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

Novel polycarbosilazanes (PCSZs) were prepared by stepwise synthesis and 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 analysis; 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 first strong weight loss was observed, which was due to the formation and 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 carbonitride. 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, yielding 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 and a second substantial weight loss. At 1600-degrees-C, the pyrolytic residue was a mixture of beta-SiC and free carbon. So, the amorphous silicon carbonitride resulting from the pyrolysis of PCSZ precursors was found to be appreciably more thermally stable than the previously reported Si-C-O ceramic obtained by pyrolysis of polycarbosilane precursors.