Isothermal oxidation behaviour of EB-PVD MCrAlY bond coat

The isothermal oxidation behaviour of EB-PVD NiCoCrAlY bond coat on nickel based superalloy at different treatment conditions were studied at 1100°C up to 100h. The reaction products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDS). It is found that after 10h oxidation, while a protective alumina layer was developed on pre-oxidized specimens, a mixed oxide layer was observed on non-pre-oxidized specimens. Only after 50h, spallation was observed on pre-oxidized specimens but did occur after 10h for a non-pre-oxidized one, in both cases it is believed that the diffusion of the substrate element Ti to the surface of the bond coat deteriorates the adhesion of protective oxide to the bond coat and hence leads to spallation. Oxide rich in Ti was found to be located on the top of column grains rather than in the column grain boundaries.