Unraveling electronic fluctuation in passivation hypothesis for ultralow friction in diamond

Abstract

Passivation hypothesis has been widely applied to interpret the outstanding tribological properties of diamond, where the ultralow friction is often attributed to the static charge repulsion between the passivated sliding surfaces. However, the action of the dynamic charge of the contact in the sliding process is still open. Here, the mechanism of passivation hypothesis is disassembled by the electronic fluctuation in the ultralow friction sliding. Typical frictional systems of diamond contact surfaces, ranging from different crystallographic structures and interfacial passivated groups, are studied by using density-functional theory (DFT) calculations. The charge density evolution along the sliding pathway is investigated to track the frictional energy dissipation in the mutual sliding between the passivated surfaces. It finds that the shallow energy corrugation synchronously varies with the charge redistribution during the slip, giving a linear dependence of ultralow friction on the electronic fluctuation acting within the sliding interfaces. The proportionality coefficient, which indicates the response of charge density in the ultralow dissipation, gives intrinsic quantity of the friction systems and is mainly determined by both the passivation groups and the lattice parameters of the sliding surfaces. Therefore, we demonstrate the dynamic fluctuation of charge density along the sliding direction underlies the mechanism of passivation hypothesis. An electronic fluctuation model is proposed to interpret the passivation hypothesis for guiding the design of superlubric diamond-like carbon (DLC). This is in contrast with the paradigm that the super-low friction is determined by the static charge repulsion.