Shear-Induced Surface-to-Bulk Transport at Room Temperature in a Sliding Metal-Metal Interface

Predictions from molecular dynamics (MD) simulations, that sliding at a metal-metal interface causes vortices in the near-surface region that transport atoms from the surface into the subsurface region, is tested experimentally. This is accomplished by rubbing a methyl thiolate overlayer grown on a clean copper foil by exposure to dimethyl disulfide at room temperature. Repeatedly rubbing a 1.27 × 10⁻² m diameter pin over a thiolate-covered copper surface at an applied load of 0.44 N and sliding speed of 4 × 10⁻³ m/s in an ultrahigh vacuum tribometer, results in the removal of sulfur from the wear track as measured using spatially resolved Auger spectroscopy. Any remaining surface species, in particular, outside the wear track, are removed by argon ion bombardment. Since sulfur is more thermodynamically stable at the surface, heating the sample causes the sulfur to resegregate to the surface only inside the wear track, thereby directly confirming the predictions from MD simulations.