Loading rate effect on the mechanical behavior of zirconia in nanoindentation

This paper reports the loading rate effect on the mechanical behavior of zirconia using nanoindentation and in situ scanning probe imaging techniques. Nanoindentation tests were performed at a peak load of 10 mN and 0.1-2 mN/s loading rates. The results show that the contact hardness increased by 31% with the loading rate while the Young׳s modulus was loading rate independent (ANOVA, p>0.05). A strain rate sensitivity model was applied to determine the strain rate sensitivity and the intrinsic contact hardness. A pressure-sensitive idealized yield criterion model was applied to analyze the pressure hardening coefficient and the intrinsic compressive yield stress. Extensive discontinuities and largest maximum and contact depths were also observed on the force-displacement curves at the lowest loading rate. These phenomena corresponded to nanoindentation-induced strain softening. The in situ scanning probe images of indentation imprints showed plastic deformation without fracture at all loading rates and dislocation-induced pileups around indentation imprints at the low loading rate. The amount of pileups decreased with increase in loading rate. Finally, these results provide scientific insight into the submicron material removal mechanisms for zirconia during sharp abrasive machining.