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. Pt-Au, a proposed stable alloy consisting of two noble metals, is shown to exhibit extraordinary resistance to wear. Ultralow wear rates, less than a monolayer of material removed per sliding pass, are measured for Pt-Au 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 mm N m , comparable to diamond-like carbon (DLC) and sapphire. Remarkably, the wear rate of sapphire and silicon nitride probes used in wear experiments are either higher or comparable to that of the Pt-Au alloy, despite the substantially higher hardness of the ceramic probe materials. High-resolution microscopy shows negligible surface microstructural evolution in the wear tracks after 100k sliding passes. Mitigation of fatigue-driven delamination enables a transition to wear by atomic attrition, a regime previously limited to highly wear-resistant materials such as DLC.

The role of water in the tribochemical mechanisms of ultralow wear polytetrafluoroethylene (PTFE) composites was investigated by studying 10 and 20 wt % polyether ether ketone (PEEK)-filled and 5 wt % αAl2O3-filled PTFE composites. These composites were run against stainless-steel substrates in humidity, water, and dry nitrogen environments. The results showed that the wear behavior of both composites was significantly affected by the sliding environment. Both composites achieved remarkably low wear rates in humidity because of tribochemically generated carboxylate end groups that anchored the polymer transfer films to the steel substrate. In nitrogen, PTFE–PEEK outperformed PTFE−αAl2O3 because of polar carbonyl groups on PEEK, which increased the surface energy of PEEK, aiding it in adhering to the substrate and resulting in a transfer film. Both composites in water exhibited high wear. The water oversaturated the functional groups at the end of the polymer chains and prevented the formation of a transfer film.

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We report an investigation of the friction mechanisms of MoS2 thin films under changing environments and contact conditions using a variety of computational and experimental techniques. Molecular dynamics simulations were used to study the effects of water and molecular oxygen on friction and bonding of MoS2 lamellae during initial sliding. Characterization via photoelectron emission microscopy (PEEM) and Kelvin probe force microscopy (KPFM) were used to determine work function changes in shear modified material within the top few nanometers of MoS2 wear scars. The work function was shown to change with contact conditions and environment, and shown by density functional theory (DFT) calculations and literature reports to be correlated with lamellae size and thickness of the basally oriented surface layer. Results from nanoscale simulations and macroscale experiments suggest that the evolution of the friction behavior of MoS2 is linked primarily to the formation or inhibition of a basally oriented, molecularly thin surface film with long-range order.

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