PVD COATINGS FOR TOOL APPLICATIONS: TRIBOLOGICAL EVALUATION

Abstract

Four different PVD coatings; CrN, TiN, (Ti, Al)N, and Ti(C,N) have been evaluated with respect to their mechanical and tribological properties. Two different tool steels were used as substrate materials. Several modern analytical methods, including glow discharge optical emission spectroscopy, and three different aboratory wear tests (erosion, abrasion, and sliding wear) have been used. The investigation shows that in order to minimise wear in erosion of coated parts, a coating–substrate composite with high substrate and coating hardness and sufficiently high coating thickness should be chosen. In particular, it was found that theinfluence of coating hardness increases with substrate hardness. A high coating fracture toughness also aids erosion resistance. The difference in intrinsic abrasive wear resistance between the investigated coatings is small. This implies that other coating properties, e.g. oxidation resistance etc., are the limiting factors whenone of these coatings is selected for a given tribological application. From the results of the sliding wear studies it was concluded that the amount of wear of the coated specimen decreases with increasing coating fracture toughness and coating hardness. To retain these properties at high sliding speed the chemical and thermal properties of the coating-substrate composite are highly important. The present work highlights the importance of the coating fracture toughness. This is a property that needs to be more closely examined and, in particular, there is a clearly defined need fordevelopment of reliable, easy to use experimental methods for fracture toughness determination. It should also be noted that the overall performance of the (Ti,Al)N coatings proved to be the best of all tested coatings, which means that if an uncertainty as to which coating material to choose arises, (Ti,Al)N is likely to be a good first choice.