Modeling of thermal residual stress in environmental barrier coated fiber reinforced ceramic matrix composites

Abstract

For SiC/SiC composites to replace metallic materials in future turbine engines, prime reliant environmental barrier coatings (EBCs) are required. However, due to the mismatch in thermal expansion and elastic modulus between the substrate and the coating, thermal residual stresses are generated in the coating after processing as well as during exposure to turbine engine operating conditions. The nature and magnitude of the thermal stresses will have a profound effect on the durability and reliability of the EBC. To estimate the magnitude of in-plane ( x- and y-directions) and through-the-thickness ( z-direction) thermal residual stresses in the coating, a finite element model (FEM) was developed. Using FEM, the residual stresses were predicted for three multilayered EBC systems considered for the SiC/SiC composites: (1) barium strontium aluminum silicate, (2) ytterbium disilicate, and (3) ytterbium monosilicate. Influence of thickness and modulus of the coating layer on the thermal residual stress were modeled. Results indicate that thermal residual stresses in the SiC/SiC composite substrate are compressive and in all the three coatings tensile. Further examination indicates that in the z-direction, tensile stresses in all three systems are negligible, but in-plane tensile stresses can be significant depending on the composition of the constituent layer and the distance from the substrate. Comparison of predicted thermal residual stresses in the three systems shows that the ytterbium monosilicate system has the highest stress (~395 MPa), while the other two systems averaged about 80 MPa in one of the coating layers. A parametric analysis conducted indicates that lowering the modulus of the coating can lower the thermal residual stresses.