to [2]. The minimum and maximum amplitude of the manufacturing-induced wavi-ness was chosen according to [10]. The limits of the roughness amplitude caused by the tool cutting edge or feed, e.g. [2], were based on arithmetic mean roughness values-(R a ) between 1.6 and 6.3-µm which can be typically measured in this context. Table 1. Defined parameter limits of shape deviations. Order of magnitude a min a max λ min λ max 2 nd 10-µm 30-µm 100a 2 1000a 2 3 rd 2.5-µm 10-µm 10a 3 100a 3 Modeling of Shape Deviations for the Development of Predictive Models of TEHD Contacts 266 24th International Colloquium Tribology - January 2024 2.2 Integration into TEHD simulation model The mathematically defined shape deviations were integrated into the TriboFEM simulation tool, which allows 2D line and 3D point contacts to be simulated. The numerical modeling was based on a fully coupled finite element approach following-[11] and-[12] and a generalized, modified Reynolds equation-[13] in its weak form considering a mass conserving cavity model-[14]. The isothermal EHD equations were further coupled with thermodynamics, as described in detail in [15]. More-over, non-Newtonian fluid properties were considered by integrating rheological models according to Roelands-[16], Dowson/ Higginson-[17] and Eyring-[18]. The TEHD contacts were simulated transiently, with an initial smooth contact and shape deviations moving into the contact. By screening, the limits of the following parameter sampling were verified using 2D simulations of line contacts. In addition to the shape deviation limits listed in Table-1, the parameter values defined included limits regarding the temperature and fluid properties, a maximum elastohydrodynamic pressure of 4-GPa, a minimum and maximum slip-roll ratio of -2 and 2 as well as a minimum and maximum cumulative velocity of 10 -1 and 10 2 -m/ s, respectively. To ensure thermo-elastohydrodynamic full-film lubrication, fully filled lubrication gaps in the deviated contacts were ensured. 2.3 Data generation and development of predictive models Within the defined parameter limits, this work will be followed by the generation of test plans using Latin-Hypercube-Sampling to generate a database for both line and point contacts using the developed deviated TEHD models. By deriving correlations between input and output variables, models such as correction factors to extend the approximation equations of Dowson/ Higginson can be developed in order to predict the central and minimum lubricant film heights as well as the maximum contact pressure in contacts subject to manufacturing deviations. The prediction models depend on the amplitudes and wavelengths of the 2 nd and 3 rd order of magnitude shape deviations, which are easy to measure in reality. 3. Conclusion In this work, transient TEHD models have been developed to determine lubricant film heights, pressures, and maximum temperatures of line (2D) and point (3D) contacts with manufacturing-related shape deviations of 2 nd (waviness) and 3 rd order (roughness). The models represent a wide range of load, speed, geometry, lubricant and temperature parameters. Thus, a basis for the development of simple but comprehensive predictive models of deviated TEHD contacts was established. References [1] Woydt, M. Material efficiency through wear protection - The contribution of tribology for reducing CO2 emissions. Wear, 2022, 488-489. [2] DIN 4760: 1982-06, Form deviations, Concepts, Classification system. [3] Simon, V. Influence of machine tool setting pareters on EHD lubrication in hypoid gears. Mech. Mach. Theory, 2009, 44, 923-937. [4] Simon, V.V. Improved mixed elastohydrodynamic lubrication of hypoid gears by the optimization of manufacture parameters. Wear, 2019, 438-439. [5] Dowson, D.; Higginson, G.R. The Effect of Material Properties on the Lubrication of Elastic Rollers. J. Mech. Eng. Sci., 1960, 2, 188-194. [6] Dowson, D.; Higginson, G.R.; Whitaker, A.V. Elasto-hydrodynamic lubrication: A survey of isothermal solutions. J. Mech. Eng. Sci., 1962, 4, 121-126. [7] Marian,- M.; - Bartz,- M.; - Wartzack,- S.; - Rosenkranz, A. Non-Dimensional Groups, Film Thickness Equations and Correction Factors for Elastohydrodynamic Lubrication: A Review. Lubricants, 2020, 8, 95. [8] Kumar, P.; Jain, S.C.; Ray, S. Study of surface roughness effects in elastohydrodynamic lubrication of rolling line contacts using a deterministic model. Tribol. Int., 2001, 34, 713-722. [9] Masjedi, M.; Khonsari, M.M. On the effect of surface roughness in point-contact EHL: Formulas for film thickness and asperity load. Tribol. Int., 2015, 82, 228- 244. [10] Brinkesmeier, E.; Sölter, J.; Grote, C. Distortion Engineering - Identification of Causes for Dimensional and Form Deviations of Bearing Rings. Annals of the CIRP, 2007, 56, 1. [11] Habchi, W.; Demirci, I.; Eyheramendy, D.; Morales-Espejel, G.; Vergne, P. A finite element approach of thin film lubrication in circular EHD contacts. Tribol. Int., 2007, 40, 1466-1473. [12] Habchi, W. Finite Element Modeling of Elastohydrodynamic Lubrication Problems. JohnWiley & Sons Incorporated, Newark, USA, 2018. [13] Reynolds, O. On the Theory of Lubrication and its Application to Mr. Beauchamp Tower’s Experiments, Including an Experimental Determination of the Viscosity of Olive Oil. Philos. Trans. R. Soc. Lond., 1886, 177, 157-234. [14] Marian, M.; Weschta, M.; Tremmel, S; Wartzack, S. Simulation of microtextured surfaces in starved EHL contacts using commercial FE software. Mater Perform Charact., 2017, 6, 165-181. [15] Weschta, M. Untersuchungen zur Wirkungsweise von Mikrotexturen in elastohydrodynamischen Gleit/ Wälz-Kontakten, Dissertation, Friedrich-Alexander-Universität Erlangen-Nürnberg, 2017. [16] Roelands, C.J.A. Correlational Aspects of the Viscosity-temperature-pressure Relationship of Lubricating Oils. Dissertation, Technical University of Delft, 1966. [17] Dowson, H.; Higginson, G.R. Elasto-hydrodynamic Lubrication, Pergamon, Oxford, UK, 1977. [18] Eyring, H. Viscosity, Plasticity, and Diffusion as Examples of Absolute Reaction Rates. Viscosity, Plasticity, and Diffusion as Examples of Absolute Reaction Rates. J. Chem. Phys., 1936, 4, 283. 24th International Colloquium Tribology - January 2024 267 Estimation of Remaining Useful Life of Greases after Thermo- Oxidative Ageing by Application of New Method DIN 51830-2 Markus Matzke 1)* , Gerd Dornhöfer 2 ) 1 Robert Bosch GmbH, Renningen, Germany 2 BMS, Leonberg, Germany * Corresponding author: markus.matzke@de.bosch.com 1. Introduction Thermo-oxidative grease degradation is a dominant mode of lubricating grease failure in high-temperature automotive applications [1]. With the new test method draft E DIN 51830 part 2 a new method was established which can determine the Arrhenius activation energy for thermo-oxidative grease degradation for specific greases in contact with application relevant metal contact materials like steels and brass [2]. Previous talks at TAE, GfT and STLE illustrated the evolution of this method and showed various use cases of this method [3]-[5]. Beside the determination of characteristic thermo-oxidative ageing parameters for the setup of Arrhenius models, quantitative data about the degree of thermo-oxidative ageing after endurance tests or field use can provide helpful insight about the robustness of a grease. 2. Scope This presentation shall focus on how the new method draft E DIN 51830-2 can be applied to evaluate the degree of accumulated thermo-oxidative degradation of a used grease sample or respectively the remaining level of protecting antioxidant additives. For purpose of simplicity the term remaining useful life (RUL) of used grease samples after thermo-oxidative ageing in static conditions or in field use is used here. 3. Experimental 3.1 Test method E DIN 51830-2 Measurements were carried out according to E DIN 51830- 2 [2]. The principle of this method is the static ageing of a grease sample at elevated temperature in a pressurized oxygen autoclave. The decisive difference to earlier methods like DIN 51808 [6] is the contact with a catalytically active material like steel or brass as in real applications. The important difference to other methods applying the Rapid Small Scale Oxidation Tester (RSSOT) is the evaluation criterion. The new method E DIN 51830-2 evaluates the inflection point of the pressure curve (Figure 1) as indicator of antioxidant depletion and consecutive rheological failure [4] which corresponds to grease failure in applications while ASTM D8206 applies an arbitrary criterion of 10% oxygen consumption [7]. All measurements of oxidation induction time were carried out with the following parameters: Contact material: Steel 1.0330 Temperature: 150 °C Sample amount: 0,5 g Initial oxygen pressure: 700 kPa Figure 1: Pressure and temperature curve in oxidation autoclave during typical measurement and evaluation of oxidation induction time 3.2 Concept of RUL determination The concept of this approach is the reduction of the antioxidant content in the grease during exposition to elevated temperatures. This affects the oxidation induction time (OIT) in measurements according to E DIN 51830-2. The characteristic inflection point is shifted to shorter OIT values by exposition to elevated temperatures. 3.3 Thermal pre-ageing To apply a defined thermal load on the greases they were exposed to a constant temperature of 150 °C with variable but measured durations from 0…885 h. The grease samples were applied on 1.0330 steel sheets with a grease thickness of 1 mm as described in [1]. 3.4 Greases under evaluation For the demonstration here three commercial fully formulated greases were selected for this investigation: Table 1: Overview of grease samples for demonstration Sample label Thickener type Base oil type NLGI consistency Li-PAO Lithium soap PAO 2 AlX-PAO Aluminium complex soap PAO 1 Li-MO Lithium soap Mineral 2 Estimation of Remaining Useful Life of Greases after Thermo-Oxidative Ageing by Application of New Method DIN 51830-2 268 24th International Colloquium Tribology - January 2024 4. Results 4.1 Impact of thermal pre-ageing on oxidation induction time Oxygen pressure curves from a series of measurements with variable duration of pre-ageing are displayed in Figure 2. Thermal pre-ageing affects the inflection point in the autoclave pressure curve from 1731 minutes for the fresh reference of grease Li-PAO and results in reduced oxidation induction times. Figure 2: Autoclave pressure curves of grease Li/ PAO after thermal pre-ageing at 150 °C on steel To quantify the correlation between duration of pre-ageing and oxidation induction time all results from grease Li/ PAO are plotted in Figure 3: Figure 3: Oxidation induction times of Li-PAO vs. duration of thermal pre-ageing at 150 °C on steel There is a linear trend for the reduction of the oxidation induction time with increasing duration of thermal pre-ageing. Including the implausible value of 1854 minutes there is a certain but not very precise correlation with an R² of 0.8215. Excluding this maverick would result in an R² of 0.8466. Despite the request for an increased R² there is an obvious reduction of the oxidation induction time which is an indicator of the antioxidant reservoir in a grease. The ratio of OIT for a used grease sample versus the fresh reference can be interpreted as indicator for the relative remaining useful life (RUL) under thermo-oxidative ageing. This concept shall also be demonstrated with a reduced number of repetitions by two further greases, labelled as AlX- PAO and Li-MO. Results of their relative oxidation induction times compared to the fresh reference are plotted in Figure 4. Both greases exhibit a similar behavior as Li-PAO with a linear decrease of their oxidation induction times along increasing duration of thermal pre-ageing with improved values of R² of 0.9388 and 0.9459. Figure 4: Oxidation induction times of Li-MO and AlX- PAO vs. duration of thermal pre-ageing at 150 °C on steel 5. Summary and conclusion The initial hypothesis of reducing oxidation induction time with increased thermal pre-ageing has been verified and measurement series with three commercial soap-thickened greases indicated a linear correlation. The concept is applicable for the evaluation of used grease samples in terms of their residual strength against thermo-oxidative failure when compared to the fresh reference. Due to scattering of results in the current state of method development the precision is still limited but results can already be used for a traffic light type categorization for the state of thermo-oxidative ageing. For an increased precision of RUL evaluation noise factors need to be identified and reduced within future work. References [1] Dornhöfer, G.: Ermittlung der Schmierfettgebrauchsdauer mit zeitraffender Prüfmethode und Übertragbarkeit auf reales Temperaturkollektiv; GfT-Fachtagung 2016 [2] E DIN 51830: 2024-04, Prüfung von Schmierfetten- - Bestimmung der Oxidationsbeständigkeit von Schmierfetten- - Teil- 2: Beschleunigte Ermittlung der Arrhenius-Aktivierungsenergie der thermo-oxidativen Degradation, in revision [3] Matzke, M.; Dornhöfer, G.; Schöfer, J.: Study of thermooxidative grease ageing and proposal of a test method standard, 22nd International Colloquium Tribology, Technische Akademie Esslingen, Ostfildern, 2020 [4] Matzke, M.- ; - Beyer-Faiß, S.; - Grebe, M.; - Höger, O.: Thermo-oxidative grease service life evaluation-- laboratory study with the catalytically accelerated method using the RapidOxy, Tribologie und Schmierungstechnik 2022, Issue 1, pp 41-49 [5] Matzke, M.; Höger, O.; Litters, T.; Fischer, J.: DIN-51830-2-- Evolution of an Advanced Method for Characterization of Thermo-Oxidative Grease Failure, 77th STLE Annual Meeting, Long Beach, CA, 2023 [6] DIN 51808: 2018-02, Prüfung von Schmierstoffen — Bestimmung der Oxidationsbeständigkeit von Schmierstoffen — Sauerstoff-Verfahren [7] ASTM D8206 (2018) Oxidation Stability of Lubricating Greases - Rapid Small Scale Oxidation Test (RS- SOT); 2018; Beuth-Verlag 24th International Colloquium Tribology - January 2024 269 Correct Lubricant Selection for Metal Forming Dr. Richard Baker 1* , Dr. Dirk Drees 2 1 TriboTonic Limited, London, UK 2 Falex Tribology, Leuven, Belgium * Corresponding author: E-mail (optional) 1. Introduction Metal forming is one of the important manufacturing processes and is split into 3 main types - sheet metal forming, bulk metal forming and sheet-bulk metal forming. Some popular metal forming processes including forging, rolling, wire drawing, extrusion, deep drawing, and bending. List and number all bibliographical references at the end of the paper. When referring to them in the text, place the reference number in square brackets [1] as presented below. 1.1 Metal Forming Lubricatn Selection In metal forming (cold or hot), a localized compressive force is applied on the workpiece through a forming tool. This force leads to generating high contact pressures at the workpiece/ tool interface and as a result the workpiece is deformed to deliver a desired shape and surface of the product. In this deformation process, high shear stresses are developed with an increase in local temperature due to both the frictional effect and plastic strain. A lubricant is typically used to control the friction at workpiece/ tool interface to help cool down the tool to avoid overheating and a drop in hardness. This leads to a very careful balance between friction and heat dissipation and in today’s energy driven environment, the systems efficiency is an ever important factor. 1.1.1 Types of Lubricants Metal Working fluids are usually categorised into the following 4 main types: • Water-based or soluble oils • Oil-based lubricants • Synthetic and semisynthetic • Solid lubricants Investigation into the effect of different lubricant formulations on friction have been carried out on various tribology test equipment including the Falex Pin & Vee and MCTT instruments - shown in figures 1 and 2. These have been based on industry used metal working fluids and give an insight into the effect of lubricant on friction and life expectancy f both the metal forming tool and the efficiency of the drawn metal test piece. 2. Conclusion Metal forming requires complicated environmentally friendly lubricants to help ensure smooth operation of the tools. Wear of the tool (or counter-face) must be kept to a minimum to ensure accurate products. Lubricants can help speed up the production process which can lead to a cost reduction. It has been shown that we can adapt existing lab instruments to match the application and hence deliver a saving tot he industry. Figure 1 - Falex Pin & Vee instrument - used extensively for looking at metal working fluids Figure 2 - Falex MCTT Instrument - adapted to look at the effect of lubricants on metal forming Weitere Informationen und Anmeldung unter www.tae.de/ go/ tribologie Besuchen Sie unsere Seminare, Lehrgänge und Fachtagungen. Reibung, Verschleiß und Schmierung Schmierstoffe und Betriebsflüssigkeit Schmierungstechnik Geschmierte Maschinenelemente Ein Großteil unserer Seminare wird unterstützt durch das Ministerium für Wirtschaft, Arbeit und Wohnungsbau Baden-Württemberg aus Mitteln des Europäischen Sozialfonds. Profitieren Sie von der ESF-Fachkursförderung und sichern Sie sich bis zu 70 % Zuschuss auf Ihre Teilnahmegebühr. Alle Infos zur Förderfähigkeit unter www.tae.de/ foerdermoeglichkeiten Triobologie, Reibung, Verschleiss und Schmierung Bis zu 70 % Zuschuss möglich Appendix 24th International Colloquium Tribology - January 2024 275 Committees - 24 th International Colloquium Tribology The program committee for the International Colloquium Tribology is made up of recognized experts from research and development, industry and practice. Its tasks include the formulation of the objectives and the definition of the main topics of the conference, the assessment and selection of the submitted proposals for the conference program and the technical advice of the organizer. Steering Committee Priv.-Doz. Dipl.-Ing. Dr. techn. Nicole Dörr AC2T Research GmbH, Wiener Neustadt, Austria Univ.-Prof. Dr.-Ing. Carsten Gachot Vienna University of Technology, Vienna, Austria Dr.-Ing. Max Marian Pontificia Universidad Católica de Chile Macul, Chile Dr.-Ing. Katharina Völkel Technical University Munich Munich, Germany Program Planning Committee Univ.-Prof. Dr.-Ing. Dr. h. c. Albert Albers Karlsruhe Institute of Technology (KIT) Karlsruhe, Germany Univ.-Prof. Dr.-Ing. Frank Bauer Institute of Machine Components (IMA) University of Stuttgart, Germany Dr. rer. nat. Martin Dienwiebel Karlsruhe Institute of Technology (KIT) Karlsruhe, Germany Dr. Arshia Fatemi Robert Bosch GmbH Stuttgart, Germany Dipl.-Ing. Gerhard Gaule Hermann Bantleon GmbH Ulm, Germany Dr.-Ing. Michael Gleß ContactEngineering Stuttgart, Germany Dr. Markus Grebe Kompetenzzentrum Tribologie KTM HS Mannheim, Germany Univ.-Prof. Dr.-Ing. Georg Jacobs RWTH Aachen University Aachen, Germany Dr. Manfred Jungk LUBEVISIO GmbH Brannenburg, Germany Dr. Thomas Kilthau Klüber Lubrication SE & Co. KG Munich, Germany Dipl.-Ing. Rüdiger Krethe OilDoc GmbH Brannenburg, Germany Dr. Markus Matzke Robert Bosch GmbH Stuttgart, Germany Dr. Johannes Müllers Robert Bosch GmbH Stuttgart, Germany Prof. Dipl-Ing. Dr. techn. Andreas Pauschitz AC2T Research GmbH Wiener Neustadt, Austria Dr. Thomas Rühle BASF SE Ludwigshafen, Germany 24th International Colloquium Tribology - January 2024 277 Index of Authors AAdler, Michael 29, 241 Agocs, Adam 29 Aha, Bernd 65 Alberdi, Alberto 221 Albrecht, Joachim 195, 237 Alemanno, Fabio 199 Algieri, Luciana 93, 213, 217 Alvis-Sanchez, Jorge 197 Asano, Yuta 169 Ayame, N. 71 BBagov, Ilia 33 Bai-o, Pedro 227 Baker, Richard 269 Bartz, Marcel 261, 265 Bäse, Mirjam 189 Bauer, Frank 177 Belkacemi, Lisa T. 107 Bellini, Marco 239 Bermúdez, Vicente 197 Besser, Charlotte 29 Beyer-Faiss, Susanne 105 Biboulet, Nans 81 Bierwisch, Nick 153 Blanco-Rodríguez, Javier 115, 157 Boidi, Guido 161 Borras, Xavier 221 Bösing, Ludger 67 Boudreau, David Sr 47 Brenner, Josef 225 Bresser, Floriane 33 Bretonnet, Amelie 69 Brodmann, Boris 97 Bruno, Marco 93, 213 Buffiere, Denis 69 Buling, Anna 91, 95 Buse, Henrik 183 CCadau, Luca 195 Casey, Brian 47 Celikbilek, Korhan 65 Chen, Yan 27 Chong, Yen Yee 55 Chretien, Christelle 39, 53 Cinca, N. 205 Codrignani, Andrea 171 Cofalka, Dominik 241 Correia Romio, P. 111 Cortada-Garcia, Marti 115, 157 Cruz, Justino A. O. 127 Dda Costa Gomes Fernandes, C. M. 111 Daum, Philipp 109 Davis, Linto 75 de Tezanos, Matias 255 De Vittorio, Massimo 93, 213, 217 Delic, Ivan 225 Dellis, Polychronis S. 191 Dienwiebel, Martin 109 Dima, Christina 257 Dini, Daniele 167 Dodos, George S. 257 Dodos, Andreas 257 Dornhöfer, Gerd 267 Dörr, Nicole 101, 167, 225 Drees, Dirk 227, 269 Dufils, Johnny 79, 81, 87 EEder, Stefan J. 167 Eisner, Patrick 251 Emrich, Stefan 125 Erdemir, Ali 51 Espallargas, Nuria 137, 163 FFabry, Dirk 135 Falk, Kerstin 165, 171 Faller, Joachim 83 Farfan-Cabrera, Leonardo 197 Fatemi, Arshia 263 Fathi-Najafi, Mehdi 63, 257 Feile, Klara 265 Feldmeth, Simon 177 Fernandes, Carlos M. C. G. 127 Fernández, Silvia 157 Flachmann, Malte 33 Frackowiak, Maria 135 Franke, Joerg W. H. 207 Frauscher, Marcella 29, 61, 247 Fritz, Janine 207 Fukushima, Shogo 169 Funamoto, Genki 95 GGachot, Carsten 51, 167 Gaiser, Uwe 241 Garabedian, Nick 33 Garabedian, Nikolay 211 Gault, Baptiste 107 Gedan-Smolka, Michaela 125 Gee, M. G. 205 Gelissen, Arjan 67 Georgiou, Emmanouil 227 Gigov, Boris 193 Gless, Michael 133 Gohs, Marco 177 Gorb, Stanislav 93 Götz, Stefan 179 Grad, Danijela 135 Graf, Simon 179 Gravemeier, Volker 151 Grebe, Markus 183, 187 Greiner, Christian 33, 107, 211 Gressier, Sylvain 201 Grün, Jeremias 177 Grünewald, Moritz 105 Grützmacher, Philipp G. 51, 167 Gumbytė, Milda 49 Güney, Didem Cansu 195, 237 HHaas, Denise 243 Habgood, Ben 185 Harrer, Philipp 251 Hasse, Alexander 143 Haupt, Stefanie 135 Havelka, Kathleen 69 He, Xin 53 Héau, Christophe 79, 87 Heiligtag, Florian Johannes 135 Hervé, Grégoire 235, 249 Hick, Hannes 29 Hofer, Alexander 85 Hoffmann, Vincent 153 Holey, Hannes 171 JJacobs, Georg 131, 155 Jech, Martin 85 KKaframani, Panorea 59 Kaframani, Nora 257 Kailer, Andreas 109 Kaiser, Fabian 145 Karonis, Dimitrios 59 Katzer, Michael 229 Kavut, Kübra 139 Kawaura, Masayuki 169 Keller, Andreas 187 Kerbrat, Marion 43 Khanmohammadi, H. 137 Kisch, Ines L. 107 Klein, R. 165 Klemenz, Andreas 89 Kley, Markus 195 Klinghart, Benjamin 131, 155 Koch, Oliver 123, 125, 179 Koch, Verena 243 278 24th International Colloquium Tribology - January 2024 Koehn, F. 147 Koenig, Timo 195 Kohnle, Marco 195 König, Tobias 109 König, Florian 131, 155 Kopnarski, Michael 125 Koschabek, Rene 253 Kracalik, Michal 193 Kreivaitis, Raimondas 49 Krenn, Stefan 241 Kruse, L. B. 165 Kübler, Andreas 85 Kubo, Momoji 169 Küchler, Stefan 201 Kuhr, Maximilian 175 Kupčinskas, Artūras 49 Kurchan, Alexei 45 Kürten, Dominik 109 LLackner, Maximilian 251 Lais, Siegfried 241 Larsson, Roland 25, 173 Larsson, J. Andreas 173 Lee, Micky 43, 55 Li, Jinxia 63 Li, Yulong 211 Liang, Hong 27 Linsler, Dominic 65 Lodhi, Ajay Pratap Singh 225 Lohmann, Peter 215 Lohner, Thomas 31 Lopes, Lais 227 Lubrecht, Thomas 81 Lubrecht, Antonius A. 81 Lucazeau, Siegfried 235 Luther, R. 165 MMacron, Etienne 79, 87 Marian, Max 159 Markert, D. 165 Markl, Erich 251 Maroto, S. 115 Marques, Pedro M. T. 127 Martini, Ashlie 167 Matsubara, K. 37, 41 Matzke, Markus 267 McClure, Ted G. 73 Mehrnia, Seyedmajid 175 Meier, Felix 145 Meller, Miłosz 33 Merk, Daniel 207 Mitterer, Stefan 219 Mohammadtabar, Karen 167 Molter, Jürgen 121 Montenegro Cortez, M. J. 111 Moody, Gareth 45, 245 Moras, Gianpietro 171 Morgan, Alexes 73 Morina, Ardian 225 Moseler, Michael 89, 165, 171 NNaeini, Vahid Fadaei 173 Najjari, Morteza 97 Nakahara, Y. 41 Namlu, Ramazan Hakkı 139 Narita, K. 37, 41 Neusser, Kevin 261 Nevosad, Andreas 85, 167, 241 Nino, Victor 199 Norrby, Thomas 61, 63 Nothnagel, Rosa-Maria 247 Nowotny-Farkas, Frans 61 Nyman, Steffen D. 233 OOehler, Manuel 123, 125 Oehme, Silvano 143 Olsson, M. 205 Ootani, Yusuke 169 Oshio, T. 71 Ozawa, Nobuki 169 PPagkalis, Konstantinos 125 Paschold, Constantin 31 Pastewka, Lars 171 Pastor, Cesar 149 Patzer, Gregor 223 Paulus, Stefan 179 Peeters, Stefan 171 Pelz, Peter F. 175 Pereira, Lúcia B. S. 127 Pessu, Frederick 225 Pichler, Jessica 247 Pirker, Franz 221 Popov, Valentin L. 99 Portaluri, Luigi 93, 213, 217 Porteiro, Jacobo 115, 157 Portron, Stephane 111, 127 Pota, Simone 239 Pratt, Clifford 57 Prost, Josef 161 Püler, Tanja 135 RRajput, Arvind K. 129 Ramkumar, P. 75 Rank, Martin 123 Rau, Julia S. 107 Rausch, J. 165 Reinicke, Stefan 65 Rich, David 229 Righi, Maria Clelia 51 Rodríguez Ripoll, Manel 167 Rogkas, Nikolaos 117 Rothammer, Benedict 261 SSavio, Daniele 145, 171 Scaraggi, Michele 93, 145, 213, 217 Scherge, Matthias 23, 83 Schmidt, Thomas 125 Schneider, Thomas 119 Schneider, Ameneh 203, 223 Schneider, Johannes 211 Schneidhofer, Christoph 61 Schwarz, Anette 133 Seabra, Jorge H. O. 111, 127 Séverac, Florence 235, 249 Shakhvorostov, Dmitriy 189 Sharma, Anutsek 95 Simón-Montero, X. 115 Singh, Vishal 129 Slabon, Adam 247 Sobisch, Titus 201 Solovyev, Sergey 149 Spaltmann, Dirk 99 Spies, Charlotte 263 Spikes, Hugh 103 Spitas, Vasilios 117 Stahl, Karsten 31, 119 Stark, Sabrina 253 Steidle, Lukas 195 Stephenson, Gemma 245 Stief, Franziska 171 Strobl, Patrick 119 Struelens, Pieter 43, 55 Svetov, Dmitrii 251 TTack, Emanuel 153 Tada, Akira 99 Tagawa, Kazuo 99 Takekawa, D. 37 Tatsumi, H. 37 Tatzgern, Fabio 193 Tavakkoli, Vahid 107 Teixeira Marques, P. M. 111 Tidona, Giuseppe 121 Tison, Sydne 45 Tiwari, Ashutosh 91 Tom, Hanife Gülen 139 Tormos, Bernardo 197 Treinytė, Jolanta 49 Tuzyna, Edith 253 Tyrovola, Theodora 209 UUhl, Arnold 201 VValaker, E. 137 Van Camp, Victoria 25 Varga, Markus 161, 193, 225 Veeregowda, Deepak Halenahally 199 Velkavrh, Igor 101 Vernon-Stroud, Edward 225 24th International Colloquium Tribology - January 2024 279 Voelkel, Katharina 119 Vorlaufer, Georg 161, 193 WWang, Yujun 155 Wannenmacher, Regina 105 Wartzack, Sandro 261, 265 Weber, Katharina 195, 237 Westbroek, René 185 Widmann, Alexander 183 Wijanarko, Wahyu 163 Williams, Daniel 185 Williams, Diarmaid 229 Witt, Thomas 105 Wong, Janet 103 Wopelka, Thomas 85 YYagishita, K. 71 Ye, Nuoyao 33 Yokoi, Mizuho 169 ZZak, Felix 203, 223 Zannikos, Fanourios 209 Zellhofer, Manuel 85 Zerrer, Jörg 91, 95 Zhang, Jie 103 Zhang, Shuo 131 Zhmud, Boris 97, 255 Zou, Weiyin 155 It is all about friction, wear and lubrication. The 24th International Colloquium Tribology aims to highlight exciting developments that affect important areas of tribology and support the development of novel technologies that will have a strong impact on future sustainable development. The conference provides an international exchange forum for the industry and the academia. Leading university researchers present their latest findings, and representatives of the industry inspire scientists to develop new solutions. Main Topics New trends in lubricants and additives Coatings, surface interactions and underlying mechanisms Machine elements and their application in tribology Computational methods and digital transformation in tribology Test and measurement methodologies Sustainability and resource efficiency This conference handbook contains the pre-submitted contributions to the presentations and provides an overview of the latest findings in the field of tribology. Target Groups Companies in the field of of lubrication, additives and tribology Research facilities ISBN 978-3-381-11831-1 www.tae.de