Precursor-derived Si-(B-)C-N ceramics:: thermolysis, amorphous state and crystallization

The preparation of silicon nitride- and carbide-based ceramics by solid-state thermolysis of polysilazanes and polysilylcarbodiimides is described. Results on the ceramization of the preceramic compounds and the architecture of the corresponding amorphous states obtained by spectroscopic means and by X-ray and neutron scattering are reviewed. Fundamental correlations between the composition and structure of the preceramic compounds and the architecture of the amorphous state are revealed. Furthermore, the crystallization behavior of the amorphous precursor-derived Si-C-N ceramics is treated. Moreover, the influence of boron on the thermal stability of the amorphous state is described. The high-temperature behavior of these Si-B-C-N solids can be correlated with their phase composition. Ceramic materials with compositions located close to the three-phase equilibrium SiC + BN + C exhibit a high temperature stability up to 2000 degreesC. This effect is accompanied by the formation of a metastable solid consisting Of Si3N4 and SiC nanocrystals that are embedded in a turbostratic B-C-N matrix phase. Based on thermodynamic considerations, a model for the high-temperature stability effect is proposed. Copyright (C) 2001 John Wiley & Sons, Ltd.