Imaging and microanalysis of liquid phase sintered silicon-based ceramic microstructures

This review paper is focussed on the characterization of the microstructural development during liquid phase sintering and post-densification crystallisation heat treatment of ceramic materials based on the Si3N4 or SiC structures. Grain shape and size distributions, assessed by quantitative microscopy in combination with stereological methods, and fine scale microstructures, investigated by electron diffraction and high resolution imaging and microanalysis in the TEM, are discussed and related to the fabrication process and the overall composition of the ceramic material. It is demonstrated that combined high resolution analytical and spatial information from chemically and structurally distinct fine scale features, such as grain boundary films of residual glass, is obtained by electron spectroscopic imaging and subsequent computation of elemental distribution images. These images reveal that residual glassy grain boundary films are rich in oxygen and cations originating from the metal oxide/nitride additives, consistent with fine probe EDX analysis in the FEGTEM. Elemental analysis with high spatial resolution has also shown that grain growth into pockets of residual liquid/glass is associated with diffusion profiles in the glass in front of the growing grain. High resolution imaging in the TEM and elemental maps computed from electron energy filtered images show that the intergranular film thickness, in general, varies within a particular silicon nitride or sialon microstructure. Furthermore, grain boundaries, apparently free from residual glass may co-exist with glass-containing grain boundaries in some silicon nitride microstructures. In addition to the choice and weight fraction of sintering additives, factors such as the ionic radius of the cations originating from the additives, the local nano-scale chemistry and the relative grain orientation have an effect on the volume fraction and morphology of the intergranular microstructure.