W. Österle, A.I. Dmitriev, H. Kloß
Tribology International, Volume 48, April 2012
The tribological properties of nanostructured surface films formed during dry sliding, for example during automotive braking, were determined by modelling using the method of movable cellular automata. Starting from a basic model structure, consisting of magnetite with 13% graphite inclusions, the impact of additional soft and hard particles of different size and volume fraction was studied systematically. It was revealed that agglomerates of soft particles decomposed and finally mixed with the oxide in the same way as single nanoparticles. On the other hand, agglomerates of hard particles mixed with the other components without decomposing. Whereas increasing the amount of soft components in the third body lowered the coefficient of friction, the opposite occurred with the hard particles. The boundary conditions for obtaining smooth sliding conditions with minor fluctuations between friction forces at successive time steps could be defined. In addition to features of the nanostructure, the applied normal pressure impacted modelling results. Within the parameter range of smooth sliding behaviour, increasing pressure induced thicker granular interface layers, which lead to a slight decrease of the coefficient of friction. Changing the amount of soft or hard particles did not change this pressure dependency but only the friction level.
A nanoscopic model is suggested to simulate the velocity accommodation between a rotating brake disc and fixed brake pad. Similar as for a lubricated contact, a velocity gradient can evolve within an interfacial layer. A prerequisite of this velocity accommodation mechanism is the transformation of the bulk material to a granular quasi-fluid layer. It has been shown by the model that oxide-based tribofilms which are furthermore mixed with soft and hard constituents of the brake pad formulation can fulfil this requirement. The figure shows a mechanically mixed granular layer of loosely bound nanoparticles of different ingredients (colours) and size between compact layers and supporting substrates (grey). The size of particles is 10 nm and the thickness of tribofilms 100 nm, in accordance with experimental observations.
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