Indentation cracks in superplastically deformed silicon nitride consisting of strongly aligned rod-shaped grains

Vickers indentation in silicon nitride, which was produced by superplastic tensile deformation of 150%, was conducted. The specimen had a highly anisotropic microstructure, where rod-like grains aligned along the tensile direction, and showed unusual crack systems. In the case of the indentation whose diagonals point to 0°/90° (0° direction is the tensile direction), two V-shaped surface cracks were propagated from each 90° edge and one crack from each 0° edge. In the case of the 45°/45° indentation, surface cracks emanated from the edges of the indent and deflected towards the tensile direction. Subsurface crack systems also showed unique shapes, differing from a cross-shaped crack system in isotropic brittle materials. Three-dimensional crack systems were estimated on the basis of the observation, and the crack evolution process was considered. Furthermore, the mechanical properties of the anisotropic silicon nitride were measured, and showed superior bending strength and fracture toughness in specific directions, compared to the undeformed material.