The anisotropies of the mechanical properties of SiC whisker (SiC (w))/Si
3N
4 composite ceramics prepared by hot pressing under 15MPa at 2073K for 7.2ks were investigated by using three kinds of specimens (VV, PV, PP), in which the tensile surface and longitudinal axis are vertical (V) or parallel (P) to the hot press direction. The results were compared with those of hot-pressed Si
3N
4 monolith (M) and SiC (p)/Si
3N
4 composite ceramics (P). The results obtained were as follows; (1) Both the flexural strength (FS) and fracture toughness (
KIC) of the composites were nearly the same for VV and PV specimens, and the properties of PP specimens were fairly lower than those of VV or PV specimens. The higher the aspect ratio of SiC (w), the larger the anisotropy of
KIC. The anisotropies of
KIC of the composites were larger than those of M and P ceramics. For both composites and monolith, the anisotropy of hardness was very small in comparison with those of FS and
KIC. (2) The anisotropy of mechanical properties of the SiC (w)/Si
3N
4 composites was ascribed to the orientation anisotropies of acicular β-Si
3N
4 grains as well as of SiC (w). (3) The anisotropy of FS was larger than that of
KIC, which was attributed to the larger size of the fracture source of PP specimens.
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