Developments in PVD tribological coatings

Progress in the development and industrial adoption of PVD tribological coatings over the past two or three decades has been considerable. In particular, TiN coatings are in widespread usage in metal-cutting and other tooling applications. However the practical uptake of PVD coatings for components in high-volume industrial sectors (such as automotive) has been relatively slow. There have been many reasons for this. One was that the coatings being developed proved to be unsuitable for the low-cost (and relatively soft) substrates which are often used in high-volume applications. The emphasis on coating development often seemed to be the pursuit of hardness at the expense of other useful characteristics. With developments in the understanding of tribological contacts has come the realisation that hardness (H) may not be the only defining property for wear resistance. Here we discuss the importance of the elastic modulus (E) and the ratio H/E in determining the endurance capability of a surface coating, especially its ability to accommodate substrate deflections under load. This is set in the context of new multi-layered and nano-composite coatings which can provide a relatively low E value whilst retaining an adequate level of hardness. Furthermore the benefits of pre-treatments and interlayers to enhance the load support available to the coating from the underlying substrate are described. Overall, the paper presents an insight into how PVD tribological coatings have now been developed to satisfy the requirement of friction and wear contact conditions in many practical applications.