The effects of step defects and of a random array of point defects (vacancies, substitutional impurities) upon the friction force which acted on a Xe monolayer film as it slid over a (111) Ag substrate were studied by means of molecular dynamics simulations. The results were compared with the results of lowest-order perturbation theory for the substrate corrugation potential. In the case of a step, the magnitude and velocity-dependence of the friction force were strongly dependent upon the direction of sliding with respect to the step, and upon the corrugation strength. When the applied force was perpendicular to the step, the film was pinned for forces that were lower than a critical value. However, motion of the film along the step was not pinned. Fluctuations in the sliding velocity with time provided evidence for both stick-slip motion and for thermally activated creep. Simulations which were performed for a substrate which contained a 5% concentration of random point defects, for various directions of the applied force, showed that the film was pinned for forces below a critical value. This critical force was still much lower than the effective inertial force which was exerted on the film by the oscillations of the substrate in experiments performed using a quartz-crystal micro-balance. The lowest-order perturbation theory for the substrate potential was shown to give results that were consistent with the simulations, and was used to give a physical picture of what could be expected for real surfaces that contained many defects.

Effects of Defects upon Friction for a Xe Film Sliding on Ag(111). M.S.Tomassone, J.B.Sokoloff: Physical Review B, 1999, 60[6], 4005-17