Microstructure and mechanical behaviour of self-reinforced Si3N4 and Si3N4-SiC whisker composites

Abstract

Abstract Monolithic Si3N4 and Si3N4-SiC whisker composites were fabricated by hot pressing or hot isostatic pressing. They were sintered in the 1600–1800°C temperature range with 6 wt% Y2O3 and 3 wt% Al2O3 as additives. Morphological aspects of whiskers were statistically determined by image analysis and different matrix microstructures were observed after chemical etching. Then, a correlation was established with mechanical properties. When the same sintering conditions are used, the composite rupture stress increases or decreases with respect to that of the corresponding monolithic Si3N4 matrices. The increase is attributed to an effective load transfer mechanism which involves stress concentration at fibre-matrix interfaces. These interfaces can become the new critical defects in the microstructure when the whiskers are too large. The resistance to short cracks was determined by indentation. The single edge precracked beam (SEPB) method allowed characterization of the resistance to long crack propagation. The toughness increases both with the aspect ratio of whiskers and/or elongated β-Si3N4 grains and with the precrack length (R-curve effect). The improvement is mainly due to the bridging of crack borders by acicular shapes or ligaments of unbroken matter. The R-curve corresponds to the enlargement of the active clamping zone as the crack extends. At a given precrack length the fracture toughness is strongly dependent upon the potential diameter of bridges, whereas the R-curve steepness rises with the density of clamping sites.