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Dense Al 2 O 3 -doped SiC ceramics with an average grain size of 1.2 μm were obtained by hot isostatic pressing. The microstructure has been analyzed by TEM, EDX, EELS, SAD and SEM. The results show that the non-crystalline secondary phase, an alumino-silicate glass, is predominantly located at triple grain junctions in Al 2 O 3 -doped SiC ceramics. The influence of the glassy phase on sintering behaviour, high-temperature strength and oxidation resistance is investigated and discussed.

α-SiC fine powder clad with special glass were sintered by HIP processing under 200 MPa at 1800 °DC. The density of material obtained is close to 97.5% of theoretical density, bending strength reached 582 MPa, which is about 50% higher than that of common pressureless sintered silicon carbide. After post-HIP treatment, density can be further increased to above 98.5% of theoretical density. The material with different composition can also be got in various atmospheres. According to thermodynamic calculation, SiC can be transformed into Si3N4 under optimum temperature and high N2-pressure. Experimental work was successful in fabricating SiC-Si3N4 gradient composite material. This kind of new SiC-Si3N4 gradient composite material were also identified by X-ray diffraction analysis. The strength and fracture toughness of this material is 900± 100 MPa and 8.4 MPa.m 1/2 respectively, which are about one times higher than those of hot-pressed SiC material. This study offered a new HIP processing technology for preparing high performance SiC-Si3N4 gradient composite ceramics directly from pure SiC powder.