Computational investigation of the mechanical and tribological responses of amorphous carbon nanoparticles

Nanoparticles are a class of materials that have seen increasing use as friction and wear reducers in tribological applications. Amorphous carbon (a-C) films have been the subject of significant scientific and industrial interest for use as solid-state lubricants. Here, we present classical molecular dynamics simulations to investigate the mechanical and tribological responses of a-C nanoparticles that are subjected to external forces between hydrogen-terminated diamond surfaces. Over the range of a-C nanoparticle diameters (2-5 nm) and hydrogenation (0%-50%) considered, the simulations predict a consistent mechanical response where each nanoparticle is highly elastic. The simulations predict that the transition from elastic to plastic response is directly related to an increase in the percentage of carbon-carbon crosslinking within the individual nanoparticles. Contrarily, the simulations also predict that the tribological response is noticeably impacted by changes in diameter and hydrogenation. This is because during friction, hydrogen passivates the unsaturated carbon atoms near the nanoparticle's surface, which prevents interfacial bond formation and allows the nanoparticle to roll within the interface. From these findings, it is demonstrated that a-C nanoparticles are able to provide good tribological performance only when sufficient chemical passivation of the nanoparticles is maintained.