Influence of Alkali Metal Oxide on CMAS Damage Progression in Air Plasma Sprayed Thermal Barrier Coatings

Abstract

Thermal barrier coatings (TBCs), which typically comprised of a yittria-stabilized zirconia (YSZ) ceramic top coat, a bond coat and a metallic substrate, have been widely used to protect the hot-section metallic components of gas turbine engines. More recently at high temperatures exceeding 1200℃, the TBCs can be damaged by calcium-magnesium-alumino-silicates (CMAS) resulting from the ingestion of siliceous minerals with the intake air and from unclean fuels. The chemical composition of CMAS is mainly comprised of CaO, MgO, Al2O3 and SiO2. In addition to that, alkali metal oxides can be interblended into CMAS constituents under some operating environments. In this study, the effect of alkali metal oxide (K2O) on CMAS damage progression on air plasma sprayed (APS) YSZ TBC specimens was investigated through simulating the CMAS damage at lab-scale employing synthetic CMAS products. Two types of synthetic CMAS products both with and without K2O were prepared. The synthetic CMAS was spread on the APSed TBC specimen, followed by isothermal exposure at different temperatures up to 1300℃ for various times. It was found that the infiltration of CMAS into the top coat was observed in the both CMAS compositions. However, the infiltration rate was significantly decreased due to the presence of K2O. In the case of CMAS without K2O, the reaction between CMAS and YSZ was significant resulting in the change in microstructural morphology, whereas it was little occurred on the CMAS including K2O. These results suggest that the addition of K2O to YSZ top coat can mitigate the CMAS damage progression.