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

Abstract

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.

Processing and Testing Re2Si207 Matrix Composites (Preprint)

“…SEM/EDS showed silica-rich dark regions with almost no yttrium in the EDS spectrum and a grey region with very similar yttrium and silica intensities. 2 The experimental hardness values were obtained from Vickers indentation and these values represent the sum of the hardnesses of the pure rare earth disilicate and silica. The hardness of pure rare earth-silicate was calculated using the rule of mixtures, assuming that the hardness of pure silica is 1GPa.…”

“…[23][24][25] To study deformation mechanisms that may operate in α, β and γ-Y 2 Si 2 O 7 , TEM foils were machined beneath indented regions of sintered Y 2 …”

“…6,[9][10] Both are soft (Vickers hardness ~6 GPa) and machineable. However, monazite decomposes in the reducing atmospheres typically used for processing SiC-SiC CMCs, and the rare earth disilicate La 2 Si 2 O powder compacts at their phase formation temperatures of 1000°C -1200°C using pressureless sintering was not successful; therefore, their hardness values were not measured. In this work, dense α, β and γ-Y 2 Si 2 O pellets were made using the field assisted sintering technique (FAST) and hardness measurements were completed using the Vickers indentation method.…”

“…The rare-earth disilicates are attractive candidates. 2 Among the rare earth disilicates, yttrium disilicate (Y 2 Si 2 O 7 ) is the most thoroughly studied. It has five polymorphs and is refractory, melting at 1775°C.…”

“…Fiber push-out experiments were conducted and the measured sliding stress values are reported and discussed. 2 O were dissolved in 50 mL of deionized water to make a solution of pH ~1. The sol pH was determined using a pH/ion meter (Corning Inc., Corning NY).…”