Investigation of CMAS resistance of sacrificial suspension sprayed alumina topcoats on EB-PVD 7YSZ layers

Thermal barrier coatings (TBCs) undergo severe degradation by interaction with molten calcium-magnesium-aluminum-silicate (CMAS) minerals that are found in volcanic ashes or desert sands. In this work sacrificial alumina coatings were sprayed on top of EB‑PVD 7YSZ layers using suspension thermal spraying starting from an aqueous Al₂O₃ suspension. The spray parameters were varied in order to produce topcoats with morphologies with a wide porosity range. The coating systems were tested under different CMAS attacks by performing infiltration experiments at 1250 °C up to 10 hours. The microstructures of the Al₂O₃ coatings as well as the chemical composition of the CMAS strongly influenced the infiltration kinetics. Furthermore, the annealing time plays a major influence on the formation of various phases. CMAS mitigation depends on the interaction between the coating morphology which dictates the driving force for infiltration, the reaction speed between alumina and the deposit, and the deposit chemistry.