Material design of environmental barrier coatings to mitigate against CMAS attack

Abstract

For eight types of environmental barrier coating (EBC) materials in the YbO_1.5-AlO_1.5, YbO_1.5-SiO₂, and YbO_1.5-AlO_1.5-HfO₂ systems, samples consisting of a Ca–Mg–Fe–Al–Si–O (CMAS) disk with a Ca/Si molar ratio of 0.77 placed on an EBC disk were treated at 1673 K for as long as 20 h. The microstructures of the reaction layers formed in the EBCs after the treatments were characterized, and the corrosion depths were measured. In computational pseudo-ternary phase diagrams consisting of an EBC and CMAS ingredients such as (SiO₂)_0.7(MgO)_0.06(FeO_1.5)_0.06(AlO_1.5)_0.18 and (CaO)_0.7(MgO)_0.06(FeO_1.5)_0.06(AlO_1.5)_0.18 at 1673 K, thermodynamic descriptors S_[i]/S₀ were defined as the ratios of the areas for specified combinations of apatite, Yb₃Al₅O₁₂, Yb₄Al₂O₉, and Yb₂Ca₂Si₂O₉ to the area enclosed by the EBC and CMAS components with Ca/Si molar ratios of 0.11–1.5 in the ingredients. The relationships between the corrosion depths of the EBCs and their corresponding S_[i]/S₀ values were evaluated. The logarithmic values of the corrosion depths linearly decreased with increasing values of the provided S_[i]/S₀, which was defined as the area of the region where apatite and garnet coexist without Yb₄Al₂O₉. YbAH, consisting of Hf-solid-soluted Yb₂O₃ and Yb₃Al₅O₁₂, exhibited the best CMAS resistance among the investigated EBCs. The solid solutioning of HfO₂ into Yb₂O₃ suppressed the decomposition of YbAH—specifically, the formation of Yb₄Al₂O₉ with a small amount of Ca and Si. In addition, the formation of crystalline compounds that incorporate a large amount of CMAS components effectively reduced the amount of residual melt.