Microstructural designing of silicon nitride related to toughness

Two ways of microstructural control were investigated with the intention to improve fracture toughness of gas pressure sintered silicon nitride : seeding and optimisation of sintering time. In order to study the effect of seeding, different amounts of large β-Si3N4 whiskers were added to different silicon nitride powders. Seeding can lead to a low toughening improvement, providing that the initial powder is free from large pre-existing β nuclei. If not, seeding has a negligible effect on the microstructure because of nuclei interaction and an unfavourable effect on densification, hence on toughness. The effect of the sintering time was studied in a case of abnormal nuclei growth. Toughness can be improved by increasing the sintering time, providing that the composition of the intergranular phase remains stable. Correlations between microstructural parameters and toughness are consistent with the reinforcing mechanisms in composites (crack bridging and crack deviation): toughness is proportional to the grain diameter square root and increases as the fraction of the large grains increases.