THE INFLUENCE OF LOAD AND COUNTERFACE ON THE EFFECTIVENESS OF THIN PHYSICALLY VAPOR-DEPOSITED TITANIUM NITRIDE COATINGS IN PROVIDING WEAR-RESISTANCE AT TEMPERATURES TO 500-DEGREES-C

A study was carried out into the tribological performance of 321 stainless steel, and similar specimens coated with a thin layer of titanium nitride, in carbon dioxide during reciprocating sliding at 20-500-degrees-C, using a pin-on-disc geometry with a spherical-ended pin of radius 50 mm, under like-on-like and like-on-unlike conditions. At 20 and 300-degrees-C, under a load Of 16.3 N, the presence of the coating on the surface of both contacting specimens results in very significant reductions in friction and wear rate compared with the corresponding uncoated specimens, by eliminating metal-to-metal contact. However, progressive thinning of the coating leads, eventually, to its failure, particularly at the higher temperature. The coating is less effective when present on one of the contacting surfaces only and is removed rapidly during sliding against an uncoated stainless steel substrate. During like-on-like sliding at 500-degrees-C, the coating ensures that metal-to-metal contact is eliminated in the early stages. Although softening of the substrate results in penetration of wear grooves through the coating after some time, oxidation of the "posed substrate ensures that low friction and wear rates are maintained. Again, coating on one of the specimens only is not successful since it is removed rapidly in the sliding test. Although the coating is reasonably effective at loads to about 20 N in giving wear protection during like-on-like sliding at 500-degrees-C, failure of the coating occurs rapidly at a load of 29.1 N and the resulting wear rate is greater than that observed for the uncoated specimens under like-on-like conditions, where the eventual development of a wear-protective oxide glaze gives relatively low friction values and wear rates.