The effect of crystal anisotropy on fracture toughness and strength of ZrB2 microcantilevers

The influence of crystal anisotropy on the micromechanical properties of ceramic grains plays an important role in the design of the macromechanical performance of bulk polycrystalline samples. To this end, the effect of crystal orientation on fracture toughness and strength was investigated by microcantilever bending experiments combined with finite element method (FEM) simulations in grains of a polycrystalline ZrB2 sample. The sample was prepared by hot pressing and the crystal orientations were determined by electron backscatter diffraction after careful surface preparation. The bending tests were carried out on notched and unnotched microcantilevers cut from specific grains along the prismatic (& BOTTOM; to c-axis), basal (& PAR; to c-axis), and intermediate (45 & DEG; to c-axis) directions of ZrB2 crystals using focused ion beam milling. The fracture toughness and strength were determined by FEM-derived analytical solutions. The fracture strength was similar with values of about 11-12 GPa in every direction. Enhanced plasticity was found in the intermediate direction of unnotched beams as compared to the other two brittle orientations. The fracture toughness was found to be 30% higher in the intermediate direction (4.1 MPa m(0.5)) than those measured for the basal (3.1 MPa m(0.5)) and prismatic directions (3.3 MPa m(0.5)). These findings were explained by the orientation dependence of slip system activations which provide a new way in the design of textured polycrystalline ZrB2 ceramics toward enhanced damage tolerance.