Tribological performance of different DLC coatings in water-lubricated conditions

Diamond-like carbon (DLC) coatings typically show low friction performance and high wear resistance. In order to evaluate how the presence of water changes the tribological performance of DLC coatings, different DLC coatings were evaluated by water-lubricated tribological tests. The different DLC coating types comprised both hydrogenated and hydrogen-free DLC films that were deposited using different techniques. The tribological tests were carried out by reciprocating sliding tests with a low sliding velocity (0.004 m/s). Alumina balls were used as the counterparts with a normal load of 5 N. The results showed that the amorphous hydrogenated carbon (a-C:H) coating could not survive in water-lubricated conditions, and was worn through during the test. The doped and multilayered hydrogenated carbon coatings exhibited improved wear resistance compared to the a-C:H coating. The hydrogen-free DLC coating deposited by vacuum arc discharge exhibited the best wear resistance in water-lubricated conditions showing only a smoothening type of wear. The friction coefficient of a-C:H film, which wore through, was high (0.7), but for the other DLC coatings the friction coefficient values were in the range of 0.04–0.1. The hydrogen-free coatings deposited by vacuum arc discharge had a high friction coefficient in the beginning of the test, but the friction coefficient decreased during the test and the lowest friction coefficient values in these tests were recorded for these coatings at the end of the tests.