Study of hard PVD coatings on strongly curved surfaces

Abstract This paper describes results of a study on the behaviour of hard coatings on the edges of tools and machine parts — or, more generally, on substrates with strongly curved surfaces. The study is concerned with coatings having high Young's modulus deposited on metal substrates, typically titanium nitride (TiN) coatings on high-speed steel (HSS) substrates. The differences in the behaviour of the coatings on flat substrates and on strongly curved substrates or edges have their origin in several reasons: effects of cleaning processes before deposition, different conditions of coating growth on edges during physical vapour deposition (PVD) processes and different operating conditions of coatings on tool edges. Because of the complexity of theoretical solutions, an experimental approach is preferred in this paper. The behaviour of TiN coatings deposited by two kinds of deposition process on two types of cutting tool was investigated: one was deposited by reactive low-voltage arc evaporation on cutting tools with almost a right angle between the face and flank, the other was deposited by reactive direct-current magnetron sputtering on a cutting tool similar to shaving blades. The degradation of the coatings during tool use was investigated as a function of the form of the edges and substrate bias during deposition. It was shown that coating integrity and tool lifetime, especially in the zones in the vicinity of edges, were significantly influenced by the bias voltage, more substantially than in case of coatings on even surfaces. In several cases a spontaneous breakdown of the coating during growth was observed.