Phase Transformation in EB-PVD Yttria Partially Stabilized Zirconia Thermal Barrier Coatings during Annealing

Electron-beam physical-vapor-deposited thermal barrier coatings consisting of ZrO₂ stabilized by 7 wt% Y₂O₃ were investigated in regard to phase transformation after annealing. Free-standing ceramic layers were heat-treated in air, for up to 200 h, in the temperature range 1200°—1400°C and then analyzed by X-ray diffractometry. Based on information obtained from the {111} and {400} peaks, the phase composition and the Y₂O₃ content in the phases were calculated. At the start of transformation, small grains of a low-Y₂O₃ t phase and a c phase formed. After >30 h at 1300°C and at 1400°C, a mixture of a t phase deficient in Y₂O₃, an m phase, and a c phase formed after cooling, with the Y₂O₃ contents in the phases roughly predicted by the phase diagrams. The results of the present study are discussed here in detail and compared with data for plasma-sprayed coatings.