DEPENDENCE OF FRACTURE-TOUGHNESS ANISOTROPY OF SI3N4 AND SIC WHISKER/SI3N4 HOT-PRESSED COMPACT ON FRACTURE-TOUGHNESS EVALUATING METHOD

In our previous report on hot-pressed Si3N4 monolith and SiC whisker (SiC(w))/Si3N4 composite compacts, it was clarifed that the fracture tougness (K-IC) measured by CSF method (the shape of pre-crack is semi-ellipse) were nearly the same for VV and PV specimens (VV specimen; the tensile surface and longitudinal direction of the specimen are both vertical to hot-press direction (HD), PV specimen; the tensile surface and longitudinal direction are parallel and vertical to HD, respectively); anisotropy of K-IC was not observed, This phenomenon was considered to be due to the fact that the average angle (theta) of the intersection or cutting direction of propagating crack to the longer axes of acicular Si3N4 grains and/or SiC(w) around the precrack is nearly the same (about 68 degrees) for both specimens, Therefore, when SEPB method is used as a K-IC evaluating method where the shape of the pre-crack in the specimen is rectangle, VV and PV specimens are presumed to show different K-IC values, because the a is different for both specimens (about 90 degrees and 45 degrees, respectively), In this study, this presumption was experimentally investigated, The result was in accordance with the presumption, and the fracture morphology (intergranular or transgranular fracture of acicular Si3N4 grain and/or SiC (w)) near the pre-crack supported the experimental results.