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 γ-polymorph of the rare-earth disilicates (RE2Si2O7) is a potential oxidation-resistant alternative to carbon or BN. The formation of γ-Y2Si2O7 and γ-Ho2Si2O7 at different temperatures and processing environments was investigated. Silica–yttrium hydroxide and silica–holmium hydroxide dispersions were made and heat treated at 1200°–1400°C for 8 h in air and argon. LiNO3 was added to the dispersions to enhance the formation of γ-Y2Si2O7 and γ-Ho2Si2O7. The effects of excess silica and LiNO3 dopant on the formation of γ-Y2Si2O7 were investigated. Coatings of Y2Si2O7 and Ho2Si2O7 were made on α-SiC plate and SCS–0 SiC fiber using these dispersions. These were heat treated in argon and argon—500 ppm oxygen mixtures at 1400°C/8 h. For coatings heat treated in argon—500 ppm oxygen mixtures, X-ray diffraction showed the formation of single phase γ-Ho2Si2O7 and a mixture of γ and β-Y2Si2O7 at 1400°C. Scanning electron microscopic image analysis gave an estimate of 18 vol% of excess silica for γ-Y2Si2O7 formed with high Si:Y ratio and ∼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.
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