Generally, ceramic materials present an elastic-brittle behaviour at room temperature, leading to fracture before any sign of plasticity. This is linked to the low mobility of dislocations, which compromises any ductility as is the case of metallic materials. However, some zirconia-based ceramics can exhibit real plasticity at room temperature, thanks to a martensitic phase transformation mechanism (tetragonal to monoclinic). This mechanism is similar to the TRIP (TRansformation Induced Plasticity) effect of certain steels and/or shape memory alloys. The phase transformation, which can be triggered by stress and/or temperature, is particularly remarkable in zirconia doped with cerium oxide, with a plastic deformation on the order of one percent before rupture, resulting in high toughness (relatively for ceramics) and lower sensitivity to the presence of defects. Still, the stress criteria for transformation onset, the effect of crystallographic orientations and the favoured lattice correspondences between the parent (tetragonal) and product (monoclinic) phases are not fully described. Therefore, this work proposes an in-depth study of the martensitic transformation in zirconia doped with ceria at the nanoscale, by combining microstructure, crystallography, and finite element analyses. To this end, in situ Laue microdiffraction combined with compression tests were performed on single-crystalline micropillars, avoiding stress mismatches caused by grain boundaries.

For microstructure and crystallography evaluation, single-crystalline micropillars with 1 µm diameter and 3 µm height and composed of 12 mol% ceria-doped zirconia, were micromachined by focused ion beam milling. Pillars with different crystalline orientations were evaluated to check for crystal orientation dependences. The micropillars were compressed using a FT-NMT04 nano-indenter (FemtoTools) equipped with a diamond flat punch and deformation was followed by Laue microdiffraction to monitor the crystal orientation before and after the phase transformation, i.e., the phase transformation correspondence. Moreover, the position and the shape of the recorded diffraction peaks allow for determining the deviatoric elastic strain and the plastic activity.

Regarding the effect of the crystal orientation, a competition between fracture and transformation was observed. Micropillars with orientations that do not favour transformation showed increased transformation critical stresses (>4.0 GPa), compared to those with orientations that accommodate the transformation more easily (~1.3 GPa). In addition, the results also showed a partial martensitic transformation at the top of the pillars that might be linked to the pillar’s geometry, which was slightly thinner at the top due to the milling process. Therefore, finite element analysis was performed to estimate the compression stress state along the pillar’s height, and compare it to the experimental critical transformation stresses here obtained.