High-temperature oxidation of yttrium silicides

Current silicon melt-infiltrated (SMI) ceramic matrix composites (CMCs) are limited by the melting temperature of silicon (1414 A degrees C) and the volatility of the thermally grown SiO2 scale in high-temperature water vapor environments. Replacement of the melt-infiltrated (MI) silicon with a rare-earth silicide offers the potential to address both limitations of SMI CMCs. This study focuses on the ability of yttrium silicides to form yttrium silicates (phases with greater stability in high-temperature water vapor than SiO2) in high-temperature oxidizing environments. Yttrium silicides with compositions of 41, 67 and 95 at.% Si-Y were fabricated using arc melting and oxidized in air at 1000 and 1200 A degrees C for up to 24 h. Oxidation resulted in the rapid formation of a non-protective Y2O3 scale and rejected Si. Additional minor oxide phases of Y-Si-O, Y2SiO5, Y2Si2O7 and SiO2 were observed to form on and beneath the specimen surface. Characterization of the microstructural evolution with time and temperature helped elucidate the diffusion mechanisms that control oxide growth rates. Replacement of MI silicon with a MI yttrium silicide would significantly compromise the high-temperature performance of a CMC due to Y2O3 CTE mismatch with SiC, high oxygen permeability and the large volume change associated with its rapid subsurface formation. Results are utilized to examine the viability of other rare-earth silicides as MI materials for CMCs.