Controlled Oxidation of MCrAlY Coatings at Low Pressures

The effect of partial oxygen pressure and a native oxide on the structure, morphology and chemical constitution of the initial oxide layer grown on a MCrAlY bond coat was investigated. To this end, a state-of-the-art Ni-19Co-18Cr-21Al-0.2Y (at.%) bond coat, as prepared by electron beam physical vapour deposition, was oxidised in a UHV reaction chamber, with or without its native oxide, at 1373 K and a partial oxygen pressure of 0.1 Pa. The structure, morphology and chemical constitution of the initially grown oxide layer, as well as the oxidation-induced changes within the coating, were characterised using a broad range of analytical techniques. Special attention was paid to differences in the distribution of Y and the presence of any non-protective oxide phases within the initially grown oxide layer. The results were compared with a corresponding oxidation treatment at atmospheric conditions in a conventional tube furnace. It is shown that the exclusive formation of a continuous, closed a-Al2O3 layer with a strong Y-enrichment at its outer surface is promoted at low partial oxygen pressure and in the absence of a native oxide.