SI-C-N CERAMICS WITH A HIGH MICROSTRUCTURAL STABILITY ELABORATED FROMTHE PYROLYSIS OF NEW POLYCARBOSILAZANE PRECURSORS .4. OXYGEN-FREE MODEL MONOFILAMENTS

Si-C-N model filaments almost free of oxygen have been prepared from a novel PCSZ precursor by melt-spinning, gamma-ray curing and pyrolysis under pure nitrogen (or argon) at a temperature theta(p) as high as 1 600-degrees-C. The organic inorganic conversion of the precursor takes place at 450 < theta(p) < 850-degrees-C. It yields an amorphous filam ent whose composition is close to SiC0.93N0.46 (with less than 2 wt % 0). No significant change in composition and microstructure occurs up to about 1400-degrees-C. Beyond 1400-degrees-C under argon, a decompos ition process takes place starting from the filament surface whereas, under nitrogen, the only observed phenomena are the growth of a skin a few nanometres thick at the filament surface and the formation of tin y beta-SiC crystals within the amorphous Si-C-N material. As theta(p) is raised, the Young's modulus at room temperature continuously increa ses to reach a value close to 220 GPa for theta(p) = 1600-degrees-C wh ereas the tensile failure stress undergoes a broad maximum close to 24 00 MPa for theta(p) = 1200-degrees-C and is still higher than 2000 MPa after ageing at 1600-degrees-C. Thus, Si-C-N filaments free of oxygen have improved stability at high temperatures with respect to Si-C-O f ilaments processed under similar conditions.