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 $F_{\rm k}$ and the non-equilibrium 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 $F_{\rm k}$. The effects of the interfaces' dimensionality, lubricant coverage, and internal degrees of freedom of lubricant particles on $F_{\rm k}$ 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.