MICROSTRUCTURE AND MECHANICAL-BEHAVIOR OF SELF-REINFORCED SI3N4 AND SI3N4-SIC WHISKER COMPOSITES

Monolithic Si3N4 and Si3N4-SiC whisker composites were fabricated by h ot pressing or hot isostatic pressing. They were sintered in the 1600- 1800-degrees-C temperature range with 6 wt% Y2O3 and 3 wt% Al2O3 as ad ditives. Morphological aspects of whiskers were statistically determin ed by image analysis and different matrix microstructures were observe d after chemical etching. Then, a correlation was established with mec hanical properties. When the same sintering conditions are used, the c omposite rupture stress increases or decreases with respect to that of the corresponding monolithic Si3N4 matrices. The increase is attribut ed to an effective load transfer mechanism which involves stress conce ntration at fibre-matrix interfaces. These interfaces can become the n ew critical defects in the microstructure when the whiskers are too la rge. 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 bot h with the aspect ratio of whiskers and/or elongated beta-Si3N4 grains and with the precrack length (R-curve effect). The improvement is mai nly due to the bridging of crack borders by acicular shapes or ligamen ts of unbroken matter. The R-curve corresponds to the enlargement of t he active clamping zone as the crack extends. At a given precrack leng th the fracture toughness is strongly dependent upon the potential dia meter of bridges, whereas the R-curve steepness rises with the density of clamping sites.