Silica activity measurements in the Gd ₂ O ₃ –SiO ₂ system via Knudsen Effusion Mass Spectrometry

Abstract Environmental barrier coatings (EBCs) are required for the use of silicon carbide‐based ceramic matrix composites in gas turbine engines. Current‐generation EBCs consist of a silicon bond coat and a rare earth (RE) silicate topcoat. The RE silicate topcoat is exposed to high‐velocity steam during engine operation, and SiO 2 within the coating can preferentially volatilize to form Si(OH) 4 gas. Therefore, the volatility of the RE silicates in a combustion environment is of interest. The volatility of the RE silicates in steam is related to SiO 2 activity (), which was measured in this work for the gadolinium silicates via Knudsen Effusion Mass Spectrometry (KEMS). The Gd 2 O 3 –SiO 2 system is of interest because gadolinium has been considered as a component in solid solution RE disilicate topcoats. Silica activities measured within the biphasic fields Gd 2 O 3 ‐Gd 2 SiO 5 () and Gd 9.33 (SiO 4 ) 6 O 2 ‐Gd 2 Si 2 O 7 () were compared to those reported for RE 2 O 3 –RE 2 SiO 5 and RE 2 SiO 5 –RE 2 Si 2 O 7 with RE = Y, Yb, Lu. The SiO 2 activity of Gd apatite (Gd 9.33 (SiO 4 ) 6 O 2 ) was determined for the first time (), and apatite phase formation was assessed in air between 1100°C and 1600°C. Experimental silica activities were also compared to those modeled by Thermo‐Calc. Implications for the Gd silicates as coating materials are discussed in the context of thermal expansion, water vapor exposure, and CaO‐MgO‐Al 2 O 3 ‐SiO 2 (CMAS) exposure.