SI-C-N CERAMICS WITH A HIGH MICROSTRUCTURAL STABILITY ELABORATED FROMTHE PYROLYSIS OF NEW POLYCARBOSILAZANE PRECURSORS .1. THE ORGANIC-INORGANIC TRANSITION

Novel polycarbosilazanes (PCSZs) were prepared by stepwise synthesis a nd thermal cross-linking of polysilasilazane (PSSZ) copolymers. Their pyrolysis under inert gas, producing Si-C-N ceramics, was investigated up to 1600-degrees-C by analyses performed on the solids (elemental a nalysis; EPMA; TGA, density; H-1, C-13 and Si-29 solid state NMR, i.r. XRD, electrical conductivity) and analyses of the evolved gases (gas chromatography and mass spectrometry). From 250 to 450-degrees-C, a fi rst strong weight loss was observed, which was due to the formation an d elimination of low-boiling-point oligomers. The weight loss closely depends on the cross-linking degree of the ceramic precursor resulting from the PSSZ/PCSZ conversion. Then, the organic/inorganic transition took place between 500 and 800-degrees-C, proceeding via evolution of gases (mainly H-2 and CH4) and yielding a hydrogenated silicon carbon itride. This residue remained stable up to 1250-degrees-C although it progressively lost its residual hydrogen as the temperature was raised . Then, crystallization occurred between 1250 and 1400-degrees-C, yiel ding beta-SiC crystals surrounded by free-carbon cage-like structures. Finally, above 1400-degrees-C, the remaining amorphous Si-C-N matrix underwent a decomposition process accompanied by nitrogen evolution an d a second substantial weight loss. At 1600-degrees-C, the pyrolytic r esidue was a mixture of beta-SiC and free carbon. So, the amorphous si licon carbonitride resulting from the pyrolysis of PCSZ precursors was found to be appreciably more thermally stable than the previously rep orted Si-C-O ceramic obtained by pyrolysis of polycarbosilane precurso rs.