Correlation of boron content and high temperature stability in Si-B-C-N ceramics II

This study treats the preparation of polyborosilazanes which are obtained b y hydroboration of oligovinylsilazane {-(H2C= CH)SiHNH=](n) with different amounts of H3B . SMe2. Pyrolysis of these precursors yields amorphous ceram ic materials, which only differ in their relative boron content whereas the ir Si/C/N ratio is very similar. High temperature thermogravimetric analysi s reveals that with increasing boron content the material's thermal degrada tion is shifted toward higher temperatures. Furthermore, the observed mass loss due to nitrogen evaporation during the material's decomposition is not iceably smaller in boron-rich materials than thermodynamically calculated, X-ray diffraction (XRD) experiments support the presence of crystalline sil icon nitride in heat treated samples with a minimum boron content of 5 at.% . The decomposition temperature of this phase strongly depends on the amoun t of boron and can exceed 2000 degreesC. However, high temperature stable m aterials are not only characterized by a definite B/N ratio, but also by th e ability to develop appropriate micro structures. (C) 2001 Elsevier Scienc e Ltd. All rights reserved.