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 c rosslinked or pyrolyzed Si-C-N powder particles, which were obtained f rom the same liquid precursor preheated at 300 degrees or 1000 degrees C, respectively, Powder compacts subsequently were annealed at 300 de grees C to crosslink the liquid precursor acting as a binder between t he powder particles, thus yielding monolithic green bodies. Heat treat ment at 1540 degrees C was performed to initiate crystallization in th e various samples. Microstructure development and, in particular, crys tallization 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 degrees C polymer powder (with oligomeric binder, also crosslinked at 300 degrees C) revealed a homogeneous amor phous microstructure after exposure to temperatures of 1540 degrees C, In contrast, the specimen containing powder particles preheated at 10 00 degrees C exhibited a high volume fraction of SiC crystallites with in regions that were previously filled by the binder; however, the Si- C-N powder particles themselves remained amorphous. SEM observations a s well as XRD studies showed the formation of idiomorphic SiC and Si3N 4 crystallites on specimen surfaces as well as along internal crack wa lls, This finding suggested that vapor-phase reactions at the surface were involved in the formation of crystalline phases at temperatures > 1250 degrees C, Moreover, NMR spectroscopy data indicated a phase sepa ration process, implying structural rearrangement prior to crystalliza tion.