Mechanism for material transfer in asperity contact

We perform a series of molecular dynamics simulations of asperity contact and separation in a model metallic system for both symmetric and asymmetric asperity geometries, for loading in the [001], [110], and [111] directions, and for systems with different works of adhesion . We examine contact morphology evolution, force-displacement relations, and the quantity of atoms transferred from one surface to the other NT upon separation with a focus on underlying physical mechanisms that control these. We find that there is a critical work of adhesion, below which no plastic deformation occurs on contact separation and a higher one in which plastic deformation occurs but no material transfer occurs. We interpret these within a model for dislocation nucleation at the crack tip. We observe abrupt changes in the amount of material transferred with increasing work of adhesion that represent thresholds for changes in deformation mechanisms. These depend on the geometry of the contact and the crystallographic orientation relative to the loading direction.