THERMAL-STABILITY AND OXIDATION RESISTANCE OF TIA1N CRN MULTILAYER COATINGS/

TiAlN/CrN multilayer films with superlattice characteristics were depo sited onto M2 high-speed steel and 304 stainless steel by reactive unb alanced magnetron sputtering involving three TiAl targets and one Cr t arget. The wavelength of the multilayer structures (ranging from 2.4 n m to 4.8 nm) and Cr content (from 10% to 30%) was controlled by varyin g 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 ra nging from 500 to 1000 degrees C and for times up to 16 h. Breakdown o f the multilayer structure was studied using such techniques as hardne ss measurements, X-ray diffraction (XRD) and cross-sectional TEM (XTEM ). The results show that both the multilayer structure and hardness ca n be maintained for as long as 16 h at 700 degrees C. This was found t o be independent of the Cr content (multilayer wavelength) of the coat ings. Oxidation behaviour involving isochronal thermogravimetric analy sis (TGA), XRD and SEM revealed that all coatings exhibited weight gai ns 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 th e Cr content, with the greatest effect occurring in the P-Cr = 12 kW s amples. Two very distinct regimes of oxidation were observed. Up to ap proximately 900 degrees C, the rate of oxidation was very low (approxi mately 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, th e 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.