Thermodynamics of the tetragonal to monoclinic phase transformation in constrained zirconia microcrystals

End-point thermodynamic analyses were made of the tetragonal to monoclinic transformation (tm) occurring in ZrO₂ precipitates in a Ca-PSZ alloy and particles in Al₂O₃-ZrO₂ composites. Calculated plots of the reciprocal critical size for transformation temperature were in excellent agreement with experimental data for both systems. Contributions to the total free energy change included bulk chemical, dilatational and residual shear strain energies and also interfacial energies. The latter term consisted of contributions from the change in the chemical surface free energy, the presence of twin boundaries in the precipitate (particle)-matrix interfacial energy. The major impediment to the transformation was the shear strain energy which could not be reduced sufficiently by twinning alone. The t m reaction proceeded spontaneously when the energy barrier was reduced by the response of the particle-matrix interface. The response comprised loss of coherency and grain boundary microcracking for the Ca-PSZ and Al₂O₃-ZrO₂ alloys, respectively. These results are in accord with recent suggestions that either a stress-free strain or a free surface is a necessary conditions for the initiation of a martensitic transformation.     

Transformation of Sc2O3-doped tetragonal zirconia polycrystals by aging under hydrothermal conditions

The phase transformation of less than 6 mol% Sc2O3-doped tetragonal zirconia polycrystals consisting of fine grains has been investigated. Dense bodies with homogeneous microstructures doped with 3.5 to 5 mol% Sc2O3 sintered at 1300°C for 1 h consisted mostly of a tetragonal phase. Within 20 h of aging under hydrothermal conditions at 180°C, the amount of monoclinic ZrO2 on the surface of the 4 to 5 mol% Sc2O3-doped specimens sintered at 1400°C was saturated and reached a constant value, and the increase in the amount of monoclinic ZrO2 showed a sigmoidal type of kinetic transformation. The apparent activation energy for the phase transformation in Sc2O3-doped zirconia was 84–91 kJ/mol. Based on the hydrothermal aging results, the possible existence of a larger two-phase (cubic+tetragonal) region is suggested, and the phase boundary between the cubic+tetragonal and cubic phase in the ZrO2-Sc2O3 system is proposed.     

Thermodynamic analysis of the tetragonal to monoclinic transformation in a constrained zirconia microcrystal

A thermodynamic analysis was made of a simple model comprising a transforming t-ZrO₂ microcrystal of sized constrained in a matrix subjected to a hydrostatic tensile stress field. The field generated a critical size range such that a t-particle transformed ifd _cl<d <d _cu. The lower limitd _cl exists because at this point the maximum energy (supplied by the applied stress) which can be taken up by the crystal is insufficient to drive the transformation. The upper limitd _cu is a consequence of the microcrystal being so large that it transforms spontaneously when the material is cooled to room temperature. Using the thermodynamic (Griffith) approach and assuming that transformation toughening is due to the dilational strain energy, this mechanism accounted for about one-third of the total observed effective surface energy in a peak-aged Ca-PSZ alloy.     

Effect of stress-induced phase transformation on the properties of polycrystalline zirconia containing metastable tetragonal phase

Polycrystalline zirconia containing a high content of metastable tetragonal phase shows high strength ( 700 MPa), high fracture toughness (K _c = 6 to 9 MN m^–3/2) and small grain size (<0.3jm). The strength and grain size remain nearly constant over a wide range of tetragonal phase content (100 to 30%). At a low concentration of tetragonal phase <30%, there is a rapid decrease in strength accompanied by a rapid increase in grain size. These results are explained by means of a stress-induced phase transformation in the metastable tetragonal phase.     

The use of X-ray line profile analysis in the tetragonal to monoclinic phase transformation of ball milled, as-sintered and thermally aged zirconia powders

The martensitic tetragonal to monoclinic (t → m) transformation in zirconia plays an important role in determining the mechanical properties and low temperature degradation of this ceramic. The analysis of X-ray data using the Warren–Averbach procedure can supply reliable information about crystallite size and microstrain under various conditions. The X-ray line profile analysis results can shed light on the martensitic transformation and the microstructural evolution of the t and m phases. The presence of m plates causing the formation of partially transformed t crystals reduces the mean size of the t crystals. The ball milling process also affects the mean size of the produced monoclinic crystals. The microstrain of the t and m phases can also be influenced by the ball milling and also by matrix constraints. Zirconia powders doped with low contents of CeO₂ exhibit an isothermal type t → m transformation during thermal annealing which affects the subsequent athermal transformation upon cooling and thus a decrease in the mean size of the m crystals is produced. This revised version was published online in November 2006 with corrections to the Cover Date.     

An in-situ investigation on the critical phase transformation stress of tetragonal zirconia polycrystalline ceramics

The popularly accepted concept of stress induced phase transformation (SIPT) for tetragonal zirconia polycrystalline (TZP) ceramics has been re-evaluated in this work using an in-situ X-ray diffraction technique that was facilitated by the use of a novel stressing fixture. At stress levels of 700 MPa, which is close to the sample's rupture strength very little of the tetragonal phase transformed to a monoclinic phase, regardless of whether a tensile or compressive stress was applied. However the intensity of the peak (2 0 2)_t, (2 2 0)_t, (1 1 3)_t, and (1 3 1)_t, compared with the peak, (1 1 1)_t did display a significant change after the tetragonal zirconia was loaded. In the fractured surface, a large amount of monoclinic phase was discovered. Thus we infer that for a homogenous TZP ceramic, the critical phase transformation stress is close to the material's rupture strength. On the basis of the observation of a non-linear deformation before the phase transformation, we suggest that the TZP material may have a four step response to an increasing applied stress. This response consists of; (i) anelastic behaviour which may be explained by ferroelastic domain switching or another anelasticity theory; (ii) t m phase transformation; (iii) microcracks emerging and then growing; (iv) final fracture of the material and a possible reverse transformation.     

Influence of a small amount of Al2O3 addition on the transformation of Y2O3-partially stabilized ZrO2 during annealing

Tetragonal-to-monoclinic phase transformations in 2 and 3 mol% Y2O3–ZrO2 ceramics and their composites with 5 vol% Al2O3 during annealing in water and in vacuum at 353–623 K were investigated to explore the effect of a small quantity of Al2O3 addition on the transformation. The dispersion of Al2O3 particles into the ZrO2(Y2O3) matrix was found to be effective to suppress the transformation directly induced by the attack of H2O during annealing in water, even though the amount was as small as 5 vol%. However, the transformation predominantly caused by thermal activation during annealing in vacuum was not affected by the limited amount of Al2O3 addition. The effect of suppression of Al2O3 on the water-induced phase transformation was considered to be realized through the hydroxydation of Al2O3 particles, by which the sample surface was effectively "protected" from further attack of H2O, which accelerated the low-temperature degradation transformation. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of process conditions on phase mixtures of sol–gel-synthesized nanoscale orthorhombic, tetragonal, and monoclinic zirconia

Zirconia particles synthesized under ambient conditions are frequently amorphous, requiring heat to produce a crystalline phase. Synthesized nanoparticles are generally found in the tetragonal phase. In this article, a room temperature sol–gel synthesis of crystalline sub-10-nm zirconia particles is described. By adjusting the acid concentration of the reaction, it is found that the particles’ crystalline phase can be modified. Under acidic conditions with moisture present, the tetragonal phase is produced, whereas under acidic conditions with low water content, 2–5-nm particles suggestive of the metastable orthorhombic phase are produced. Subsequent heat treatment of all powders produced with this technique resulted in their transformation first to the tetragonal phase, and ultimately to the monoclinic phase. The extent of the transformation to the monoclinic phase depends upon the atmosphere, however, suggesting that oxygen vacancies play a significant role in the stabilization and determination of the resulting phase.     

Role of oxygen phonons in the martensitic phase transformation of yttria stabilized tetragonal zirconia polycrystals

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