Dependence of Fracture Toughness Anisotropy of Si₃N₄ and SiC Whisker/Si₃N₄ Hot-Pressed Compact on Fracture Toughness Evaluating Method

Abstract

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 (θ) 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°) 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 θ is different for both specimens (about 90° and 45°, 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.