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HEXOLOY{reg_sign} SX-SiC, fabricated with Y and Al containing compounds as sintering aids, has been shown to possess significantly improved strength and toughness over HEXOLOY{reg_sign}SA-SiC. This study was undertaken to establish and benchmark the complete mechanical property database of a first generation material, followed by a process optimization task to further improve the properties. Mechanical characterization on the first generation material indicated that silicon-rich pools, presumably formed as a reaction product during sintering, controlled the strength from room temperature to 1,232 C. At 1,370 C in air, the material was failing due to a glass-phase formation at the surface. This glass-phase formation was attributed to the reaction of yttrium aluminates, which exist as a second phase in the material, with the ambient. This process was determined to be a time-dependent one that leads to slow crack growth. Fatigue experiments clearly indicated that the slow crack growth driven by the reaction occurred only at temperatures >1,300 C, above the melting point of the glass phase. Process optimization tasks conducted included the selection of the best SiC powder source, studies on mixing/milling conditions for SiC powder with the sintering aids, and a designed experiment involving a range of sintering and post-treatment conditions. The optimization study conducted on the densification variables indicated that lower sintering temperatures and higher post-treatment pressures reduce the Si-rich pool formation, thereby improving the room-temperature strength. In addition, it was also determined that furnacing configuration and atmosphere were critical in controlling the Si-rich formation.

High purity, submicron silicon carbide powders were produced via gas phase synthesis using a hydrogen/argon plasma. Two test facilities were constructed, a bench-scale unit and a larger pilot scale reactor. Three candidate silicon sources were evaluated:silicon tetrachloride (SiCl{sub 4}). dimethyldichlorosilane (CH{sub 3}){sub 2}(SiCl{sub 2}) and methyltrichlorosilane (CH{sub 3}SiCl{sub 3}). Product powders were evaluated on the basis of pressureless sinterability, surface area, agglomeration, particle size distribution, phase distribution and chemistry. Three commercial powders, Starck A10, Starck B10, and Carborundum submicron alpha silicon carbide, were also evaluated for comparison to the product powders. Powders were reproducibly synthesized at a rate of one pound per hour for standard run times of five hours. Product powders exhibited chemical and physical properties equal to or exceeding the commercial powders evaluated. In limited attempts to pressureless sinter the product powders, densities of 91% of theoretical were obtained with as-produced powder. Post-processing permitted densities in excess of 97% of theoretical. X-ray diffraction of the product indicates that the product powders are primarily beta poly-types, with traces of alpha present. Increased production rates to a target level of seven pounds per hour were not possible due to current transients produced by the pilot scale power supply. Extensive unsuccessfulmore » efforts to reduce or eliminate the transients are described. Low recovered product yields resulted from a failure of a product collection filter that was not discovered until the completion of the project.« less