Matrix morphology and mechanical properties of whisker reinforced Si3N4 ceramics

The development of microstructure in Si3N4 ceramics fabricated with additions of metal oxides and/or whiskers has been characterized by analytical electron microscopy and quantitative microscopy. The different microstructures have been related to strength and indentation fracture toughness. The Si3N4 ceramics were hot isostatically pressed without metal oxide sintering additives or with the addition of 2.5 wt% Y2O3 and 0.2 wt% Fe2O3. 25 wt% SiC or ss-Si3N4 whiskers were added to these two matrix compositions. The addition of the metal oxides resulted in the development of a fibrous ss-Si3N4 matrix microstructure with concomitant increase in room temperature strength and toughness. Additions of SiC whiskers reduced Si3N4 grain growth while an addition of ss-Si3N4 whiskers resulted in a coarsening of the microstructure. Fracture surfaces and cracks from Vickers indentation marks showed that an increased Si3N4 mean grain section area and an apparently larger ss-Si3N4 aspect ratio, as well as the addition of SiC whiskers, resulted in an increase in fracture resistance. The results demonstrate that metal oxides as well as whisker additives affect Si3N4 matrix microstructure and thereby the matrix contribution to the mechanical properties of whisker reinforced Si3N4 ceramics.