Stick slip and wear on metal surfaces

Molecular dynamics (MD) simulations of ploughing friction are performed on fcc crystalline Ag and the results compared to experiments on polycrystalline Ag. For the simulations, a sharp pyramidal diamond tip is inserted into the (1 0 0) surface at various depths and for different temperatures and pulled across the surface. The mechanisms behind the stick slip and wear events are investigated. The forces and friction coefficients vary with tip orientation and linear distance travelled by the tip over the surface. The simulations show that dislocations propagate in the 〈1 1 0〉 direction from the tip as the scratch develops. Dislocations can also be linked to the stick events, with the emission of a dislocation in the substrate region near the tip, when slip occurs after stick. The pile-up patterns are shown to be in good qualitative agreement with experimental observations in the scanning force microscope. Higher temperature simulations in silver show less pronounced dislocation emission than at 0K and pile up which is spread further away from the scratch.