Si-(B-)C-N ceramics derived from preceramic polymers: Stability and nano-composite formation

Inorganic ceramic materials of the system Si-(B-)C-N which are produced from preceramic polymers by solid state thermolysis have gained much interest during the last years because of their high temperature stability (up to 2000 degrees C), their good oxidation resistance, and their mechanical and electrical properties. After thermolysis, the ceramics are amorphous, however, phase separated in nano-scaled domains. During annealing at temperatures above 1450 degrees C nano- crystalline (and also micro-crystalline) particles precipitate, resulting in the formation of completely or partially crystallized composites (e. g., SiC/Si3N4 or SiCxNy/SiC). The present paper gives an overview of our work (in context to literature) on the complex chemical and physical processes which determine the in-situ formation of Si-(B-)C-N nano-composites and the thermal stability of the nano- structured state. These processes are: de-mixing of the homogenous amorphous matrix in phase separated nano- domains, crystallization of SiC and Si3N4 mediated by nucleation and diffusional growth, crystallite growth due to coarsening, and special solid state decomposition reactions.