Corrosion Behavior and Mechanism of Aluminum-rich CMAS on Rare-earth Silicate Environmental Barrier Coatings

Environmental barrier coatings (EBCs) are expected to be applied to the hot-section components of a new generation of high thrust-to-weight ratio aero-engines. Rare-earth silicates have been acknowledged as promising alternatives to EBC materials due to their superior high-temperature phase stability, suitable coefficient of thermal expansion, and long durability in water vapor. However, the calcium-magnesium-alumino-silicates (CMAS) molten salt corrosion under service conditions has become a bottleneck that limits the application of rare-earth silicates in EBCs. Factors such as the composition of CMAS and the crystal structures of rare-earth silicates have a significant impact on their corrosion behavior. In this paper, X1-Gd2SiO5 and X2-RE2SiO5 (RE=Y, Er) coatings with different crystal structures, were prepared by atmospheric plasma spraying (APS) technique. Their corrosion behaviors and mechanisms of the three kinds of coatings under CMAS melt environment at 1400 degrees C were explored. Results showed that the corrosion resistance of X2-RE2SiO5 coatings were better than that of X1-Gd2SiO5 coating due to their phase compositions and stability of crystal structure. After corrosion by CMAS, X1-Gd2SiO5 coating dissolved in CMAS melt and formed apatite phase, while the X2-RE2SiO5 coatings not only formed apatite phase, but also formed garnet phase from reaction of the RE2O3 in the coatings with Al2O3 in CMAS. Formation of generate garnet phase could increase relative content of CaO and SiO2 in CMAS, and promote formation of dense apatite layer, thereby improving corrosion resistance. This study provides a strategy for designing EBC systems with excellent CMAS corrosion resistance.