  The effects of a step defect and a random array of point defects (such as
vacancies or substitutional impurities) on the force of friction acting on a
xenon monolayer film as it slides on a silver (111) substrate are studied by
molecular dynamic simulations and compared with the results of lowest order
perturbation theory in the substrate corrugation potential. For the case of a
step, the magnitude and velocity dependence of the friction force are strongly
dependent on the direction of sliding respect to the step and the corrugation
strength. When the applied force F is perpendicular to the step, the film is
pinned forF less than a critical force Fc. Motion of the film along the step,
however, is not pinned. Fluctuations in the sliding velocity in time provide
evidence of both stick-slip motion and thermally activated creep. Simulations
done with a substrate containing a 5 percent concentration of random point
defects for various directions of the applied force show that the film is
pinned for the force below a critical value. The critical force, however, is
still much lower than the effective inertial force exerted on the film by the
oscillations of the substrate in experiments done with a quartz crystal
microbalance (QCM). Lowest order perturbation theory in the substrate potential
is shown to give results consistent with the simulations, and it is used to
give a physical picture of what could be expected for real surfaces which
contain many defects.
