Microstructural evolution and degradation modes in cyclic and isothermal oxidation of an EB-PVD thermal barrier coating

Thermal barrier coatings constituted of a partially stabilised zirconia layer (similar to 100 mu m thick) deposited by EB-PVD on top of a platinum-modified aluminide bondcoat to protect a single crystal superalloy substrate, have been studied in cyclic and isothermal oxidation at 1,100 degrees C in air. Close examination by high resolution SEM (with field emission gun) of the oxide scale and of the metallic surfaces after various durations, up to 600 hours at 1,100 degrees C, show several morphological features which may affect the integrity of the systems: partial decohesion regions with large grain areas at the inner surface of the scale, subsequently replaced by a cellular morphology, cavities within the oxide scale as well as at the zirconia-alumina interface. The nature and distribution of these features depend on the type of test (isothermal or cyclic oxidation) and on the systems studied (standard superalloy or modified to lower the sulphur content). Scenarios are proposed to explain the observations reported.