A COMPARISON OF THE MICROSTRUCTURE OF SILICON NITRIDE-SILICON CARBIDE COMPOSITES MADE WITH AND WITHOUT DEOXIDIZED STARTING MATERIAL

摘要

Two different types of silicon carbide (SiC) matrix composites, with either 10wt% or 20wt% silicon nitride (Si3N4) reinforcement, were fabricated to investigate the effect of pretreatment on the resulting composite microstructure. The first type of composite was prepared from as-received alpha-SiC and alpha-Si3N4 powders, while the second type was prepared from powder compacts that had been deoxidized to eliminate surface silica on the powder particles. The composites were hot isostatically pressed in tantalum cans at 2373K for Ih under a pressure of 200 MPa. Density measurements showed that full theoretical density was achieved for the composites prepared from the as-received powders, while much lower densities were obtained for the composites prepared from the deoxidized green compacts. Almost all of the alpha-SiC transformed into beta-SiC, and almost all the alpha-Si3N4 transformed into beta-Si3N4 in the composites made from the as-received powders, while in the composites made from the deoxidized material the alpha-SiC remained untransformed and both alpha-Si3N4 and beta-Si3N4 phases were present in significant quantities. High-resolution transmission electron microscopy and Fresnel fringe imaging were used to identify the grain boundary and interphase boundary structure, Most interfaces were found to be covered with approximate to 1nm thick amorphous intergranular films in the composites prepared from as-received powders, whereas most interfaces were found to be free of such amorphous intergranular films in the composites prepared from the deoxidized material. Taken together, the presence of intergranular films at the interfaces and the results from density measurements are consistent with the densification and reverse alpha-->beta-SiC transformation taking place in the composites made from as-received powders by a liquid-phase sintering route, An incomplete liquid-phase sintering mechanism is also able to explain the microstructure observed in the composites made from the deoxidized material. [References: 27]