How grain size controls friction and wear in nanocrystalline metals

Using molecular simulations we investigated the dependence of friction and wear on grain size in nanocrystalline copper. We found that effects of grain size are coupled to the effects of contact size, resulting in a transition from grain size--sensitive regime to grain size--insensitive regime in friction. This transition occurs because for small tips, friction-induced easy-shear planes can be entirely accommodated in a single grain, rendering grain boundaries less relevant to sliding resistance. Trends in friction do not follow trends in hardness, which is sensitive to grain diameter in the entire grain size regime considered in this study. We have also discovered that coupling of the effects of grain diameter and contact size leads to an optimum grain size that minimizes formation of wear chips on the surface.