ABSTRACT
One of the first requirements for the application of thermal barrier coatings on hot stage turbine components is a high bond strength between the metallic bond coat and the ceramic undercoat. In the case of yttria partially stabilized zirconia top coat plasma sprayed on a NiCrA1Y bond coat, oxide scale growth at the ceramic/metal interface is a major contribution to the enhancement of interfacial thermomechanical resistance. In order to better understand this phenomenon, microstructural observations of the alumina scales formed at 1100° C and 1200° C under air, between low pressure plasma sprayed NiCrAlY and air plasma sprayed ZrO2 – 8. 5 wt. % Y2O3, have been performed by classical and analytical transmission electron microscopy on transverse thin foil specimens. The evolution of the oxide grain morphology from the metal/oxide to the oxide/oxide interface has been studied. The oxide grains are small and equiaxial near the oxide/zirconia interface, coarser and elongated near the MCrAlY/oxide interface, suggesting that the scale growth principally takes place at the latter interface. Segregation of yttrium at oxide grain boundaries has been detected either as a solute or as Y2Al5O12 garnet precipitates. Significant quantities of zirconium are also present inside the oxide grains. The oxide growth seems to be dominated by a classical grain boundary oxygen diffusion mechanism. The presence of zirconium inside the alumina grains also suggests that a secondary growth mechanism has taken place: Al2O3 partially forms at the oxide/zirconia interface by chemical reduction of ZrO2 by Al. The comparison between the microstructures observed and that of alumina scales grown under similar conditions on bare MCrAlY alloys gives some insight on how the ceramic topcoat modifies NiCrAlY high temperature oxidation mechanisms.
