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Tribologie und Schmierungstechnik EDITOR IN CHIEF MANFRED JUNGK 5 _ 25 VOLUME 72 Tribology—Lubrication Friction Wear An Official Journal of Gesellschaft für Tribologie An Official Journal of Österreichische Tribologische Gesellschaft An Official Journal of Swiss Tribology Issue 5 | 2025 Volume 72 Editor in chief: Dr. Manfred Jungk Tel.: +49 (0)177 1902330 eMail: jungk@verlag.expert www.mj-tribology.com Editorial director: Ulrich Sandten-Ma Tel.: +49 (0)7071 97 556 56 / eMail: sandten@verlag.expert Editor: Patrick Sorg Tel.: +49 (0)7071 97 556 57 / eMail: sorg@verlag.expert Dr. rer. nat. Erich Santner Tel.: +49 (0)2289 616136 / eMail: esantner@arcor.de Contributions marked with the author’s initials or full name represent the author’s opinion, not necessarily that of the editorial office. We take no responsibility for unsolicited contributions. The author is responsible for obtaining the rights to pictures. 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Receiving the journal for a reduced price obligates the subscriber to purchase the whole volume. If the subscription is terminated prematurely, the unit price will be charged. Higher power cancels delivery obligation. Place of performance and jurisdiction: Tübingen. ISSN 0724-3472 ISBN 978-3-381-13801-2 Imprint Tribologie und Schmierungstechnik Tribology—Lubrication Friction Wear An Official Journal of Gesellschaft für Tribologie | An Official Journal of Österreichische Tribologische Gesellschaft | An Official Journal of Swiss Tribology Editorial 1 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0024 According to a new study commissioned by the Bertelsmann Foundation Germany is a global leader in circular economy technologies. Between 2010 and 2024, companies and research institutions worldwide filed nearly 62,000 technological patents in the field of the circular economy, for example, for the recycling of metals and plastics or the production of chemical products from renewable raw materials. In this study for the first time global technological developments in this future-oriented field were evaluated. Germany accounts for nearly 17 percent of global patent applications during this period, with 10,700 patents. Only the United States performs better, with 14,000 patents. Japan ranks third globally with 8,600 patent applications. France, Italy, and the United Kingdom, as well as China and South Korea, follow. Germany's leading position is increasingly under pressure, as China, South Korea, and Japan are rapidly catching up. In recent years, these countries have demonstrated above-average growth in patenting innovations in the field of the circular economy. For example, in China, almost five times as many patents were successfully filed in 2021 as in 2010. South Korea and Japan also significantly outperformed Germany, with three and two times as many patents, respectively, in the same years. In 2021, only 1.3 times as many patents were registered in Germany as in 2010 - the lowest figure among the top-performing nations. However, the sheer number of patents is not the only indicator of how well Germany and other countries are technologically positioned for the circular economy; their quality and economic relevance are also crucial. As an indicator for patent quality, a study by Prognos AG, commissioned by the Bertelsmann Foundation, examined how frequently a country's patents are cited in subsequent patent applications, i.e., whether they are relevant for new innovation cycles. In this respect, Germany currently leads the way. German patents have been cited almost 15,000 times in the past 15 years. Japan follows in second place with 9,960 citations, practically tied with the United States (9,900 citations). Italy, France, and the United Kingdom follow at a considerable distance. South Korea (1,734) and China (1,386) lag even further behind. The study cites two examples of important innovation fields in the German economy. Battery technologies and recycling, and circular construction. Battery recycling is the only technology field considered in which the German share of global patents has recently increased. With 20 percent of global patents, Germany holds the highest market share in the technology field of the circular construction. It would be interesting to know how many patents and patent citations exist in the field of tribology with respect to circular economy. It is well known, that lubricants per se at the moment fall into categories of recycled, converted to energy, landfilled and unknown. Thus, trying to press lubricants into a circularity will be difficult. The focus should be on how much energy can be saved with optimized tribological systems, either by reducing friction in the use phase of a component or extending the life of it. The German Tribological Society (GfT) has published studies on sustainability where one can find that Tribology is everywhere. Your editor-in-chief Manfred Jungk Germany is a world leader in patent quality of Circular Economy Technologies Events 2 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 Events We look forward to your contribution! The scientific journal Tribologie und Schmierungstechnik (TuS) is one of the leading publications for tribological research in Germany, Austria and Switzerland. As the official journal of the Society for Tribology (GfT) in Germany, the Austrian Tribological Society (ÖTG) and Swiss Tribology, the issues provide information on research from industry and science, current events and developments in the specialist community. Further information on the journal and publication: https: / / elibrary.narr.digital/ xibrary/ start.xav? zeitschriftid=tus&lang=en Date Place Event ► 11.04. - 14.04.26 Rome, Italy ELGI Annual General Meeting ► 21.04. - 22.04.26 Stuttgart, Germany UNITI Mineral Oil Technology Congress ► 17.05. - 21.05.26 New Orleans, Louisiana (USA) 80 th STLE Annual Meeting & Exhibition ► 03.06. - 04.06.26 Bilbao, Spain 10 th Lubmat Conference ► 08.06. - 11.06.26 Palm Springs, CA (USA) NLGI Annual Meeting ► 02.09. - 04.09.26 Valpre, France 51 th Leeds Lyon Symposium ► 08.09. - 11.09.26 Gdansk, Poland 44 th Polish Tribology Conference Contents 3 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 Tribologie und Schmierungstechnik Tribology - Lubrication Friction Wear An Official Journal of Gesellschaft für Tribologie An Official Journal of Österreichische Tribologische Gesellschaft An Official Journal of Swiss Tribology Volume 72, Issue 5 March 2026 5 J. Torben Terwey Real Contact Area and Pressure Distribution in Mixed Lubricated Rolling Contacts under Consideration of the Real Rheology 20 Justus Rüthing, Frank Haupert, Regine Schmitz, Mirek Göbel Analysing the friction behaviour of paper machine forming fabrics under water lubricated hydrodynamic conditions and systematical test parameter variation in three defined wear depths using a pin-on-disc test assembly 27 Juliane Heydemann, Emil Elbæk, Thomas Lohner Key Survey Results on Tribological Optimization Opportunities, Barriers, and Research Needs 1 Editorial Germany is a world leader in patent quality of Circular Economy Technologies 2 Events Science and Research 47 News Gesellschaft für Tribologie Förderpreise: Benedict Rothammer Amorphous carbon coatings for extending the service life of total knee replacements Gerrit W. Schnelle Modelling the Stick-slip Effect in Matlab/ Simulink to Identify Influencing Parameters Columns Preface For authors Authors of scientific contributions are requested to submit their manuscripts directly to the editor, Dr. Jungk (see inside back cover for formatting guidelines). Anzeige 4 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 Tribologie und Schmierungstechnik Tribology—Lubrication Friction Wear The scientific journal Tribologie und Schmierungstechnik (TuS) is one of the leading publications for tribological research in Germany, Austria and Switzerland. As the official journal of the Society for Tribology (GfT) in Germany, the Austrian Tribological Society (ÖTG) and Swiss Tribology, the issues provide information on research from industry and science, current events and developments in the specialist community. Subscription service: phone: +49 (0)89 85 85 38 81 eMail: abo-service@narr.de Further information on the journal and publication: www.narr.digital/ tus 2 starke Komponenten für die Tribologie Damit alles läuft, wie geschmiert! Die Zeitschrift SCHMIERSTOFF + SCHMIERUNG bietet einen umfassenden Überblick über alle Themen der Schmierstoffbranche. Dabei werden neueste Trends und Technologien ebenso behandelt, wie grundlegendes Basiswissen und wirtschaftliche Entwicklungen. SCHMIERSTOFF + SCHMIERUNG richtet sich insbesondere an Leser: innen aus der Praxis. Anwender von Schmierstoffen und Hersteller von Schmiermitteln erhalten durch unsere Zeitschrift ebenso fundierte Fachinformationen wie Dienstleister im Bereich Öl sowie jene, die in Schmierstofflaboren und Industrieservice-Unternehmen tätig sind. Auch Tätige des Handels und des Außendienstes in der Schmierstoffbranche finden hier eine aufschlussreiche Lektüre. Hier können Sie die Zeitschrift kostenlos abonnieren: www.sus.expert SCHMIERSTOFF SCHMIERUNG Ein Fachmagazin des 1 Introduction When two bodies come into contact, their surfaces do not engage uniformly across the entire nominal (Hertzian) contact area. Instead, contact occurs at discrete asperities due to surface roughness. Consequently, the real contact area, which transmits the normal force F N from the base to the opposing body, consists of numerous micro-contact regions. The cumulative area of these micro-contacts is significantly smaller than the apparent Hertzian contact area. Due to the equilibrium of forces, the local force acting on each asperity is typically much higher than the equivalent force distributed over a fullsurface contact, as illustrated schematically in Figure 1.1. Analogous to the mean contact pressure, a real contact pressure can be defined as the ratio of the normal force to the sum of all micro-contact areas. This real contact pressure is generally much higher than the apparent pressure can be resolved at a local level. Due to the concentrated forces and small size of the micro-contact areas, Science and Research 5 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0025 Real Contact Area and Pressure Distribution in Mixed Lubricated Rolling Contacts under Consideration of the Real Rheology J. Torben Terwey* submitted: 7.10.2024 accepted: 30.10.2025 (peer review) Presented at GfT Conference 2024 * Dr.-Ing. J. Torben Terwey ORCID: https: / / orcid.org/ 0009-0009-5038-6654 thyssenkrupp rothe erde Germany GmbH Beckumer Straße 87, 59555 Lippstadt, Germany Tribology in machine elements is a major challenge to achieving optimum performance and efficiency. It can be reduced to single component contact, such as between rolling elements and the raceway in a rolling bearing. Contrary to the intuitive notion of full-surface contact between these machine element components, real interactions are characterized by a much more complex interplay. Surface asperities, microscopic peaks and valleys on the contacting surfaces, create a limited “real” contact area comprised of numerous microcontact areas. These microcontact areas collectively transmit the contact force between the contacting components and are responsible for the friction in dry contacts. In practical applications, however, machine elements rarely operate in dry conditions. Therefore, the interplay between these microscopic asperity contacts is further enhanced by the presence of a surrounding lubricant film, introducing the complexities of mixed lubrication. In this regime, force transmission is shared between the asperity contacts and the lubricant reservoirs trapped between them. Lubricant viscosity and density, which are crucial for film formation, are themselves influenced by a variety of factors, including tem- Kurzfassung perature, pressure, and the relative speed of the contacting surfaces. The objective of this work is to obtain an accurate understanding of the real contact area and pressure distribution within these mixed lubricated contacts. A half-space based numerical contact model of the surface asperities is coupled to an analytic film formation model that considers the rheological properties of the lubricant under high pressure and shear conditions. Lubricant properties are derived from high pressure measurements. The application of the proposed model is demonstrated through a practical example: the contact between two discs within a dedicated test apparatus. This allows to validate the ability of the model to predict the real contact area and pressure distribution with a high degree of accuracy. This work was mainly carried out at Leibniz University Hannover. Keywords mixed lubrication, pressure distribution, rolling contacts, numerical model, elastohydrodynamic lubrication (EHL) faces and experimentally obtained surface topographies in contact scenarios involving a sphere and a flat surface. Theoretical and simulation-based approaches for incorporating surface roughness into the formation of pressure profiles in lubricated contacts have also been explored. P ATIR and C HENG (1978) [10] used the Reynolds equation to model fluid flow around asperities, showing how the geometry and distribution of asperities disrupt local flow and pressure formation. One advantage of their approach is the calculation of “flux factors” for representative surface elements, which can be applied across the entire contact surface. However, the approach lacks sufficient local resolution, yielding only integral pressure curves. Extensions of this method to elastohydrodynamic lubrication have been presented by T RIPP and H AMROCK (1985) [11], E FFENDI (1987) [12], Z HU and C HENG (1988) [13], and S ADEGHI and S UI (1989) [14]. Further studies into individual asperities and sinusoidal roughness structures were conducted by C HANG (1995) [15], with extensions to real surface profiles performed by J IANG et al. (1999) [16], H U and Z HU (2000) [17], and R EDLICH et al. (2000) [18]. These works aimed to quantify the area fractions of solid and fluid contacts, force transmission proportions, and resulting lubricant film thickness. W EINHAUER (1996) [19] calculated the frictional contributions of solid-state contacts based on the energetic deformation of asperities. A combination of the flow factor model of R EDLICH and C HENG and the solid-state contact model of G REENWOOD and W ILLIAMSON [1] was proposed by M AKINO et al. (1999) [20]. G ELINCK (1999) [21] employed stochastic models to calculate Stribeck curves, while further work based on flow factor models was conducted by E LROD (1979) [22], T EALE and L EBECK (1980) [23], T RIPP (1983) [24], and R IENÄCKER (1995) [25]. The objective of this study is to develop a deterministic model for calculating the real contact area and pressure distribution in mixed lubricated contacts, incorporating measured non-Newtonian and high-pressure fluid properties. This work was primarily carried out at Leibniz University Hannover, Germany, and published in T ER- WEY (2020) [26]. Science and Research 6 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0025 the local contact pressures increase significantly. These micro-contact pressures interact with the local deformations of the asperities, which, in turn, influence each other. Unlike the Hertzian contact problem, which can be solved analytically, the relationship between microcontact pressures and asperity deformations requires numerical methods for resolution. The real contact area and the resulting load distribution can be determined using either stochastic or deterministic approaches. For instance, the stochastic model developed by G REENWOOD and W ILLIAMSON (1966) [1] examined the effect of different surface profiles on the real contact area formation. A deterministic model by W EBSTER and S AYLES (1986) [2] incorporated real surface topographies to analyze load distribution and elastic deformation. Further deterministic models, such as those by B AILEY and S AYLES (1991) [3] or S AYLES (1996) [4], focused on calculating pressure distribution in rough contacts, with an emphasis on subsurface stress distributions and their impact on the service life of tribological contacts under pure rolling stress. R EN and L EE (1994) [5] explored rough contacts with a focus on mean gap height and micro-contact pressure, performing numerical studies on a range of artificially generated roughness profiles. G LEß (2009) [6] conducted further studies on pressure and stress distribution in rough rolling contacts. B RECHER et al. (2016) [7] introduced a method to calculate locally resolved contact pressure in large-area rough contacts, emphasizing the feasibility of performing such calculations with limited computational resources. Their method discretizes the contact surface into fine and coarse regions, allowing detailed analysis of small areas while maintaining computational efficiency. This Method of Combined Solutions was also used by T ERWEY et al. (2020) [8] to investigate the real contact surface size under dry and mixed friction conditions. Their study also simulated the effect of varying proportions of solidstate friction on adhesive wear. W ANG and S CHIPPER (2020) [9] examined real contact surfaces and pressure distributions in rough contacts using a semi-analytical elastoplastic half-space model. They analyzed both numerically generated rough sur- 𝐴 𝑖 𝐴 𝑖 𝐴 Ge ge nkörper Grundkörpe r 𝐴 0 𝐹 N 𝐴 𝑖 Base body Counter body Figure 1.1: Hertzian (A 0 ) vs. real (A i ) contact, formed by multiple micro-contacts. [26] 2. Methodology This study investigates the contact behavior between two discs in a two-disc test rig designed to measure frictional characteristics. The simplicity of the setup allows precise control and definition of key tribosystem parameters, including contact geometry, relative and entrainment velocities, and normal force. The formation of the lubrication film, which separates the contact surfaces, is described analytically considering the lubricant’s measured rheological properties. The test rig used in this study has been previously employed in the works of S CHMIDT (1985) [27], M EYER (2010) [28], W ANG (2015) [29], and B ADER (2018) [30]. A schematic of the rig’s structure is shown in Figure 2.1 (left). It consists of two independently driven shafts, each powered by separate electric motors. Mounted on each shaft is a test disc with a diameter of 120 mm. Disc 1 features a crowning radius of 100 mm. The rig enables rotational speeds of up to 2500 rpm, corresponding to a circumferential speed of 15 m/ s. Motor 2 is mounted on a foundation and can be adjusted horizontally and radially via a linear guide. This allows the application of force through a hydraulic cylinder, which can generate a normal load of up to 20 kN, resulting in a Hertzian contact pressure of up to 3000 MPa. The lubricant is externally heated and introduced directly into the contact area via a nozzle, with inlet temperature and flow rate controlled by a valve. The discs used in the tests were ground and polished. Photographs of the test discs are presented in Figure 2.1 (right). The surface condition of the discs is a critical parameter influencing the formation of the lubricating film and the extent of mechanical contact in the mixed friction regime. Consequently, the surfaces were examined using optical methods, and roughness was measured with a tactile surface tester using the tactile section method in both transverse and longitudinal directions. The roughness values measured were combined into a common roughness characteristic using the Euclidean norm (R q = (R 2q,1 + R 2q,2 ) 1/ 2 ). For Disc 1, the roughness R q in the transverse direction was 192 nm, and 108 nm in the longitudinal direction, yielding a combined roughness of 220 nm. Disc 2 exhibited a transverse roughness of 360 nm, and a longitudinal roughness of 25 nm, resulting in a combined roughness of 361 nm. These results are summarized in Figure 2.2 (top). A more detailed examination of the disc surfaces was performed using a Keyence VK-X100 laser microscope, which offers the advantage of capturing a three-dimensional map of the surface, improving the resolution and allowing for direct integration into numerical contact models. The lenses used have a magnification factor of 100, yielding in element widths of 0,139 µm. The scanned surfaces were approximately 570 µm x 428 µm = 0,244 mm 2 . The resolution in depth (z direction) is 0,1 nm. The measured profiles of the discs are illustrated in Figure 2.2 (bottom). Due to the manufacturing process, a wavy surface structure developed along the xaxis. For the calculation of the specific lubrication film thicknesses, the roughness parameters obtained through the tactile method were utilized in this study. These values provided the foundation for determining the surface characteristics relevant to lubrication. In the numerical contact simulations, the three-dimensional surface profiles captured by laser microscopy were incorporated to achieve a more detailed and accurate representation of the surface topography. The two-disc contact is lubricated with two test oils: a synthetic oil based on polyalkylene glycol (“Glycol”), and Fuchs Dexron Titan ATF 6000 SL (“Dexron”). Gly- Science and Research 7 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0025 Figure 2.1: Left: Schematic of the test rig (Leibniz University Hanover) [28]. Right: Test discs. [26] Öle inla s s R60 R50/ R100 Sche ibe 1 Sche ibe 2 Krafts e nsor Hydraulikzylinde r Krafts e nsor Motor 1 Motor 2 Ge häus e Oil inlet Force sensor Hydraulic cylinder Force sensor Disc 2 R60 Disc 1 Housing R50/ R100 Science and Research 8 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0025 Figure 2.2: Top: Quadratic mean roughness measured through tactile surface tester. Bottom: Three-dimensional surface profiles measured with laser microscope. [26] Figure 2.3: Top: Viscosities and Rodermund parameters of the test oils (measured at ITR). Bottom: Pressure and temperature dependence of the dynamic viscosities of the test oils: experiment and Rodermund model. [26] col is predominantly used as a hydraulic and circulating oil, exhibiting a dynamic viscosity of 44.2 mPas at 40 °C and 9.1 mPas at 100 °C, with a viscosity index of 194. Dexron is commonly applied in automatic transmissions, having a viscosity of 22.5 mPas at 40 °C and 5.2 mPas at 100 °C, with a viscosity index of 173. Both serve as bases for various lubricant formulations. To characterize the temperature dependence of both oils’ viscosities, measurements were conducted at ITR TU Clausthal across a range of temperatures. From these measurements, the parameters for the Rodermund equation [31] were derived, and the results are shown in Figure 2.3 (top). Figure 2.3 (bottom) presents a logarithmic plot of viscosity versus temperature for both oils. In this figure, the solid lines correspond to the Rodermund equation approximation, while the plotted diamonds indicate the experimentally measured values. The dotted lines represent for a comparison the Vogel-Barus equation results. 3 Simulation Model 3.1 Dry Contact The simulation of locally resolved contact pressure is performed using the theory of the elasto-plastic halfspace. This theory assumes an infinitely extended body in two spatial directions, implying an ideally smooth surface for the half-space. To account for the actual measured rough surface profiles of both contact partners, these profiles are superimposed to the flat half space, synthesizing a generic combined surface. Despite the inherent inaccuracy caused by the random spatial orientation of the two roughness profiles relative to each other, this study assumes that the resulting combined surface reasonably approximates the contact behavior of the real surfaces. The numerical determination of the locally resolved pressure for large contact surfaces, while considering surface roughness, is highly computationally demanding due to the large number of elements in the influence matrix, which grows quadratically with the number of surface elements. In this work, the Method of Combined Solutions (MOCS) [7] was employed to address this challenge. The implementation of the contact pressure calculation was carried out in MATLAB, following the algorithm outlined in the program flowchart shown in Figure 3.1 (top). The process begins with the input of contact geometries (1), material parameters (2), and external loads (3). The mesh is then defined (4), with the total number of elements determined by the size of the Hertzian contact area and the element width d. The iterative parameters Ω 1 and Ω h are prescribed for the calculation of deformation and pressure based on the elasto-plastic half-space model, according to [32]. After calculating additional parameters for the mesh (5) and the tribosystem (6), the contact area (7) and the Hertzian pressure distribution are analytically determined (8). In order to apply the Method of Combined Solutions, different irregular influence matrices C irr S are required for each calculation step s, which would otherwise require extensive computation. Therefore, following the approach of B RECHER et al. (2016) [7], an irregular global influence matrix C irr ges is calculated once at step (9) of the flow chart. This matrix contains a finely discretized element in the center and is larger than a simple irregular influence matrix. However, it only needs to be computed once and contains all necessary irregular influence matrices, which can be accessed via simple index shifts for the subsequent steps. Once the irregular global influence matrix is determined, the influence matrix for the coarse mesh is calculated (10), followed by the calculation of an initial deformation w H (11). Before starting the iteration loop (16), initial values for the Hertzian pressure, residual, Hertzian solid overlap, and height distribution are set (12-13). The iteration loop for deformation and pressure calculation (16) is repeated n max times (14-15), where n max is chosen to minimize the relative error ϵ. Each iteration step n begins by setting (*) n -1 = (*), where n -1 denotes the previous iteration step (or the initial condition for n = 0). The placeholder (*) represents the pressure p, residual r, gap height h, or solid overlap h 0 . The pressure change Δp from step n - 1 to n is calculated using the residual, parameter Ω 1 , and the matrix C, following [7]. Numerically calculated negative pressures are set to zero according to the Hertz-Signorini-Moreau condition. The yield criterion is satisfied by limiting the pressure to a predefined maximum value. The pressure for step n is the sum of the pressure in step n -1 and the calculated pressure change. The iteration is controlled by adjusting the solid overlap, with the adjustment based on comparing the force derived from the pressure distribution with the applied normal force. For each pressure step n, the deformation and gap height distribution are recalculated. Any non-physical gap heights h i < 0 are set to zero (also under the Hertz-Signorini-Moreau condition), and the residual is calculated for step n. Finally, the values (*) = (*) n are set as initial conditions for the next iteration step n + 1. The deformation and pressure calculations within step (16) are executed in a subprogram where the Method of Combined Solutions is implemented. The corresponding program flowchart is schematically illustrated in Figure 3.1 (bottom). For each iteration step n, the entire contact area is scanned in Z el = M̃ · Ñ steps (a). In each step s ≤ Z el , the corresponding irregular influence matrix is read from the irregular global influence matrix (b). The displacement is then determined using the half-space model for irregular grids (c), with the finely discretized section being written out and temporarily stored (d). Once the partial displacements for each step s are calculated, they are assembled into the total displacement (e), resulting in the final pressure and deformation fields. Science and Research 9 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0025 Science and Research 10 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0025 Figure 3.1: Flowchart. Top: Calculation of contact pressure (dry contact). Bottom: Sub-model, calculation of deformation and pressure (MOCS). [26] ENDE 𝑛 ≤ 𝑛 max 𝑛 = 1 ℎ 0 = 𝑤 H,𝑖 = ge + H + ℎ 0 = H = H Le s e Werte a us Schritt 𝑛 − 1 𝑛− = 𝑛− = 𝑛− = ℎ 0 𝑛 − = ℎ 0 𝑛 = 𝑛− + Δ Be re chne Pre ssung für Schritt 𝑛 ℎ 0 𝑛 = ℎ 0 𝑛 − − 𝐹 − 𝐹 / 𝐹 Be re chne Einfe de rung für Schritt 𝑛 Ände rung und Korre ktur Δ = 𝑛 − / 𝐶 𝑛− 𝑝 𝑖 < 0 = 0 𝑛− 𝑝 𝑖 𝑝 𝑖 = 𝑝 𝑖 Be re chne Kräftegleichgewicht 𝐹 = 𝑝 𝑖 𝑛 𝑖 Be re chne Ve rformung für Schritt 𝑛 𝑛 ℎ 𝑖 < 0 = 0 𝑛 = + 𝑛 + ℎ 0 𝑛 Be re chne Spalthöhe für Schritt 𝑛 𝑛 = − + 𝑛 + ℎ 0 𝑛 Be re chne Re siduum für Schritt 𝑛 Se tze W erte aus Schritt 𝑛 = 𝑛 = 𝑛 = 𝑛 ℎ 0 = ℎ 0 𝑛 𝑛 = 𝑛 + 1 Ve rformungs- und Pre s sungsberechnung MoCS Irre guläre Ges amt- Einflus smatrix be rechnen START Kontaktge ometrie e inle s en Be la stung e inle sen Gitte rde finition e inle s en Triboparameter be re chnen Konta ktflä che be re chnen H ERT Z ‘s che Pressungsverteilung be re chnen Ne tzparameter be re chnen H = g H Ma te ria lpa rameter e inle s en Grobe Einflus sma trix be re chnen (1) (2) (3) (4) (7) (5) (6) (8) (9) (10) (12) (13) (14) (15) (16) (11) Read contact geometry Read material parameter Read loads Read grid definition Calculate grid parameters Calculate tribo parameters Calculate contact area Calculate Hertzian pressure distribution Calculate irregular influence matrix Calculate coarse influence matrix Read parameters from step n-1 Change and correction Calculate pressure for step n Calculate equilibrium of forces Calculate deflection for step n Calculate deformation for step n Calculate gap heigth for step n Calculate residuum for step n Set parameters for step n i = i i i i ges i ≤ 𝑍 el 𝑠 = 1 𝑠 = 𝑠 + 1 ja ne in Ve rformungs- und Pre s sungsberechnung (MOCS) 𝑛 = (a ) (b) (c) (d) (e ) Calculation of deformation and pressure yes no 3.2 Lubricated Contact In lubricated contacts, the normal force acting on the tribosystem is partially absorbed by the pressure generated within the lubricating film. As the film thickness increases, the proportion of the force transmitted through the lubricating film also increases, since fewer asperities make direct contact. When the film becomes thick enough that no asperities come into direct contact, the lubricant alone transmits the load between the two bodies, signifying the regime of full lubrication. However, it is assumed that asperities continue to interact with each other through the lubricant, even when they are not in direct contact. A complete separation of the surfaces, without asperity interaction, is typically assumed to occur at a lubrication film thickness ratio of approximately λ ≥ 3 [33]. Various researchers have proposed ranges for this transition between liquid and mixed friction. H AMROCK et al. (2004) [34] suggest a range of λ = 3 … 5, while L UDEMA (1984) [35] identifies a range of λ = 0.25 … 2.5. G UANGTENG and S PIKES (1997) [36] report a transition at λ = 2, and B ARTEL (2000) [37], based on numerical calculations, proposes a range of λ = 1.95 … 4.09. The calculation process for contact in the mixed friction regime is based on the flowchart for dry contact, with some modifications. In contrast to step (16), the gap height distribution is not iteratively recalculated in each step but is instead coupled with the analytically computed central lubrication film thickness h c according to H AMROCK and D OWSON (1981) [38]. The temperature of the lubricating film is determined by the oil inlet temperature. In each computational step, the arithmetic mean of the gap height distribution (i.e., the mean h¯ of the deformed surface profile) is compared to the analytically calculated film thickness. If the calculated mean gap height h¯ is smaller than the central film thickness h c , a lower deflection is applied in the next iteration step. Conversely, if h¯ is larger, a greater deflection is specified. At the end of the calculation, the sum of the locally acting forces can be used to determine the proportion of the load carried by asperity contact in relation to the total system force (Figure 3.2). Similarly, the area fraction of contacting asperities can be calculated. The hydrodynamic pressure between contacting asperities follows a qualitative pattern similar to Hertzian pressure distribution, but the maximum value corrected from Hertz’s prediction. This maximum value is determined iteratively so that the integrated hydrodynamic pressure over the contact area corresponds to the total force transmitted by the lubricant. If the analytically calculated lubrication film thickness is less than the gap between the deformed surface profiles in dry contact, the lubricant does not contribute to load transmission. 4. Results 4.1 Test conditions The contact pressure can be varied by adjusting the radial load. The mean (Hertzian) contact pressures considered in this work are 850 (1275), 1000 (1500), and 1250 (1875) MPa. The oil supplied to the tribological contact is preheated before being introduced into the test chamber. The target temperature is varied between 40, 60, and 80 °C. The entrainment velocity is varied between 5, 10, and 15 m/ s, resulting in 27 different parameter combinations, each yielding different central lubricant film thicknesses. The specific lubricant film thickness is λ = h c / R q . Since all experiments were conducted with two different oils (Glycol and Dexron), a total of 54 experiments were performed, along with 3 additional dry tests. An overview of all experiments, along with the corresponding specific film thicknesses, is provided in Figure 4.1 (top). Values in the region 2 < λ ≤ 3 are highlighted in light gray, while values in the region λ < 2 are marked in dark gray. Figure 4.1 (bottom) provides a graphical representation of the central specific lubricant film thicknesses as a function of temperature. Values for a combined velocity of 15 m/ s are shown in red, those for 10 m/ s in green, and those for 5 m/ s in blue. Solid lines represent a mean pressure of 850 MPa, dashed lines correspond to 1000 MPa, and dotted lines represent 1250 MPa. The entrainment velocity and the oil inlet temperature have a significantly greater influence on the lubricant film thickness than the contact pressure. On the other hand, within the considered range, pressure variation did not significantly impact the lubrication regime for any velocity-temperature combination. Science and Research 11 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0025 ℎ c = 2.69 𝑅 𝑈 0.67 𝐺 0.53 𝑊 p−0.067 (1 − 0.61 −0.73𝜅 ) ℎ̅ = 1 𝑍 el ∫ ℎ 𝑖 𝑖 𝐴 𝑖 𝐴 𝑖 𝐴 Ge ge nkörper Grundkörpe r 𝐴 𝑖 ℎ c = ℎ̅ Base body Counter body Figure 3.2: Center plane of the deformed surfaces as the central lubricating film height. [26] pressure distribution for smooth surfaces, as predicted by Hertz’s theory, providing a basis for comparison. It is evident that a significant number of individual roughness asperities exceed the limiting pressure, resulting in the upper portion of the pressure profile being cut off. The ratio of the limiting pressure to the Hertzian pressure is approximately p lim / p Hertz ≈ 3.8, highlighting the localized increase in pressure due to surface roughness. If all locally acting solid-state forces (f i FK ) are summed up for the dry contact, this total sum is identical to the normal force F N applied to the system from the outside. In lubricated contact scenarios (mixed friction), the force transmission is facilitated by a pressurized lubricating film. By specifying a corresponding separation distance between the mid-planes of the roughness profiles, the calculation based on the halfspace model yields a cumulative force significantly lower than the externally applied normal force indicating a reduction in the contribution of force transmitted through solid-state contacts. In this mixed friction scenario, the solid-state force accounts for only 8.8 % of the total force transmission. The difference between the externally applied normal force and the sum of the solid-state forces corresponds precisely to the force transmitted by the lubricating film, which can be calculated accordingly. Science and Research 12 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0025 4.2 Calculated contact pressure and contact area distributions Contact pressure for dry contact and exemplary Glycol mixed lubrication regime In Figure 4.2 (left), the calculated gap height distribution between the combined deformed surface (black pattern) and the reference half-space plane (zero line) for a dry contact with F = 6850 N respectively p = 1250 MPa is presented in a side view. The gap height g (in the zdirection) is plotted on the ordinate in micrometers (µm), while the abscissa (in the x-direction) corresponds to the long semi-axis of the Hertzian contact ellipse. The data on the abscissa is limited to the Hertzian contact length a Hertz . Figure 4.2 (right) depicts the same load scenario but includes the effect of an elastohydrodynamic lubricating film for Glycol (p = 1250 MPa, T = 60 °C, v = 5 m/ s). The horizontal dotted blue centerline of the deformed profile aligns with the calculated film height, illustrating that the asperities are more widely separated compared to a), thereby reducing the proportion of force transmitted through direct solid contacts. Figure 4.3 represents the contact pressures normalized to Hertzian pressure. The red dashed lines indicate the Figure 4.1: Test matrix (specific film thicknesses). [26] / MPa Glycol Dexron 40 °C 60 °C 80 °C 40 °C 60 °C 80 °C 15 850 4.39 2.72 1.98 3.11 2.04 1.48 1000 4.25 2.63 1.92 3.01 1.97 1.44 1250 4.06 2.51 1.83 2.88 1.89 1.37 10 850 3.34 2.07 1.51 2.37 1.55 1.13 1000 3.24 2.00 1.46 2.29 1.51 1.09 1250 3.09 1.91 1.39 2.20 1.44 1.04 5 850 2.10 1.30 0.94 1.49 0.97 0.71 1000 2.03 1.26 0.91 1.44 0.94 0.68 1250 1.94 1.20 0.87 1.38 0.90 0.65 Temperature T Spec. Film Thickness Temperature T Spec. Film Thickness Contact area In addition to analyzing the force contributions from solid-state and EHL contacts, evaluating the contact area distribution may also be of interest. This can be achieved by visualizing the contact pressure from a top-down perspective. In Figure 4.4, the contact zones for both dry and lubricated conditions are depicted. The abscissa represents the x-direction along the major semi-axis, while the ordinate corresponds to the y-direction along the mi- Science and Research 13 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0025 Figure 4.2: Simulated gap height. Left: dry contact. Right: Mixed lubricated contact (Glycol). (Glycol, T = 60 °C, v = 5 m/ s). [26] a) b) c) d) Figure 4.3: Simulated pressure distribution. 1250 MPa, dry and mixed lubricated contact (Glycol, T = 60 °C, v = 5 m/ s). [26] mixed friction, with an area fraction of 0.01 %, indicating that only a few individual roughness asperities remain in mechanical contact. 4.3 Classification of results into friction regimes In Figure 4.5 the force and surface area proportions for 57 calculations are summarized as a function of the specific film thickness λ. The applied pressure is represented by different symbols: “o” for 850 MPa, “+” for 1000 MPa, and “x” for 1250 MPa. The subdivision of the tribosystem into different friction states based on the specific film thickness, as already discussed, is also reflected in the analysis of the results presented here. For example, the specific film thicknesses at the boundaryto-mixed friction transition are λ = 0.35 / 0.27 / 0.20 for pressures of p = 850 / 1000 / 1250 MPa, respectively. When the analytically determined specific film thickness Science and Research 14 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0025 nor semi-axis. Both axes are normalized to their respective Hertzian semi-axes. It is evident that, in the case of dry contact, nearly the entire Hertzian contact area is engaged (as indicated by the red dotted line). Additionally, the cyclically repetitive surface waviness is also apparent in the contact pressure pattern. The proportion of the contact area, that is in mechanical contact relative to the total Hertzian contact area, is determined based on the number of contacting roughness asperities for dry contact. In the case of mixed friction, some roughness asperities partially lift off from each other, leading to a reduction in the number of contacting roughness asperities. As a result, the proportion of solid-state contacts relative to the Hertzian contact surface decreases. In addition to the results at a temperature of 60 °C (area fraction: 18.76 %), the area fractions for a lower temperature of 40 °C (0.01 %) and a higher temperature of 80 °C (39.64 %) are also shown. The case at 40 °C is mathematically close to a transition away from significant Temperature T Area share Figure 4.4: Contact surface. Dry contact and contacts in mixed friction (Glycol, T = 60 °C, v = 5 m/ s, p = 1250 MPa. [26] Area Share Spec. Film Thickness Area share Load Share Spec. Film Thickness Load share Figure 4.5: Area (left) and force (right) fraction of solid contacts. The entrainment velocity is represented by the color (blue: 5 m/ s; green: 10 m/ s; red: 15 m/ s). [26] is below these values, the surfaces do not approach each other further, indicating that the system has reached the boundary friction state, where the force and area contributions to load transmission resemble those of dry contact. In Figure 4.5, the regions corresponding to different friction states are shaded as follows: dark gray represents the area of boundary friction, medium gray denotes the mixed friction zone, and light gray highlights the transition zone. As the specific lubricating film height increases, both the force and surface area ratio decline significantly. The end of the true mixed friction regime is distinctly observed at λ=2, beyond which no roughness asperities remain in mechanical contact. Beyond this value, no further roughness asperities make mechanical contact, marking the end of true mixed friction. However, due to assumed continuing interactions between the surface structures via the lubricating film, additional microdeformations of the asperities and the formation of micro-EHL contacts may still occur. Consequently, the actual lubricating film in a real system tends to be thicker than what is predicted by analytical models (which assume smooth contact surfaces). This is accounted for by defining a distinct transition zone in the friction regime classification. Thus, the friction regime based on this study is classified as follows: • 0 < λ ≤ 0.35 boundary friction, • 0.35 < λ ≤ 2 real mixed friction, • 2 < λ ≤ 3 transition zone, • λ > 3 pure fluid friction. 4.4 Analysis of the results The Influence of yield stress The choice of yield stress for the elasto-plastic half space plays a crucial role in influencing the simulation results. In this study, the value of p lim = 7.13 GPa was chosen, which corresponds to a Vickers hardness of 674 HV and a Rockwell hardness of 59 HRC. The conversion from surface hardness to applied pressure follows the approach where the ratio of the force applied during the Vickers hardness test to the residual impression area due to plastic deformation is equivalent to a pressure. To evaluate the effect of different yield stress assumptions on the simulation results, the pressure distribution was simulated for a selected load case under varying ex- Science and Research 15 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0025 Area share (dry) Figure 4.6: Top: Influence of the yield stress on the surface fraction (850 MPa). Bottom: Area fraction of solid-state contacts as a function of the yield stress. [26] The influence of mesh fineness In addition to the yield stress, the choice of the discrete element width - and consequently the mesh fineness - has a significant impact on the simulation results. In this work, an element width of 2 µm was selected to balance the need for accurate representation of roughness asperities in the micrometer range while avoiding excessive computational demands. To assess the influence of mesh fineness, simulations were conducted for element widths ranging from 0.8 µm to 10 µm on a selected case, and the results were compared based on the area fraction of solid-state contacts. To make the simulations computationally efficient, especially for the smallest element width of 0.8 µm, the analysis was simplified by considering contact between two similar discs instead of the previously modeled two-disc test rig. Both discs were provided with a profiling of 50 mm, which significantly reduced the contact area and made the calculation feasible. The results of these calculations are shown in Figure 4.7 (left). In Figure 4.7 (right), all results are summarized as a function of the element width. Generally, there is a noticeable trend where the area fraction of solid-state contacts increases as the element width increases. A significant jump in results is observed at the step of d = 1.8 μm. The range of results spans from 27.92 % (at d = 1.8 μm) to 50.41 % (at d = 4.5 μm), nearly doubling the area fraction over this range of element widths. The mesh fineness has a significant impact on the simulation results and must be carefully considered when interpreting the outcomes. Variations in element width can lead to substantial differences Science and Research 16 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0025 trusion conditions. Specifically, the contact was modeled at an mean pressure of 850 MPa and for yield stresses of 1000 MPa, along with 1, 2, and 4 times the yield value, as well as without any specification (assuming purely elastic deformation). The results of the contact area formation for these scenarios are illustrated in Figure 4.6 (top). The graphs depict the calculated contact areas for rough surfaces in black, and the corresponding Hertzian contact areas are indicated by a red dashed line. In the case of an yield stress of 1000 MPa, the contact area is larger than the corresponding Hertzian contact area by a factor of 1.64. There are no gaps within the contact surface. Since the yield stress is lower than the mean pressure, a larger contact area is required to sustain the applied force. However, when the extrusion is increased to 2 GPa, the contact area becomes smaller than the corresponding Hertzian contact area (reaching 85.35 % of the Hertzian contact surface). Although some regions outside the Hertzian contact surface remain engaged, gaps start forming within the contact area. As the yield stress increases further, these gaps expand, reducing the solid-state contact area even more. A further increase in yield stress towards infinity shows only marginal changes in the area fraction of solid-state contacts. Figure 4.6 (bottom) plots the results, showing the area fraction of solid contacts as a function of the yield stress. The yield stress has a significant impact on the obtained simulation results. However, for materials typically used in tribologically loaded machine elements, it can be assumed that a yield stress of at least 4000 MPa is consistently applicable. As shown, the deviation in the results for the present case is relatively minor for a yield stress of 4000 MPa. Area share (dry) Element width d Area share y/ b Hertz y/ b Hertz y/ b Hertz x/ a Hertz x/ a Hertz x/ a Hertz x/ a Hertz 0,80 μm 1,00 μm 1,30 μm 1,50 μm 1,80 μm 2,00 μm 2,50 μm 3,00 μm 3,50 μm 4,00 μm 4,50 μm 10,00 μm Figure 4.7: Left: Influence of mesh fineness on the area fraction (850 MPa). Right: Area fraction of solid-state contacts as a function of element width. [26] in the calculated area fractions of solid-state contacts, highlighting the importance of selecting an appropriate mesh resolution for accurate analysis. 5 Conclusion and outlook This study has developed a deterministic model to calculate the real pressure distribution in mixed lubricated contacts, incorporating measured high-pressure fluid properties. The results demonstrate that the proportion of load carried by the lubricant film increases significantly with film thickness, reducing the number of asperities in direct contact. As the specific film thickness increases, the tribosystem transitions through different friction regimes - from boundary to mixed, and ultimately to fluid friction as the asperities lose mechanical contact. The findings highlight the critical role of lubricant viscosity and film thickness in determining the load distribution and contact mechanics within the system, which is relevant for subsequent analyses like friction and wear determination. The transition zones identified provide valuable insights into the behavior of lubricated contacts under varying conditions, offering a robust framework for optimizing tribological performance in practical applications. Future publications will therefore focus on further refining the model to account for more complex surface interactions and extending its applicability to a wider range of tribosystems. They will also include numerical friction predictions and validation by experimental friction measurements on a two-disc apparatus. Literature [1] G REENWOOD , J.A.; W ILLIAMSON , J.B.P. (1966): Contact of Nominally Flat Surfaces. In: Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character 295 (1442), S. 300- 319. [2] W EBSTER , M.N.; S AYLES , R.S. (1986): A Numerical Model for the Elastic Frictionless Contact of Real Rough Surfaces. In: Journal of Tribology 108 (3), S. 314-320. [3] B AILEY , D.M.; S AYLES , R.S. (1991): Effect of Roughness and Sliding Friction on Contact Stresses. In: Journal of Tribology 113 (4), S. 729-738. [4] S AYLES , R.S. (1996): Basic Principles of Rough Surface Contact Analysis Using Numerical Methods. In: Tribology International 29 (8), S. 639-650. [5] R EN , N.; L EE , S.C. (1994): The Effects of Surface Roughness and Topography on the Contact Behavior of Elastic Bodies. In: Journal of Tribology 116 (4), S. 804- 810. [6] G LEß , M. (2009): Wälzkontaktermüdung bei Mischreibung. Dissertation, Otto-von-Guericke-Universität Magdeburg. [7] B RECHER , C.; R ENKENS , D.; L ÖPENHAUS , C. (2016): Method for Calculating Normal Pressure Distribution of High Resolution and Large Contact Area. In: Journal of Tribology 138 (1), S. 156. [8] T ERWEY , J.T.; F OURATI, M.A.; P APE , F.; P OLL , G. (2020): Energy Based Modelling of Adhesive Wear in the Mixed Lubrication Regime. In: Lubricants (Open Access) 8 (2). [9] W ANG , C.; S CHIPPER , D.J. (2020): A Numerical-Analytical Approach to Determining the Real Contact Area of Rough Surface Contact. In: Tribology, S. 1-11. [10] P ATIR , N.; C HENG , H.S. (1978): An Average Flow Model for Determining Effects of Three-Dimensional Roughness on Partial Hydrodynamic Lubrication. In: Journal of Lubrication Technology 100 (1), S. 12-17. [11] T RIPP , J.H.; H AMROCK , B.J. (1985): Surface Roughness Effects in Elastohydrodynamic Contacts - NASA Technical Paper 2488., Cleveland (USA). [12] EFFENDI , J. (1987): Die numerische Lösung der elastohydrodynamischen Kontaktprobleme unter Berücksichtigung der Oberflächenrauheiten. Dissertation, RWTH Aachen. [13] Z HU , D.; C HENG , H.S. (1988): Effect of Surface Roughness on the Point Contact EHL. In: Journal of Tribology 110 (1), S. 32-37. [14] S ADEGHI , F.; S UI , P.C. (1989): Compressible Elastohydrodynamic Lubrication of Rough Surfaces. In: Journal of Tribology 111 (1), S. 56-62. [15] C HANG , L. (1995): Deterministic Modeling and Numerical Simulation of Lubrication Between Rough Surfaces - A Review of Recent Developments. In: Wear 184 (2), S. 155-160. [16] J IANG , X.; H UA , D.Y.; C HENG , H.S.; A I , X.; L EE , S.C. (1999): A Mixed Elastohydrodynamic Lubrication Model With Asperity Contact. In: Journal of Tribology 121 (3), S. 481-491. [17] H U , Y.-Z.; Z HU , D. (2000): A Full Numerical Solution to the Mixed Lubrication in Point Contacts. In: Journal of Tribology 122 (1), S. 1-9. [18] R EDLICH , A.; B ARTEL , D.; S CHORR , H.; D ETERS , L. (2000): A Deterministic EHL Model for Point Contacts in Mixed Lubrication Regime. In: Proceedings of the 26th Leeds- Lyon Symposium on Tribology 38, S. 85-93. [19] W EINHAUER , D. (1996): Reibung und Verschleiß einer hochbeanspruchten Reibpaarung unter Mischreibungsbedingungen. Dissertation, Otto-von-Guericke-Universität Magdeburg. [20] M AKINO , T.; M OROHOSHI , S.; S AKI , K. (1999): The Effect of Roughness Orientation on Mixed Friction. In: Proceedings of the 25th Leeds-Lyon Symposium on Tribology 36, S. 355-365. [21] G ELINCK , E.R.M. (1999): Mixed Lubrication of Line Contacts. Dissertation, Enschede (NL), Universität Twente. [22] E LROD , H.G. (1979): A General Theory for Laminar Lubrication With Reynolds Roughness. In: Journal of Lubrication Technology 101 (1), S. 8-14. [23] T EALE , J.L.; L EBECK , A.O. (1980): An Evaluation of the Average Flow Model for Surface Roughness Effects in Lubrication. In: Journal of Lubrication Technology 102 (3), S. 360-366. [24] T RIPP , J.H. (1983): Surface Roughness Effects in Hydrodynamic Lubrication - The Flow Factor Method. In: Journal of Lubrication Technology 105 (3), S. 458-463. Science and Research 17 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0025 [32] V ENNER , C.H.; L UBRECHT , A.A. (2000): Multilevel Methods in Lubrication. Elsevier, Amsterdam (NL). [33] B ARTEL , D. (2010): Simulation von Tribosystemen - Grundlagen und Anwendungen. Habilitationsschrift, Otto-von-Guericke-Universität Magdeburg. Vieweg+ Teubner, Wiesbaden. [34] H AMROCK , B.J.; S CHMID , S.R.; J ACOBSON , B.O. (2004): Fundamentals of Fluid Film Lubrication. Marcel Dekker, New York (USA) - Basel (CH), 2. Auflage. [35] L UDEMA , K.C. (1984): A Review of Scuffing and Running-In of Lubricated Surfaces, with Asperities and Oxides in Perspective. In: Wear 100 (1-3), S. 315-331. [36] G UANGTENG , G.; S PIKES , H.A. (1997): Elastohydrodynamic Film Thickness in Mixed Lubrication. In: First World Tribology Congress, London (UK), S. 649. [37] B ARTEL , D. (2000): Berechnung von Festkörper- und Mischreibung bei Metallpaarungen. Dissertation, Ottovon-Guericke-Universität Magdeburg. [38] H AMROCK , B.J.; D OWSON , D. (1981): Ball Bearing Lubrication - The Elastohydrodynamics of Elliptical Contacts. John Wiley & Sons, New York (USA). Science and Research 18 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0025 [25] R IENÄCKER , A. (1995): Instationäre Elastohydrodynamik von Gleitlagern mit rauhen Oberflächen und inverse Bestimmung der Warmkonturen. Dissertation, RWTH Aachen. [26] T ERWEY , J.T. (2020): Näherungslösungen für Reibung und Verschleiß in ölgeschmierten Wälzkontakten unter Berücksichtigung der realen Rheologie. Dissertation, Universität Hannover. DOI: https: / / doi.org/ 10.15488/ 10299 [27] S CHMIDT , U. (1985): Die Schmierfilmbildung in elastohydrodynamisch beanspruchten Wälzkontakten unter Berücksichtigung der Oberflächenrauheit. Dissertation, Universität Hannover. [28] M EYER , C. (2010): Reibung in hoch belasteten EHD- Wälzkontakten. Dissertation, Gottfried Wilhelm Leibniz Universität Hannover. [29] W ANG , D. (2013): Wirkungsgradoptimiertes Getriebe. Abschlussbericht, Forschungsvereinigung Verbrennungskraftmaschinen" Frankfurt am Main. [30] B ADER , N. (2018): Traction in EHL-Contacts - The Influence of Local Fluid Rheology and Temperatures. Dissertation, Gottfried Wilhelm Leibniz Universität Hannover. [31] R ODERMUND , H. (1975): Beitrag zur elastohydrodynamischen Schmierung von Evolventenzahnrädern. Dissertation, TU Clausthal. Science and Research 19 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 MEDIENTIPP Erik Kuhn On the Tribology of Lubricating Greases An energetic approach to post-modern tribology Tribologie - Schmierung, Reibung, Verschleiß 1. Auflage 2025, 204 Seiten ISBN print 978-3-381-14171-5 ISBN eBook 978-3-381-14172-2 DOI 10.24053/ 9783381141722 Ladenpreis print €[D] 118,00 Ladenpreis eBook €[D] 94,99 This monograph takes a new look at tribology with its basic concepts of friction and wear using the example of lubricating greases. The consideration of the phenomenon of occurring instabilities and the introduction of the entropy concept into lubricating grease tribology provide a new perspective on known phenomena. The second part of this book presents a wide range of experimental possibilities for investigating lubricating greases. Contents Introduction to Instability and Postmodern Tribology - On the Phenomenon of Self - Organization - Postmodern Grease Tribology - Lubricating Grease - Rheological behavior of Lubricating greases - A Selected Traditional Wear Model - The Extension of the Wear Concept expert verlag - Ein Unternehmen der Narr Francke Attempto Verlag GmbH + Co. KG Dischingerweg 5 \ 72070 Tübingen \ Germany \ Tel. +49 (0)7071 97 97 0 \ info@narr.de \ www.narr.de ning options to reduce friction in the forming section of a paper machine. Conducting tests on newly developed or optimised forming fabrics on industrial paper machines, however, presents a challenge, due to the high cost of the forming fabric product with widths in the range of 10 m and lengths of 70 m, and the machine production losses. Therefore, the development of a new test method to test model forming fabrics on a smaller scale is desirable for the Science and Research 20 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0026 1 Introduction Paper and its affiliated products are produced by dewatering a paper stock suspension using a paper machine (see Figure 1) in an energy intensive process. In the first section of the paper machine, this suspension is applied onto a revolving water-permeable multilayer technical fabric known as the forming fabric. During this process, the paper stock suspension is continuously being dewatered using varying kinds of dewatering elements positioned underneath the forming fabric as the paper stock is retained on top and conveyed through the paper machine. The relative motion between the moving forming fabric and the stationary dewatering elements is known to be a major cause of friction and wear and therefore contributes significantly to the energy consumption of the paper production process [1]. A reduction of the occurring friction is thus desirable for the paper machine operator due to potential savings in energy costs and the accompanying reduction of CO 2 emissions [2]. As paper machine operating parameters and stationary dewatering elements are often pre-set to produce a particular paper, optimising the forming fabric remains as one of the remai- Analysing the friction behaviour of paper machine forming fabrics under water lubricated hydrodynamic conditions and systematical test parameter variation in three defined wear depths using a pin-on-disc test assembly Justus Rüthing, Frank Haupert, Regine Schmitz, Mirek Göbel* submitted: 27.09.2024 accepted: 7.01.2025 (peer review) Presented at GfT Conference 2024 The production of paper on modern paper machines is associated with significant electrical energy consumption, a considerable amount of which is required to overcome friction between paper machine dewatering elements and the paper conveying forming fabric. In this study, an optimised pin-on-disc test method is used to investigate the coefficient of friction (COF) of two paper machine forming fabrics in three different wear depths and varying water-lubricated conditions. The results show a dependence of COF on lubrication rate, with a maximum COF under lubricant-starved conditions, a minimum at intermediate lubrication, and increasing COF at higher lubrication rates. It is shown that the two tested forming fabrics can be differentiated based on their lubricationand wear-depth-dependent frictional behaviour by applying the devised test metrics. Keywords pin-on-disc, water lubrication, hydrodynamic, forming fabrics, wear, friction Abstract * M. Sc. Justus Rüthing Orcid-ID: https: / / orcid.org/ 0000-0001-7615-4979 Prof. Dr. -Ing. Frank Haupert Orcid-ID: https: / / orcid.org/ 0000-0002-3312-6844 Prof. Dr. -Ing. Mirek Göbel Orcid-ID: https: / / orcid.org/ 0009-0007-6203-6585 Dr.-Ing. Regine Schmitz Orcid-ID: https: / / orcid.org/ 0000-0002-4510-2559 Hamm-Lippstadt University of Applied Sciences, Marker Allee 76, 78, 59063 Hamm producers of paper machine forming fabrics and the paper machine operators as the primary beneficiaries. As of today, there are many known tribological test methods to test the wear of paper machine forming fabrics [3-8]. However, a method to reliably investigate the frictional behaviour of these forming fabrics is still limited. Specifically, the influence of the progressive paper suspension dewatering on the coefficient of friction (COF) has not been addressed in existing component scale tests. To address this gap, this model study applies an optimised pin-on-disc test rig to investigate the frictional behaviour of two forming fabrics under varying and controlled lubrication rates to model the progressive paper suspension dewatering in the forming section. The coefficient of friction is measured under defined lubrication ranging from fully lubricated to lubricant starved conditions and in three defined wear depths representing the life cycle of a forming fabric during machine operation to assess the influence of lubrication and wear-depth on the frictional, and therefore energetic, performance of paper machine forming fabrics. 2 Materials 2.1 Pin-on-disc test rig utilised for forming fabric characterisation For the characterisation of the paper machine forming fabrics an optimised, in-house designed and built test rig in the pin-on-disc configuration described in [10, 11], is used (see Figure 2). Normal force is applied by a mechatronic controlled load and test unit. The load and test unit are able to apply precise force to the specimen in the range of 1 to 200 N. During measurement a precise normal force application (± 0.3 N) is realised using a feedback control system and spindle drive. Utilising a load cell, the friction force is measured. To measure the weardepth of the forming fabric specimen, a µm-accurate laser deflection sensor (Micro-Epsilon optoNCDT ILD1700-10) is used to measure the wear of the forming Science and Research 21 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0026 Figure 1: The VOITH Paper Huatai PM12 Paper Machine in China (paper machine forming fabric in blue) [9]. Figure 2: Optimised pin-on-disc test assembly for the tribological characterisation of paper machine forming fabrics with peristaltic pump (right) and close up of the contact area (left). by a defined wet grinding process consisting of a two times two-minute grinding procedure with the SiC wet abrasive paper grain sizes of 360 and 600. To ensure the quality of the conducted experiments the counterpart surface roughness metrics of Ra of 0.11 ± 0.02 µm, Rpk of 0.38 ± 0.15 µm, Rvk of 0.43 ± 0.16 µm, Rk of 0.23 ± 0.03 µm are measured and controlled in defined intervals using a white light interferometer (FRT Mirco-Prof ® ). 2.3 Specimen - polymer forming fabrics Paper machine forming fabrics are woven multi-layer technical fabrics made from varying polymer fibre materials, fibre thicknesses and weave patterns with an overall thickness of approx. 1000 µm (see Figure 3). A modern Self Support Binding (SSB) forming fabric is made up of two interwoven parts: the upper paper side and the bottom machine side (see Figure 4). The upper side of the forming fabric is designed to define the surface properties of the paper product. The machine side is designed to resist the tribological stresses, good wear resistance and minimal power consumption, during paper machine operation [12]. To test the general applicability of the optimised test rig, two forming fabrics with two differing kinds of machine side materials where selected, investigated and compared in this study with the methods described in the following chapter. 3 Methods 3.1 Parameters for the COF measurement of forming fabrics To model the paper machine forming section on a model test rig, an analysis of a paper machine forming section Science and Research 22 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0026 fabric specimen by detecting changes in specimen thickness that occur as a function of test time during measurement. Counterpart rotation is realised by a servomotor, enabling stable test rig operation at sliding speeds of 5.0 m/ s. Temperature sensors are located underneath the counterpart and are used to monitor the counterpart’s overall temperature. To conduct measurements under lubricated conditions, a programmable peristaltic pump (Ismatec ® Reglo ICC) is used. With this set up, lubricants can be applied directly onto the counterpart’s surface with lubrication rates ranging from 0.01 to 5.70 ml/ min over the water injection attachment. Surrounding the counterpart, there is a liquid enclosure protecting the tribometers electronics. All of the features of the described test rig are controlled by a MATLAB ® (The MathWorks, Inc.) based in-house designed operating system which is operated using a Graphical User Interface (GUI). During the conducted tests, applied normal force F N in [N], friction force F X in [N], speed v in [m/ s], specimen wear distance s in [µm] and temperature T in [°C] are measured and displayed as a function over test time t in [min]. Hereby F N and F X are used, to calculate the steady-state coefficient of friction µ [-]. 2.2 Counterpart - dewatering element alumina To model the paper machine dewatering elements on the pin-on-disc test rig, an Alumina (Al 2 O 3 ) that is used on the stationary dewatering elements is employed as the rotating counterpart material (see Figure 2). Before the start of each experiment, the counterpart disc is prepared Figure 4: View of the bottom machine side (left) and top paper side (right) of a paper machine forming fabric [9]. Figure 3: Side view of a polymer forming fabric with the upper paper side (in blue) and the bottom machine side (in white) [9]. has been conducted. To derive the tribosystem parameters from that analysis the following paper machine operating parameters were considered: dewatering element vacuum (normal force), paper machine operating speed (sliding speed) and paper suspension dewatering (lubrication rate). As other parameters of influence like temperature and humidity could not be controlled using the current test set up, described in 2.1, they were not included and could not be considered. Therefore, all experiments were conducted in between the temperature range of 24 -25 °C, of which no adjustments were made during the experiments. Further wear occurs on the machine side of the paper machine forming section with an increase in its run-time. As the contact area of the individual machine side fibres increases with wear depth, it is also being considered and tested as a parameter of influence on the COF. Normal Force: The selected normal force was derived from the average load induced by the vacuum aided dewatering elements in addition to the load applied by the paper suspension. Using the sample size of 400 mm 2 to test on the test rig currently, the normal force was calculated to be 8 N with every conducted test. Sliding Speed: Modern paper machines are operating under high speeds of up to 33 m/ s. Due to limitations in the test rig drive-unit, a sliding speed of 5.0 m/ s was chosen to ensure good test rig run ability and repeatability of the generated test results. Lubrication Rate: As the paper is constantly being dewatered while being conveyed through the forming section, several lubrication rates ranging from 2.50 to 0.00 ml/ min (see Figure 5) were chosen to model the dewatering on the test rig scale. Wear Depth: Wear, increasing with paper machine run time, can be observed on the paper machine forming fabric machine side contact area. As the contact area of the fibres used to create the forming fabric machine side change with added wear, three defined wear depths are being tested to investigate the influence on the COF. 3.2 Experimental procedure To investigate the COF of forming fabrics in the paper machine forming section, an experimental procedure has been devised. Using the testing parameters described in 3.1, the first test is conducted using a new forming fabric sample (wear depth 1) and a lubrication rate of 2.50 ml/ min. After a test time of 50 min, the same sample is being tested with a lubrication rate of 1.00 ml/ min for a further 50 min continuing this progression as displayed in Figure 7, until the lubrication rate of 0.00 ml/ min (dry conditions) is reached. With the lubrication rate of 0.00 ml/ min, this test is being run until the next wear depth (wear depth 2) is reached. With the targeted wear depth reached, the test procedure is repeated starting with a lubrication rate of 2.50 ml/ min and the last wear depth (wear depth 3) has been tested. To ensure repeatability of the conducted experiments, this test procedure is repeated three times using three new samples from the same batch in total to give the mean and standard deviation of the COF. Science and Research 23 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0026 Figure 5: Display of the lubrication rates used to model the dewatering of the paper machine forming fabric. Figure 6: Display of the three defined wear depths tested on the measured wear curve (left) and its depth equivalent in the paper machine forming fabric machine side. Figure 7: Representation of the devised test procedure to measure the COF of paper machine forming fabrics by 1. Measuring the COF using six defined lubrication rates; 2. Wearing down the sample by conducting dry measurements until the next wear depth is reached. This procedure is repeated two times until the all three wear depths have been measured. To ensure repeatability, this procedure is repeated three times. attributed to the onset of mixed lubrication. The introduction of lubricant reduces the fabric-counterbody contact, and thereby lowers friction. The minimum COF observed at approximately 0.50 ml/ min corresponds to a balance between boundary friction and hydrodynamic effects, where sufficient lubrication is present to reduce surface interaction without yet introducing significant viscous drag. This minimum is observed consistently across all investigated wear depths, with the exemption of fabric B wear depth 2. At lubrication rates exceeding 0.50 ml/ min, the COF increases for both fabrics, indicating an increasing contribution of hydrodynamic friction. In this regime, the added lubricant can no longer fully enter into the contact, leading to increased drag effect. This hydrodynamic contribution becomes particularly pronounced for forming fabric B, where wear depths 2 and 3 show a steep increase in COF with increasing lubrication rate. The stronger sensitivity of fabric B to lubrication rate is most likely related to differences in fibre material compared to fabric A. The influence of wear depth at higher lubrication rates further supports this interpretation. For forming fabric B, increasing wear depth leads to a widening of the fibres and an associated increase in contact area, which enhances the hydrodynamic contribution to friction. In contrast, forming fabric A exhibits a more stable COF response at higher lubrication rates, with overlapping COF values across all wear depths. This suggests a lower sensitivity of fabric A to hydrodynamic drag. At low lubrication rates, wear depth differentiation remains weak for both fabrics due to the dominance of boundary lubrication and the associated data scatter. However, for forming fabric A, wear depth 3 exhibits a consistently higher COF compared to wear depths 1 and 2. This behaviour can be attributed to an increased effective surface area and reduced local surface pressure, which promotes higher friction under boundary-dominated conditions. Science and Research 24 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0026 4 Findings The results of the experimental procedure are presented as the COF plotted against lubrication rate for forming fabric A (Figure 8) and forming fabric B (Figure 9). For both fabrics, the data are grouped according to the three investigated wear depths, referred to as wear depth 1 (green), wear depth 2 (yellow), and wear depth 3 (red). The following analysis of the research findings presents a scientific interpretation of observed trends and is grounded in the measured COF data. Across all tested conditions, both forming fabrics exhibit a similar qualitative dependence of COF on lubrication rate. Independent of fabric type and wear depth, the COF decreases with the introduction of lubrication, reaches a minimum at a lubrication rate of approximately 0.50 ml/ min, and subsequently increases as the lubrication rate is further increased to the maximum tested value of 2.50 ml/ min. This behaviour resembles a Stribeck-like curve, characterised by a transition from boundary-dominated friction to mixed lubrication and an increasing contribution of hydrodynamic friction at higher lubrication rates. At a lubrication rate of 0.00 ml/ min, the contact can be considered starved of lubricant following the interruption of lubricant supply. Under these conditions, boundary lubrication dominates the frictional behaviour due to direct contact between the forming fabric fibres and the counterbody. In this regime, both forming fabrics A and B exhibit overlapping COF values across all wear depths, with relatively high scatter (see Figure 8 and 9) in the measured data. This overlap indicates that weardepth-dependent effects are largely masked by boundary-dominated friction, limiting the ability to differentiate wear states based on COF alone. The observed scatter further highlights a limitation of the measurement procedure under dry conditions. With increasing lubrication rate from 0.00 to 0.50 ml/ min, the COF decreases for both fabrics, which can be Overall, the results demonstrate that the measured COF arises from a combination of boundary and hydrodynamic friction components, with their relative contributions varying as a function of lubrication rate, fabric structure, and wear depth. While the absence of direct film thickness measurements limits a fully quantitative hydrodynamic analysis, the observed trends are consistent with established tribological concepts. The findings emphasise that both lubrication regime and fabric geometry must be considered when evaluating the frictional performance of forming fabrics. 5 Summary In this study, two different forming fabrics were characterised using the COF obtained with the devised test method, defined test parameters, and an optimised pin-ondisc test rig. Analysis of the experimental results identified the lubrication rate as a parameter of significant influence on the COF compared to the additionally investigated wear depth. The highest COF values were measured under dry conditions at a lubrication rate of 0.00 ml/ min while the introduction of lubrication led to a reduction in COF, followed by an increase at higher lubrication rates. This behaviour can be explained by a transition from boundary-dominated friction under dry conditions to mixed lubrication at intermediate lubrication rates, and an increasing contribution of hydrodynamic friction at higher lubrication rates. In this regime, viscous drag becomes increasingly significant and contributes to the observed rise in COF. Forming fabric B exhibits a pronounced increase in COF with increasing lubrication rate and wear depth, indicating a higher sensitivity to hydrodynamic friction effects as the test progresses. This suggests that forming Science and Research 25 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0026 Figure 8: Comparison of the COF of forming fabric Sample A over the lubrication rate in the three wear depths. Figure 9: Comparison of the COF of forming fabric Sample B over the lubrication rate in the three wear depths. Literature [1] J. H. Tylczak und F. J. Friedersdorf, “A new test for pulp and paper forming fabric materials”, Wear, Jg. 302, 1-2, S. 1082-1087, 2013, doi: 10.1016/ j.wear.2012.10.026. [2] K. Holmberg, R. Siilasto, T. Laitinen, P. Andersson und A. Jäsberg, “Global energy consumption due to friction in paper machines”, Tribology international, Jg. 62, S. 58- 77, 2013, doi: 10.1016/ j.triboint.2013.02.003. [3] G. A. Hemstock und H. B. Neubold, “Effect of pigment properties on the wear of plastic fourdrinier wire”, Tappi Journal, Jg. 71, Nr. 5, S. 127-132, 1988. [4] H. Einlehner, “Abrasion testing apparatus,” US4633701A. USA. [5] R. Pitt, “Fillers and fabric life in alkaline papermaking”, Tappi Journal, Jg. 67, Nr. 4, S. 96-98, 1984. [6] G. J. Gill, J. E. Holton und M. M. Kazanzian, “Using the Valley Abrasion Tester to determine the wearing effect of a paper filler on plastic forming fabric”, TAPPI, Jg. 65, Nr. 7, S. 54-56, 1982. [7] H. B. Neubold, P. Sennet und H. H. Morris, “Abrasiveness of pigments and extenders”, Tappi Journal, Jg. 65, Nr. 12, S. 90-93, 1982. [8] J. Vlossak, “Improving the accuracy and flexibility of the valley clay tester”, tappi, Jg. 57, Nr. 3, S. 123-125, 1974. [9] VOITH, Mediendatenbank. [10] J. Rüthing, F. Haupert, R. Schmitz, M. Sigrüner und N. Strübbe, “A new approach for the friction and wear characterisation of polymer fibres under dry, mixed and hydrodynamic sliding”, T+S, Jg. 69, eOnly Sonderausgabe 2, S. 18-25, 2022, doi: 10.24053/ TuS-2022-0034. [11] R. Schmitz, F. Haupert, J. Rüthing, M. Sigrüner und N. Strübbe, “Tribologische Charakterisierung von Polymerfasern unter Trockenreibung, Mischreibung und Hydrodynamik mittels einer optimierten Pin-on-Disc-Prüfmethode”, T+S, Jg. 68, 3-4, 2021, doi: 10.24053/ TuS-2021- 0015. [12] S. Adanur, Paper Machine Clothing: Key to the Paper Making Process. Boca Raton, FL: CRC Press, 2017. [Online]. Verfügbar unter: https: / / permalink.obvsg.at/ Science and Research 26 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0026 fabric B performs more favourably in earlier test stages, while its frictional performance deteriorates with increasing wear. In contrast, forming fabric A shows a higher initial COF across all lubrication rates but a comparatively stable frictional response with increasing wear, indicating a lower susceptibility to wear-induced changes in hydrodynamic friction. While no clear distinction between forming fabrics A and B could be established at low lubrication rates between 0.00 and 0.25 ml/ min, a clear differentiation becomes possible at lubrication rates of 0.50 ml/ min and above, with the strongest contrast observed at 2.50 ml/ min. Using the optimised pin-on-disc test rig and the applied test methodology, it was demonstrated that forming fabrics can be differentiated with respect to their lubrication-dependent and wear-dependent frictional behaviour based on COF measurements. It is noted that the test rig in its current development stage cannot fully replicate the industrial paper machine operating conditions being limited by its test speed of 5.0 m/ s and without temperature and humidity control. Therefore, this study emphasized the methodological validation with the development of next generation test rig capable of higher sliding speeds and controlled environmental conditions already in progress. Collectively, these findings show that the presented method enables a systematic, forming fabric specific comparison of the COF under varying operating conditions providing a foundation for optimising forming fabric selection and design to reduce the energy consumption in paper machines. 6 Acknowledgements The authors thank the German federal ministry of research, technology and space and research (BMFTR) and J.M. Voith SE & Co. KG for the funding of this study as part of the FH-Kooperativ project TriboMath26 - 13FH010KX1. Introduction Tribological systems, which involve the control and understanding of friction, wear, and lubrication, are key enablers of mechanical efficiency and durability across industrial sectors (Gesellschaft für Tribologie e.V 2002). From rotating machinery to transport systems, tribological optimization has the potential to significantly reduce energy losses, extend component lifespans, and improve product performance (Woydt et al. 2019; Woydt et al. 2021; Woydt et al. 2023). Considering rising environmental and economic demands, such as energy efficiency, resource conservation, and life-cycle cost reduction, interest in tribology as a strategic lever for industrial optimization is growing. Despite this growing awareness, tribological solutions often remain underutilized in practice. While technical innovations in coatings, materials, and lubrication systems continue to advance, their systematic integration into industrial design, production, and maintenance routines lags. This indicates a gap between research-driven innovation and real-world application and raises important questions: What is the current state of tribological practice in industry? What are the main barriers to broader implementation? Where should future research efforts be focused to accelerate progress? To address these questions, an expert survey was conducted in the context of the 65 th Tribology Conference of the German Society for Tribology (German: Gesellschaft für Tribologie, GfT) and the accompanying research initiative EE4InG2. The joint project EE4InG2 serves as an accompanying research project to the Research Network on Industry and Commerce (Forschungsnetzwerk Industrie und Gewerbe). Its core objective is the scientific cross-evaluation of applied energy efficiency research and funding within the Federal Energy Research considering past developments, the current state, and future perspectives. This program is commissioned by the Federal Ministry for Economic Affairs and Energy (BMWE). A further aim is to foster exchange among key actors of the innovation system - industry, science, and policy - through a coordination office established within the project. Building on the groundwork of the preceding project EE4InG, the initiative continues to support networking, research activities, and the dissemination of results. This paper presents and analyzes the findings of the expert survey, highlighting perceived opportunities, limitations, and strategic research needs for tribological technologies in industry. Methodology The survey was introduced during the 65 th Tribology Conference organized by the German Society for Tribology (GfT) in 2024. It targets experts and stakeholders from fields such as industry, academia, public instituti- Science and Research 27 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 Key Survey Results on Tribological Optimization Opportunities, Barriers, and Research Needs Juliane Heydemann, Emil Elbæk, Thomas Lohner* Presented at GfT Conference 2025 This article presents the key results of an expert survey conducted as part of the 65 th GfT Tribology Conference in 2024. The survey investigates opportunities and barriers to tribological optimization in industrial practice, as well as perceived research needs in the field. A total of 46 fully completed responses are evaluated. The findings show a high potential for tribological methods to improve energy efficiency and extend service life of components. However, respondents also identified significant knowledge gaps, organizational barriers, and a lack of education and awareness that currently hinder broader implementation. The article concludes with concrete suggestions for future research directions and educational initiatives. Keywords Tribology, Industrial Optimization, Survey, Energy Efficiency, Barriers, Research Needs, Engineering Practice Abstract * M. Sc. Juliane Heydemann M. Sc. Emil Elbæk ETA-Solutions GmbH Darmstädter Str. 239, 64625 Bensheim, Deutschland Dr.-Ing. Thomas Lohner Forschungsstelle für Zahnräder und Getriebesysteme (FZG), Technische Universität München (TUM) Boltzmannstraße 15, 85748 Garching bei München prises (41 %), followed by representatives from consulting services, public sector organizations, and other professional domains (see Figure 1). Q2 - Q5: Industrial sector. Among industry respondents, mechanical engineering is the dominant sector (53 %), followed by vehicle manufacturing (42 %) and metal industry (37 %). The chemical industry accounts for 16 %, while plastics and rubber, glass and ceramics, and food/ feed industry each represent 5 % (see Figure 2). Multiple responses were allowed. The high participation from the mechanical engineering and automotive sectors indicates that the topic of tribology plays an above-average important role in these industries. Beyond the sectoral distribution, an even clearer pattern is evident at the functional level: 95 % of industry respondents are employed in research and development, whereas 5 % are engaged in marketing and communications (see Figure 3, left). No responses are recorded for other operational roles such as production or quality management. In the industrial sector 63 % of the participants identify as manufacturers of products/ solutions for tribological system optimization, while 37 % are users of such solutions (see Figure 3, right). Among the industrial respondents, 84 % work in companies developing products with tribological requirements. Of these, 37 % focus on manufacturing tribologically optimized materials or coatings, 32 % are active in lubricant development, and 26 % are involved in designing tribological components such as bearings or seals, as well as in consulting and process optimization. Maintenance and servicing of equipment with tribological components account for 16 %, while 5 % provide testing services and procedures for tribological systems (see Figure 4). Of all respondents, 94 % report possessing good knowledge and practical experience in tribology, with 24 % even rating their expertise as very good and comprehensive. Combined with the fact that participants are expli- Science and Research 28 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 ons, and technical consulting and focuses on tribological technologies in industrial applications. A total of 108 responses were received, with 46 fully completed and included in the final evaluation. The questionnaire comprises 21 questions and is designed to be completed in approximately 15 minutes. Conditional logic is used in several sections to tailor follow-up questions based on earlier responses. The questionnaire consisted of four thematic blocks: • Participant background, • Opportunities for tribological optimization, • Barriers to implementation and • Research needs. The key findings from the 21 questions, labeled Q1 through Q21, are presented below. A complete version of the questionnaire can be found in the appendix. The evaluation is carried out using standard statistical methods. The results of the single and multiple-choice questions are given as percentages, whereby the percentages refer to the proportion of the respective answer categories in the total number of answers given to a question. The exact number of responses is indicated separately in each case. Responses to free-text questions are presented in a clear, listed form. Results This section presents the results of the four main subchapters, following the structure of the survey: background information, opportunities, barriers, and future research needs. 1 Background Information Q1: Participant background information. Participants represented a broad cross-section of relevant stakeholder groups in the tribology field. The largest proportion of respondents comes from academia and research institutions (46 %) and industrial and commercial enter- 𝑛 1 =46 4 1 % 4 6 % 9 % 2 % 2 % Industry and manufacturing Academic research and education Consulting and services Public sector and government agencies Others Figure 1: Participant background information (Q1) citly selected based on their domain expertise, the survey can therefore be classified as an expert survey (cf. Häder, 2014). 2 Opportunities for Tribological Optimization To identify and assess the opportunities for tribological optimization, the survey includes a series of targeted questions addressing the most significant perceived benefits (Q7), current and future final energy consumption saving potentials across different industrial sectors (Q8.1 and Q8.2), current and future economic potentials by sectors (Q9.1 and Q9.2), current and future final energy consumption saving potentials in mobility applications (Q10.1 and Q10.2), as well as untapped application areas (Q11), potentials in familiar production processes (Q12), and tribology-related innovations implemented or under preparation (Q13). The corresponding results are presented in the following sections. Q7: Greatest benefits of tribological optimization. Respondents see the greatest potential in two primary Science and Research 29 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 9 5 % 5 % Research and development (R&D) Marketing/ communications 𝑛 3 =19 6 3 % 3 7 % Manufacturer of products/ solutions for optimizing tribological systems User of products/ solutions for optimizing tribological systems 𝑛 4 =19 𝑛 4 =19 Figure 3: Role of the company (Q3, left) and department of the participant (Q4, right) 8 4 % 3 2 % 2 6 % 3 7 % 1 6 % 5 % 2 6 % 0% 20% 40% 60% 80% 100% Development of products with tribological requirements Development of lubricants Development of tribotechnical components Manufacture of tribologically optimized materials or coatings Maintenance and repair of systems with tribological components Testing services and test procedures for tribological systems Consulting and optimization of tribological processes 𝑛 5 =19 Figure 4: Specific categories of the represented companies (Q5) 5 % 1 6 % 5 % 5 % 3 7 % 5 3 % 4 2 % 0% 10% 20% 30% 40% 50% 60% Food and feed industry Chemical industry Plastics and rubber industry Glass and ceramics industry Metal industry Mechanical engineering Vehicle manufacturing 𝑛 2 =19 Figure 2: Represented sectors in the survey (Q2) Q8.2: Potential for final energy consumption saving through tribological optimization in 10 years. Experts regard tribological optimization as a growing instrument for improving energy efficiency. While its potential is already considered relevant today, a marked increase is expected across nearly all sectors over the next ten years. The future savings potential is rated particularly high in mechanical engineering (43 % high, 30 % very high) and vehicle manufacturing (20 % high, 43 % very high), whereas industries such as chemicals (14 % very high), metals (14 % very high), and plastics (19 % very high) also exhibit considerably more optimistic assessments for the coming decade (see Figure 7). Even in sectors currently perceived as less relevant (e.g., textiles, wood and paper), a noticeable increase in potential is anticipated. Q9.1: Current economic potential of tribological optimization. Already today, the economic potential of Science and Research 30 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 areas: energy savings (33 %) and increased service life (22 %, see Figure 5). Other benefits mentioned include performance improvement (11 %), enhanced functionality under extreme conditions (9 %), improved product quality (9 %), cost reduction (7 %), reduced downtime (4 %), increased environmental compatibility (4 %), and improved production cost efficiency (2 %). Q8.1: Current potential for final energy consumption saving through tribological optimization. The highest current potential is seen in mechanical engineering (39 % high, 22 % very high) and vehicle manufacturing (37 % high, 33 % very high). This aligns with the strong representation of respondents from these sectors in the survey, reflecting the prominent role tribological considerations play in their operations. The high ratings and the participant profile thus reinforce each other, suggesting that both perceived potential and industry engagement are closely linked. The chemical, metal, and plastics/ rubber industries are rated mostly in the mid-range, while glass/ ceramics, food/ feed, and textiles receive predominantly lower ratings (see Figure 6). 2 2 % 9 % 9 % 3 3 % 4 % 1 1 % 2 % 7 % 4 % 0% 5% 10% 15% 20% 25% 30% 35% Increased service life Improved functionality (in ext. cond.) Improved product quality Energy savings Reduced downtime Increased performance Cost efficiency in production Cost reduction Environmental friendliness 𝑛 7 =45 Figure 5: Greatest benefits of tribological optimization (Q7) 10% 7% 15% 20% 7% 5% 11% 11% 35% 30% 33% 38% 33% 44% 34% 29% 32% 28% 17% 40% 32% 28% 29% 24% 22% 29% 41% 44% 39% 37% 21% 30% 28% 19% 26% 15% 15% 15% 22% 33% 5% 7% 9% 5% 10% 5% 7% Mechanical engineering Vehicle manufacturing Chemical industry Metal industry Plastics and rubber industry Glass and ceramics industry Electrical industry Food and feed industry Textile and clothing industry Wood and paper industry Other industries 1 2 3 4 5 Scale: 1 = no potential; 5 = very high potential: 𝑛 8.1 =41-46 ∅ = 3,72 ∅ = 3,87 ∅ = 2,95 ∅ = 3,09 ∅ = 3,09 ∅ = 2,71 ∅ = 2,98 ∅ = 2,51 ∅ = 2,46 ∅ = 2,85 ∅ = 2,12 Figure 6: Potential for final energy consumption savings through tribological optimization today (Q8.1) 1 1 With “Other industries” we mean “Other manufacturing industries”. tribological optimization is assessed as high to very high in many industries. Mechanical engineering leads with 40 % very high and 35 % high ratings, followed by vehicle manufacturing (32 % very high, 34 % high). The chemical industry is mainly rated medium (46 %), with balanced lower and higher ratings. Metals show a mixed profile (27 % very high, 24 % low), while plastics/ rubber is split between low (41 %) and high or very high (43 %). Glass/ ceramics, food/ feed, and textiles/ clothing receive predominantly lower scores (see Figure 8). Q9.2: Economic potential of tribological optimization in 10 Years. Projections remain overall stable, with slight increases in the highest ratings for mechanical engineering (44 % very high) and for plastics and rubber (25 % very high, up from 15 %). By contrast, vehicle manufacturing shows a slight decline, while sectors such as glass and ceramics, food and feed, as well as textiles and clothing continue to receive comparatively lower scores. The economic assessment thus does not fully mirror the energetic one: although similar overall tendencies can be observed, in some cases, such as vehicle manufacturing, opposing developments are expected (see Figure 9). Q10.1: Current potential for final energy consumption savings through tribological optimization in various mobility applications. In summary, the greatest potential is currently seen in vehicles with combustion engines, closely followed by vehicles with electric drivetrains (see Figure 10, top). Hydrogen based propulsion follows closely. According to the respondents, other mobility applications also show considerable potential, although the distribution of ratings is somewhat more widely spread. Science and Research 31 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 5% 7% 5% 5% 5% 8% 8% 13% 15% 8% 5% 7% 7% 22% 24% 36% 38% 39% 38% 31% 33% 30% 14% 20% 46% 22% 15% 21% 18% 18% 23% 31% 33% 35% 34% 15% 22% 28% 21% 18% 18% 21% 15% 15% 40% 32% 12% 27% 15% 13% 16% 13% 10% 13% 18% Mechanical engineering Vehicle manufacturing Chemical industry Metal industry Plastics and rubber industry Glass and ceramics industry Electrical industry Food and feed industry Textile and clothing industry Wood and paper industry Other industries 1 2 3 4 5 𝑛 9.1 =39-43 Scale: 1 = no potential; 5 = very high potential: Figure 8: Economic potential of tribological optimization (Q9.1) 7% 5% 7% 14% 5% 15% 15% 7% 5% 7% 7% 26% 30% 23% 31% 31% 34% 37% 34% 17% 17% 24% 37% 25% 14% 21% 21% 27% 27% 27% 44% 43% 20% 21% 27% 37% 29% 26% 20% 17% 20% 24% 30% 43% 14% 14% 19% 5% 17% 5% 5% 12% 10% Mechanical engineering Vehicle manufacturing Chemical industry Metal industry Plastics and rubber industry Glass and ceramics industry Electrical industry Food and feed industry Textile and clothing industry Wood and paper industry Other industries 1 2 3 4 5 Scale: 1 = no potential; 5 = very high potential: 𝑛 8.2 =41-46 ∅ = 3,93 ∅ = 3,87 ∅ = 3,19 ∅ = 3,16 ∅ = 3,37 ∅ = 2,79 ∅ = 3,19 ∅ = 2,66 ∅ = 2,61 ∅ = 2,95 ∅ = 3,17 Figure 7: Potential for final energy consumption savings through tribological optimization in 10 years (Q8.2) using any of the listed innovations, and 4 % indicated no knowledge of them. An additional 9 % mentioned other relevant technologies for their specific applications, including surface structuring, triboconditioning, lubricantfree polymer bearings, and testing techniques and methods (see Figure 11). 3 Barriers to Implementation Q14: Barriers to the practical Implementation of tribological optimization. The survey results indicate that the main obstacles to the widespread implementation of tribological optimization are less technical in nature but Science and Research 32 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 Q10.2: Potential for final energy consumption savings through tribological optimization in various mobility applications in 10 years. Respondents expect an increasing potential for energy savings in all categories but combustion engines (see Figure 10, bottom). The greatest potential is expected within hydrogen drivetrains. Q13: Tribology innovations implemented or in preparation. The most frequently adopted or planned innovations include optimized materials and coatings (76 %), machine elements such as gears, bearings or seals (72 %), and lubricants (65 %). Only 7 % reported not 5% 9% 5% 5% 5% 8% 5% 10% 13% 8% 5% 9% 25% 20% 25% 33% 34% 31% 28% 26% 26% 14% 25% 38% 29% 13% 23% 21% 26% 26% 33% 28% 33% 25% 18% 20% 33% 26% 24% 21% 18% 15% 21% 44% 32% 15% 27% 25% 10% 16% 13% 15% 18% 23% Mechanical engineering Vehicle manufacturing Chemical industry Metal industry Plastics and rubber industry Glass and ceramics industry Electrical industry Food and feed industry Textile and clothing industry Wood and paper industry Other industries 1 2 3 4 5 𝑛 9.2 =39-43 Scale: 1 = no potential; 5 = very high potential: Figure 9: Economic potential of tribological optimization in 10 years (Q9.2) 16% 4% 5% 5% 18% 13% 14% 7% 13% 16% 9% 24% 22% 29% 30% 36% 31% 38% 43% 29% Vehicles with combustion engines Vehicles with electric mobility Vehicles with hydrogen drives Other mobility applications 1 2 3 4 5 𝑛 10.2 =42-45 Scale: 1 = no potential; 5 = very high potential: 9% 11% 11% 14% 14% 18% 27% 20% 26% 47% 27% 25% 36% 22% 33% 32% 21% Vehicles with combustion engines Vehicles with electric mobility Vehicles with hydrogen drives Other mobility applications 1 2 3 4 5 𝑛 10.1 =42-45 Scale: 1 = no potential; 5 = very high potential: Figure 10: Potential for final energy consumption savings through tribological optimization in various mobility applications today (Q10.1, top) vs. in 10 years (Q10.2, bottom) rather linked to knowledge and organizational issues. The most frequently cited barriers are a lack of awareness about possibilities and potentials (76 %) and insufficient knowledge among engineers and designers (67 %), highlighting deficits in education, training, and knowledge transfer. Economic considerations are another major concern: 43 % of respondents emphasize high investment costs, 41 % point to an unfavorable cost- benefit ratio, and 22 % mention long payback periods. Similarly, 43 % referred to insufficient support or prioritization at the management level, underlining the role of corporate decision-making structures. Structural and institutional factors are also identified, including resistance to change or innovation (35 %), lack of standardization and norms (26 %), and unclear or difficult-to-measure advantages (26 %). By contrast, technological and practical barriers such as insufficient technology and equipment (13 %), lack of supply (11 %), or integration challenges (15 %) appear to play a secondary role, while regulatory or legal restrictions (2 %) and high operating costs (2 %) are of negligible importance (see Figure 12). Overall, the findings suggest that the diffusion of tribological optimization is hindered primarily by limited knowledge, weak management commitment, and economic uncertainties rather than by technological limitations, underscoring the need for targeted information campaigns, training initiatives, and a clearer communication of economic benefits. 4 Research needs Q15: Measures to foster the practical application of tribological optimization. Greater integration of tribo- Science and Research 33 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 6 5 % 7 2 % 7 6 % 7 % 4 % 9 % 0% 20% 40% 60% 80% Optimized lubricants Optimized tribotechnical components (e.g., bearings, seals) Optimized materials or coatings Nothing Unknown Other: 𝑛 13 =45 Figure 11: Tribology Innovations Implemented or in Preparation (Q13) 4 3 % 2 % 2 2 % 4 1 % 1 1 % 7 6 % 6 7 % 1 3 % 1 5 % 4 % 4 1 % 2 6 % 3 5 % 4 3 % 2 6 % 1 3 % 2 % 0% 20% 40% 60% 80% High investment costs High operating costs Payback period Price-benefit ratio Lack of supply Lack of knowledge about potential Lack of knowledge among engineers Lack of or insufficient tech. and equip. Interference with tech./ install. space Efficiency Product acceptance/ importance Lack of standardization and norms Resistance to change or innovation Insufficient support in management Unclear or unmeasurable advantages Lack of suitable training opportunities Regulatory or legal restrictions 𝑛 14 =46 Figure 12: Barriers to the practical implementation of tribological optimization (Q14) are also considered relatively important. Other areas considered less relevant include the performance and temperature range (24 %), information processing and market analysis (15 %), and the modularity and standardization of products and assistance with market integration, both of which are mentioned by 13 % of respondents. In the “Other” category, 11 % of respondents also mention topics such as “chemical interaction,” “improvement of simulation,” “electrical properties of tribological contacts,” “data analysis/ use of AI,” and “research on reusability/ circular economy/ life cycle assessment (LCA 2 ).” Q18: Availability of information on innovative tribological solutions. These results illustrate that many respondents perceive the information available on tribology as needing improvement. Only a smaller proportion of respondents consider it to be good or very good (see Figure 15). Science and Research 34 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 logy into teaching and training (80 %) is regarded as the most effective measure, followed by the publication of studies and best practice examples (63 %) and government funding for implementation (57 %). Companylevel audits and consulting has the lowest score (43 %). The results emphasize that education and demonstrative evidence are key drivers for a wider uptake of tribological optimization (see Figure 13). Q16: Research needs for tribological solutions to enhance energy efficiency. The most frequently cited priority is the optimization of lubricants and lubrication systems, which is identified as a key research need by 67 % of respondents. Improving surface coatings (63 %) and developing new tribological materials and integrating tribological solutions into existing systems are also highly rated, with 61 % of respondents citing each of these areas (see Figure 14). Topics such as knowledge of industrial processes and materials technology (37 %), cost-benefit analysis of investments (35 %) and the expansion of measurement and analysis techniques (28 %) 4 3 % 6 3 % 5 7 % 4 % 0% 10% 20% 30% 40% 50% 60% 70% 80% Audit/ consulting to identify tribological potential within your own company Publication of studies/ best practice examples to illustrate the potential of tribo. optimization Government funding for the implementation of tribological optimization Greater integration of tribology into teaching and training Other 𝑛 15 =45 80% Figure 13: Measures to foster the practical application of tribological optimization (Q15) 6 7 % 6 1 % 6 3 % 2 8 % 6 1 % 1 3 % 2 4 % 1 3 % 1 5 % 3 7 % 3 5 % 1 1 % 0% 20% 40% 60% 80% Optimization of lubricants and lubrication systems Development of new tribological materials Improvement of surface coatings Expansion of measurement and analysis techniques Integration of tribological solutions into existing systems Modularity and standardization of products Performance and temperature range Assistance with market integration Information processing and market analyses Knowledge of industrial processes and technology Cost-benefit analysis of investments Other 𝑛 16 =46 Figure 14: Research needs for tribological solutions to enhance energy efficiency (Q16) 2 Life Cycle Assessment Q20: Representation of tribology in teaching. The current representation of tribology in teaching is viewed critically by the majority. 63 % rate its presence with 2 points, which indicates considerable deficits. Only 4 % of respondents consider its representation to be good or very good (4 or 5 points, see Figure 16). Discussion The survey results reveal a dual narrative: on the one hand, there is strong optimism about the potential of tribological optimization to improve industrial performance. On the other hand, real-world implementation remains hampered by structural knowledge gaps and educational shortcomings. The gap between available scientific insights and their uptake in industry is particularly concerning. Even among technically literate professionals, there appears to be a lack of tools and institutional frameworks for effectively integrating tribological principles into design and maintenance workflows. One promising approach to address this is the integration of tribology into core engineering curricula - both at the university level and in vocational training. Furthermore, cross-sectoral demonstration projects could help validate technologies and create reference points for practitioners. The 10-year forecasts should be interpreted with caution. Actual developments may depend weakly on the research field's own innovative strength. Energy prices, and economic developments, however, will have a strong influence. The survey shows that there is no common consensus among experts regarding the final energy-saving potential across industries. Though, on average, the respondents see an increasing potential in all sectors, but automotive (ø Automotive,now = 3,87; ø Automotive,10a = 3,87). The answers to question 10 may provide an explanation for this. Here, only drivetrains based on combustion engines are expected to have a decreasing potential. Consequently, the participants rate the final energy-saving potential of BEVs 3 and FCEVs 4 significantly lower. Nevertheless, when asked for further areas with untapped potential the responses mention electric vehicles and their drive components multiple times. Thus, the experts still seem uncertain about the energy-saving potential of new drivetrain options. When interpreting the potential, it should also be taken into account that the respondents mainly come from the mechanical engineering, vehicle manufacturing, and metalworking industries. At the same time, mechanical engineering and vehicle manufacturing are considered to have the highest potential in questions 8 and 9. This could be related to the in-depth knowledge of industrystandard processes and plant components. Consequently, Science and Research 35 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 7 % 6 3 % 2 6 % 2 % 2 % 0 % 1 0 % 2 0 % 3 0 % 4 0 % 5 0 % 6 0 % 7 0 % 1 2 3 4 5 𝑛 20 =46 Figure 16: Representation of tribology in teaching (1 = not represented at all; 5 = very well represented, Q20) 2 % 3 2 % 4 3 % 1 4 % 9 % 0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 1 2 3 4 5 𝑛 18 =44 Figure 15: Availability of information on innovative solutions in the field of tribology in general (1 = very bad; 5 = very good, Q18) 3 BEV: Batterie Electric Vehicle 4 FCEV: Fuel Cell Electric Vehicle such as high investment costs (43 %) and an inadequate price-benefit ratio (41 %) were also cited as significant obstacles. A lack of management support (43 %) was likewise identified as a complicating factor. Respondents attach particularly little importance to regulatory hurdles (2 %) and high operating costs (2 %), which indicates that economic and technical challenges play a greater role for them. Measures that could contribute to tribological optimization becoming more widely used in practical applications include greater integration of the topic into teaching and training (80 %). The publication of studies and best practice examples illustrating the potential of tribological optimization in specific products and industries was also considered to be of great importance (63 %). Future research needs. In the field of research related to increasing energy efficiency, respondents see a considerable need for the optimization of lubricants and lubrication systems (67 %), the improvement of surface coatings (63 %), and the development of new tribological materials (61 %). The integration of tribological solutions into existing systems (61 %) is also considered particularly promising. When asked where they see a need for research in the field of tribological solutions outside the area of energy efficiency, service life and wear were mentioned most frequently, with 10 and 8 mentions respectively. Currently popular tools like simulation, digital twins and AI were not mentioned. The existing information available on innovative solutions in the field of tribology in general was rated by most respondents (43 %) with 3 out of 5 points, with a decreasing trend. 48 % of respondents gave the importance of tribology in teaching a rating of 4 out of 5, and 35 % even gave it a rating of 5 out of 5. However, 63 % rated the current representation of tribology in teaching with only 2 points. Here only 2 % of respondents gave ratings of 4 and 5 points. Summary The results show that experts regard tribological optimizations as a means of saving energy, extending service life, and improving product quality in numerous industrial sectors. Nevertheless, obstacles remain, particularly in the areas of expertise and economic efficiency. According to the respondents, future research activities should therefore focus both on the further development and dissemination of tribological technologies in teaching and on the publication of studies and best-practice examples. Acknowledgments The results presented in this paper are based on the joint project “EE4InG2 - Accompanying Scientific Research for Energy Efficiency in Industry and Commerce 2.0” (funding code: 03EN2107B), funded by the German Federal Ministry for Economic Affairs and Energy (BMWE) and managed by the Project Management Jülich (PtJ). The authors gratefully acknowledge the support provided by the BMWE and PtJ. Science and Research 36 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 it is possible that the potential of other industries is underestimated due to a possible lack of knowledge about processes and systems. To obtain a more reliable assessment for underrepresented industries in the future, the respective industry representatives should be contacted directly and encouraged to participate in the survey. Surprisingly, digital trends such as simulation, digital twins, and AI received the lowest approval ratings in question 16. These technologies are regarded and used in many industries as tools for optimization and the development of innovative solutions. The survey results suggest that experts are skeptical about the added value in the field of tribology. In view of the widespread use of digital applications, future surveys should ask about the reasons for this skepticism and the respondent’s knowledge about simulation, digital twins, and AI. Conclusion The results of the expert survey show that tribological optimization is believed to have significant potential for increasing efficiency and sustainability in industry. However, this study also identifies existing obstacles and future research needs, hampering the broader application of tribological optimization technologies to date. Opportunities for tribological optimization. Respondents see significant advantages in tribological optimization, particularly in the areas of energy savings (33 %) and increased component service life (22 %). Aspects such as improved product quality (9 %) and greater environmental friendliness (4 %) were also highlighted. Particularly high energy-saving potential is seen in mechanical engineering (39 % high potential, 22 % very high potential) and vehicle manufacturing (37 % high potential, 33 % very high potential). Relevant savings potential is also expected for other industries such as the chemical industry, metal processing and the plastics industry. In all areas except vehicle manufacturing saving potentials were estimated to be higher ten years from now. However, it should be considered, that the knowledge of these areas might be limited, as most respondents have a background in mechanical engineering and automotive. The saving potentials for alternative propulsion systems, such as electromobility and hydrogen-based drive systems are also expected to be high. At 40 %, the majority of respondents rated the current final energy-saving potential through tribological optimization in production areas known to them at 4 out of 5 points. The most frequently used tribology innovations of the last ten years (76 %), which are already in use or in preparation among the respondents, are optimized materials or coatings. Barriers to tribological optimization. According to the respondents, the biggest challenges to the spread of tribological optimizations include a lack of knowledge about technological potentials (76 %) and a lack of expertise among engineers and designers (67 %). Economic factors Science and Research 37 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 Attachment: Results raw data Q02 In welchem Industriezweig ist Ihr Unternehmen tätig? [ Mehrfachnennungen möglich ] Nahrungs- und Futtermittelindustrie 1 Textil- und Bekleidungsindustrie 0 Holz- und Papierindustrie 0 Chemische Industrie 3 Kunststoff- und Gummiindustrie 1 Glas- und Keramikindustrie 1 Metallindustrie 7 Maschinenbau 10 Elektroindustrie 0 Fahrzeugbau 8 Sonstiges verarbeitendes Gewerbe 0 Summe = 19 Q03 In welcher fachlichen Rolle/ Abteilung sind Sie in Ihrem Unternehmen tätig? Forschung und Entwicklung (F&E) 18 Marketing/ Kommunikation 1 Geschäftsführung/ Management 0 Summe = 19 Q01 In welchem Bereich ordnen Sie sich aus fachlicher Perspektive primär ein? Industrie und Gewerbe 19 Wissenschaft/ Forschung 21 Beratungswesen/ Dienstleistung 4 Öffentlicher Sektor/ Behörden 1 Sonstiges: Weiterbildung 1 Summe= 46 Q04Welche Perspektive trifft auf Ihr Unternehmen primär zu? Hersteller von Produkten/ Lösungen zur Optimierung tribologischer Systeme 12 Anwender von Produkten/ Lösungen zur Optimierung tribologischer Systeme 7 Summe = 19 Science and Research 38 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 Q05 Welche Kategorie trifft auf Ihr Unternehmen zu? [ Mehrfachnennungen möglich ] Entwicklung von Produkten mit tribologischen Anforderungen (z.B. Maschinen- und Anlagenbau, Automotive) 16 Entwicklung von Schmierstoffen 6 Entwicklung tribotechnischer Komponenten (z.B. Lager, Dichtungen) 5 Herstellung von tribologisch optimierten Werkstoffen oder Beschichtungen 7 Wartung und Instandhaltung von Anlagen mit tribologischen Komponenten 3 Prüfdienstleistungen und Testverfahren für tribologische Systeme 1 Beratung und Optimierung tribologischer Prozesse 5 Summe = 19 Q06 Wie schätzen Sie Ihre Kenntnisse zum Fachgebiet Tribologie ein? Keine - Keine Kenntnisse oder Erfahrungen im Bereich Tribologie 0 Gering - Grundkenntnisse ohne praktische Erfahrung 1 Durchschnittlich - Grundlegende Kenntnisse, aber begrenzte praktische Erfahrung 2 Gut - Solide Kenntnisse mit einigen praktischen Erfahrungen in Tribologie 11 Sehr gut - Umfassende Kenntnisse und praktische Erfahrung in Tribologie 32 Summe = 46 Q07 Wo sehen Sie die größten Nutzeneffekte durch eine tribologische Optimierung? Erhöhung der Lebensdauer 10 Verbesserung der Funktionalität unter extremen Bedingungen 4 Verbesserte Produktqualität 4 Energieeinsparung 15 Reduzierung von Ausfallzeiten 2 Leistungssteigerung 5 Kosteneffizienz bei der Produktion 1 Kostenreduktion 3 Umweltfreundlichkeit 2 Summe = 46 Science and Research 39 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 Q08 Bewerten Sie das vorliegende Endenergieeinsparpotenzial durch tribologische Optimierung in einzelnen Industriezweigen auf einer Skala von 1 (keine Bedeutung) bis 5 (sehr große Bedeutung). [ Mehrfachnennungen möglich ] aktuell. Skalenpunkte: 1 2 3 4 5 Summe Maschinenbau 0 5 13 18 10 46 Fahrzeugbau 1 5 8 17 15 46 Chemische Industrie 0 15 17 9 2 43 Metallindustrie 1 13 14 13 3 44 Kunststoff- und Gummiindustrie 1 14 12 12 4 43 Glas- und Keramikindustrie 4 16 12 8 2 42 Elektroindustrie 3 14 10 11 4 42 Nahrungs- und Futtermittelindustrie 6 18 9 6 2 41 Textil- und Bekleidungsindustrie 8 14 12 6 1 41 Holz- und Papierindustrie 3 12 17 6 3 41 Sonstiges verarbeitendes Gewerbe 2 13 18 0 1 41 Q08 Bewerten Sie das vorliegende Endenergieeinsparpotenzial durch tribologische Optimierung in einzelnen Industriezweigen auf einer Skala von 1 (keine Bedeutung) bis 5 (sehr große Bedeutung). [ Mehrfachnennungen möglich ] in 10 Jahren. Skalenpunkte: 1 2 3 4 5 Summe Maschinenbau 1 3 8 20 14 46 Fahrzeugbau 3 3 11 9 20 46 Chemische Industrie 1 11 16 9 6 43 Metallindustrie 2 13 11 12 6 44 Kunststoff- und Gummiindustrie 3 10 6 16 8 43 Glas- und Keramikindustrie 6 13 9 12 2 42 Elektroindustrie 2 13 9 11 7 42 Nahrungs- und Futtermittelindustrie 6 14 11 8 2 41 Textil- und Bekleidungsindustrie 6 15 11 7 2 41 Holz- und Papierindustrie 3 14 11 8 5 41 Sonstiges verarbeitendes Gewerbe 2 7 18 10 4 41 Science and Research 40 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 Q09 Bewerten Sie das vorliegende volkswirtschaftliche Potenzial durch tribologische Optimierung in einzelnen Industriezweigen auf einer Skala von 1 (geringes Potenzial) bis 5 (sehr großes Potenzial). [ Mehrfachnennungen möglich ] aktuell. Skalenpunkte: 1 2 3 4 5 Summe Maschinenbau 2 3 6 15 17 43 Fahrzeugbau 3 3 9 15 14 44 Chemische Industrie 2 9 19 6 5 41 Metallindustrie 2 10 9 9 11 41 Kunststoff- und Gummiindustrie 2 14 6 11 6 39 Glas- und Keramikindustrie 3 15 8 8 5 39 Elektroindustrie 3 15 7 7 6 38 Nahrungs- und Futtermittelindustrie 5 15 7 7 5 39 Textil- und Bekleidungsindustrie 6 12 9 8 4 39 Holz- und Papierindustrie 3 13 12 6 5 39 Sonstiges verarbeitendes Gewerbe 2 12 13 6 7 40 Q09 Bewerten Sie das vorliegende volkswirtschaftliche Potenzial durch tribologische Optimierung in einzelnen Industriezweigen auf einer Skala von 1 (geringes Potenzial) bis 5 (sehr großes Potenzial). [ Mehrfachnennungen möglich ] in 10 Jahren. Skalenpunkte: 1 2 3 4 5 Summe Maschinenbau 2 2 6 14 19 43 Fahrzeugbau 4 4 11 11 14 44 Chemische Industrie 2 10 15 7 6 41 Metallindustrie 2 8 12 8 11 41 Kunststoff- und Gummiindustrie 2 10 5 13 10 39 Glas- und Keramikindustrie 3 13 9 10 4 39 Elektroindustrie 2 13 8 9 6 38 Nahrungs- und Futtermittelindustrie 4 12 10 8 5 39 Textil- und Bekleidungsindustrie 5 11 10 7 6 39 Holz- und Papierindustrie 3 10 13 6 7 39 Sonstiges verarbeitendes Gewerbe 1 10 11 8 9 40 Q10 Bewerten Sie das vorliegende Endenergieeinsparpotenzial durch tribologische Optimierung in verschiedenen Mobilitätsanwendungen auf einer Skala von 1 (kein Potenzial) bis 5 (sehr großes Potenzial). [ Mehrfachnennungen möglich ] aktuell Skalenpunkte: 1 2 3 4 5 Summe Fahrzeuge mit Verbrennungsmotoren 1 5 8 21 10 45 Fahrzeuge mit Elektromobilität 1 5 12 12 15 45 Fahrzeuge mit Wasserstoffantrieben 4 6 9 11 14 44 Sonstige Mobilitätsanwendungen 1 6 11 15 9 42 Science and Research 41 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 Q10 Bewerten Sie das vorliegende Endenergieeinsparpotenzial durch tribologische Optimierung in verschiedenen Mobilitätsanwendungen auf einer Skala von 1 (kein Potenzial) bis 5 (sehr großes Potenzial). [ Mehrfachnennungen möglich ] in 10 Jahren Skalenpunkte: 1 2 3 4 5 Summe Fahrzeuge mit Verbrennungsmotoren 7 8 6 10 14 45 Fahrzeuge mit Elektromobilität 2 6 7 13 17 45 Fahrzeuge mit Wasserstoffantrieben 2 6 4 13 19 44 Sonstige Mobilitätsanwendungen 2 3 10 15 12 42 Q12 Bewerten Sie wie hoch das vorliegende Endenergieeinsparpotenzial durch tribologische Optimierung in Ihnen bekannten Produktionsbereichen auf einer Skala von 1 (geringes Anwendungspotenzial) bis 5 (sehr großes Anwendungspotenzial) ist. • Sport • Sportgeräte • Fettschmierung • Schmierung und Beschichtungsverfahren/ arten in Wasserstoffanwendungen (z.B. H2- Verbrennungsmotoren) • Logistik • Flugtriebwerksbau • Airspace • Mechatronische Systeme • Getriebe (2x genannt) • Pumpen (2x genannt) • Hydraulische Pumpen • Wälzlager • Windkraft Produkte mit geringen Stückzahlen (2x genannt) • Automobilindustrie • Automobile • Off Road Fahrzeuge • E-Fahrzeuge • Thermomanagement bei E-Fahrzeugen • Schmierung in der Elektromobilität • Antriebsstränge • Antriebstechnik • Electric motors • Industriemotoren • Schienenverkehr (3x genannt) • Bearbeitungsmaschinen • Produktionsanlagen • Fertigungsprozesse, insbesondere Umformen Metallbe- und -verarbeitung • Polymere Dichtungen • Polymere Werkstoffe sowohl in der Antriebstechnik als auch in der generellen Kraftübertragung • Kompositwerkstoffe (metallischer und/ oder polymerer Herkunft) in Kompressoren, Ventilen und neu erschließbaren Anwendungen • Kompressoren • Gaskompressoren • Kolbenmaschinen aller Art (Kälte-, Wärme-, Klimaanlagen ...) • Strömungsmaschinen, z.B. Klimageräte • Klimageräte • E-Fuels Q12 Bewerten Sie wie hoch das vorliegende Endenergieeinsparpotenzial durch tribologische Optimierung in Ihnen bekannten Produktionsbereichen auf einer Skala von 1 (geringes Anwendungspotenzial) bis 5 (sehr großes Anwendungspotenzial) ist. 1 4 2 4 3 10 4 18 5 9 Summe = 45 Science and Research 42 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 Q14 Welche Hemmnisse sehen Sie hinsichtlich einer Verbreitung tribologischer Optimierung in der Praxis? Hohe Investitionskosten 20 Hohe Betriebskosten 1 Amortisationszeit 10 Preis-Benefit-Verhältnis 19 Fehlendes Angebot 5 Mangel an Wissen über Möglichkeiten/ Potenzial 35 Fehlendes Wissen der Ingenieure/ Konstrukteure 31 Fehlende oder unzureichende Technologie und Ausrüstung 6 Eingriff in Technik/ Bauraum 7 Wirkungsgrad 2 Produktakzeptanz/ Stellenwert 19 Fehlende Standardisierung und Normen 12 Widerstand gegenüber Veränderung oder Innovation 16 Unzureichende Unterstützung oder Priorität im Management 20 Unklare oder nicht messbare Vorteile 12 Mangel an geeigneten Schulungs- und Weiterbildungsangeboten 6 Regulatorische oder rechtliche Einschränkungen 1 n_14 = 46 Q13 Nennen Sie Tribologie-Innovationen der letzten 10 Jahre, die bei Ihnen bereits im Einsatz sind oder deren Einsatz bei Ihnen bereits in laufender Vorbereitung ist. n= Optimierte Schmierstoffe 30 46 Optimierte tribotechnische Komponenten (z.B. Lager, Dichtungen) 33 46 Optimierte Werkstoffe oder Beschichtungen 35 46 Nichts 3 46 Nicht bekannt 2 46 Sonstiges: (Oberflächenstrukturierungen, Triboconditioning, schmierstofffreie Kunststofflager, Prüftechniken & -methoden) 4 46 Science and Research 43 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 Q15 Welche Maßnahmen können dazu beitragen, dass tribologische Optimierung verstärkt Einzug in die praktische Anwendung finden? Audit/ Beratung zur Identifikation tribologischer Potenziale im eigenen Unternehmen 20 Veröffentlichung von Studien / Best-Practice-Beispielen, welche die Potenziale einer tribologischen Optimierung in konkreten Produkten / Branchen verdeutlichen 29 Staatliche Förderung zur Umsetzung tribologischer Optimierung 26 Verstärkte Integration des Themenfeldes Tribologie in Lehre und Ausbildung 37 Sonstiges: (Wirtschaftlichkeit tribologischer Optimierungen verbessern, Generationswechsel befördern) 2 n_14 = 46 Q16 Wo sehen Sie Forschungsbedarf im Bereich tribologischer Lösungen in Bezug auf die Steigerung der Energieeffizienz? Optimierung von Schmierstoffen und -systemen 31 Entwicklung neuer tribologischer Materialien 28 Verbesserung der Oberflächenbeschichtungen 29 Erweiterung der Mess- und Analysetechniken 13 Integration von tribologischen Lösungen in bestehende Systeme 28 Modularität und Standardisierung von Produkten 6 Leistungs- und Temperaturbereich 11 Hilfe zur Marktintegration 6 Informationsaufbereitung und Marktanalysen 7 Kenntnisse über Industrieprozesse und Werkstofftechnik 17 Kosten-Nutzen-Analyse von Investitionen 16 Sonstiges: (Chemische Wechselwirkung, Elektrische Eigenschaften tribologischer Kontakte, Verbrsserung der Simulation, Datenanalyse/ KI-Einsatz, Forschung zur Wiederverwendbarkeit/ Kreislaufwirtschaft/ LCA) 5 n_16 = 46 Science and Research 44 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 Q17: Wo sehen Sie Forschungsbedarf im Bereich tribologischer Lösungen außerhalb des Themenfeldes Energieeffizienz? • Tieftemperaturanwendung • Geringe Reibung: Super Schmierstoffe • Weltraumanwendung (2x genannt) • Extreme Bedingungen • Produktionsrelevante Tribologie (Gesamtkostenanalyse) • Restlebensdauer • Vorhersagbarkeit eines Ausfalls • Fehlerbzw. Ausfallmechanismen • Erweiterung bestehender Leistungsgrenzen • Normung, Standardisierung und Harmonisierung tribologischer Charakterisierungen, Daten und Methodiken • Schmierstofflebensdauermodelle (sowohl flüssig, als auch Fette) • Aufbau einer öffentlich zugänglichen Ontologie zur Repräsentierung tribologischer Systeme • Entwicklung und Testung tribologischer Lösungen unter realen Anwendungsbedingungen (Berücksichtigung von Schmutz, Alterung und Missbrauchslasten) • Ermittlung von geeigneten Messverfahren zur Bestimmung von Stoffeigenschaften tribologischer Komponenten (Material und Schmierstoff) • Restlebensdauer • Vorhersagbarkeit eines Ausfalls • Weiterentwicklung von Simulationsmethoden • Lebensdauer (10x genannt) • Verschleißverhalten von neuen und im Einsatz befindlichen Materialien • Verschleiß (-minimierung) (8x genannt) • Verschleißminimierung bei hohen Temperaturen • Kreislaufwirtschaft (2x genannt) • Ressourceneffizienz • Nachhaltigkeit und Kreislaufwirtschaft von Schmierstoffen • sparsamer Umgang mit Schmierstoffen • CO2 Emission • LCA (2x genannt) • Leistungsfähigkeit von Recyclaten • Nachhaltige Werk- und Schmierstoffe • Kombination nachhaltiger Schmierstoffe mit Konstruktionselementen • Materialsubstitution (z.B. PEFAS- und Cr6-Verbot, etc.) • Ressourceneffizienz • Berücksichtigung von Cradle-to-Cradle während der Produktentwicklung • Parasitärer Stromdurchgang in Windenergieanlagen und Elektroautos • tribokorrosive Beanspruchung identifizieren + Optimierung entwickeln • Wälz- und Gleitkontakte mit Stromdurchgang • Tribokorrosion • Interaktion zwischen Schmierstoffen und Beschichtungen • Elektrisch induzierte Ströme und Schäden Q18 Bewerten Sie wie gut Ihrer Einschätzung nach das Informationsangebot zu innovativen Lösungen im Bereich Tribologie im Allgemeinen ist. 1 1 2 14 3 19 4 6 5 4 n_18 = 44 Science and Research 45 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0027 Q19 Wie wichtig schätzen Sie die Bedeutung der Tribologie in der Lehre ein? Bitte bewerten Sie auf einer Skala von 1 (keine Bedeutung) bis 5 (sehr große Bedeutung). 1 0 2 2 3 6 4 22 5 16 n_19 = 46 Q20 Wie gut ist die Tribologie Ihrer Meinung nach aktuell in der Lehre repräsentiert? Bitte bewerten Sie auf einer Skala von 1 (gar nicht vertreten) bis 5 (sehr gut vertreten). 1 3 2 29 3 12 4 1 5 1 n_20 = 46 Literature Gesellschaft für Tribologie e.V (2002): Tribologie. GfT-Arbeitsblatt 7. Verschleiß, Reibung. Definitionen, Begriffe, Prüfung. Online verfügbar unter https: / / gjetc.org/ wp-content/ uploads/ 2023/ 01/ Topical-Paper-Waste-Heat.pdf, zuletzt geprüft am 31.07.2025. Woydt, Mathias; Bock, Eberhard; Hosenfeldt, Tim; Bakolas, Vasilios; Luther, Rolf; Wincierz, Christoph (2023): Wirkungen der Tribologie auf die CO2-Emissionen in der Nutzungsphase von Produkten. Beiträge der Tribologie zur Defossilisierung. Hg. v. Gesellschaft für Tribologie e.V. Online verfügbar unter https: / / www.gft-ev.de/ wp-content/ uploads/ GfT-Studie-Wir kungen-der-Tribologie.pdf, zuletzt geprüft am 12.08.2025. Woydt, Mathias; Gradt, Thomas; Hosenfeldt, Tim; Luther, Rolf; Rienäcker, Adrian; Wetzel, Franz-Josef; Wincierz, Christoph (2019): Tribologie in Deutschland: Querschnittstechnologie zur Minderung von CO2-Emissionen und zur Ressourcenschonung. Hg. v. Gesellschaft für Tribologie e.V. Online verfügbar unter https: / / www.gft-ev.de/ wp-content/ uploads/ GfT-Studie-Tribologie-in-Deutschland.pdf, zuletzt geprüft am 12.08.2025. Woydt, Mathias; Hosenfeldt, Tim; Luther, Rolf; Scholz, Christian; Bäse, Mirjam; Wincierz, Christoph; Schulz, Joachim (2021): Tribologie in Deutschland: Verschleißschutz und Nachhaltigkeit als Querschnittsherausforderungen. Hg. v. Gesellschaft für Tribologie e.V. Online verfügbar unter https: / / www.gft-ev.de/ wp-content/ uploads/ GfT-Studie-Ver schlei%C3%9Fschutz-und-Nachhaltigkeit.pdf, zuletzt geprüft am 12.08.2025. Science and Research 46 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 MEDIENTIPP Marcus Schulz, Tinka Meier Project Management A Practical Guideline for Today’s Project Managers 3 rd edition 2025, 258 Seiten ISBN print 978-3-381-13481-6 ISBN eBook 978-3-381-13482-3 DOI 10.24053/ 9783381134823 Ladenpreis print €[D] 44,99 Ladenpreis eBook €[D] 35,99 Recommended for non-German speaking IPMA certification candidates, for work package managers, as well as experienced project managers For years, advocates of professional project work have stressed the growing shift towards project-oriented work structures. This has now become a reality in the daily routines of many employees and managers. Consequently, strong project management skills are becoming increasingly vital to business success. Following the five project management phases of DIN 69901: 2009 and supplemented by chapters on crossphase competencies and agile methods, this book offers a clear and professionally sound presentation of the modernised ICB 4.0 framework (effective from January 1, 2024). Its structured content, illustrated by a consistent project example, not only guides readers but also ensures they are well-prepared to meet the IPMA ICB 4.0 examination requirements. This book equips anyone seeking to engage in professional project management with the knowledge and tools needed to successfully apply current best practices. UVK Verlag - Ein Unternehmen der Narr Francke Attempto Verlag GmbH + Co. KG Dischingerweg 5 \ 72070 Tübingen \ Germany \ Tel. +49 (0)7071 97 97 0 \ info@narr.de \ www.narr.de News 47 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 GfT award Disseration 2025 1 Introduction Despite the optimal adaptation of the natural biotribological system knee joint to its physiological environment, progressive joint wear may necessitate the implantation of an artificial knee joint. On the one hand, such endoprosthetic procedures are increasingly common due to the aging population worldwide. On the other hand, younger, active individuals also require endoprosthetic treatment at an early stage due to above-average intense sporting activity. Especially in younger, active patients, the service life of an artificial knee joint is insufficient when subjected to increased stress. Accordingly, implant-associated wear, together with the particles worn off, can be made responsible for premature prosthesis failure due to aseptic prosthesis loosening as a result of osteolysis. Reducing wear can be achieved, for example, by using low-wear material combinations or surface coatings. The application of biotribologically effective amorphous carbon (diamond-like carbon, DLC) to both articulating surfaces of a metallic-polymeric total knee replacement (TKR) is a promising approach to enhancing wear resistance. [I 1 ] Despite the significant progress that has been made with regard to endoprostheses and implant materials, challenges remain with respect to the interaction between articulating surfaces and the interaction of released particles and wear products with the human body, thus requiring further development. In the following section 2, the need for action, the objectives within the research questions, and the corresponding approach for successfully clarifying the research questions are shown. [I] Amorphous carbon coatings for extending the service life of total knee replacements Benedict Rothammer* The topic was submitted for the GfT Sponsorship Award 2025 in the category “dissertation or similar theses”. The award took place at the GfT conference in September 2025. The durability of a total knee endoprosthesis - especially in younger, active patients - is due to increased higher stresses insufficient. Accordingly, implantassociated wear particles lead to aseptic endoprosthesis loosening as a result of osteolysis and thus to premature failure of the endoprosthesis. Amorphous carbon coatings can meet the high requirements for implants. They combine tribologically effective behavior with high biocompatibility and excellent mechanical properties. The focus was on the application and the mechanical-biotribological preclinical in vitro investigation of amorphous carbon coatings on metallic/ polymeric articulation partners of endoprostheses. Using rheological investigations and numerical simulations, an application-related, biotribological test chain consisting of a ball-on-three-pin tribometer, pin-on-disk tribometer and fully kinematic knee simulator was established. This test chain allowed a comprehensive wear analysis of numerous contact combinations in a reasonable time with respect to dominant wear mechanisms, typical particle sizes and morphologies, as well as their correlation across the test links. In all test links, the biotribological effectiveness of the amorphous carbon coatings was shown, indicating a significant increase in the service life of endoprostheses. Keywords Service Life, TKRs (Total Knee Replacements), Amorphous Carbon Coatings, Coating Hardness, Biotribological Performance, Knee Simulation, Wear Protection Abstract * Dr. Benedict Rothammer Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) Lehrstuhl für Konstruktionstechnik KTmfk Martensstraße 9 | 91058 Erlangen 1 Since this contribution is a presentation of the author’s dissertation for the GfT Award in Category 1, this contribution refers to the original literature, which is clearly cited in the dissertation at the relevant points. News 48 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 2. Need for action, objectives, methodology There are no sufficiently detailed descriptions of coating processes or follow-up medical studies in literature. Nevertheless, from the literature already published, it is clear that amorphous carbon coatings have been used successfully in preclinical studies for load-bearing implants. This is mainly due to the excellent physical, chemical, and structural properties of amorphous carbon coatings, which enable biocompatibility and wear protection when sufficient adhesion to the substrate is ensured. [I] Currently, femoral components made of cobalt-chromium-molybdenum alloys such as Co28Cr6Mo (CoCr) are the standard, although titanium alloys such as Ti6Al4V (Ti64) in combination with tribologically effective amorphous carbon coatings may replace CoCr as a substrate material in the intermediate-term. However, understanding the biotribological system better than in the past is crucial to the reliable use of these coatings in systems subjected to high mechanical and biotribological stress, such as artificial knee joints in particular. Based on this, the coating adhesion needs to be improved to avoid the cell biological consequences of coating delamination. It’s also vital to do a comprehensive investigation of the mechanical properties, cell biological behavior, and biotribological behavior of the amorphous carbon coatings on metallic and polymeric specimens and on components of TKRs. In my research, I intend to transfer the progress made in recent years at the model level to more application-relevant test conditions and subsequently to real components of TKRs. [I] The long-term goal is to significantly extend the in vivo service life of knee implants made of Ti64 and ultrahigh molecular weight polyethylene (UHMWPE) compared to the state of the art by using tribologically effective, biocompatible amorphous carbon coating systems, thus creating an advantageous alternative to existing CoCr 2 implants. Furthermore, a coating on the polymeric tibial inlay should minimize wear-related, aseptic TKR loosening while providing high biocompatibility. In particular, the main weakness of existing carbon coatings, i.e., insufficient coating adhesion for load-bearing implants, should be remedied. For this reason, amorphous carbon coatings are applied to metallic and polymeric implant surfaces - i.e., CoCr, Ti64, and UHMWPE - using plasma-enhanced chemical vapor deposition (PECVD) and physical vapor deposition (PVD) technologies. Requirements regarding biocompatibility, mechanical properties, good coating adhesion, and desired tribological behavior must be met. Additionally, the key factors influencing biocompatibility, coating adhesion, and tribological behavior need to be identified and their interactions understood. [I] With regard to a framework for future coating development processes for implants, a systematic investigation should ultimately be conducted to determine the extent to which, under what stresses, and under what environmental conditions during preclinical in vitro testing, simpler model tests and more complex component tests can provide reliable information on wear behavior. Consequently, the following key research question: [I] Do polymeric and metallic contacts coated with amorphous carbon in TKRs significantly improve biotribological effectiveness compared to conventional (uncoated) metallic-polymeric contacts? [I] and derived sub-research questions regarding the following topics had to be answered: 1. Substitute medium: Which rheological properties are required for a suitable artificial synovial fluid that is as similar as possible to natural synovial fluid in order to shorten application-oriented or person-specific, biotribological investigations of implant materials at various levels of abstraction, such as the model level, under peri-endoprosthetic boundary conditions? [I] 2. Loading and stresses: Can application-oriented or person-specific stresses be predicted with the aid of a musculoskeletal biomechanic simulation and an elastohydrodynamic simulation and can these be transferred to biotribological investigations of implant materials at different levels of abstraction, such as the model level, under peri-endoprosthetic boundary conditions? [I] 3. Coating development: Can amorphous carbon coatings be designed in such a way that they meet the requirements for biocompatibility, mechanical properties, coating-substrate adhesion and thus tribological behavior and can be applied to polymeric and metallic implant materials by means of plasma coating? [I] 4. Biotriobology: For which contact partners can an improvement in the biotribological effectiveness of amorphous carbon coatings compared to conventional implant materials be already demonstrated in model tests and can this be transferred to more complex knee wear simulator tests? [I] The approach in the scope of this work was characterized by the close interlocking between coating development of biocompatible, low-defect, mechanically excellent and adherent amorphous carbon coatings and biotribological preclinical in vitro testing. Prior to establishing a 2 Ti64 is considerably preferable to CoCr in terms of high biocompatibility, but the unfavorable tribological behavior of Ti64 excludes its use as a direct sliding partner. News 49 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 tribological test chain, the periendoprosthetic environmental conditions as well as the in vivo contact conditions and stresses had to be adequately represented. Therefore, in this work, the artificial synovial fluid had to be primarily investigated for its rheological suitability and proper load collectives had to be derived by means of a musculoskeletal simulation coupled with an elastohydrodynamic simulation for an application-like biotriobological testing. The tribological testing followed a stringent chain of tribological testing procedures, combining a ball-on-three-pins tribometer, a pin-on-disk tribometer and a fully kinematic knee simulator. This allowed for a comprehensive wear analysis of numerous contact pairings in a reasonable time with respect to prevailing wear mechanisms, typical particle sizes and morphologies as well as their correlation across the links of the test chain. Thus, the biotribological interactions could be described integrally. The approach pursued in this PhD thesis to answer the research questions and thus to increase the service life of TKRs by coating them with amorphous carbon is illustrated in Figure 1. [I] Initially, amorphous carbon coatings were applied to metallic and polymeric implant surfaces by PVD and PECVD processes. The biotribologically effective amorphous carbon coatings were developed to exhibit good mechanical properties, in particular sufficiently good adhesion, as a fundamental requirement for further examinations with regard to their biocompatibility and tribological behavior. If, additionally, the coating proved suffi- Figure 1: Procedure for extending the service life of TKRs by coating with amorphous carbon and classification of the corresponding publications. [I] © B. Rothammer News 50 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 ciently good biocompatibility and tribologically favorable behavior in ball-on-three-pins tests, it could be used for further tribological tests in the pin-on-disk tribometer and also in the knee simulator. The comparison with the uncoated references allowed for a classification of the mechanical properties, the biocompatibility and the biotribological effectiveness of the coating. The ability to test a large number of coatings with regard to their biotribological behavior in the first two links of the test chain also enables the extent to which and the factors influencing this behavior to be determined. The sequence of different stages - from coating to mechanical characterization to biotribological testing - initially shown linearly in Figure 1 allows for iterations at the individual “gates” (blue boxes). For instance, testing the biocompatibility or adhesion of a first series of coatings can provide insights that result in the adjustment of coating parameters to produce a beneficial second series of coatings. [I] 3 Summary of results and discussion Amorphous carbon coatings can make a significant contribution to increasing the service life of TKRs. These coatings meet the requirements for implant materials and surfaces as well as for their cell biological environment. Meaning, they combine tribologically effective behavior with high biocompatibility and excellent mechanical properties. Due to the close link between the mechanical properties, cell biological and tribological behavior, and the characteristic chemical-structural properties, these properties of amorphous carbon coatings can be influenced by selectively adjusting the coating process parameters. Therefore, the overarching, longterm goal was to significantly extend the in vivo service life of Ti64 and UHMWPE knee implants compared to the state of the art by using tribologically effective, biocompatible amorphous carbon coating systems. This should create an attractive alternative to existing CoCr implants. In particular, research groups dealing with issues relating to the specific influencing of friction and wear reduction of amorphous carbon coatings for TKRs should be provided with an understanding of the biotribological mechanisms involved. An efficient framework for the design of coated metallic femoral shields and polymeric tibial inlays was made available for this purpose. The focus was on the application and mechanical-biotribological investigation of amorphous carbon coatings on endoprosthetic materials and TKRs. [I] The approach within the scope of my work was characterized by the close interlinking of coating development of biocompatible, defect-free, mechanically excellent, and adherent amorphous carbon coatings and biotribological preclinical in vitro testing. Prior to establishing a tribological testing chain, it was necessary to adequately simulate the periendoprosthetic environmental conditions as well as the in vivo contact conditions and stresses. [I] Therefore, in this work, the artificial synovial fluid had to be primarily investigated for its rheological suitability and proper load collectives had to be derived by means of a musculoskeletal simulation coupled with an elastohydrodynamic simulation for an application-like biotribological testing. The tribological testing followed a stringent chain of tribological testing procedures, combining a ball-on-three-pins tribometer, a pin-on-disk tribometer and a fully kinematic knee simulator. This allowed for a comprehensive wear analysis of numerous contact pairings in a reasonable time with respect to prevailing wear mechanisms, typical particle sizes and morphologies as well as their correlation across the links of the test chain. Thus, the biotribological interactions could be described integrally. [I] Initially, amorphous carbon coatings were applied to metallic and polymeric implant surfaces by PVD and PECVD processes. The biotribologically effective amorphous carbon coatings were developed to exhibit good mechanical properties, in particular sufficiently good adhesion, as a fundamental requirement for further examinations with regard to their biocompatibility and tribological behavior. If, additionally, the coating proved sufficiently good biocompatibility and tribologically favorable behavior in ball-on-three-pins tests, it could be used for further tribological tests in the pin-on-disk tribometer and also in the knee simulator. The comparison with the uncoated references allowed for a classification of the mechanical properties, the biocompatibility and the biotribological effectiveness of the coating. [I] Derived from the first sub-research question concerning a suitable substitute medium for tribological testing, the influence of temperature, shear rate and pressure on the density and viscosity of an artificial synovial fluid was investigated in Pub I by means of rheological measurements. Thereby, a temperature dependence of both the density and the viscosity could be observed with both values decreasing at higher temperatures. The temperature dependence of the viscosity within the range from room to human core temperature could be adequately approximated by an ARRHENIUS model. Furthermore, shear-thinning properties could be demonstrated, allowing for the synovial fluid to be simulated and fitted well to a CROSS model, which has also been described in studies on human synovial fluid in the literature. However, the non-NEWTONian behavior was more pronounced at lower temperatures and less pronounced at higher temperatures, the latter of which being more relevant to the human body. An anomaly in the pressure dependence of the viscosity was also present, which correlates with the behavior of pure water as the main constituent. At lower temperatures, viscosity first decreased to a minimum and then increased again at higher pressures. Although further research is needed on the influences of the individual components of the artificial synovial fluid, it can be concluded that the examined synovial fluid can be used for wear tests of total joint replace- News 51 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 ments and for comparing different implant materials, since the most important properties of the human synovial fluid are adequately mimicked. The rheological data as well as the adjustments to the model provide useful formulas for numerical modeling of similar fluids or for studies in different environments. Since most rheological tests on synovial fluid behavior and many tribological experiments on the performance of total joint replacements are performed under ambient conditions, Pub I emphasized the importance of realistic test conditions in terms of temperatures, pressures and shear rates to ensure or even shorten transferability and comparability. [I] Fundamental knowledge about the in vivo contact conditions at the articulating surfaces of TKRs for predicting and optimizing the behavior of implant systems could be obtained in Pub II, answering the second subresearch question. The contact stresses prevailing in TKRs cannot be accurately determined by conventional in vivo measurement methods. In turn, in silico modeling allowed the prediction of loads, velocities, deformations, stresses, and lubrication conditions across the scales during an entire gait cycle. Thus, in this work, a combination of musculoskeletal and tribocontact modeling was performed. In a first step, reliable contact forces were calculated, with low kinematic errors and very low residual forces and torques, which showed sufficient correlation with literature data, even though some of the values were higher. This could be explained by the approach of this investigation. The aim was to determine the contact forces during healthy/ physiological gait of young subjects rather than the best possible reproduction of the forces measured in vivo in much older subjects performing a non-physiological gait. In a second step, the derived data were used as input data for an elastohydrodynamic model to predict subject-specific lubrication conditions, contact pressures, deformations, and stresses. Thus, Pub II has the potential to further stimulate and accelerate research and optimization of the biomechanical and biotribological behavior of artificial synovial joints, such as TKRs. [I] In the context of the third sub-research question, several investigations on the coating topography and structure, cell-biological interaction, mechanical properties and coating-substrate adhesion of (tetrahedral) amorphous carbon coatings on Ti64, CoCr and UHMWPE substrates were conducted in Pub III - V. The coatings exhibited a morphology and composition typical of (tetrahedral) amorphous carbon coatings. The roughness of the coatings was higher than that of the substrates, especially on UHMWPE. Initial studies with contact angles and surface tensions as well as indirect and direct in vitro biocompatibility tests on (tetrahedral) amorphous carbon coatings showed comparable behavior to the substrates. The surface modifications exhibited no cytotoxic effects, confirming the potential for biomedical application. The developed coatings proved excellent mechanical properties with a substantial improvement in indentation hardness/ indentation modulus (H ITx / E ITy ) ratios, indicating favorable biotribological wear behavior. The adhesion of the coatings to the substrates Ti64, CoCr and UHMWPE could be considered at least sufficient - to very good - for use in TKRs. The findings lead to the assumption that the coatings presented in this work are capable of outperforming uncoated metallic-polymeric reference pairings under biotribological loading. [I] The assumption derived from the third sub-research question was being tested in Pub IV - VI and PrePub to answer the fourth sub-research question regarding the biotribological effectiveness of amorphous carbon coatings. In this context, different test categories were used to confirm the biotribological effectiveness of amorphous carbon coatings in terms of a significant increase in the service life of TKRs. In general, the favorable biotribologically effective behavior of amorphous carbon coatings could be successfully demonstrated across the links of the tribological test chain, from simple model tests to complex knee simulator tests, and transferred between the links individually. The possibility of testing numerous coatings with respect to their biotribological behavior in the first two links of the test chain (ball-onthree-pins and pin-on-disk tribometer tests) also allowed for determining to which extent and by what means this behavior can be influenced. In addition, the last link, the knee simulator, already represented an operational test (category III) and thus a considerable extension of the test chain in this work. In this near-application test the first, current results on the excellent biotribologically effective behavior of TKRs coated with amorphous carbon were confirmed repeatedly by the evaluation of the wear mass of the polymeric tibia inlay. Such an audit trail could possibly be taken up as a proposal for legal requirements for conformity and validation of medical devices. [I] In Pub IV, screening tests were performed on the biotribological behavior of amorphous carbon coatings in a ball-on-three-pins configuration, mimicking the conditions of TKRs during gait. Thus, the influence on complete and single-sided coating of TKR components (metallic and/ or polymeric) and the variation thereof as well as the differences between a higher and a moderate load case with uncrosslinked and conventionally crosslinked UHMWPE, respectively, were investigated. Although the coatings predominantly resulted in a friction increase due to the considerably higher roughness, the wear was substantially reduced. In particular, the coating of the polymeric component is of crucial importance for improving the wear behavior and increasing the service life of load-bearing implants. In addition, a one-sided coating led to higher wear of the uncoated counterpart. Accordingly, coating systems that are applied to both joint surfaces should be pursued further and specifically modified. [I] News 52 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 In Pub V and Pub VI, the wear behavior and mechanisms of (tetrahedral) amorphous carbon coatings were investigated for the evaluation of the biotribological behavior of TKRs using pin-on-disk tests. A numerical elastohydrodynamic contact simulation was used in Pub VI to assess the influence of coatings on contact and lubrication conditions for TKRs. The wear behavior was analyzed using LM, LSM, SEM and Raman spectroscopy. This allowed for the targeted selection of a suitable coating combination and the isolated observation of wear phenomena. Nevertheless, the subsequent transfer to component tests remained essential. The macrogeometric elastic deformation behavior and the contact area of TKRs remained almost unchanged due to the coating (Pub VI). However, the higher roughness of the coated specimens resulted in the increase of friction compared to the uncoated references. Despite the higher friction, all coating combinations contributed to a substantial wear reduction on the metallic pins and especially on the UHMWPE disks (Pub V and Pub VI). Thus, the UHMWPE disk was still protected by an intact ta-C layer without signs of adhesive or abrasive wear (Pub V). However, crack networks and first signs of near-surface fatigue appeared after the completion of the entire test duration (Pub V). In contrast, the a-C: H layer showed no delamination or spalling of larger wear particles, but rather continuous and slow wear (Pub VI). At the same time, the UHMWPE disk was still protected by a largely intact coating with no signs of cracking or fatigue after the entire test period (Pub VI). The particle analysis revealed that nanometer-sized particles were released by all tested groups, their sizes being comparable. In general, it can be assumed that the particles of the coated groups exhibit a more favorable cell biological behavior than pure PE particles (Pub V and Pub VI). This suggests that the service life of the femoral shield and in particular of the tibial inlay is significantly prolonged by adherent amorphous carbon coatings (Pub V and Pub VI). However, the time of failure of the ta-C coating can barely be predicted due to the prevailing failure mechanisms (Pub V). [I] To protect TKRs from continuous abrasive and adhesive wear, a sufficiently high coating hardness is preferred. At the same time, the coating hardness has to be sufficiently low to allow for the deformation of the soft substrate without near-surface fatigue of the coating. [I] The current results of the experimental in vitro testing of both uncoated and coated TKRs presented in PrePub showed that a substantial reduction in the polymeric wear mass of about 57 % compared to CoCr/ CPE and about 62 % compared to Ti64/ CPE could be achieved by coating both articulating counterparts, so that the service life of TKRs can in general be significantly extended by amorphous carbon coatings. The wear results obtained initially were in accordance with the findings of Pub IV - Pub VI and confirmed the biotribological effectiveness of amorphous carbon coatings. Due to the updating of ISO 14243-1, a quantitative comparison with previously published results with regard to wear rates is not yet expedient. In addition, the limitations of a component test rig under ideal conditions must be considered in order to derive accurate, realistic wear predictions. Nevertheless, based on the investigations carried out within the scope of this work, it can be stated that the service life of TKRs could be significantly increased by biotribologically effective amorphous carbon coatings. However, the current investigations from the knee simulation must be fully continued and consolidated in order to make a holistic statement regarding the biotribological performance of coated TKRs. [I] 4 Prospect The biotribological findings obtained from this work can be used in the future for a targeted development of advanced amorphous carbon coatings for TKRs. Thereby, modifications of the coating system can be made, for instance in the form of a multilayer functional coating, in order to adjust the occurrence of wear, the particle size and to control possible cell biological consequences. Likewise, the influence of a medical functionalization of the top layer, such as SiO or Ag, on the antibacterial and tribological behavior can be investigated. Furthermore, the addition of clinically relevant occurances, such as knee flexion or three-body wear tests to the proceedings of the knee simulator is rendered useful for evaluating the biotribological behavior of the endoprosthesis under the account of extreme conditions. The interaction of an amorphous carbon coating exhibiting high wear resistance even under extreme test conditions with a medical functionalization can minimize infection-related and wear-associated failure causes of TKRs and thus lead to a significant holistic extension of the service life. Additionally, this approach can be investigated on the next experimental level - in vivo small animal experiments - in order to enable a transfer from preclinical to clinical testing and finally to clinical usage. [I] Literature [I] ROTHAMMER, B., Dissertation: Amorphous carbon coatings for extending the service life of total knee replacements. 2024. News 53 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 Introduction Stick-slip is a frequently occurring effect in which sticking and sliding alternate repeatedly on a contact surface. This unsteady movement usually results in considerable wear as well as unwanted heat and noise generation. Stick-slip is responsible, for example, for the squeaking of brakes or the squeaking of chalk on a blackboard. When playing a violin, the effect is produced in such a controlled manner that music can also be created. State of the art The stick-slip effect is usually illustrated in the literature using one of two common models. A rigid body of mass m (counter body) is pulled at a constant speed v 2 over a rigid base (base body) (Figure 1, left). A spring of stiffness c is placed between the pulling point and the base body. This spring can be perceived as a circuit of different springs, including, for example, the tangential con- GfT award Bachelorthesis 2025 Modelling the Stick-slip Effect in Matlab/ Simulink to Identify Influencing Parameters Gerrit W. Schnelle* The topic was submitted for the GfT Sponsorship Award 2025 in the category “bachelor or similar theses”. The award took place at the GfT conference in September 2025. In this paper, the stick-slip phenomenon will be discussed, taking into account the velocity-dependent friction according to Stribeck. The underlying modelling approach and its numerical implementation will be considered. In addition, different effect characteristics and a dimensionless mapping of the system behaviour will be presented. Keywords Stick-slip, Stribeck Curve, Stability Map, Stability, Frictional Dampening, Matlab, Simulink, Stable Oscillation Behaviour, Instable Oscillation Behaviour, Stick Slip Abstract * Gerrit W. Schnelle B. Sc., Orcid-ID: https: / / orcid.org/ 0009-0006-0940-2809 Universität Paderborn Chair of Design and Drive Technology Warburger Str. 100, 33098 Paderborn Figure 1: Modelling the stick-slip effect according to [Pop15] (Model 1) and Modelling the stick-slip effect according to [Sau18] (Model 2, frictional force F μ = F f , 1 counter body, 2 base bodies) News 54 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 tact stiffness of the base and counter body as well as other elasticities in the system. Friction μ prevails between the base body and counter body, which depends on the relative speed in the contact. Sticking occurs as long as the adhesive force F μ is greater than the spring force F c . At the beginning, spring force F c and adhesive force F μ balance each other out. The counter body remains behind the acting force for this period (see a, b remaining in position i). If the spring force F c becomes greater than the holding force F μ by tensioning the spring with continuous tension v 2 , sliding begins. During sliding (“slip”), the spring releases its stored energy and the counter body catches up (c to d, catching up from i to ii). After the slip phase, the spring force is again less than the adhesive force and the counter body remains stationary (d). Position d) corresponds to position a) and the characteristic, periodic stuttering of the body on the surface occurs. In the second model, instead of sideways movement on a rigid base, it is assumed that a treadmill moves under the counter body (Figure 1, Model 2). This allows the oscillation around a rest position x 0 to be considered. [Sau18] also introduces damping parallel to the spring. It should be noted that the relative velocity at the contact surface is now v rel = v 2 - x˙ 1 . To understand the effect, the equation of motion of the second model is considered: (1) x 1 m counter body position x˙ 1 m/ s counter body velocity ẍ 1 m/ s 2 counter body acceleration m kg counter body mass c N/ m spring stiffness (> 0) d Ns/ m viscous damping (> 0) F μ N friction force F d N viscous damping force F c N spring force In the case of sticking (v rel = 0), the spring is charged as an energy store until the spring and damper force are greater than the adhesive force, then the spring releases its energy and the mass slides. Either until it sticks to the belt again, or a smooth glide occurs in the rest position x 0 > 0. The case of quiet sliding is called “stable to the rest position”, the case of periodic adhesion “stable to the limit cycle”. In the case of smooth sliding (x˙ 1 = 0, ẍ 1 = 0), there is a permanent equilibrium of forces between spring force F c and frictional force F μ . According to C OULUMB ’s law of friction, the frictional force (tangential force) F t is the product of the normal force F N and the coefficient of friction μ: 𝑚 ∙ 𝑥̈+ 𝑑 ∙ 𝑥̇ ⏟ 𝐹 d + 𝑐 ∙ 𝑥 ⏟ 𝐹 c = 𝐹 μ (𝑣 2 − 𝑥̇) (2) F t N tangential force F N N normal force μ 1 coefficient of friction v rel m/ s relative velocity between two surfaces The frictional force is always opposite to the direction of movement, the switching function in equation (2.2) changes sign accordingly. For hydrodynamic bearings, S TRIBECK showed in 1902 the particular dependence of the coefficient of friction μ, on the relative speed v rel of the contact surfaces, with the S TRIBECK curve [Str02]. The S TRIBECK curve typically characterizes a greater static friction than dynamic friction. Modelling stick-slip The equation of motion (1) of the model (Figure 1, Model 2) was rearranged so that it can be represented in Simulink. Simulink’s solver ODE3 (B OGACKI -S HAMPINE ) with a step size of 10 -4 was used for the solution. Only linear spring stiffness and damping are considered in the model. Friction is idealized as solely dependent on relative velocity. The work is based on characteristic S TRIBECK curves using the following equation (3) and parameters (Figure 2): (3) μ(v rel ) 1 coefficient of friction a bis g 1 shaping coefficients v rel m/ s relative velocity Discussion of results The stick-slip effect can be described well using a v-x diagram, as well as an associated t-x diagram. A “normal” occurrence of the effect can be seen in Figure 3 a) with phases of adhesion marked green. In systems with the same parameterization, the placement of the counter body can determine whether stickslip occurs or not. Both solutions coexist until the start condition is defined. Five cases are defined for more precise differentiation: • local stability denotes the oscillation of the system with small disturbances around the rest position x 0 • local instability refers to the oscillation of the system with small perturbations around the rest position x 0 • global stability to the rest position refers to the system’s oscillation down to the rest position regardless of any disturbances 𝐹 t = −𝜇 ∙ 𝐹 N ∙ 𝑣 el |𝑣 el | ⏟ sign switch 𝜇(𝑣 el ) = (𝑒 ∙ r𝑏 + (1 − 𝑒 𝑐∙ r𝑑 ) ∙ ) ∙ 𝑔 News 55 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 • global stability to the limit cycle denotes achievement of the stick-slip effect with suitable disturbances • complete compensation describes the fragile state of permanent oscillation when friction damping and viscous damping balance each other out Figure 2: Parameters and progression of the defined S TRIBECK curve. (v min , v max ) is the low point of the curve, ( μ min / μ max )[GS1.1] is the ratio of the minimum to maximum coefficient of friction Figure 3: from a) to e): “normal” occurrence, decay, aperiodic limit case, overshoot and compensation News 56 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 In Figure 4 (left and right) is an example in which two different systems are each started with a small and large disturbance x. All four simulations in Figure 4 belong to the same S TRIBECK curve and were simulated at low viscous damping d. The left system is locally stable (green), as it decays for small perturbation and lies in the range dμ/ dx˙ (v 2 ) > 0. However, if the same system is started with a larger disturbance, the global stability to the limit cycle of the system is recognizable (orange). Both states can coexist. The grey dashed line shows the limit at which the system can oscillate in a fragile state (compensation). The system on the right is in the range dμ/ dx˙ (v 2 ) < 0 and is therefore locally unstable (small perturbation, yellow) and globally stable to the limit cycle (large perturbation, purple). From a linearization around the rest position [Pop15], the compensation of viscous damping and frictional damping can be traced back to a critical normal force F N,crit . This represents the limit between upswing and downswing when touching down on the rest position: (4) F N,crit N critical normal force dμ/ dx˙ s/ m gradient of the S TRIBECK curve d Ns/ m viscous damping v 2 m/ s belt speed If a normal force F N > F N,crit , the influence of friction is increased and the system becomes unstable, for F N < F N,crit it stabilises. Systems of local stability to the rest position and global oscillation to stick-slip (Figure 4, left) pass through smaller and smaller gradients dμ/ dx˙ (v 2 ) with increasing amplitude and thus inevitably pass the state of compensation. The following dimensionless numbers are taken from K EN N AKANO and S ATORU M AEGAWA [NM10]: 𝐹 N,c it = − 𝑑 𝑑𝜇 𝑑𝑥̇(𝑣 2 ) Figure 4: Local and global stability of two systems. Both systems are parameterized identically with blue S TRIBECK curve, only the belt speed varies on the left and right. left green: locally stable (positive slope) left orange: globally stable to limit cycle, oscillates to stick-slip left grey: complete compensation at the transition from stability to instability. right yellow: locally unstable, oscillates to stick-slip (negative slope) right purple: globally stable to limit cycle, oscillates to stick-slip News 57 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 (5) (6) ζ 1 dimensionless damping λ 1 stick-slip parameter v 2 m/ s belt speed m kg counter body mass c N/ m spring stiffness d Ns/ m viscous damping F N N normal force [NM10] assume a friction model with static friction and lower, constant sliding friction. Under this assumption, they find two asymptotes in the double logarithmic: ζ -1 λ 2 = 4 π, and ζ = 1. Both asymptotes are superimposed on the simulink results in Figure 5. 𝜆 = 𝐹 N 𝑣 2 √𝑐𝑚 𝜁 = 𝑑 2√𝑐𝑚 In future, the aim is to implement the model in dimensionless form. This reduces numerical errors and enables better-resolved images with acceptable computing times. Furthermore, a separate consideration of the tangential stiffness (depending on force, material and contact type) and other elasticities in the system, as well as an extended consideration of the S TRIBECK curve as a function of pressure, viscosity, roughness and sliding speed by means of the S CHIPPER number [Sch91] in order to take into account locally different friction coefficients, are conceivable. References [NM10] Nakano, K.; Maegawa, S.: Occurrence limit of stickslip: dimensionless analysis for fundamental design of robust-stable systems. Lubrication Science, (22)1: 2010, S. 1-18. Figure 6: Significance of the critical normal force. Figure 5: λ-ζ diagram: S TRIBECK curve #2, v 2 = 0.25 · v min yellow: stick-slip, blue: smooth glide a) Oscillation due to low friction and viscous damping b) Stick-slip because the frictional damping compensates for the viscous damping The stick-slip effect is also clearly recognizable for ζ > 1 (Figure 5, b). The simulation suggests that the viscous damping is compensated for at these points. Summary The following findings were obtained in this work: For global stability to the rest position in systems without viscous damping, local stability to the rest position is a necessary condition. In viscous damped systems, local instability can be compensated depending on the critical normal force. Simulation results of various S TRIBECK curves indicate a clear gradientand disturbance-dependent occurrence of the stick-slip effect. Particularly in operating ranges with rising S TRIBECK curves, stick-slip cannot generally be ruled out under the assumptions made (Figure 6). News 58 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 [Pop15] Popov, V. L.: Kontaktmechanik und Reibung. Springer Berlin Heidelberg, Berlin, Heidelberg: 2015. [Sau18] Sauer, B.: Konstruktionselemente des Maschinenbaus 2. Springer Berlin Heidelberg, Berlin, Heidelberg: 2018. [Sch91] Schipper, D. J.: Prediction of Lubrication Regimes of Concentrated Contacts. Lubrication Science, 3: 1991, S. 191-200. [Str02] Stribeck, R. H.: Die wesentlichen Eigenschaften der Gleit- und Rollenlager. Zeitschrift des Vereines Deutscher Ingenieure, 46: 1902, S. 1241-1348. 59 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 MEDIENTIPP Marc Sens (ED.) 6 th International Conference on Ignition Systems for SI Engines 7 th International Conference on Knocking in SI Engines 1. Auflage 2024, 386 Seiten ISBN print 978-3-381-12991-1 ISBN eBook 978-3-381-12992-8 DOI 10.24053/ 9783381129928 Ladenpreis print €[D] 189,00 Ladenpreis eBook €[D] 151,99 In addition to the indisputably necessary electrification of the transport sector, which is currently being ramped up, internal combustion engines will still be urgently needed in the future. Otherwise, the demand for mobility in the on-road, off-road and non-road sectors cannot be met. There is no doubt that these internal combustion engines will have to be improved regarding efficiency plus lower emissions and nowadays more and more important upgraded for zero and low carbon fuels. Even though Spark Ignition (SI) engines have been around for more than a century, there is still a lot of room for improvement, particularly in terms of power density, ignition, combustion control, and preventing uncontrolled combustion. To offer all interested developers an inspiring exchange platform for the latest developments, IAV established two exciting conferences more than two decades ago, which are now held under the heading “Two Conferences - One Goal”. This volume brings together the contributions to this conference. expert verlag - Ein Unternehmen der Narr Francke Attempto Verlag GmbH + Co. KG Dischingerweg 5 \ 72070 Tübingen \ Germany \ Tel. +49 (0)7071 97 97 0 \ info@narr.de \ www.narr.de 60 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 MEDIENTIPP Nicole Dörr, Carsten Gachot, Max Marian, Katharina Völkel 24th International Colloquium Tribology Industrial and Automotive Lubrication Conference Proceedings 2024 1. Auflage 2024, 279 Seiten ISBN print 978-3-381-11831-1 ISBN eBook 978-3-381-11832-8 DOI 10.24053/ 9783381118328 Ladenpreis print €[D] 148,00 Ladenpreis eBook €[D] 188,00 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 - Trends lubricants and additives - Automotive and transport industry - Industrial machine elements and wind turbine industry - Coatings, surfaces and underlying mechanisms - Test methodologies and measurement technologies - Digitalisation in tribology - Digital Tribological Services: i-TRIBOMAT - Sustainable lubrication www.verlag.expert www.narr.de MEDIENTIPP Sierk A. Horn Intercultural Leadership Humanistic Perspectives 1. Auflage 2024, 552 Seiten ISBN print 978-3-8252-6186-3 ISBN eBook 978-3-8385-6186-8 DOI 10.36198/ 9783838561868 Ladenpreis print €[D] 44,90 Ladenpreis eBook €[D] 43,99 For many of us, connecting with people across the world is now easy and commonplace. But coming into contact with different ways of doing things means losing our superpower of giving meaning to what is happening around us, interacting skilfully and building rapport. In the first part of this book, Sierk Horn shows how intercultural interactions set in motion psychological processes. The second part deals with the behavioural determinants of intercultural communication. The third part examines our social environment and how we deal with cultural differences. The book wants to make you curious about intercultural leadership. It invites you to explore humanistic perspectives in everyday communication. A wealth of exercises will accompany you on your learning journey. An extensive range of online resources and learning and teaching materials accompany the textbook. 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Torben Terwey Real Contact Area and Pressure Distribution in Mixed Lubricated Rolling Contacts under Consideration of the Real Rheology Justus Rüthing, Frank Haupert, Regine Schmitz, Mirek Göbel Analysing the friction behaviour of paper machine forming fabrics under water lubricated hydrodynamic conditions and systematical test parameter variation in three defined wear depths using a pin-on-disc test assembly Juliane Heydemann, Emil Elbæk, Thomas Lohner Key Survey Results on Tribological Optimization Opportunities, Barriers, and Research Needs