Kinetic friction and atomistic instabilities in boundary-lubricated systems

The contribution of sliding-induced, atomic-scale instabilities to the kinetic friction force is investigated by molecular dynamics. For this purpose, we derive a relationship between the kinetic friction force F(k) and the nonequilibrium velocity distribution P(v) of the lubricant particles. P(v) typically shows exponential tails, which cannot be described in terms of an effective temperature. It is investigated which parameters control the existence of instabilities and how they affect P(v) and hence F(k). The effects of the interfaces' dimensionality, lubricant coverage, and internal degrees of freedom of lubricant particles on F(k) are studied explicitly. Among other results, we find that the kinetic friction between commensurate surfaces is much more susceptible to changes in (i) lubricant coverage, (ii) sliding velocity, and (iii) bond length of lubricant molecules than incommensurate surfaces.