Friction mechanism in the running-in stage of copper: From plastic deformation to delamination and oxidation

The friction mechanisms during the running-in stage and its effect on the subsequent steady-state were investigated by examining the evolution of surface topography and subsurface microstructures in pure copper sliding against a WC-Co ball. Scanning electron microscopy observations indicated that plastic deformation induced grain refinement occurs after only several sliding passes. A transition in the dominant friction mechanism from plastic deformation to delamination and oxidation was found in the running-in stage, corresponding to a peak coefficient of friction and onset of detectable wear.