Friction and wear of DLC films in hydrogen gas

In order to investigate the effects of hydrogen on friction and wear of DLC (Diamond-like carbon), reciprocating sliding tests were conducted with a-C:H (hydrogenated amorphous carbon) and ta-C (tetrahedral amorphous carbon) DLC films in controlled gas including hydrogen, air, argon and vacuum. The specimens were transferred to XPS (X-ray photoelectron spectrometer) for surface analysis without exposure to ambient air. It was found that ta-C DLC exhibited lower coefficient of friction and specific wear in hydrogen than in other environments, while a-C:H DLC showed lower friction in air and hydrogen. These results were discussed with the surface analysis with XPS. It was suggested that the low coefficient of friction depended on oxygen content on the sliding surfaces. It was also suggested that graphitization affected the friction, and that ta-C DLC graphitized less in hydrogen than in the other environments, which resulted in the low friction.