Simulations of atomic-scale sliding friction

Simulation studies of atomic-scale sliding friction have been performed for a number of tip-surface and surface-surface contacts consisting of copper atoms. Both geometrically very simple tip-surface structures and more realistic interface necks formed by simulated annealing have been studied. Kinetic friction is observed to be caused by atomic-scale stick and slip which occurs by nucleation and subsequent motion of dislocations preferably between close-packed {111} planes. Stick and slip seems to occur in different situations. For single crystalline contacts without grain boundaries at the interface the stick and slip process is clearly observed for a large number of contact areas, normal loads, and sliding velocities. If the tip and substrate crystal orientations are different so that a mismatch exists in the interface, the stick and slip process is more fragile. It is then caused by local pinning of atoms near the boundary of the interface and is therefore more easily observed for smaller contacts. Depending on crystal orientation and load, frictional wear can also be seen in the simulations. In particular, for the annealed interface necks which model contacts created by scanning tunneling microscope/atomic force microscope tip indentations the sliding process involves breaking contacts leaving tip material behind on the substrate.