High Resolution Electron Microscopy Studies on Silicon Nitride Ceramics

Abstract

High-resolution electron microscopy (HREM) was used to study interfaces in Si3N4 materials on a sub-nanometer scale. Amorphous secondary phases are present at multigrain junctions, owing to the liquid-phase sintering process involved. In addition, all Si3N4-grains are separated by a continuous amorphous intergranular film. A control of these amorphous films is most desirable and hence emphasis was placed on the characterization of both structural and compositional information of interganular films. Different TEM techniques can be applied for the detection and determination of intergranular-film thickness, however, the most accurate results are obtained by the lattice-imaging technique. Statistical analyses of a number of grain-boundary films provided an experimental verification that the film thickness is constant. High-purity Si3N4 materials with SiO2 as the only additive revealed a film thickness of 1.0 nm, independent of SiO2-volume fraction. Si3N4 materials with different metal-oxide additions showed differences in thickness and chemical composition of the intergranular films, as revealed by analytical electron microscopy (AEM) with high spatial resolution. The results on film thickness (HREM) strongly suggest a dependence of film thickness on its chemical composition. So far, no experimental observations suggest a dependence of film thickness on additive-volume fraction. During an appropriate post-sintering heat treatment crystalline secondary phases form at triple-grain regions. Depending on the secondary phases formed, interface chemistry can be altered and hence grain-boundary film thickness be changed.