High temperature oxidation of thermal barrier coating systems on RR3000 substrates: Pt aluminide bond coats

The isothermal oxidation behaviour of RR3000, a single-crystal Ni-base superalloy, coated with one of two Pt aluminide bond coats: a single-phase coat or a two-phase coat, has been investigated in dry air at 1100 and 1200 degreesC. The main oxide formed on the bond coats at both temperatures was found to be alpha -Al2O3; this was more susceptible to spallation on cooling from 1200 degreesC than from 1100 degreesC. In addition to alpha -Al2O3, gamma -Al2O3 was formed at 1100 degreesC on the two-phase bond coat, which is less rich in aluminium than the single-phase bond coat. The thermal-cycling oxidation behaviour of the RR3000 superalloy, coated with the two-phase Pt aluminide bond coat and an yttria partially-stabilised zirconia top coat, has been studied in dry air at 1200 degreesC. Failure of the thermal barrier coating system was found to occur after similar to 240 one-hour-cycles. Spallation occurred mainly at the bond coat/oxide scale interface. The microstructures of the bond coats were found to change during the oxidation process, with significant interdiffusion of the substrate and bond coat elements.