Mechanical Properties and Behaviour of BSAS/Mullite-Based Environmental Barrier Coatings Exposed to High Temperature in Water Vapour Environment

Si-based ceramics (e.g., SiC and Si₃N₄) are known as promising high-temperature structural materials in various components where metals/alloys reached their ultimate performances (e.g., advanced gas turbine engines and structural components of future hypersonic vehicles). To alleviate the thickness recess that Si-based ceramics undergo in a high-temperature environmental attack (e.g., H₂O vapour), appropriate refractory oxides are engineered as environmental barrier coatings (EBCs). Presently, the state-of-the art EBCs comprise multilayers of silicon (Si) bond coat, mullite (Al₆Si₂O₁₃) intermediate layer and BaO-SrO-Al₂O₃-SiO₂ (BSAS) top coat. Evaluating and understanding their mechanical properties, such as, the elastic modulus (E) and the strain-stress relationship is essential for their practical application and reliable employment. It was investigated via depth-sensing indentation the role of high-temperature treatment (1300°C), performed in H₂O vapour environment (for time intervals up to 500 h), on the mechanical behaviour of air plasma sprayed Si/mullite/BSAS layers deposited on SiC substrates. Laser-ultrasonics was employed to evaluate the E values of as-sprayed coatings and to validate the indentation results. The fully crystalline, crack-free and near crack-free as-sprayed EBCs were engineered under controlled deposition conditions. The (i) absence of phase transformation and (ii) stability of the low elastic modulus values (e.g., ~60-70 GPa) retained by the BSAS top layers even after harsh environmental exposures provides a plausible explanation for the almost crack-free coatings observed. The measured mechanical properties of the EBCs and their microstructural behaviour during the high-temperature exposure are discussed and correlated.