Decomposition-Crystallization of Polymer-Derived Si-C-N Ceramics

Monolithic polymer‐derived Si‐C‐N ceramics were processed by blending an oligomeric Si‐C‐N precursor (liquid polysilazane) with 70 vol% of crosslinked or pyrolyzed Si‐C‐N powder particles, which were obtained from the same liquid precursor preheated at 300° or 1000°C, respectively. Powder compacts subsequently were annealed at 300°C to crosslink the liquid precursor acting as a binder between the powder particles, thus yielding monolithic green bodies. Heat treatment at 1540°C was performed to initiate crystallization in the various samples. Microstructure development and, in particular, crystallization behavior were characterized by X‐ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and preliminary nuclear magnetic resonance (NMR) spectroscopy. The material containing 300°C polymer powder (with oligomeric binder, also crosslinked at 300°C) revealed a homogeneous amorphous microstructure after exposure to temperatures of 1540°C. In contrast, the specimen containing powder particles preheated at 1000°C exhibited a high volume fraction of SiC crystallites within regions that were previously filled by the binder; however, the Si‐C‐N powder particles themselves remained amorphous. SEM observations as well as XRD studies showed the formation of idiomorphic SiC and Si3N4 crystallites on specimen surfaces as well as along internal crack walls. This finding suggested that vapor‐phase reactions at the surface were involved in the formation of crystalline phases at temperatures >1250°C. Moreover, NMR spectroscopy data indicated a phase separation process, implying structural rearrangement prior to crystallization.