The role of aluminum additions in the oxidation and wear of a TaC:C low-friction coating

A very attractive feature possessed by many carbon-based coatings is the ability to provide a combination of lowered friction and high wear resistance in various sliding contact situations. The flair for low friction is commonly attributed to large amounts of surplus carbon present in the microstructure. However, the surplus carbon also makes these coatings rather susceptible to oxidation. To expand the field of application for carbon-based low-friction coatings it is important to improve their oxidation resistance. Of course this must not be accomplished by impairing their main function, of giving low friction. In an attempt to increase the oxidation resistance of a TaC:C coating without sacrificing its extremely good tribological behaviour, Al was added in increasing amounts at the expense of carbon. The effects of Al addition was evaluated on as-deposited coatings from a mechanical and tribological standpoint. The oxidation resistance of the coatings was evaluated in air at temperatures up to 500 °C and the resulting oxides were analyzed as to their chemistry and thickness.The oxidation rate was found to decrease quite rapidly with increasing amounts of Al. Moreover, it was found that the Al addition could be optimized to give low friction and reduced oxidation rate at only a moderate increase in wear rate.