Effect of Uni-directional Orientation of SiC Whiskers on Mechanical Properties of SiC Whisker/Si<SUB>3</SUB>N<SUB>4</SUB> Composite Ceramics

Effects of uni-directional orientation of SiC whisker (SiC(W)) on the transverse-rupture strength (TRS) and fracture toughness (KIC) of SiC(W)/Si3N4 composite ceramics were mainly investigated. Two kinds of specimens (WL, WT) were prepared by powder extrusion and hot-pressing; The longer axes of SiC(W) in WL and WT specimens are aligned longitudinally or transversely to the specimen axis, respectively. The results were compared with those on the monolith and WR specimen in which the longer axes of SiC(W) are aligned randomly to the specimen axis, but vertically to the tensile surface.The results obtained are as follows: (1) The TRS was increased from 1000 MPa (WR specimen) to about 1150 MPa (WL specimen) by the uni-directional orientation of SiC(W), although TRS of the WT specimen was as low as 780 MPa. The KIC was also increased from 6.0 MPam1⁄2 up to 9.4 MPam1⁄2 (about 57% increase) by the uni-directional orientation, although it was decreased to 5.1 MPam1⁄2 in the WT specimen. The KIC of the WL specimen was 95% larger than the monolith. (2) The degree of crack deflection in the WL and WT specimens were small, compared with that in the WR specimen; there was no correlation between the degree of crack deflection and the KIC value, indicating that the high KIC of the WL specimen could be ascribed to the increase of the load transfer effect by the uni-directional orientation of SiC(W), etc. (3) The effect of the uni-directional orientation of SiC(W) was presumed to be promoted by the preferential orientation of acicular β-Si3N4 grains which were induced by the uni-directional orientation of SiC(W).