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 calciamagnesia-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 degrees C. Additionally, a 'model' experimental study was conducted by characterizing 7YSZ ceramic powders immersed in molten CMAS glass at 1300 degrees 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. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.