SI-C-N CERAMICS WITH A HIGH MICROSTRUCTURAL STABILITY ELABORATED FROMTHE PYROLYSIS OF NEW POLYCARBOSILAZANE PRECURSORS .5. OXIDATION-KINETICS OF MODEL FILAMENTS

Oxidation tests have been performed on model Si-C-N-O filaments prepar ed from a new polycarbosilazane (PCSZ) precursor, according to a spinn ing-oxygen-curing-pyrolysis procedure. Thermogravimetric analyses have been run under a flowing atmosphere of dry oxygen (P = 100 kPa) at 10 00 < T < 1400-degrees-C. The oxidation treatment results in the growth of a silica layer (passive oxidation regime) and simultaneously the r elease of carbon oxides and nitrogen, with an overall weight increase. At a given test temperature, the kinetics of growth of the silica sca le obeys a parabolic law. The parabolic rate constant is thermally act ivated with an apparent activation energy of 170 kJ mol-1. The rate-li miting step is diffusion-controlled. The ex-PCSZ filaments exhibit a b etter oxidation resistance than the related Si-C-O fibres prepared fro m polycarbosilane precursors according to a similar procedure.