The Effects of High Temperature Exposure on the Durability of Thermal Barrier Coatings

Abstract

Abstract Yttria-stabilized Zirconia (YSZ) coatings deposited by electron beam physical vapor deposition on platinum aluminide and NiCoCrAlY bond coats on single crystal superalloy substrates have been oxidized at temperatures between 1000 and 1200°C in air. The cyclic oxidation lives of the systems with platinum aluminide bond coats were substantially longer than those with NiCoCrAlY bond coats. The thermally grown oxide (TGO) that develops between the bond coat and the TBC during oxidation, as well as the bond coat and the TBC adjacent to the TGO, have been examined in detail using optical metallography, scanning electron microscopy (SEM), and cross-sectional transmission electron microscopy (XTEM). The YSZ is observed to undergo significant amounts of sintering. The TGO grows by the inward diffusion of oxygen and the outward diffusion of aluminum. In some cases, the outward growth component incorporates some of the TBC into the TGO. The depletion of aluminum results in phase changes in the bond coats. Failure of the TBCs occurs after fixed amounts of oxidation which result in increasing amounts of elastic energy being stored in the TGO and YSZ as well as degradation of the TGO-bond coat interface. The fracture path changed as a function of exposure time and temperature with larger amounts of separation occurring at the TGO/BC interface for higher temperatures and longer exposures in dry air. Failure can be accelerated in the presence of water vapor, particularly if spinel formation is induced. Fracture occurs primarily in the oxides, in this case. The fracture surface for systems with platinum aluminide bond coats often contains precipitates, which are rich in refractory metals. These features do not appear to be prevalent with NiCoCrAlY bond coats.