High temperature oxidation of (Ti1-xAlx)N coatings made by plasma enhanced chemical vapor deposition

(Ti1-xAlx)N coatings were deposited by the plasma enhanced chemical vapor deposition method using a gas mixture of TiCl4, AlCl3, NH3, H-2 and Ar, and the oxidation behavior at high temperatures as well as the oxidized structure of the coatings were examined. The oxidation rate of the coatings at temperatures below 1000 degrees C was found to fit well to a parabolic time dependence. It has also been found that up to 900 degrees C (Ti1-xAlx)N coatings at X greater than or equal to 0.25 showed considerably improved oxidation resistance compared to TiN, due to the formation of a dense alpha-Al2O3 layer at the surface of the coatings. At X<0.25, however, this dense protective layer could not form, and the coatings were fully oxidized after a short period of time. When the temperature reached 900 degrees C, Fe in the substrate diffused out through the coating by pipe diffusion as well as through the cracks that were produced by thermal stresses, and oxidized rapidly at the surface of the coating. (C) 1999 American Vacuum Society. [S0734-2101(99)04101-9].