Abradable ytterbium disilicate environmental barrier coatings: A story of CMAS and combined CMAS-erosion performance

Abradable environmental barrier coatings (EBCs) can be implemented to realise crucial gains in gas turbine efficiency. The aim of this study was to better understand how abradable coatings perform when exposed to molten calcium magnesium alumino-silicates (CMAS), one of the key challenges facing current EBC design, and how this exposure affects the mechanical properties of the abradable coatings. In this study, three ytterbium disilicate (Yb₂Si₂O₇) abradable EBCs containing 8, 15 and 22 % porosity were deposited using atmospheric plasma spraying. These coatings were then exposed to CMAS at high temperatures for 0.5 h, 4 h and 100 h. The results show that increasing the overall level of porosity had minimal impact on the degree of CMAS infiltration and interaction observed in the coatings during exposure. Reaction with the CMAS occurred by a dissolution-precipitation mechanism, with a reprecipitated ytterbium disilicate phase and Yb-apatite (Ca₂Yb₈(SiO₄)₆O₂) crystals noted as the only reaction products. After 100 h CMAS exposure, the erosion resistance of the coatings was investigated. For all the coatings, ductile failure was the main erosion mechanism. The change in phase composition and microstructure after CMAS exposure led to an increase in erosion resistance for all the coatings.