Reversible Stress-Induced Martensitic Transformation in ZrO2

Optical observations and X-ray measurements were used to detect the martensitic transformation of tetragonal ZrO₂ to the monocline structure during room-temperature stressing of Mg-PSZ. The transformation occurs at stresses as low as ∼200 MPa and is reversible. Additional observations of transformation around Vickers indentations and cracks suggest that application of much higher stresses prevents the reverse transformation on unloading.     

Metallographic Observation of the Monoclinic-Tetragonal Phase Transformation in ZrO2

The monoclinic-tetragonal phase transformation of ZrO₂ was examined with a vacuum hot-stage microscope. Polished sections of vacuum-sintered 99.7% pure ZrO₂ were observed from room temperature to 1300°C. The rapid formation of platelet substructure within ZrO₂ grains in the temperature range 1050° to 1150°C was associated with the heating transformation. Photomicrographs and motion pictures were taken of the specimen as the transformation progressed. The surface deformation was irreversible, preventing observation of the phase inversion on cooling. From these hot-stage studies, supplementary DTA, X-ray, and thermal expansion data, and other existing information, it is concluded that the transformation is of the diffusionless, athermal type, characteristic of Fe-Ni martensitic transformations.     

In Situ Martensitic Transformation in a Ternary MgO-Y2O3-ZrO2 Alloy: I, Transformation in Tetragonal ZrO2 Grains

The martensitic transformation in tetragonal ZrO₂ grains in a ternary MgO-Y₂O₃-ZrO₂ alloy has been studied using in situ observations in the transmission electron microscope. Transformation occurred by the nucleation and growth of monoclinic laths; thermoelastic equilibrium can be maintained at different extents of transformation by continuously varying the applied stress. The product phase was always twinned, but two twinning mechanisms were found-sequential formation of twin-related variants and posttransformation deformation twinning. In one example, a (401)/(410) pair of martensite habit planes led to a (100) conjunction twinning plane.     

In Situ Martensitic Transformation in a Ternary MgO-Y2O3-ZrO2 Alloy: II, Transformation in Tetragonal ZrO2 Precipitates

The stress-induced martensitic transformation of t -ZrO₂ precipitates in a ternary MgO-Y₂O₃-ZrO₂ alloy has been studied in situ in the transmission electron microscope. The transformation occurs autocatalytically and takes place by piecewise growth of two twin-related m -ZrO₂ variants. Unloading causes retransformation of partially transformed precipitates, but this reverse ( m → t ) transformation of fully transformed precipitates only occurs on heating. The martensitic transformation in this system is clearly thermoelastic .     

Lattice Defects in Tetragonal ZrO2

The activity of zirconium in the tetragonal ZrO₂ phase was measured at 2200° to 2300°K as a function of composition by studying the reaction

From the variation of the zirconium activity with composition, the pairwise interaction energy E₁₁ between vacancies in the oxygen lattice was evaluated at -4.6 ± 1.0 kcal per mole.     

X-Ray Determination of Transformation Depths in Ceramics Containing Tetragonal ZrO2

An X-ray method to determine the transformation zone size in ceramics containing tetragonal ZrO₂ is described. Results obtained on plane fracture surfaces of various composites were correlated to mechanical property data and TEM observations and were found to be in good agreement. This indicates that the method presented is a useful tool for estimating stress-induced transformation zone sizes and for checking the effectiveness of tetragonal particles introduced in a ceramic matrix.     

Transformation of Yttria-Doped Tetragonal ZrO2 Polycrystals by Annealing in Water

Phase changes and the microstructure resulting from low-temperature annealing of yttria-doped tetragonal ZrO₂ polycrystals in water were investigated at 65° to 120°C. Tetragonal ZrO₂ on the surface of the sintered body transformed to the monoclinic phase, accompanied by microcracking. The transformation rate in water, which was much greater than that in air, was first order with respect to surface concentration of tetragonal ZrO₂. Nonaqueous solvents with a molecular structure containing a lone-pair electron orbital opposite a proton donor site also greatly enhanced the transformation.     

Stabilization of Tetragonal ZrO2 in ZrO2–SiO2 Binary Oxides

Gel-glasses of various compositions in the x ZrO₂^.(10 – x )SiO₂system were fabricated by the sol–gel process. Precipitation due to the different reactivities between tetraethyl orthosilicate (TEOS) and zirconium(IV) n -propoxide has been eliminated through the use of 2-methoxyethanol as a chelating agent. Thermal treatment of these gels produced crystalline ZrO₂particles. While monoclinic is the stable crystalline phase of zirconia at low temperatures, the metastable tetragonal phase is usually the first crystalline phase formed on heat treatment. However, stability of the tetragonal phase is low, and it transforms to the monoclinic phase on further heat treatment. In this study, it has been found that the transformation temperature increases as the SiO₂content in the ZrO₂–SiO₂ binary oxide increases. The most significant results were from samples containing only 2 mol% SiO₂, where the metastable tetragonal phase formed at low temperatures and remained stable over a broad temperature range. X-ray diffraction, transmission electron microscopy, and Fourier transform infrared spectroscopy were used to elucidate the structure of these binary oxides as a function of temperature.     

Thin-Foil Phase Transformations of Tetragonal ZrO2 in a ZrO2-8 wt% Y2O3 Alloy

Tetragonal zirconia ( t -ZrO₂) grains in an annealed ZrO₂ 8 wt% Y₂O₃ alloy transformed to orthorhombic ( o ) or monoclinic ( m ) symmetry by stresses induced by localized electron beam heating in the transmission electron microscope. Different transformation mechanisms were observed, depending on foil thickness and orientation of individual grains. In thicker grains (≥150 nm), the transformation proceeded by a burst-like growth of m laths, and this is believed to approximate bulk behavior. In thinner grains near the edge of the foil, usually those with a [100], orientation perpendicular to the thin-foil surface, "continuous" growth of an o or m phase with an antiphase-boundary-containing microstructure was observed. The o phase is believed to be a high-pressure poly-morph of ZrO₂, which forms (paradoxically) as a thin-foil artifact because it is less dense than t -ZrO₂, but more dense than m -ZrO₂. In some very thin grains, the t → m transformation was thermoelastic. Furthermore, a mottled structure often occurred just before the t → m or t → o transformation, which is attributed to surface transformation. Aside from the lath formation, the observed transformation modes are a result of the reduced constraints in thin foils.     

Deformation of Monoclinic ZrO2 Polycrystals and Y2O3-Stabilized Tetragonal ZrO2 Polycrystals below the Monoclinic–Tetragonal Transition Temperature

Ultrafine-grained monoclinic ZrO₂ polycrystals (MZP) and 3-mol%-Y₂O₃-stabilized tetragonal ZrO₂ polycrystals (3Y-TZP) were obtained by hot isostatic pressing (HIP). Both MZP and TZP were "high-purity" materials with impurities less than 0.1 wt%. The deformation behavior was studied at 1373 K, which was lower than the monoclinic ↔ tetragonal transition temperature. The stress exponent of 3Y-TZP with grain size of 63 nm was 3 in the higher stress region, and increased from 3 to 4 with decreasing stress. The deformation of MZP was characterized by a stress exponent of 2.5 over a wide stress range. The strain rate of 3Y-TZP was slower than that of MZP by 1 order of magnitude. It was suggested that either the doped yttrium or the difference in the crystal structure affected the diffusion coefficients of ZrO₂.     

Chemical Interactions Promoting the ZrO2 Tetragonal Stabilization in ZrO2–SiO2 Binary Oxides

Metastable tetragonal ZrO₂ phase has been observed in ZrO₂–SiO₂ binary oxides prepared by the sol–gel method. There are many studies concerning the causes of ZrO₂ tetragonal stabilization in binary oxides such as Y₃O₂–ZrO₂, MgO–ZrO₂, or CaO–ZrO₂. In these binary oxides, oxygen vacancies cause changes or defects in the ZrO₂ lattice parameters, which are responsible for tetragonal stabilization. Since oxygen vacancies are not expected in ZrO₂–SiO₂ binary oxides, tetragonal stabilization should just be due to the difficulty of zirconia particles growing in the silica matrix. Furthermore, changes in the tetragonal ZrO₂ crystalline lattice parameters of these binary oxides have recently been reported in a previous paper. The changes of the zirconia crystalline lattice parameters must result from the chemical interactions at the silica–zirconia interface (e.g., formation of Si–O–Zr bonds or Si–O⁻ groups). In this paper, FT-IR and ²⁹Si NMR spectroscopy have been used to elucidate whether the presence of Si–O–Zr or Si–O⁻ is responsible for tetragonal phase stabilization. Moreover, X-ray diffraction, Raman spectroscopy, and transmission electron microscopy have also been used to study the crystalline characteristics of the samples.     

Transformation Strengthening of β"Al2O3 with Tetragonal ZrO2

The solid sodium electrolyte β"-Al₂O₃ (Li-stabilized) was strengthened with additions of tetragonal ZrO₂ (15 vol%). The conductivity of this composite material, measured in an Na/Na cell, was 7.7 Ω· at 300°C. Average values of strength and the critical stress intensity factor were 350 MPa and 4.5 MPa·m_1/2, respectively, for the sintered composite material.     

Characterization and Stabilization of Metastable Tetragonal ZrO2

Fine single-domain and polydomain particles of tetragonal ZrO₂ were prepared by hydrothermal and heat treatment of ZrO₂· n H₂O. The particles were characterized by X-ray diffraction, electron microscopy, NMR, mass spectrometry, and ir spectroscopy. The main impurity in the samples was 1 to 2 wt% OH⁻ ions, most of which were concentrated on the particle surfaces or at domain boundaries; some were also distributed in the lattice. Fine single-domain tetragonal particles were strain-free, but polydomain particles had large strains. The single-domain tetragonal particles were transformed much more easily than the polydomain particles by mechanical treatment. The stablization of metastable tetragonal ZrO₂ cannot be explained adequately by the surface-energy theory. An explanation based on the concept of a martensitic transformation may be more reasonable.     

Morphology of Tetragonal Precipitates in Partially Stabilized ZrO2

Tetragonal ( t ) ZrO₂ precipitates in Mg-, Ca-, and Y-partially-stabilized ZrO₂ (Mg-PSZ, Ca-PSZ, Y-PSZ) have different habit planes and different morphologies. These differences arise because of differences in lattice parameters of precipitate and cubic ( c ) ZrO₂ matrix in the three systems. The approximate {001} habit plane and oblate spheroid precipitate morphology observed in Mg-PSZ are explained in terms of anisotropic elasticity using the theory of Khachaturyan. The aspect ratio of ∼5 of these particles is used to calculate a c/t interfacial energy of ∼0.15 J·m^–2. The aligned equiaxed precipitates observed in Ca-PSZ and the twinned colonies observed in Y-PSZ can also be explained using this theory and arise from interactions between strain fields during coarsening; the aligned particles in Ca-PSZ may actually represent an intermediate state before the formation of colonies in this system.     

Stability of Tetragonal ZrO2 Particles in Ceramic Matrices

The stability of tetragonal ZrO₂ particles in ceramic matrices was considered, with particular reference to Al₂O₃-ZrO₂ composites and to partially stabilized ZrO₂. In both systems, particles above a "critical" size transform martensitically to monoclinic symmetry on cooling to room temperature. The critical factors that could affect the size dependence of the transformation temperature—surface and strain energy effects, the chemical free energy driving force, and the difficulty of nucleating the martensitic transformation—were considered. Nucleation arguments are probably the most important.     

Transmission Electron Microscopy of Annealed ZrO2+8 Mol% Sc2O3

The influence of heat treatment (800°C for 200 h) on the micro-structure of 8 mol% Sc₂O₃-ZrO₂ was investigated by XRD and TEM. The starting material was initially characterized and found to contain predominantly cubic-fluorite phase grains, with <5% of the grain containing the rhombohedral β phase. The β phase was positively identified by the analysis of electron diffraction pattern and by quantitative energy dispersive X-ray analysis. On aging, the relative amount of β phase was found to increase to about 15 to 20% by XRD measurement; this was confirmed by TEM observation. The orientation relation between the different variants of the β phase within a grain was determined to be (100)_r||(010)_r, and the geometrical arrangement of these variants within the grain was deduced using a 2¹/₂D imaging electron microscopy technique.     

Effect of Densification Conditions on the Stabilization of the Tetragonal Phase in ZrO2 Polycrystals

The stability of the tetragonal phase in tetragonal zirconia polycrystals consolidated at 830 MPa for 2 to 5 s at 1400°C was studied. Consolidation under such conditions stabilizes the tetragonal phase because of the resulting grain size. Subsequent heat treatment of the samples for relatively long periods, however, tends to promote the tetragonal→monoclinic transformation.     

Toughening of Dental Porcelain by Tetragonal ZrO2 Additions

The effect on mechanical behavior of ZrO₂ additions to a dental porcelain was investigated. The ZrO₂ was introduced into the glassy matrix phase of the porcelain by refritting the all-glass porcelain constituent. X-ray diffraction indicated that a sizeable fraction of the ZrO₂ was retained in the tetragonal form after the porcelain was fired. Zirconia additions to the porcelain produced substantial improvements in fracture toughness, strength, and thermal shock resistance.