Effect of iron and silicon addition on the densification, microstructure and mechanical properties of silicon nitride

In this work, attention was concentrated on the microstructure optimization of normal sintering of silicon nitride by interference on the α → β-Si3N4 transformation and the morphology and spatial distribution of β-Si3N4 grains, aiming for improvement of the mechanical properties in these non-oxide based ceramics. Iron has a significant influence on the α → β-Si3N4 transformation, allowing the formation of a great number of β-phase nuclei. Under appropriate conditions of temperature and time, a more homogeneous microstructure results, in which the grains have a high aspect ratio and exhibit a low degree of abnormal grain growth, achieving an in situ reinforcement. The refinement of the microstructure was accompanied by an increase of fracture toughness values (KIC) for samples sintered under the best densification conditions.