Transient phase evolution during oxidation of a two-phase NiCoCrAlY bond coat

The formation of the transient, metastable theta-alumina ({theta}-Al{sub 2}O{sub 3}) and its conversion to the stable alpha-alumina ({alpha}-Al{sub 2}O{sub 3}) protective oxide during the oxidation of a two-phase NiCoCrAlY bond coat is reported. The alumina phases were identified using their characteristic photo-stimulated Cr{sup 3+} luminescence spectra. The luminescence was also used to measure the residual stress in the {alpha}-Al{sub 2}O{sub 3} phase. It is concluded that {alpha}-Al{sub 2}O{sub 3} and {theta}-Al{sub 2}O{sub 3} initially form on different phases in the bond coat and that there is then a slow transformation during oxidation of the {theta}-Al{sub 2}O{sub 3} phase to {alpha}-Al{sub 2}O{sub 3} that preserves the platelet morphology characteristic of the original {theta}-Al{sub 2}O{sub 3} phase. The {theta}-Al{sub 2}O{sub 3} phase is detectable for more than 50 h at 1000 C and 2 h at 1100 C. The consequences on the life of thermal barrier coatings deposited on the two-phase bond coat are discussed. (orig.)