  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.
