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

Two different types of silicon carbide (SiC) matrix composites, with e ither 10wt% or 20wt% silicon nitride (Si3N4) reinforcement, were fabri cated to investigate the effect of pretreatment on the resulting compo site 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 su rface silica on the powder particles. The composites were hot isostati cally pressed in tantalum cans at 2373K for Ih under a pressure of 200 MPa. Density measurements showed that full theoretical density was ac hieved for the composites prepared from the as-received powders, while much lower densities were obtained for the composites prepared from t he deoxidized green compacts. Almost all of the alpha-SiC transformed into beta-SiC, and almost all the alpha-Si3N4 transformed into beta-Si 3N4 in the composites made from the as-received powders, while in the composites made from the deoxidized material the alpha-SiC remained un transformed and both alpha-Si3N4 and beta-Si3N4 phases were present in significant quantities. High-resolution transmission electron microsc opy and Fresnel fringe imaging were used to identify the grain boundar y 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 interf aces were found to be free of such amorphous intergranular films in th e composites prepared from the deoxidized material. Taken together, th e presence of intergranular films at the interfaces and the results fr om density measurements are consistent with the densification and reve rse alpha-->beta-SiC transformation taking place in the composites mad e from as-received powders by a liquid-phase sintering route, An incom plete liquid-phase sintering mechanism is also able to explain the mic rostructure observed in the composites made from the deoxidized materi al.