Abstract
Automotive brake pads consist of many components and the role of each of the elements of this complex composition in respect to specified regimes of sliding is not yet completely clear. This is due to mutual interactions and multi-scale mechanisms realized during the friction. In this work we have attempted to partly answer this question using computer simulations. Since the simulation allows us to consider various combinations of the structure of the system being simulated ceteris paribus, it becomes possible to study the impact of each ingredient separately. According to observations by Godet a dry friction is largely determined not by the properties of friction materials of contacting pair, but the characteristics of structure and composition of the thin film that forms on the surface of both bodies as a result of compaction of wear product, its chemical composition and oxidation. This layer, also named as a third body or friction film, differs in composition and microstructure from the first two bodies – a pair of contacting materials, which also may be modified as a result of plastic deformation. We considered a single contact for the steady state sliding when the structure and composition of friction films already are formed. From pad side it was considered cold-worked ferritic steel representing a reinforcing ingredient of the brake pad. From disc side - pearlitic steel representing the major constituent of a gray cast iron brake disc. Both substrates were covered by nanocrystalline iron oxide of type magnetite representing the major phase of the third body and graphite particles representing a typical solid lubricant inclusion in the third body. As a modeling tool we used the method of movable cellular automata (MCA), which has well proven itself in solving of such tasks. This method belongs to methods of discrete approach and is a synthesis of conventional cellular automata and particle method. We investigated the influence of modification of the structure and composition of the third body on the features of system behavior at friction. The following parameters were varied: mechanical properties of inclusions, their concentration, their size, uniformity of distribution in an iron oxide matrix, and the structure of the oxide matrix itself. The results show the influence of the presence of both soft and hard inclusions of different size on the frictional characteristics of the system. To assess the adequacy of the numerical model experimental studies with an artificial third body were also carried out. Comparison of simulation results with experimental data was done on those tests where it was possible from the side of the experimental study. The simulation results are in good agreement with those experimental data.
KEYWORDS – third body, dry sliding, mechanically mixed layer, computer simulation