On the scale dependence of coefficient of friction in unlubricated sliding contacts

Friction at different force, length, and time scales is of great interest in tribology. The mechanical, chemical, and physical (atomic) interactions, each operating at their own force and length scale, make friction a highly scale dependent event. This work is an attempt to trace important mechanisms of friction on commonly used engineering materials over a normal force scale ranging from nN up to N, and thereby altering the contact size from nm2 up to mm2. The relevance of existing theories on friction is verified on different engineering surfaces taking into account mechanical (hard/hard, hard/soft), chemical, and physical aspects of the sliding surfaces. The applicability of Amontons’ law is experimentally investigated. For rough surfaces it was found that the existence of a constant coefficient of friction over a wide force and length scales is only a special case. For a hard/hard tribosystem (like DLC/Si3N4), a linear dependence of friction on normal force was observed, whereas a non-linear relationship was more evident on hydrophilic surfaces and hard/soft couples. Irrespective of the material system considered, the dependence of friction on normal force can be altered by modifying the surface roughness or texture of the material couple. In all, changes in the force and length scales bring about significant changes in the governing mechanisms of friction. The experimental findings were in good agreement with recent elasto-plastic and fractal contact mechanical theories on friction.