Friction and adhesion of fluorine containing hydrophobic hydrogenated diamond-like carbon (F-H-DLC) coating against magnesium alloy AZ91

Tribological tests were performed on hydrogenated diamond-like carbon (H-DLC) and F containing H-DLC (F-H-DLC) coatings sliding against a magnesium alloy (AZ91) in air with relative humidity (RH) ranging between 0% and 75%. Pin-on-disk type friction tests typically exhibited high running-in COF, followed by a steady state regime with a low and constant coefficient of friction (μS). At 32% RH both the F-H-DLC and the H-DLC showed similar friction behavior, but μS of H-DLC increased with an increase in RH while μS of F-H-DLC decreased. Carbonaceous transfer layers were formed on the AZ91 surfaces and incorporated F transferred from F-H-DLC. The transfer layers were passivated by OH molecules, as detected by the Fourier transform infrared and micro-Raman spectroscopy. A first principles interface model that examined the hydrophobic interactions between F and OH terminated diamond (111) surfaces showed that high repulsive electrostatic forces would contribute to low friction in high humidity atmospheres.