Indentation Cracking in Mono and Polycrystalline Cubic Zirconia: Methodology of an Apparent Fracture Toughness Evaluation

Indentation-induced cracking with sharp tips is studied on mono- and polycrystalline cubic zirconia. Tests at different applied loads are carried out, ranging below and above a threshold load above which visible surface cracking is noted after testing. This threshold load does not appear to be correlated to any discontinuity in the evolution of either the hardness or the irreversible energy ratio with applied load. The crystal orientation and the presence of grain boundaries clearly affects the shape of the cracks generated around the residual imprint. Polycrystalline samples show significantly shorter indentation induced cracks and therefore a higher apparent toughness than single crystals for the same load. In addition, for single crystals, cracks are deflected towards {110} type planes, appearing to be the most susceptible to cracking, and thus logically presenting the lowest apparent toughness. Tests with a Berkovich tip leads to an average of different crystal orientations, while tests with a Vickers tip can be used to test a specific crystal orientation in the case of a cubic crystal such as zirconia.