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

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 loadsensitive 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.