Kinetics of Thermally Grown Oxide at Interface Between Thermal Barrier Coatings and MCrAlY Bond Coatings

Abstract

Abstract In recent years, thermal barrier coatings (TBC) have been used in advanced gas turbine plants for improved durability and performance. Typically, TBCs consist of an inner layer of metallic bond coating (MCrAlY) and an outer layer of ceramic top coating (8wt% yttria stabilized zirconia (YSZ)). According to several studies, the failure of coating is induced by thermal stress due to formation of oxides at the interface between YSZ and MCrAlY. Therefore, it is important to investigate kinetics of oxidation at the interface. In this work, the interface between YSZ and MCrAlY is studied and characterized. TBC specimens are thermally aged at 1000 deg. C to simulate the surface temperature of first rotating blades. After aging, thermally grown oxide (TGO) is formed at the interface. The TGO has two layers of different contrasts. One layer is black, and is closer to MCrAlY; the other layer is gray, and is closer to YSZ. The black layer is identified as Al2O3 by energy dispersive X-ray spectroscopy (EDX) and electron probe micro analyzer (EPMA). The EDX and EPMA spectra of the gray layer contain various peaks of Al, Cr, Co, Ni, and O, which suggests that the layer can be mixed oxide which is a combination of NiO, CoO, Cr2O3, and Ni(Cr, Al)2O4. However, the outer layer of mixed oxide contains only Cr and O. Accordingly, Cr2O3 forms at the outer layer of mixed oxide, and the other oxides are distributed at the inner layer of mixed oxide. The thickness of the two oxide layers increases with aging time. While the formation of mixed oxide layer obeys a parabolic law, the formation of an alumina layer cannot be expressed in terms of a parabolic law. Due to the formation of protective mixed oxide on the alumina layer, the oxidation rate of alumina decreases as the thickness of mixed oxide increases.