Effects of surface defects on friction for a thin solid film sliding over a solid surface

Abstract

Effects of the most common surface defects, namely, vacancies, substitutional impurities, steps and facet boundaries, on the force of friction acting on a sliding film are studied theoretically using lowest-order perturbation theory in the force exerted on film due to the surface on which the film is sliding. It is found that the contribution to the force of friction due to point defects, such as vacancies and substitutional impurities, is velocity-independent (i.e., there is ``dry friction''). In contrast, for straight line defects, such as steps and facet boundaries, for nearly all orientations of the defect relative to the crystallographic axes, the viscous friction law generally observed in the microbalance experiment is found. Implications for the interpretation of the experimental results are discussed. The range of parameters for which lowest-order perturbation theory is valid is tested by calculating the mean-square atomic displacement and requiring that it be much smaller than the square of an atomic spacing.