Effects of crystal structure and cation size on molten silicate reactivity with environmental barrier coating materials

Abstract

Abstract Rare earth (RE) disilicates are utilized in environmental barrier coatings to protect Si‐based engine components from destructive reactions with water vapor and other combustion species. These coating materials, however, degrade when exposed to molten silicate deposits in the engine. Four RE‐disilicates (RE 2 Si 2 O 7 , RE = Er, Dy, Gd, Nd) are analyzed herein in thermochemical interactions with glassy calcium‐magnesium‐aluminosilicate (CMAS) compositions at 1400°C. Crystalline reaction products included RE 2 Si 2 O 7 , SiO 2 , and a Ca 2+ y RE 8+ x (SiO 4 ) 6 O 2+3 x /2+ y apatite‐type silicate. RE 2 Si 2 O 7 formation was favored in interactions with CMAS having low CaO:SiO 2 ratios. Increased reactivity was observed for higher CaO:SiO 2 ratios in CMAS combined with larger RE 3+ cation size, resulting in apatite formation of varying stoichiometry and changes in lattice parameters. The crystallization of SiO 2 was dependent on both thermodynamic equilibrium at low CaO:SiO 2 ratios and sequestration of silicate modifiers at higher CaO:SiO 2 ratios, although residual amorphous content after CMAS exposure in both cases was still substantial.