Simulations of friction between clean, atomically flat surfaces indicate that the friction force per unit area decreases to zero with increasing area. In previous work, we showed that the presence of mobile atoms between surfaces suppresses this superlubric behavior. The current paper examines the effect of atomic scale roughness. Both single asperities and random self-affine surfaces are considered in two and three-dimensional geometries. We have also examined the effect of commensurability, and elastic or plastic deformation within the bounding solids. The friction force on single asperities decreases to zero as the contact area and radius of curvature increase, but can still be significant at the scale of scanning probe tips. Introducing mobile atoms leads to friction forces that rise linearly with area in large contacts. Frictional forces on random self-affine surfaces can be quite complex. If self-affine scaling extends down to atomic dimensions, the average contact between elastic solids only contains a few atoms and the friction can be large. Plasticity increases contact dimensions and may lower the friction.

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