Environmental Barrier Coatings for SiC<sub>f</sub>/SiC: Fid-Bau Portal

Environmental Barrier Coatings for SiC_f/SiC

Lee, Kang N.

Ceramic matrix composite (CMCs) are not viable in gas turbine engines without mitigating rapid surface recession resulting from water vapor–induced vaporization of silica scale. Currently, external barrier coatings, known as environmental barrier coatings (EBCs), are the most promising approach to protecting CMCs from surface recession. The first generation EBCs, silicon mullite or mullite + BSAS BSAS, developed in the NASA HSCT‐EPM Program enabled the world's first successful engine test of a CMC component in a Solar Turbines industrial gas turbine with one engine completing a 14 000‐hour field test. The second generation EBCs with temperature capabilities >1482°C (2700°F) were developed in the NASA UEET Program. The first generation and second generation EBCs laid the foundation for next generation EBCs. APS is currently the most successful and widely used process to fabricate EBCs. Other potential coating processes include EB‐PVD, DVD, PS‐PVD, CVD, and slurry‐based processes. Non‐line‐of‐sight EBC processes are desirable for complex‐shaped CMC components because EBC is a prime reliant coating. Three key components for validating EBCs are water vapor, gas velocity, and temperature gradient. Currently combinations of steam cycling rig, high velocity–high steam burner rig, laser rig, and high pressure burner rig are used to validate EBCs. Sources for EBC degradation can be categorized into three groups: chemical, mechanical, and physical. Strong interactions between various degradation modes are expected, and therefore, a model that incorporates all these factors needs to be developed to successfully project the life of EBC. An industry standard or a model system, which is relevant to various versions of EBCs, needs to be established on which the life modeling effort can be focused.