Rare-Earth Disilicates As Oxidation-Resistant Fiber Coatings for Silicon Carbide Ceramic-Matrix Composites

Current SiC-based ceramic-matrix composites (SiC-SiC CMCs) rely on carbon or boron nitride fiber-matrix interphases for toughness and flaw tolerance. However, oxidation of these interphases can be performance limiting in many CMC applications. The gamma-polymorph of the rare-earth disilicates (RE2Si2O7) is a potential oxidation-resistant alternative to carbon or BN. The formation of gamma-Y2Si2O7 and gamma-Ho2Si2O7 at different temperatures and processing environments was investigated. Silica-yttrium hydroxide and silica-holmium hydroxide dispersions were made and heat treated at 1200 degrees-1400 degrees C for 8 h in air and argon. LiNO3 was added to the dispersions to enhance the formation of gamma-Y2Si2O7 and gamma-Ho2Si2O7. The effects of excess silica and LiNO3 dopant on the formation of gamma-Y2Si2O7 were investigated. Coatings of Y2Si2O7 and Ho2Si2O7 were made on alpha-SiC plate and SCS-0 SiC fiber using these dispersions. These were heat treated in argon and argon-500 ppm oxygen mixtures at 1400 degrees C/8 h. For coatings heat treated in argon-500 ppm oxygen mixtures, X-ray diffraction showed the formation of single phase gamma-Ho2Si2O7 and a mixture of gamma and beta-Y2Si2O7 at 1400 degrees C. Scanning electron microscopic image analysis gave an estimate of 18 vol% of excess silica for gamma-Y2Si2O7 formed with high Si:Y ratio and similar to 5 vol% excess silica for material formed with lower Si:Y ratio. Transmission electron microscopy of samples directly beneath indentations showed both extensive dislocation slip and fracture.