Thermal and thermo-mechanical properties of Ti–Al–N and Cr–Al–N coatings

Abstract

Metastable Ti–Al–N and Cr–Al–N coatings have been proven to be an effective wear protection due to their outstanding mechanical and thermal properties. Here, a comparative investigation of mechanical and thermal properties, for Ti–Al–N and Cr–Al–N coatings deposited by cathodic arc evaporation with the compositions (c-Ti0.52Al0.48N, c/w-Ti0.34Al0.66N and c-Cr0.32Al0.68N) widely used in industry, has been performed in detail. The hardness of Ti0.52Al0.48N and Ti0.34Al0.66N coatings during thermal annealing, after initially increasing to the maximum value of ~34.1 and 38.7GPa with Ta up to 900°C due to the precipitation of cubic Al-rich and Ti-rich domains, decreases with further elevated Ta, as the formation of w-AlN and coarsening of precipitated phases. A transformation to Cr2N and finally Cr via N-loss in addition to w-AlN formation during annealing of the Cr0.32Al0.68N coating occurs, and thus results in a continuous decrease in hardness. Among our coatings, the mixed cubic-wurtzite Ti0.34Al0.66N coating exhibits the highest thermal hardness, but the worst oxidation resistance. The Cr0.32Al0.68N coating shows the best oxidation resistance due to the formation of dense protective α-Al2O3-rich and Cr2O3-rich layers, with only ~1.4μm oxide scale thickness, after thermal exposure for 10h at 1050°C in ambient air, whereas Ti–Al–N coatings are already completely oxidized at 950°C.