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 optimiz ation of normal sintering of silicon nitride by interference on the al pha-->beta-Si3N4 transformation and the morphology and spatial distrib ution of beta-Si3N4 grains, aiming for improvement of the mechanical p roperties in these non-oxide based ceramics. Iron has a significant in fluence on the alpha-beta-Si3N4 transformation, allowing the formation of a great number of beta-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 refine ment of the microstructure was accompanied by an increase of fracture toughness values (K-IC) for samples sintered under the best densificat ion conditions.