Role of transfer films and interfacial cracking in metallic sliding wear

The origin of wear particles in metallic sliding contacts remains debated. Classical views based on cold-welded junctions suggest that plastic yielding in the real contact area should lead to large wear coefficients, in apparent contradiction with the small values typically measured for metals. Here, we argue that this discrepancy can be resolved if most junctions do not directly produce wear particles but instead cause material transfer and the formation of a weakly bound transfer film. Wear then occurs intermittently when fragments of this film detach due to crack propagation at the interface between the transfer film and the underlying bulk metal. We performed unlubricated reciprocating sliding experiments on nominally smooth stainless steel, brass, and aluminum. For steel on steel, the wear mass loss shows an initial stage with negligible mass change up to a sliding distance of ∼2.4m, followed by a nearly linear regime, while the friction coefficient changes much less over the same sliding-distance range. For dissimilar-metal contacts, transfer-film formation is evidenced by optical imaging, net mass gain of the steel slider, and energy-dispersive x-ray spectroscopy. The results also show strongly asymmetric wear for some dissimilar-metal pairs. In addition, the collected wear debris is flake-like. These observations support a transfer-film-controlled wear mechanism associated with cold-welded junctions.