Atomic effect and mechanism of different hydrogen content on superlubricity properties of H-DLC films

Abstract

Hydrogen atoms play a pivotal role in achieving superlubricity in DLC films. Nevertheless, the inherent limitations in analytical testing techniques obscure the mechanisms by which hydrogen atoms achieve superlubricity in DLC films. Furthermore, direct evidence supporting the widely accepted mechanism of hydrogen passivation remains elusive. Therefore, we used ReaxFF MD simulations to investigate the effects of different hydrogen content on the tribological and wear properties of hydrogen-doped H-DLC films. Hydrogen atoms diffuse from the DLC films and gradually accumulate on the friction surface during the sliding, effectively passivating and inhibiting the C–C bonds at the interface. This reduces cross-linking between H-DLC films, consequently loweringfrictiontemperature significantly during the sliding. A significant release of hydrogen atoms during the friction process weakens the strength of machinery of H-DLC films, potentially causing structural failures under hydrogen-rich conditions, with the released hydrogen atoms subsequently occupying structural voids within films. At the microscopic scale, the friction of H-DLC film decreases with the increase of hydrogen content, but when the hydrogen content reaches 40 % or above, the local collapse of the system will be caused, resulting in the failure of superlubricity. Finally, the friction and wear of H-DLC film under high hydrogen conditions will rebound.