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Uniform submicron crystals of B,C, boron-enriched boron carbide, and B4C/

Dense samples of several compositions in the system Sic-AIN were fabricated by hot-pressing. The SIC-AIN powder was prepared by carbothermal reduction of an intimate mixture of alumina, silica, and carbon in a nitrogen atmosphere. X-ray diffraction and electron and optical microscopy were used to determine the chemical and microstructural characteristics of the hot-pressed specimens. Materials with bulk compositions between 15 and 75 wt% AIN were found to be nonhomogeneous when hot-pressed below 2100°C. These materials were determined to be a mixture of Sic-AIN solid solutions with different compositions. The observed compositional variations were distinctly bimodal. The source of the inhomogeneity was the starting Sic-AIN powder. The powders, as well as the hot-pressed samples, consisted of a mixture of small crystals rich in S i c and large AIN-rich crystals. Compositions outside the 15 to 75 wt% AIN region were found to be single phase and to have the wurtzite structure. Hot-pressing SIC-AIN in the intermediate composition range at 230OOC produced an optically and chemically homogeneous material. The precipitation of an Sic-rich phase from a 75 wt% AIN solid solution and the precipitation of an AIN-rich phase from a 47 wt% AIN alloy when annealed at 1700°C are strong indications that a miscibility gas exists in the system Sic-AIN.