CMAS degradation of environmental barrier coatings

Environmental barrier coatings (EBCs) based on Ba1 − xSrxAl2Si2O8 (BSAS) have demonstrated potential for the protection of Si-based ceramic matrix composites (CMCs) against moisture-induced degradation in gas turbines. However, EBCs are susceptible to attack by calcium–magnesium alumino-silicate (CMAS) melts produced when siliceous debris is ingested with the intake air and deposits on component surfaces. The mechanism involves the dissolution of BSAS into CMAS and re-precipitation as a modified Celsian phase incorporating Ca, as well as secondary crystalline silicates that may degrade the durability and efficiency of the EBC. The process is aggravated by grain boundary penetration of CMAS into the polycrystalline BSAS. The mechanisms and their potential implications for durability are discussed.