24th International Colloquium Tribology - January 2024 167 Computational Modeling of Tribological Systems: Insights into Grinding Processes, Materials Tribology and Tribofilm Formation through Molecular Dynamics Stefan J. Eder 1,2,* , Philipp G. Grützmacher 2 , Manel Rodríguez Ripoll 1 , Andreas Nevosad 1 , Karen Mohammadtabar 3 , Ashlie Martini 3 , Nicole Dörr 1 , Daniele Dini 4 , Carsten Gachot 2 1 AC2T research GmbH, Viktor-Kaplan-Straße 2/ C, 2700 Wiener Neustadt, Austria 2 Institute of Engineering Design and Product Development, TU Wien, Lehárgasse 6 - Objekt 7, 1060 Vienna, Austria 3 Department of Mechanical Engineering, University of California Merced, 5200 N. Lake Road Merced, CA 95343, USA 4 Department of Mechanical Engineering, Imperial College London, South Kensington Campus, Exhibition Road, London SW7 2AZ, UK * Corresponding author: stefan.j.eder@tuwien.ac.at 1. Introduction Molecular dynamics (MD) simulation has long been a fancy academic toy for solid-state physicists that suffered from limited credibility due to the system size restrictions and its inability to capture chemical bond breaking and formation. The past 10 years have seen an enormous increase in manageable system sizes and the ability to treat chemical reactions without the necessity to consider electrons explicitly. In this contribution, we will present three examples from different corners of tribology that showcase how this simulation method has matured to a level that makes it interesting to surface processing and lubrication experts, materials tribologists, and mechanical engineers. 2. Surface finishing and abrasive processes The first example introduces an atomistic approach for modeling and analyzing a finishing process, offering a glimpse into the microscopic aspects of workpiece development that are often elusive in experiments, see Figure 1. The method involves four key system parameters: workpiece material, abrasive shape, temperature, and infeed depth [1]. The model’s validation demonstrates its potential, accurately reproducing critical process parameters and even predicting surface quality deterioration due to tool wear, despite considerable differences in grain sizes between simulation and macroscopic experiments. Utilizing the validated model, we delved into time-resolved stress profiles within the ferrite/ steel workpiece and mapped microstructural changes near the surface [2]. This investigation highlighted the approach’s capability to unveil the intricate and dynamic effects of blunt abrasives at elevated temperatures on near-surface microstructure and stresses, where multiple processes vie for dominance. Figure 1: Overview of a nanoscopic model designed to study surface finishing and abrasion processes. 3. Materials tribology of high-velocity sliding Numerous contemporary applications rely on the reliable operation of high-speed sliding components, such as those in e-mobility, high-speed manufacturing, or impact-resistant materials. While extensive research has explored the impact of mechanical energy and heat on the friction and wear behavior of dry metallic interfaces [3], the effect of extreme speeds on near-surface deformation mechanisms in polycrystalline metals has remained elusive [4]. This example addresses this crucial issue by employing large-scale molecular dynamics simulations, investigating sliding velocities 168 24th International Colloquium Tribology - January 2024 Computational Modeling of Tribological Systems: Insights into Grinding Processes, Materials Tribology and Tribofilm Formation through Molecular Dynamics ranging from 10 to 2560 m/ s in CuNi alloys, see Figure 2. Four distinct deformation regimes emerge, with microstructural responses remaining consistent up to 40 m/ s but undergoing significant changes between 150 and 500 m/ s, depending on composition. The decrease in plastic deformation at the highest speeds is attributed to a substantial rise in contact temperature, approaching or surpassing the bulk melting point. This phenomenon results in a reduced contact area and diminished sliding resistance. Understanding this nonlinear and non-monotonic material response to increasing sliding velocities offers valuable insights for selecting materials and designing durable components for high-speed sliding and wear applications. Figure 2: Dependence of several tribological key indicators on the sliding speed, with results obtained from large-scale molecular dynamics modeling. 4. Reactive molecular modeling Molecular modeling also comes in a “reactive flavor” that allows the elucidation of tribochemical processes [5]. For example, lubricated mechanical components subjected to extreme conditions rely on protective films that develop through the presence of additives in lubricants. The formation of these films is influenced by heat, load, and shear forces within the sliding interface, yet the specific impact of each factor remains poorly understood. Here, reactive molecular dynamics simulations were applied to untangle the effects of heat, load, and shear force on chemical reactions between di-tertbutyl disulfide, an extreme-pressure lubricant additive, and Fe(100), a model representing the ferrous surfaces of mechanical components [6]. The study characterizes the reaction pathway in terms of the number of chemisorbed sulfur atoms and the release of tert-butyl radicals during various heat, load, and shear stages in the simulation, see Figure 3. It was observed that chemisorption is constrained by the accessibility of reaction sites, with shear promoting the reaction by facilitating the movement of radicals to available sites. By plotting the reaction yield of the rate-limiting reaction for tribofilm formation, we provide a tool for surface engineers to optimize the growth of protective sulfurous films. Figure 3: Representative example of the power of reactive modeling. The reaction pathway for tribofilm formation and its bottleneck-reaction are identified, then the reaction yield of that reaction is evaluated as a function of shear stress and temperature. Acknowledgements This work was supported by the Austrian COMET program (project K2 InTribology, no. 872176). D.D. acknowledges the support of the Engineering and Physical Sciences Research Council (EPSRC) via his Established Career Fellowship EP/ N025954/ 1. References [1] Eder, S. J., Grützmacher, P. G., Spenger, T., Heckes, H., Rojacz, H., Nevosad, A., & Haas, F. (2022). 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