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

Abstract

Novel polycarbosilazanes (PCSZs) were prepared by stepwise synthesis and thermal crosslinking of polysilasilazane (PSSZ) copolymers. Their pyrolysis under inert gas, producing Si-C-N ceramics, was investigated up to 1600 °C by analyses performed on the solids (elemental analysis; EPMA; TGA, density; 1H, 13C and 29Si solid state NMR, i.r. XRD, electrical conductivity) and analyses of the evolved gases (gas chromatography and mass spectrometry). From 250 to 450 °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 °C, proceeding via evolution of gases (mainly H2 and CH4) and yielding a hydrogenated silicon carbonitride. This residue remained stable up to 1250 °C although it progressively lost its residual hydrogen as the temperature was raised. Then, crystallization occurred between 1250 and 1400 °C, yielding β-SiC crystals surrounded by free-carbon cage-like structures. Finally, above 1400 °C, the remaining amorphous Si-C-N matrix underwent a decomposition process accompanied by nitrogen evolution and a second substantial weight loss. At 1600 °C, the pyrolytic residue was a mixture of β-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.