Structure, mechanical properties, and thermal stability of arc evaporated (Al1-xCrx)2O3 coatings

Cathodic arc evaporated (Al1-xCrx)2O3 coatings, prepared from powder-metallurgically produced Al0.75Cr0.25, Al0.70Cr0.30, Al0.50Cr0.50, and Al0.25Cr0.75 cathodes using an industrial-type batch facility were investigated with respect to their microstructure, mechanical properties, and structural evolution upon annealing in vacuum and oxidising atmosphere. Both Al-rich coatings crystallise in a mixed microstructure consisting of a corundum-type solid solution and fine-crystalline metastable cubic phase fractions. The latter irreversibly phase transform into α-(Al1-xCrx)2O3 between 800 and 1050 °C, which either proceeds via a γ- or θ-structured intermediate stage depending on the annealing environment (i.e., vacuum or flowing O2). Both Cr-rich coatings initially crystallise in a corundum-type solid solution, and are therefore not affected from any phase transition upon annealing to at least 1500 °C. Hardness and indentation moduli of our coatings are, next to the phase composition, strongly related to the microstructure but also the substrate material used, showing a relatively large spread from 11 to 23 GPa for H and 190 to 375 GPa for E, respectively.