Ferroelastic Domain Switching in Polydomain Tetragonal Zirconia Single Crystals

As‐received, yttria‐doped (4.2 wt% Y2O3) single crystals of zirconia were heated to ≥2100°C in air. Cube‐shaped samples with faces perpendicular to 〈100〉 axes on the basis of the pseudocubic symmetry were cut from the crystals. X‐ray and electron diffraction indicated that the crystals are polydomain with [001] axes, on the basis of the tetragonal symmetry, in three mutually orthogonal directions (perpendicular to the cube faces). The cube‐shaped crystals were tested in compression at temperatures as high as 1400°C. X‐ray diffraction indicated that ferroelastic domains underwent reorientation (switching) in compression. Subsequently, notched samples with the long direction of the beams along 〈100〉 on the basis of the pseudocubic symmetry were fractured in three‐point bending at temperatures as high as 1000°C. X‐ray diffraction from fracture surfaces showed that domain reorientation had occurred and that no monoclinic phase was observed on fracture or ground surfaces. The fracture toughness at room temperature and at 1000°C was ∼12 and ∼8 MPa · m1/2, respectively. Preliminary experiments on polycrystalline tetragonal zirconia samples containing 5.4 wt% Y2O3 and sintered at ≥2100°C also showed no evidence of the monoclinic phase on fracture or ground surfaces. The toughness of the polycrystalline samples was typically 7.7 MPa · m1/2. These results indicate that ferroelastic domain switching can occur during fracture and may contribute to toughness.