Analysis of a-C:H running-in behavior in hydrogen-containing atmosphere

The running-in of hydrogenated amorphous carbon (a-C:H) surfaces in hydrogen-containing gaseous atmospheres is not yet fully understood, despite its significance in tribological applications. This study analyzes the running-in of a-C:H coatings against a reciprocating steel specimen using tribological experiments and quantum chemical simulations. The influence of temperature and coating hardness on the tribological performance was investigated experimentally. A non-explosive environment with 5 % hydrogen and nitrogen as the carrier gas was used. The results show that higher temperatures reduce the running-in distance and lower the averaged coefficient of friction (COF) after running-in, while wear volumes do not show a clear temperature dependency. Experiments with a-C:H coatings of distinct hardness indicate that the harder a-C:H coating needs more sliding distance of the moving steel counterbody to reach a low and stable COF and experiences more wear in this interval, but shows less wear after the running-in phase compared to a softer a-C:H coating. It is shown that the transfer film changes its structure during the running-in. Results from quantum chemical simulations suggest that the running-in of the investigated tribological systems is controlled by dissociative chemisorption of hydrogen molecules on a-C:H surfaces. While we can conclude that physisorption of molecules on the surfaces does not play an important role in the running-in process, a possible effect of surface passivation by aromatization cannot be ruled out.