Hot corrosion of RE₂SiO₅ with different cation substitution under calcium–magnesium–aluminosilicate attack

Rare earth (RE) silicates have been applied as advanced environmental barrier coatings (EBCs) to protect silicon carbide fibers reinforced silicon carbide ceramic matrix from water vapor and molten salt corrosion in engines. This process, however, is limited by volcanic ash corrosion as assessment of ash-induced corrosion is anecdotal and quantitative data are insufficient. In this account, the corrosion behavior of RE monosilicates (RE₂SiO₅, RE = Y, Lu, Yb, Eu, Gd, and La) by calcium–magnesium–aluminosilicate (CMAS), with similar composition as volcanic ash, was comprehensively investigated. Results indicated that RE₂SiO₅ could react with CMAS at 1200 °C at the interface, where the products crystallized in CMAS glass. RE₂Si₂O₇ was formed by the reaction between RE₂SiO₅ and silica (SiO₂) in CMAS, which was followed by corrosion of RE₂Si₂O₇ by CMAS. RE₂SiO₅ with Type B structure showed better resistance toward CMAS than RE₂SiO₅ with Type A structure. Moreover, RE₂SiO₅ with larger radii of RE³⁺ cations led to easy formation of oxyapatite phase; however, RE₂SiO₅ with smaller radii of RE³⁺ cations easily formed garnet phase. Besides, smaller radii RE³⁺ cations induced slower reactions. These findings can contribute to identifying, preventing, and minimizing the damage to matrix components with EBCs caused by volcanic ash.