Effect of thermal exposure on the microstructure and properties of EB-PVD gradient thermal barrier coatings

The effects of thermal exposure on the microstructure, properties and failure of electron beam physical vapor deposited Al₂O₃-yttria stabilized zirconia (YSZ) gradient thermal barrier coatings (GTBCs) were studied. The GTBCs, with lifetimes of more than 500 h for 1-h cycles and 13 h for 0.25-h cycles at 1100 °C, exhibited a better thermal shock resistance than two-layered-TBCs consisting of a NiCoCrAlY bond coat and a YSZ topcoat. The GTBCs also showed a relatively low oxidation rate under cyclic exposure, due to the formation of a pre-deposited Al₂O₃ film on the bond coat. The oxidation of the bond coat is dominated by the selective oxidation of aluminum. The values of hardness and modulus for the Al₂O₃-YSZ graded layer are in the range of 2.0-3 and 170-230 GPa, respectively, whereas after 500 h exposure a considerable reduction in the mechanical properties occurred in a rich Al₂O₃ area of the graded layer. Micro-cracks initiated and propagated in the rich Al₂O₃ area, and finally resulted in the spallation failure of the coating. The thermal conductivity of a GTBC was found to be 1.7 W/mK, which was marginally lower than that of the two-layered-TBC at 2.2 W/mK. However, the value of thermal conductivity increased up to 2.0 W/mK after 500 h exposure.