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.