Mechanical properties, structure and oxidation behaviour of Ti1-xAlxN-hard coatings deposited by pulsed dc plasma-assisted chemical vapour deposition (PACVD)

Wear resistant Ti1-xAlxN hard coatings with different compositions (0 less than or equal to x less than or equal to 1) were deposited using d.c. plasma-assisted chemical vapour deposition on high-speed steel. TiCl4 and AlCl3 were used as metal precursors. The influence of the AlCl3 partial pressure on the properties of the coatings was examined. The aluminum content in the coating increases linearly with the AlCl3 partial pressure. The micro-hardness of the Ti1-xAlxN coatings is about 2000 HV 0.01 for low aluminum contents (x<0.5), but decreases to a value of 1300 HV 0.01 for higher aluminum contents. The adhesion, measured with the scratch test, decreases with increasing aluminum content. XRD measurements reveal the fee structure up to an aluminum content of x=0.8. For x>0.8 a weak (201) peak of the hexagonal Wurtzite structure is visible. But the Wurtzite structure is confirmed only for x=1. The oxidation experiments were carried out in air at 873 K. XPS depth profiles show a better oxidation resistance of Ti1-xAlxN as compared to TiN. The oxidation resistance increases with increasing aluminum content. The aluminum content near the surface of Ti1-xAlxN coatings increases during the oxidation. Aluminum atoms were found to diffuse up to the surface. (C) 1998 Elsevier Science S.A. All rights reserved.