Effect of Y2O3 Content on the Phase Structure and Thermal Conductivity of Plasma-Sprayed YSZ Coatings

The thermal insulation performance of thermal barrier coatings (TBCs) is dominated by their thermal conductivity. With thermally sprayed YSZ coatings, their thermal conductivity is a sensitive function of Y₂O₃ content and phase structure besides the total porosity and lamellar bonding within the coating. In this study, 8-wt.%Y₂O₃-ZrO₂ (8YSZ), 13-wt.%Y₂O₃-ZrO₂ (13YSZ), and 20-wt.%Y₂O₃-ZrO₂ (20YSZ) coatings were deposited by plasma spraying. The amounts of monoclinic (M), tetragonal (T), and cubic (C) phase of coatings were estimated by x-ray diffraction and Raman analyses. Results revealed that the 8YSZ coating was a mixture of 80% T-phase and some M-phase, the 13YSZ coating consisted of T-phase and a small quantity of C-phase, while the 20YSZ coating was mainly composed of C-phase and a small amount of T-phase. Moreover, it was found that thermal conductivity of the coatings decreased with the increase of Y₂O₃ content. Those results suggest that the change of phase composition and the increase of oxygen vacancies in the coatings with the change of Y₂O₃ content were effective in reducing thermal conductivity.