Calcia-magnesia-alumino-silicate (CMAS)-induced degradation and failure of air plasma sprayed yttria-stabilized zirconia thermal barrier coatings

Thermal barrier coatings (TBCs) used in gas-turbine engines experience severe degradation by calcia-magnesia-alumino-silicate (CMAS) deposits during high-temperature operation. The present study identified and evaluated the chemical and microstructural changes in air plasma-sprayed (APS) 7 wt.% Y₂O₃ stabilized ZrO₂ (7YSZ) TBCs caused by CMAS attack under isothermal conditions at 1340°C. Additionally, a 'model' experimental study was conducted by characterizing 7YSZ ceramic powders immersed in molten CMAS glass at 1300°C for different exposure times. The combined results from both studies highlight the importance of local CMAS glass composition on the 7YSZ/CMAS interaction. Specifically, low Y-content in the glass, caused by a relatively large glass 'sink,' produces Y-depleted ZrO₂ grains that undergo tetragonal (t) → monoclinic (m) phase transformation upon cooling. Alternatively, small pockets of Y-enriched glass induce the formation of t″-ZrO₂, a phase characterized by its high stabilizer content. After prolonged high-temperature exposure, solution-reprecipitation induces the formation of both m-ZrO₂ and t″-ZrO₂ throughout the APS 7YSZ TBC in accordance with the phase diagram. Using a thermomechanical model it is shown that the strain associated with the martensitic t→m phase transformation plays an important role in the delamination failure of TBCs attacked by CMAS.