High-Resolution Transmission Electron Microscopy Study of the Microstructural Development of a Silicon Carbonitride Nanocomposite

Nanocrystalline ceramics are expected to possess enhanced superplasticity over their microcrystalline counterparts. In this effort of producing nanocomposites of silicon nitride and silicon carbide, amorphous Si-C-N derived from pyrolysis of a polysilazane precursor was sintered with yttria as an additive. High-pressure sintering at different temperatures resulted in sintered materials ranging from amorphous to nanocrystalline. High-resolution transmission electron microscopy was conducted to characterize the development and grain-boundary features of the nanocrystalline microstructure. The results provide a preliminary understanding of the process of the formation of the nanocrystalline structure from an amorphous matrix, under the condition of high pressure and relatively low temperature. The wide variation in the thickness of grain-boundary phases observed in this material suggests a nonequilibrium state of the grain boundary, which might be related to the processing conditions.