Elevated temperature tribological behavior of non-hydrogenated diamond-like carbon coatings against 319 aluminum alloy

Diamond-like carbon (DLC) coatings are promising candidates for dry machining of aluminum alloys since in ambient conditions aluminum has much less tendency to adhere to the DLC coating surfaces compared to other hard coatings such as TiN, TiAlN and CrN. In an attempt to better understand the tribological properties of the DLC coatings for cutting tool coating applications, non-hydrogenated DLC coatings were produced by magnetron sputtering and their elevated temperature friction and wear behavior were studied. DLC coated M2 tool steel discs were tested against 319 Al pins using a high-temperature tribometer. Counterface materials of tungsten carbide (WC) and sapphire (Al2O3) balls were also tested for comparison. Tests were done at 25, 120, 300 and 400 °C in air. The wear resistance of the DLC coating has been found to be poor at elevated temperatures. The mechanisms that provide the high wear resistance of the coating in ambient temperature cease to operate at temperatures as low as 120 °C. Annealing at elevated temperatures in air causes oxidation of the coating. Although much softer than WC and sapphire, the 319 Al alloy causes the most severe wear to the DLC coating. A two- and three-body abrasive wear model is proposed to explain this surprising observation.