High-resolution analytical electron microscopy and creep deformation of silicon nitride ceramics

The typical microstructure of silicon nitride consists of rigid Si₃N₄ grains and intergranular amorphous films associated with the liquid phase sintering process involved during densification. The presence of the amorphous films may affect the creep behaviour of silicon nitride ceramics at elevated temperatures. The advent of high resolution transmission electron microscopy (HRTEM) coupled with fine-probe chemistry analysis enables us to investigate the structure and chemical composition of the nano-scale grain boundary amorphous films and the role they play in creep deformation of silicon nitride.; The materials investigated consist of β-Si₃N₄ grains with and without secondary crystalline phases. All grains were covered with a thin intergranular amorphous film at both homophase and heterophase boundaries. It was found that these amorphous films have a characteristic value of thickness, independent of grain misorientation, but dependent on the chemical composition of the film and the grains on either side of the film. The creep behaviour of the materials were evaluated by compressive and tensile testing. The grain-boundary film thickness distribution was measured before and after creep using both high-resolution lattice imaging technique and Fresnel fringe imaging technique. The results show a narrow range of film widths in the uncrept material but a bimodal distribution after creep. This provides, for the first time, direct evidence for the occurrence of viscous flow of intergranular amorphous films during creep deformation of silicon nitride. Finally, a model is developed to describe the viscous flow process in multi-phase Si₃N₄ materials in contrast to prior models which are only applicable to “pure” Si₃N ₄ materials. The creep response predicted by the model is consistent with the experiment.