Heinz Kloß

Numerical Simulation of Mechanically Mixed Layer Formation at Local Contacts of an Automotive Brake System

Processes taking place at local contacts in an automotive brake system are analyzed on the basis of computer simulation with the method of movable cellular automata. The conditions of a mechanically mixed layer (MML) formation on the tribosurfaces and the influence of the layer on the friction coefficient are investigated. The results show that the MML formation leads to the stabilization of the coefficient of friction at a convenient range (0.3–0.4) for brake application. The presence of graphite particles in the MML decreases the critical value of local normal stress for switching from stick-slip to smooth relative motion with MML formation.

Assessment of Sliding Friction of a Nanostructured Solid Lubricant Film by Numerical Simulation with the Method of Movable Cellular Automata (MCA)

Tribofilms formed during dry sliding usually exhibit a nanocrystalline structure and complicated composition. In the present study, tribofilms consisting mainly of a solid lubricant, namely graphite nanoparticles, are considered. Systems providing such tribofilms are candidates for anti-friction applications. Since sliding action always leads to mixing of the materials at both sides of the tribological interface, it was of major interest to study the impact of different amounts of a hard constituent, SiC in the considered case, within the soft matrix systematically. Furthermore, the impact of normal pressure was considered. A mechanically mixed layer was observed for the whole range of normal pressures and SiC volume fractions. The calculated coefficient of friction decreased significantly with increasing thickness of this layer but was only marginally affected by SiC volume fraction, which is good news for anti-friction applications.

Does ultra-mild wear play any role for dry friction applications, such as automotive braking?

Nanostructured third body films and/or storage of wear debris at the surfaces of the first bodies are deemed as prerequisites of sliding under ultra-mild wear conditions. Since such features have been observed experimentally on brake pads and discs, attempts were undertaken to study their sliding behaviour by modelling on the nanoscopic scale with an approach based on Movable Cellular Automata (MCA). The model rendered the possibility to study the influence of different nanostructures systematically and to assess the impact of different brake pad ingredients on the sliding behaviour, velocity accommodation and friction force stabilization at a sliding contact. Besides providing a review on previously published modelling results, some additional new graphs enabling better visualization of dynamic processes are presented. Although ultra-mild wear conditions were considered to be essential for achieving the desired tribological properties, transitions to mesoscopic and macroscopic wear mechanisms were studied as well. The final conclusion is that ultra-mild wear and corresponding smooth sliding behaviour play an important role during automotive braking, even though temporarily and locally events of severe wear may cause friction instabilities, surface damage and release of coarse wear particles.

Some results of numerical study on the role of tribofilms formed during automotive braking

As it was shown in our experimental observation tribofilms generated during braking usually are 100 nm thick and exhibit a multiphase nanocrystalline structure. The objective of our modeling was to obtain a better understanding of the sliding behavior and associated friction properties and to study the impact of internal and external parameters on these properties. The third bodies were considered as aggregates of linked nanoparticles which may decompose and form a layer of granular material. The basic model structure which consists of Fe3O4 nanoparticles with 13% graphite inclusions was used. In order to assess the robustness of the model the following parameter studies were performed. The mechanical properties of the oxide were varied between brittle and ductile behaviour corresponding to room temperature and high temperature behavior. The mechanical properties of the soft ingredient were varied ±50% of the properties of graphite. The influence of a variety of model parameters on the evolution of the fr...