INDENTATION CRACKS IN SUPERPLASTICALLY DEFORMED SILICON-NITRIDE CONSISTING OF STRONGLY ALIGNED ROD-SHAPED GRAINS

Vickers indentation in silicon nitride, which was produced by superpla stic tensile deformation of 150%, was conducted. The specimen had a hi ghly anisotropic microstructure, where rod-like grains aligned along t he tensile direction, and showed unusual crack systems. In the case of the indentation whose diagonals point to 0 degrees/90 degrees (0 degr ees direction is the tensile direction), two V-shaped surface cracks w ere propagated from each 90 degrees edge and one crack from each 0 deg rees edge. In the case of the 45 degrees/45 degrees indentation, surfa ce cracks emanated from the edges of the indent and deflected towards the tensile direction. Subsurface crack systems also showed unique sha pes, differing from a cross-shaped crack system in isotropic brittle m aterials. Three-dimensional crack systems were estimated on the basis of the observation, and the crack evolution process was considered. Fu rthermore, the mechanical properties of the anisotropic silicon nitrid e were measured: and showed superior bending strength and fracture tou ghness in specific directions, compared to the undeformed material. (C ) 1998 Elsevier Science S.A. All rights reserved.