Si-C-N ceramics with a high microstructural stability elaborated from the pyrolysis of new polycarbosilazane precursors

Si-C-N model filaments almost free of oxygen have been prepared from a novel PCSZ precursor by melt-spinning, -ray curing and pyrolysis under pure nitrogen (or argon) at a temperature _p as high as 1600 °C. The organic-inorganic conversion of the precursor takes place at 450 < _p < 850 °C. It yields an amorphous filament whose composition is close to SiC_0.93N_0.46 (with less than 2 wt % O). No significant change in composition and microstructure occurs up to about 1400 °C. Beyond 1400 °C under argon, a decomposition 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 tiny -SiC crystals within the amorphous Si-C-N material. As _p is raised, the Young's modulus at room temperature continuously increases to reach a value close to 220 GPa for _p=1600 °C whereas the tensile failure stress undergoes a broad maximum close to 2400 MPa for _p =1200 °C and is still higher than 2000 MPa after ageing at 1600 °C. Thus, Si-C-N filaments free of oxygen have improved stability at high temperatures with respect to Si-C-O filaments processed under similar conditions.