Anisotropy in the indentation fracture piezoelectric titanate ceramics

The mechanical anisotropy of several poled and unpoled well characterised lead-titanate based piezoelectrics with different tetragonal distortions was studied using indentation techniques, with applied loads both in perpendicular and parallel orientation to the polarization axis. Values of hardness, H-v, H-k, and Young's Modulus, E, showed no dependance on poling state, and therefore no anisotropy. However, the indentation fracture toughness, K-Ic, was affected by poling, with a significant anisotropy in the poled samples, with lower and higher values for crack lengths in the directions parallel and perpendicular to the poling axis. To observe the behavior of the cracks caused by the indentations the damage profiles below the indented surfaces were studied for the unpoled and three different orientations of the poled samples. The origin of the observed anisotropy in fracture behavior is thought to be the consequence of a toughening mechanism associated with domain switching. However, the effect appears to be exaggerated by anisotropic plastic deformation at the indenter tip and the associated anisotropic stress field that results.