Microstructure and stability of polymer-derived ceramics; The Si-C-N system

Abstract

Monolithic polymer-derived Si-C-N ceramics were processed by blending 70 vol% of both cross-linked and pyrolyzed Si-C-N powder particles with an oligomeric Si-C-N precursor (liquid polysilazane). The respective Si-C-N powder particles were prepared from the same liquid precursor, however, pre-heated at 300 and 1000°C. Powder compacts were annealed at 300°C, in order to crosslink the liquid precursor that acts as a binder phase between the powder particles. After cross-linking, an additional heat treatment was performed at 1540°C to transform both the binder phase and the particles into a homogeneous ceramic matrix. Microstructure development and, in particular, crystallization behavior of the monoliths were characterized by transmission electron microscopy (TEM). In general, the two starting materials, which only differ with respect to the pre-heat treatment of the powder particles, evolved markedly different inicrostructures, The material prepared with 300°C polymer powder and oligonieric binder revealed a homogeneous amorphous microstructure with only a small fraction of crystallized spherical inclusions after exposure to 1540°C. In contrast, blending the powder particles annealed at 1000°C with the same binder yielded a high degree of SiC crystallization within regions that were formerly filled by the polymeric binder. The Si-C-N powder particles, however, remained amorphous. As will be shown, the observed microstructure variations are closely related to the residual porosity of the system. Moreover, phase separation in the amorphous matrix can also affect the overall stability of such polymer-derived ceramics, when exposed to high temperatures. A distinction between open and closed systems allows to explain the observed microstructure variations and, more importantly, a correlation with the high-temperature stability of the materials.