Contact, nanoindentation, and sliding friction

This paper presents an atomic-scale study of contact, indentation, and subsequent pulling and dry sliding of a sharp and blunt metal tip on a metal surface. The evolution of atomic structure and the variation of perpendicular and lateral forces are calculated by molecular-dynamics methods using an empirical potential based on the embedded-atom model. The sharp tip experiences multiple jumps to contact in the attractive force range. The contact interface grows discontinuously mainly due to disorder-order transformation leading to disappearance of a layer and hence abrupt changes in the normal-force variation. Atom exchange occurs in the repulsive range. During the pulling off, the connective neck is reduced discontinuously; however, not all the abrupt changes of the pulling force are associated with the creation of a new layer in the neck. The sliding of the sharp tip (or single asperity) induces two consecutive structural transformations that occur periodically, but end with the wear of a layer. The situation for a blunt tip is, however, quite different.