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The influence of temperature on the friction and wear of thin ceramic coatings in carbon dioxide

An investigation has been carried out into the effects of thin titanium nitride coatings, produced by several PVD and CVD processes, on the sliding friction and wear behaviour of 321 stainless steel in carbon dioxide under reciprocating sliding conditions at temperatures to 500°C. At 20°C and 300°C there was a very considerable reduction in both the coefficient of friction and the wear rate for the coated surfaces compared with the uncoated steel during the early stages of like-on-like sliding. However, after some time, which depended on the type of coating and the temperature, the friction and wear rate of the coated surfaces increased significantly. In some cases, this was accompanied by gross coating failure, followed by a severe rate of wear, similar to that observed for the uncoated steel. The general mechanism of coating failure involved thinning, leading to cracking and fragmentation, which produced areas of exposed substrate. The wear resistance of the coated surfaces was improved at 500°C. Although thermal softening resulted in deformation of the substrate through the coating, oxidation of the exposed alloy prevented severe damage resulting from metal-metal contact. Possible explanations for this behaviour are discussed in relation to the recorded tribological data and morphological features of the wear scars.