Effects of diffused hydrogen atoms on thermomechanical properties and contact behavior of a diamond-like carbon film

Hydrogen atoms are doped to diamond-like carbon (DLC) to improve its thermomechanical properties and tribological performance as a surface protective coating. In this study, molecular dynamics (MD) simulations are performed to investigate the impacts of diffused H atoms on the mechanical stiffness, surface energy, specific heat, and thermomechanical contact behavior of DLC. The hydrogenated DLC (a-C:H) is prepared by adding H atoms to a fixed amount of C atoms (method 1) and by replacing C atoms in DLC with H atoms (method 2). The atomic percentage of hydrogen (at. % H) in DLC is varied from 0 to 8.6%. From the systematic MD simulation results, it is observed that the DLC's mechanical stiffness increases with at. % H due to the increasing density with a higher sp(3)%, but it shows a decreasing trend for method 2 due to the decreasing density. During the sliding contact with a hemispherical diamond tip, the a-C:H samples show a lower coefficient of friction (COF) than the hydrogen-free DLC (ta-C) sample for method 1 but a higher COF for method 2, which can be attributed to the changes in density and surface energy with respect to hydrogen contents in DLC.