Mechanical properties and oxidation resistance of Al-Cr-N/Ti-Al-Ta-N multilayer coatings

Arc evaporated Al-Cr-N and Ti-Al-N are widely used protective coating materials due to their outstanding properties. While Al-Cr-N is known for its excellent oxidation resistance, Ti-Al-N exhibits interesting thermo-mechanical properties based on age hardening effects accompanying the spinodal decomposition upon annealing. Previous studies showed that the addition of Ta to Ti-Al-N further enhances the mechanical properties and oxidation resistance. In this work, we combine the beneficial properties of both systems (Ti-Al-Ta-N with a Ta content of about 7 at.% on the metal sublattice) in nano-scaled multilayer films with different bilayer periods of 15, 24, and 31 nm and total film thicknesses of 3 and 20 mu m, respectively. We found significantly higher hardness values of up to 35 +/- 2 GPa for the Al-Cr-N/Ti-Al-Ta-N multilayer coatings as compared with their monolithically prepared single phase face-centered cubic structured counterparts, Al-Cr-N with 28 +/- 2 GPa and Ti-Al-Ta-N with 32 +/- 2 GPa. All coatings exhibit a single phase face-centred cubic structure in the as-deposited state. Due to the formation of an Al-rich oxide scale, the high temperature oxidation resistance of the multilayer coating is nearly as good as that of Al0.62Cr0.38N, withstanding oxidation at 900 degrees C easily for >20 h with a parabolic growth rate constant of k(p) similar to 0.01 mu m(2)/h. Increasing the total film thickness to 20 mu m increases the measured hardness to up to 42 +/- 2 GPa and at the same time provides additional protection of the coated parts against oxidation.