Direct Observation of Layer-by-Layer Wear

Wear at the macroscopic scale has been well documented to involve the deformation of sliding interfaces. The mechanical behavior of single asperities in sliding contact, however, is not fully understood. Classical wear mechanisms have not been necessarily substantiated by direct studies of events at the contacting interface and have often contradicted post facto investigations. Wear processes, particularly those under mild loads, are not well understood at the nanoscale to submicron scale. We report here the observation, via in situ transmission electron microscopy, of wear of M₂₃C₆ carbides in a CoCrMo alloy sliding against a single silicon asperity. Each slide of the asperity under a normal stress of ~20 MPa resulted in removal of one atomic layer of the carbide. This represents a new type of wear process, which differs from what one would predict from models based on fracture of brittle materials and what has been called atomic wear. For context, we compare the processes and conditions used to those relevant for metal-on-metal hip implants, concluding that these severe wear processes are probably not important for the head/cup junction but could be relevant to fretting wear and corrosion at modular junctions.