Achieving Ultralow Wear with Stable Nanocrystalline Metals

Recent work suggests that thermally stable nanocrystallinity in metals is achievable in several binary alloys by modifying grain boundary energies via solute segregation. The remarkable thermal stability of these alloys has been demonstrated in recent reports, with many alloys exhibiting negligible grain growth during prolonged exposure to near-melting temperatures. In this paper, we show that PtAu, a proposed stable alloy consisting of two noble metals, exhibits extraordinary resistance to wear. Ultra-low wear rates, less than a monolayer of material removed per sliding pass, were measured for PtAu thin films at a maximum Hertz contact stress of up to 1.1 GPa. This is the first instance of an all-metallic material exhibiting a specific wear rate on the order of 10-9 mm3/N-m, comparable to diamond-like carbon and sapphire. Remarkably, the wear rate of sapphire and silicon nitride probes used in wear experiments were either higher or comparable to that of the PtAu alloy, despite the substantially higher hardness of the ceramic probe materials. High-resolution microscopy showed negligible surface microstructural evolution in the wear tracks after 100k sliding passes. Finally, mitigation of fatigue-driven delamination enabled a transition to wear by atomic attrition, a regime previously limited to highly wear resistant materials such as diamond-like carbon.