Si/C/N amorphous bulk ceramics by axial pressing of polyhydridomethylsilazane at elevated temperature

Plastic forming (warm pressing) was studied as a shaping method for the preparation of amorphous Si/C/N ceramics from a commercial poly(hydridomethyl)silazane precursor. Pressures up to 630 MPa and temperatures between 70 and 380°C were applied during axial pressing of the polymeric powders. Transparent or translucent, virtually pore free rigid green bodies were prepared at temperatures between 300 and 360°C. The temperature and pressure applied during warm pressing promote the formation of chemical bonds among adjacent polymeric particles and thus increase the degree of crosslinking of the body. Pyrolysis of transparent green bodies in Ar at 1100°C yields black, crack free and nearly fully dense amorphous Si/C/N ceramics (porosity < 2%) with low mechanical strength. Extensive crack formation was observed during grinding and polishing of the pyrolysed bodies. The formation of cracks results from the superposition of stresses generated by machining and those from insufficient outgassing of the specimen during pyrolysis.