Optimal rare-earth disilicates as top coat in multilayer environmental barrier coatings

Abstract

In last decade, rare-earth disilicates have been considered as candidate materials for top coat in multilayer environmental barrier coatings (EBCs). However, optimal coating material has not yet been found. Previous studies revealed that parabolic rate constant of thermally grown silica scale (silica-TGO) beneath EBCs played a key role in the durability of SiC matrix ceramic composites coated with EBCs. To determine optimal coating material, in this study, Lu2Si2O7 and Sc2Si2O7 coatings were first corroded under quasi-static water-vapor corrosion conditions. The evolution of silica-TGO beneath coatings was then examined in detail. Accordingly, parabolic rate constants of silica-TGO growth were calculated. Compared with parabolic rate constants of silica-TGO growth beneath barium-strontium aluminosilicate (BSAS), Yb2Si2O7, and Y2Si2O7-BSAS coatings in previous studies, Y2Si2O7-BSAS coating shows the lowest parabolic rate constant and growth rate of silica-TGO. Crystal structure of rare-earth disilicates was also studied. Results showed that bond lengths of Y–O and Si–O in Y2Si2O7 were short, indicating that it possessed low ionic oxygen permeability. This resulted in its low parabolic rate constant of silica-TGO growth. Therefore, Y2Si2O7 is the optimal material as top coat in multilayer EBCs.