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 by hydroboration of oligovinylsilazane {-(H2C= CH)SiHNH=](n) with different amounts of H3B . SMe2. Pyrolysis of these precursors yields amorphous ceramic materials, which only differ in their relative boron content whereas their Si/C/N ratio is very similar. High temperature thermogravimetric analysis reveals that with increasing boron content the material's thermal degradation is shifted toward higher temperatures. Furthermore, the observed mass loss due to nitrogen evaporation during the material's decomposition is noticeably smaller in boron-rich materials than thermodynamically calculated, X-ray diffraction (XRD) experiments support the presence of crystalline silicon nitride in heat treated samples with a minimum boron content of 5 at.%. The decomposition temperature of this phase strongly depends on the amount of boron and can exceed 2000 degreesC. However, high temperature stable materials are not only characterized by a definite B/N ratio, but also by the ability to develop appropriate micro structures. (C) 2001 Elsevier Science Ltd. All rights reserved.