NANOSTRUCTURED SIC CERAMICS PREPARED BY A NEW PROCESS - CRYSTALLIZATION OF INTERFACIAL GLASS

Large bulk and fully dense SiC based nanoceramics with average grain s ize of 50 nm and 20-30 wt.% nanometer sized alpha-Sialon, AlxSi3-xO6 a nd alpha-SiO2 interfacial phases were prepared by a new process, cryst allization of interfacial glass, using LMAS glass-coated SiC powder as starting material. The process involves two major steps: densificatio n by hot pressing, and crystallization of interfacial glass by anneali ng treatment. The densification was controlled by interfacial glass co ntent, hot pressing temperature, and hot pressing pressure; density 99 .8% theoretical being reached for SiC/30 wt.% glass nanoceramics hot-p ressed at 1520 degrees C and 22 MPa for 30 min. The crystallization wa s complete and nearly all the interfacial glass was transformed into n anocrystalline phases after 800 degrees C and 900 degrees C for 5 h an nealing treatments. Plastic flow and rearrangement of particles and in terfacial glass infiltration are densification Mechanisms. A large num ber of nanometer sized SiC powder particles serve as nucleating agents , e.g. hetero-nucleation, and are responsible for interfacial glass cr ystallization. A characteristic of the present process is that there i s no SiC grain growth during densification and interfacial glass cryst allization.