SiO2 Activity Measurements in the Gd2O3-SiO2 System Via Knudsen Effusion Mass Spectrometry (KEMS)

Environmental barrier coatings (EBCs) are required for the use of silicon carbide (SiC)-based ceramic matrix composites (CMCs) in gas turbine engines, as they protect the SiO2 thermally grown oxide (TGO) formed at the CMC/coating interface from volatilization in a combustion environment. 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 SiO2 within the coating can preferentially volatilize to form Si(OH)4 gas species, albeit at a lesser degree than SiO2 alone. Therefore, it is of interest to investigate the stability of these materials in steam as a function of their RE. In this study, SiO2 activity, which can be related to the partial pressure of Si(OH)4, was measured across the Gd2O3-SiO2 phase diagram, which includes Gd2SiO5, Gd9.33(SiO4)6O2, and Gd2Si2O7 as line compounds, via Knudsen Effusion Mass Spectrometry (KEMS). Values obtained for the Gd2O3-SiO2 system were compared to those of other RE2O3-SiO2 systems and contextualized in terms of EBC viability.