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Automotive braking is based on dry friction between fixed pads and a rotating disc. Besides macroscopic thermo-physical properties, the development of topographic features on the mesoscopic scale and the nanostructure of the third body formed by wear processes, determine brake performance properties. Whereas modelling on the atomistic scale is suitable to understand mechanisms leading to nanocrystalline surface films, the properties of such films can be assessed best with a model based on movable cellular automata (MCA). It turned out that the presence of at least 10% of soft nanoinclusions is most essential in respect to smooth sliding conditions. It made no major difference whether graphite or copper particles were assumed as soft nanoinclusions. The third body material is not only the stuff which spreads over contact areas, but it also contributes to contact size by wear particle compaction and formation of secondary contact areas. The evolution of contact size is the major feature of mesoscopic modelling and thus it is capable to model and explain dynamic changes of the coefficient of friction (COF) during certain brake operations. Although it is still ambiguous in many cases which feature has the major impact on friction behaviour, the following conclusions can be drawn. The reinforcing ingredients of the pad material serve as primary contact sites and thus define the starting condition for mesoscopic simulations. A certain amount of wear is necessary to provide a third body which is capable to form secondary contact sites and friction layers screening the first body materials. The composition and nanostructure of the third body is important as well, because it determines the friction level and is responsible for smooth sliding conditions.
Does ultra-mild wear play any role for dry friction applications, such as automotive braking?
(2012)
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.
The frictional behavior at local contacts in an automotive brake system was analysed on the basis of computer simulation by movable cellular automata method. The boundary conditions of the model were adjusted to experimental observations obtained by TEM. The model proved to be adequate for simulating mechanical mixing and velocity accommodation at the pad-disc interface. Dynamics of particle interaction were visualized by showing rotation angles and velocity vectors. The model provided information on the development of plastic deformation for metal-on-metal contacts and on crack formation at graphite lamellae of cast iron disc. Results are in agreement with conventional friction theories.