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

摘要

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 y-polymorph of the rare-earth disilicates (RE_2 Si_2O_7) is a potential oxidation-resistant alternative to carbon or BN. The formation of γ-Y_2Si_2O_7 and γ-Ho_2Si_2O_7 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. LiNO_3 was added to the dispersions to enhance the formation of y-γ_2Si_2O_7 and γ-Ho_2Si_2O_7. The effects of excess silica and LiNO_3 dopant on the formation of γ-Y_2Si_2O_7 were investigated. Coatings of Y_2Si_2O_7 and Ho_2Si_2O_7 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-Ho_2Si_2O_7 and a mixture of γ and β-Y_2Si_2O_7 at 1400 C. Scanning electron microscopic image analysis gave an estimate of 18 vol% of excess silica for γ-Y_2Si_2O_7 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.