Material design of environmental barrier coatings to mitigate against CMAS attack

For eight types of environmental barrier coating (EBC) materials in the YbO 1.5 -AlO 1.5 , YbO 1.5 -SiO 2 , and YbO 1.5 -AlO 1.5 HfO 2 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 (SiO 2 ) 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] /S 0 were defined as the ratios of the areas for specified combinations of apatite , Yb 3 Al 5 O 12 , Yb 4 Al 2 O 9 , and Yb 2 Ca 2 Si 2 O 9 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] /S 0 values were evaluated. The logarithmic values of the corrosion depths linearly decreased with increasing values of the provided S [i] /S 0 , which was defined as the area of the region where apatite and garnet coexist without Yb 4 Al 2 O 9 . YbAH, consisting of Hf-solid-soluted Yb 2 O 3 and Yb 3 Al 5 O 12 , exhibited the best CMAS resistance among the investigated EBCs. The solid solutioning of HfO 2 into Yb 2 O 3 suppressed the decomposition of YbAH—specifically, the formation of Yb 4 Al 2 O 9 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. The logarithmic values of the experimental corrosion depths decreased linearly with increasing S [i] / S 0 , defined based on CALPHAD as the area of the region where apatite and garnet coexist without Yb 4 Al 2 O 9 .