On wear of TiAlN coated tools with and without NbN overlayer in machining titanium alloys.

Finding a wear resistant coating for cemented carbide cutting tools in the machining of difficult to cut Ti alloys is a challenge due to their high strength and chemical reactivity. Tool manufacturers recommend physical vapor deposited (PVD) Ti x Al 1-x N (x = 0.4–0.7), and an extra NbN overlayer has shown promising potential. This study explores wear mechanisms of PVD Ti 0.45 Al 0.55 N with and without NbN overlayer and its WC-Co substrate in machining Ti alloys. To achieve an accurate understanding of tool-chip-workpiece interaction and related wear mechanisms, several approaches were employed. Tests with controlled variation of cutting speeds were complemented by process freezing experiments using the quick stop method and imitational experiments of diffusion couples. Advanced microscopy techniques were employed for accurate detection of wear products and phenomena across length scale. Findings reveal that any new design of coatings for Ti machining must combine both high mechanical integrity and resistance to diffusional dissolution and oxidation. Observed diffusional loss of Al and N from the coating results in a TiN layer which is mechanically weaker than the original coating, while the NbN overlayer reduces the Al diffusion rate, but NbN is subjected to diffusional dissolution itself. On dissolution, Nb stabilizes β-Ti and thus facilitating loss of Al, but the observed formation of intermetallic Nb 3 Al at the NbN–Ti interface works as a diffusion barrier. However, brittle Nb 3 Al can be more easily removed during machining. It was found that the coating retains longest on the edge line and protects the tool edge from failure because substrate cemented carbide wears at a faster rate than the coating with outward diffusion of C from WC grains and Co binder. [Display omitted] • Degradation of Ti 0.45 Al 0.55 N ± NbN WC-6%Co is governed by mechanical and diffusion wear. • Diffusion of Al from Ti 0.45 Al 0.55 N govern its wear forming a weaker TiN layer. • Diffusion of NbN overlayer causes its wear and forms diffusion barrier Nb 3 Al. • A novel combination of methods to explore tool wear mechanisms is introduced. • New coating designs need improved diffusional resistance for higher performance. [ABSTRACT FROM AUTHOR]