Thermal stability and oxidation resistance of TiA1N/CrN multilayer coatings

TiAlN/CrN multilayer films with superlattice characteristics were deposited onto M2 high-speed steel and 304 stainless steel by reactive unbalanced magnetron sputtering involving three TiAl targets and one Cr target. The wavelength of the multilayer structures (ranging from 2.4 nm to 4.8 nm) and Cr content (from 10% to 30%) was controlled by varying the power supplied to the Cr target (P-Cr) from 2 kW to 12 kW. These coatings were then heated in an ambient atmosphere at temperatures ranging from 500 to 1000 degrees C and for times up to 16 h. Breakdown of the multilayer structure was studied using such techniques as hardness measurements, X-ray diffraction (XRD) and cross-sectional TEM (XTEM). The results show that both the multilayer structure and hardness can be maintained for as long as 16 h at 700 degrees C. This was found to be independent of the Cr content (multilayer wavelength) of the coatings. Oxidation behaviour involving isochronal thermogravimetric analysis (TGA), XRD and SEM revealed that all coatings exhibited weight gains of as little as 5% of that of TiN under similar conditions at 1000 degrees C. The resistance to oxidation was found to be dependent on the Cr content, with the greatest effect occurring in the P-Cr = 12 kW samples. Two very distinct regimes of oxidation were observed. Up to approximately 900 degrees C, the rate of oxidation was very low (approximately 0.2 gm(-2) in the case of the P-Cr = 4 kW samples and 0.1 gm(-2) in the case of the P-Cr = 12 kW samples). Above this temperature, the oxidation rates increased markedly for all samples, with that of the P-Cr = 12 kW being the least severe. (C) 1997 Elsevier Science S.A.