Improved Ti₁–xAl_xN PVD Coatings for Dry High Speed Cutting Operations

Abstract

Titanium aluminium nitride (Ti₁–_xAl_xN) has been shown to enhance the performance of cutting tools in dry, high speed machining of cast iron and die steels when compared with TiN and TiCN coatings. The ultimate benefit of Ti₁–_xAl_xN coatings depends on two main criteria: the microroughness of the coating surface has to be minimised, and the oxidation resistance has to be maximised. The microroughness is governed by the choice of deposition method whereas the oxidation resistance is controlled by the composition of the coating. In particular, the generation of macroparticles during the PVD process has to be suppressed and the Al content of the film metallic constituents has to be kept between 40 and 60 at.-%. To achieve these demands supplementary to excellent adhesion, a PVD deposition process has been developed based on combined steered arc and unbalanced magnetron sputtering techniques. Using Cr in the steered arc sustained metal ion etching process the desired adhesion criteria may be exceeded and the surface roughness is kept as low as R_a = 0·04 μm. Unbalanced magnetronsputtering takes place from 50: 50 (at.-%) TiAI alloy targets. Further improvement has been achieved by incorporation of 3 at.-%Cr and, in particular, about 2 at.-% Y into the TiA1N coating, leading to an optimised film composition of Ti_0·42Al_0.53Cr_0.03 Y0.02N. Cross-sectional TEM analysis, SNMS depth profiling, and thermogravimetric evaluation reveal grain refinement, interface stabilisation, and reduced oxidation at temperatures up to 950°C. These coatings allow enhanced dry drilling performance in GG 25 grey cast iron at a cutting speed of 60 m min^–1 and feed of 0·2 mm rev^–1, and dry milling of 56 HRC die steel with a 300 m min^–1 (12000 rev min^–1) cutting speed and linear feeds up to 6 m min^–1, with a total lifetime of more than 20 h for this latter application.