Factors affecting the tribological behaviour of thin hard TiN and TiC coatings

Abstract

The use of thin hard coatings, often of ceramic materials, appears to offer the possibility of controlled improvement of the tribological behaviour, surface damage resistance and corrosion protection of many engineering components. However, it is only recently that the full complexities of the behaviour of coated systems are being appreciated. The mechanical response of a coated surface to scratching and asperity penetration is complex, involving some properties intrinsic to the coating ( e.g. coating material and thickness), some controlled by processing ( e.g. coating microstructure and residual stress state), some determined by the substrate ( e.g. flexure relative to the coating and deformation in response to stresses transmitted through the coating) and some controlled by interfacial adhesion. Also, raising the temperature of coated systems, for example either in genuine high temperature applications or by frictional heating during ambient temperature service, can be detrimental to coating performance and integrity in that it may alter the microstructure, create pores which allow environmental ingress, relax the residual stress state of the coating and affect interfacial bonding. Based on the authors' own work with TiN and TiC coated steels, this paper discusses these factors and highlights the problems of tailoring coatings to particular applications, predicting coating performance and assuring quality and reliability.