Currently, intensive research effort is focused all over the world on engineering novel nonmetallic structural materials that would combine high mechanical strength, fracture toughness, wear resistance, and chemical inertness with high stability in aggressive media in a wide temperature range. One such material is partially stabilized zirconia (PSZ) crystals — a zirconia-based solid solution containing small additions of stabilizing oxides.

The PSZ single crystals were grown by directional crystallization technique with direct RF-heating in the cold container (skull melting).The mechanical properties of PSZ crystals depend primarily on the synthesis procedure and conditions. PSZ single crystals possess very attractive mechanical properties owing to their unusual, twin domain structure.

PSZ crystals growing during melt synthesis initially have a cubic structure, and a phase transformations occur during cooling in the solid state. As the temperature decreases, the cubic phase becomes unstable and transforms to a tetragonal modification. The slight atomic shifts, mainly affecting the oxygen ions, distort the symmetry of the initial structure. The oxygen ions shift relative to the perfect fluorite lattice positions (1/4, 1/4, 1/4). Generally, the tetragonal phase lattice is slightly elongated along the c axis as compared to the cubic phase lattice.

X-ray diffraction from the PSZ phase showed that the material has two tetragonal phases (fig. 1). The simultaneous occurrence of the (006) and (600) reflections in the diffraction pattern is accounted for by twinning as will be shown below in the discussion of the transmission electron microscopy results. Phase constituent study of zirconia doped to different Y₂O₃ concentrations (2.8–5.0 mol.%) showed that all the specimens, regardless of the stabilizing impurity content, have two tetragonal zirconia modification phases with varying degrees of tetragonality. Both phases have a slightly distorted fluorite structure and differ in the ratio of their lattice parameters. For one tetragonal phase – t, the c/a ratio was 1.014–1.015, and for the other tetragonal phase – t’, the c/a ratio was close to 1, i.e. 1.004–1.005. The yttrium rich t’ phase is not transformable unlike the lower yttrium content t phase which undergoes a martensitic transformation to the monoclinic state under mechanical stress: this transformation may suppress the sources of stress concentration and increase the fracture toughness of the material.

Fig. 1.  X-ray diffraction patterns of the PSZ 2.8 mol.% Y₂O₃

TEM study of the PSZ crystals showed that all the specimens had a well developed twin domain structure. The twined structure forms due to the polymorphic transformation from the cubic to the tetragonal phase which occurs during single crystal cooling. Fig. 2 shows a typical example of the twined structure for the PSZ crystals. It can be seen that most domains have an elongated shape. The twinning plane is {110}. The primary twin plates also undergo secondary twinning to form a parquet-like structure consisting of twin domains. The twinning can occur on along the planes inclined to the fourth order axis c. Twinning may occur along the (101) and (011) planes cannot occur along the (110) plane which is parallel to the c axis.

Fig. 2. Bright-field image of twin domains in the ZrO₂ 2.8 mol.% Y₂O₃ sample, with selected area diffraction pattern in <100> orientation.

Study of the PSZ crystals with different stabilizing impurity concentrations showed that an increase in Y₂O₃ concentration controls the type and size of the twin domains. The twin structure also changes depending on the stabilizing impurity concentration. At Y₂O₃ concentrations from 2.8 to 3.2 mol.% twinning occurs first in larger domains which in turn also undergo twinning. High resolution study of the fine twin domain structure showed that nanosized twins only occur in specimens with the Y₂O₃ concentrations of up to 3.2 mol.%. At higher stabilizing impurity concentrations (3.7–5 mol.% Y₂O₃), no twinning hierarchy was observed, atomic plane traces inside twin domains were not broken, and minimum twin domain sizes could be identified in diffraction contrast images. This suggests that twinning occurs simultaneously and is localized within small volumes. According to the phase diagram, the transition from the single phase cubic region to the two phase one during cooling occurs at lower temperatures if the yttrium oxide concentration is higher, and this controls the twin structure pattern.

The work was supported by the grant № 12-02-31751of the Russian foundation for basic research (RFBR) and the program of basic researches of Presidium of the Russian Academy of Sciences “Bases of basic researches nanotechnologies and nanomaterials”

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