  It has long been recognized that the key to understand kinetic friction force
$F_k$ is the analysis of microscopic instabilities that lead to sudden
irreversible "pops" of certain degrees of freedom. In this Letter, the nature
of such instabilities is characterized with an emphasis on boundary lubricants.
It is shown that there are certain critical values of the parameters defining
our model Hamiltonian, where the behavior of the instabilities changes
qualitatively. Simultaneously, the functional dependence of $F_k$ on the
sliding velocity $v_0$ changes. The relevant parameters studied here are
dimensionality of the interface, degree of commensurability, first higher
harmonic in the lubricant wall potential, and temperature. Molecular dynamics
simulations are carried out to test whether the predictions made on the basis
of the simple model also hold in less idealized circumstances.
