Nanoindentation-induced plasticity in cubic zirconia up to 500 °C

Orientation-dependent crystal plasticity in cubic zirconia ceramics (8 mol% yttria-stabilized zirconia) was investigated from 25 °C to 500 °C, which is below the macroscopic brittle–ductile transition temperature. Nanoindentation experiments and subsequent electron microscope analysis revealed that dislocation activities on {001} planes predominantly governed small-scale plasticity at the grain interior for each crystal orientation in this temperature range. The maximum shear stress at yielding, τ _max , necessary for plastic yielding, was comparable to the theoretical shear strength, τ _th , estimated from the measured elastic modulus at low temperatures, while their ratio, τ _max/ τ _th , almost monotonically decreased at elevated temperatures. This suggests that plastic yielding was predominantly mediated by atomistic shear processes at low temperatures and increasingly affected by thermally assisted processes at elevated temperatures. The orientation-dependent behavior switched in the vicinity of ~25° from <001> impression axes. In the near-[001] axis, plastic yielding occurred more smoothly at lower stresses, which can be attributed for the surface nucleation and forest-cutting interaction of dislocations on multiple {001} slip planes along and across the impression axes. In near-[101] and [111] axes, on the other hand, yielding was more burst-like and typically accompanied by pop-in events at higher stresses, which could result from the homogeneous dislocation nucleation and free-gliding motion of dislocations on {001} planes with considerable Schmid factors for the impression axes.