Evaluation of Environmental Barrier Coatings for SiC/SiC Composites

SiC fiber reinforced SiC matrix (SiC/SiC) composites are one of the most promising materials for high temperature structural applications such as power generation and propulsion systems. SiC/SiC composites are, however, susceptible to accelerated attacks in water vapor environments through oxidation and volatilization reaction, so Environmental Barrier Coatings (EBCs) are indispensable. We have investigated some oxides and rare-earth silicates as topcoat candidates for EBCs. Topcoat materials must be stable in the high-water-vapor pressurized environments at high temperatures. Also, it is important that the thermal expansion coefficient of topcoat materials is similar to that of SiC/SiC composites. In this study, rare-earth silicates such as lutetium silicates, 8YSZ and ZrO2-SiO2 were selected as topcoat candidates. They were exposed in the water containing atmosphere at a temperature 1673K for 100 hr under a total pressure 0.96 MPa and under an atmospheric pressure. Mass changes, structure of crystals and microstructures were investigated after the exposure experiments in order to evaluate the thermal stability of those materials. After this evaluation, lutetium silicates were considered to be promising for topcoat materials. Lutetium silicates were coated on SiC/SiC composites, and their fracture toughness and microstructures were investigated and reported in this paper.