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 comme rcial polysilazane, poly(ureamethylvinyl)silazane (PUMVS; Ceraset((TM)), Al lied Signal Composites Inc., USA), into silicon-based ceramics. The precurs or was thermally cross-linked and pyrolyzed between 200 and 1700 degreesC u nder argon or nitrogen atmosphere and the products were investigated by spe ctroscopic techniques (FTIR and Raman spectroscopy, solid-state NMR), eleme ntal analysis and simultaneous thermal analysis coupled with mass spectrome try. Upon heating under argon, the starting liquid precursor transformed in to an infusible solid polymer at T > 250 degreesC with a conversion yield o f > 95 wt%. The crosslinking solidification occurred predominantly through hydrosilylation or addition reaction involving vinyl groups. Subsequent pyr olysis of the cross-linked products around 1000 degreesC in argon yielded a morphous silicon carbonitride ceramics with a composition of SiN0.82C0.86. The overall ceramic yield (with respect to the starting PUMVS) was around 7 0 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 carbon itrides contain predominately CSiN3 units. There is evidence for the format ion of free amorphous carbon between 700 and 800 degreesC. Graphitic phases were detected by X-ray diffraction in the samples heated to T > 1000 degre esC at high heating rates. Upon annealing at T > 1500 degreesC, the excess carbon reacted completely with the silicon (carbo)nitride to form SiC and n itrogen. The final ceramics contained a large amount of crystalline SiC (si milar to 90 wt%), and were free of excess carbon or silicon. Therefore, PUM VS is an ideal precursor for the formation of high-quality SiC-based cerami cs. Copyright (C) 2001 John Wiley & Sons, Ltd.