  A tribological system consisting of two contacting blocks has been
considered. One of them is arranged between two springs, the other is driven
periodically. The kinetics of the system has been studied in the boundary
friction mode, when an ultrathin lubricant film is contained between the
atomically smooth surfaces. In order to describe the film state, the expression
for the free energy density is used in the form of an expansion in a power
series in the order parameter, the latter being reduced to the shear modulus of
a lubricant. The stick-slip mode is shown to be realized in a wide range of
parameters, being a result of the periodic first-order phase transitions
between kinetic friction regimes. The behavior of the system governed by
internal and external parameters has been predicted.
