Extreme contact pressure-induced in-situ structural evolution of nanoclusters governing macroscopic superlubricity in a-C:H films

Abstract

Though hydrogenated amorphous carbon (a-C:H) films can provide macroscale superlubricity states in vacuum, their self-lubricating behaviors are highly dependent on the applied loads. The mechanisms of loss of superlubricity under ultra-low or extremely high contact pressure remain unclear. In this work, the origin of load-sensitive superlubricity of a-C:H films was revealed based on spatially resolved structural analyses of the sliding interfaces. The results highlighted the key role of contact pressure-induced diversified nano-structural evolution of transfer films in controlling superlubricity. To achieve superlubricity, a sufficiently high contact pressure was required to trigger the structural evolution of transfer films from polymer-like disordered bonding network structure towards locally ordered, layered-like sp² nanoclustering structures. Robust superlubricity can still be maintained under extremely high peak Hertz contact pressure up to 4.87 GPa, which is the highest value reported for macroscopic superlubricity in carbon-based materials. Nevertheless, excessively high contact pressure can cause an increase in the interfacial shear strength due to the pressure-induced generation of heterogeneous transfer films with thin, poor-hydrogenated, over-graphitized local regions embedded with enriched ironic sub-micro debris and nanoparticles, which inhibited further decrease of friction coefficient under extremely high contact pressure. These findings will enable more effective space applications of superlubricious a-C:H films under extreme conditions.