THE EVOLUTIONARY PROCESS DURING PYROLYTIC TRANSFORMATION OF POLY(N-METHYLSILAZANE) FROM A PRECERAMIC POLYMER INTO AN AMORPHOUS-SILICON NITRIDE CARBON COMPOSITE

The pyrolytic evolution of poly(N-methylsilazane),-[H(2)SiNMe](x)-, fr om preceramic polymer to ceramic product is followed by heating sample s of the partially cross-linked polymer, in 200 degrees C increments, from ambient temperature to 1400 degrees C. The intermediate products are characterized by chemical analysis, diffuse reflectance Fourier tr ansform IR spectroscopy (DRIFTS), Raman spectroscopy, and Si-29 and C- 13 magic-angle spinning (MAS) solid-state NMR. Spectroscopic character ization indicates that the 1400 degrees C pyrolysis products are amorp hous silicon nitride mixed with amorphous and graphitic carbon (as det ermined by Raman spectroscopy), rather than silicon carbide nitride, a s expected based on the presence of up to 20 mol% retained carbon. Eff orts to crystallize the silicon nitride through heat treatments up to 1400 degrees C do not lead to any crystalline phases, as established b y transmission electron microscopy (TEM) and small-area electron diffr action (SAD). It appears that the presence of free carbon, along with the absence of oxygen, strongly inhibits crystallization of amorphous silicon nitride, These results contrast with the isostructural poly(Si -methylsilazane), -[MeHSiNH](x)-, which is reported to form silicon ca rbide nitride on pyrolysis.