Enhanced crystallization of α-(Cr, Al)2O3 coatings with increasing oxygen pressure

The synthesis of corundum structured α-Al2O3 coatings at relatively low temperatures has obsessed researchers for decades. Here, we propose an approach to promote the crystallization of α-(Cr, Al)2O3 by increasing the oxygen pressure during the deposition process. Specifically, cathodic arc evaporated (Cr, Al)2O3 coatings, synthesized from Al0.70Cr0.30 alloy targets under oxygen pressures of 0.8, 1.6, and 3.2 Pa, were investigated with reference to their microstructure, thermal stability, and mechanical properties. All three oxides exhibit an amorphous, cubic, and α-(Cr, Al)2O3 mixed structure. The latter crystallizes progressively with the increased oxygen pressure. Nevertheless, the (Cr, Al)2O3-3.2 coating containing most corundum constituents has the lowest hardness due to its uncompacted feature. Additionally, the transformation from metastable rock salt-type cubic-(Cr, Al)2O3 to α-(Cr, Al)2O3 upon annealing is accelerated with the elevated oxygen pressure, which leads to a strengthening in indentation hardness. This strengthening, together with the thermal-induced structural densification, results in a significant increase in hardness from ~19.0 to ~22.8 GPa for the corundum-containing coating after annealing at 1100 °C.