Residual stresses and their effects on the durability of environmental barrier coatings for SiC ceramics

Qualitative residual stresses in current environmental barrier coatings (EBCs) were inferred from the curvature of EBC-coated SiC wafers, and the effects of EBC stresses on the durability of EBC-coated SiC were evaluated. The magnitude of substrate curvature correlated fairly well with the EBC-SiC coefficient of thermal expansion (CTE) mismatch, EBC modulus, and thermally induced physical changes in EBC. BSAS (1-xBaO.xSrO.Al2O3.2SiO(2), 0 <= x <= 1) components in the current EBCs, i.e., Si/mullite or mullite+BSAS/BSAS or yttria-stabilized zirconia (YSZ: ZrO2-8 wt% Y2O3), were the most beneficial for reducing the EBC stress in as-sprayed as well as in post-exposure EBCs. The reduced stress was attributed to the low modulus of BSAS. The addition of a YSZ top coat significantly increased the substrate curvature because of its high CTE and sintering in thermal exposures. There were clear correlations between the wafer curvature and the EBC durability. The Si/mullite+20 wt% BSAS/BSAS EBC maintained excellent adherence, protecting the SiC substrate from oxidation, while the Si/mullite+20 wt% BSAS/YSZ EBC suffered delamination, leading to severe oxidation of the SiC substrate, after a 100 h -1300 degrees C exposure in a high-pressure burner rig.