Thermal cross-linking and pyrolytic conversion of poly(ureamethylvinyl)silazanes to silicon-based ceramics

The aim of this work was to study the pyrolytic conversion of a novel commercial polysilazane, poly(ureamethylvinyl)silazane (PUMVS; Ceraset((TM)), Allied Signal Composites Inc., USA), into silicon-based ceramics. The precursor was thermally cross-linked and pyrolyzed between 200 and 1700 degreesC under argon or nitrogen atmosphere and the products were investigated by spectroscopic techniques (FTIR and Raman spectroscopy, solid-state NMR), elemental analysis and simultaneous thermal analysis coupled with mass spectrometry. Upon heating under argon, the starting liquid precursor transformed into an infusible solid polymer at T > 250 degreesC with a conversion yield of > 95 wt%. The crosslinking solidification occurred predominantly through hydrosilylation or addition reaction involving vinyl groups. Subsequent pyrolysis of the cross-linked products around 1000 degreesC in argon yielded amorphous silicon carbonitride ceramics with a composition of SiN0.82C0.86. The overall ceramic yield (with respect to the starting PUMVS) was around 70 wt%, which was found to be independent of the initial crosslinking step. Solid-state NMR (Si-29 and C-13) revealed that the amorphous silicon carbonitrides contain predominately CSiN3 units. There is evidence for the formation of free amorphous carbon between 700 and 800 degreesC. Graphitic phases were detected by X-ray diffraction in the samples heated to T > 1000 degreesC at high heating rates. Upon annealing at T > 1500 degreesC, the excess carbon reacted completely with the silicon (carbo)nitride to form SiC and nitrogen. The final ceramics contained a large amount of crystalline SiC (similar to 90 wt%), and were free of excess carbon or silicon. Therefore, PUMVS is an ideal precursor for the formation of high-quality SiC-based ceramics. Copyright (C) 2001 John Wiley & Sons, Ltd.