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.     

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.     

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.     

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.     

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.     

Tetragonal—Monoclinic Phase Transition Enthalpy and Temperature of ZrO2-CeO2 Solid Solutions

DSC (differential scanning calorimeter) measurements were performed to investigate the tetragonal-monoclinic ( t-m ) transition enthalpy of the compositionally homogeneous ZrO₂- X mol% CeO₂ solid solution of X = 0, 4, 8, and 12. The transition enthalpy decreases linearly with an increase of CeO₂ content. The m → t transition enthalpy on heating agreed well with the inverse t → m one obtained during cooling. With increasing X , the DSC peak broadens and the transition temperature distribution of each sample increases, while the thermal hysteresis is almost independent of X .     

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.     

Monoclinic Crystal Structures of ZrO2 and HfO2 Refined from X-ray Powder Diffraction Data

The crystal structures of ZrO₂ and HfO₂ (P2₁/c) were refined in detail from X-ray powder diffraction data. The precision obtained for the atom positions for both compounds is comparable to that of previous single-crystal work.     

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.     

Hydrothermal Preparation of Ultrafme Monoclinic ZrO2 Powder

Ultrafine powder of single-phase manoclinic ZrO₂ was prepared by hydrothermal treatments of amorphous hydrated zirconia with 8 wt% KF solution under 100 MPa at 200° to 500°C for 24 h. The process yielded well-crystallized particles 16 nm and 22 nm in size at 200° and 500°C, respectively .     

Anisotropic Thermal Expansion of β-Ca2SiO4 Monoclinic Crystal

Crystals of β-Ca₂SiO₄ (space group P 12₁/ n 1) were examined by high-temperature powder X-ray diffractometry to determine the change in unit-cell dimensions with temperature up to 645°C. The temperature dependence of the principal expansion coefficients (α_i) found from the matrix algebra analysis was as follows: α₁= 20.492 × 10⁻⁶+ 16.490 × 10⁻⁹ ( T - 25)°C⁻¹, α₂= 7.494 × 10⁻⁶+ 5.168 × 10⁻⁹( T - 25)°C⁻¹, α₃=−0.842 × 10⁻⁶− 1.497 × 10⁻⁹( T - 25)°C⁻¹. The expansion coefficient α₁, nearly along [302] was approximately 3 times α₂ along the b -axis. Very small contraction (α₃) occurred nearly along [     01]. The volume changes upon martensitic transformations of β↔α_L' were very small, and the strain accommodation would be almost complete. This is consistent with the thermoelasticity.     

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.     

Conversion from β-Ce2O3·11Al2O3 to α-Al2O3 in Tetragonal ZrO2 Matrix

Composites of β-Ce₂O₃·11Al₂O₃ and tetragonal ZrO₂ were fabricated by a reductive atmosphere sintering of mixed powders of CeO₂, ZrO₂ (2 mol% Y₂O₃), and Al₂O₃. The composites had microstructures composed of elongated grains of β-Ce₂O₃·11Al₂O₃ in a Y-TZP matrix. The β-Ce₂O₃·11Al₂O₃ decomposed to α-Al₂O₃ and CeO₂ by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce₂O₃·11Al₂O₃ divided into several grains of α-Al₂O₃ and ZrO₂ doped with Y₂O₃ and CeO₂. High-temperature bending strength of the oxygen-annealed α-Al₂O₃ composite was comparable to the β-Ce₂O₃·11Al₂O₃ composite before annealing.     

X-Ray Microanalysis of ZrO2 Particles in ZrO2-Toughened A1203

Analytical electron microscopy of ZrO₂ particles in a zirconia-toughened alumina was performed. Significant solute heterogeneities in these particles were found but did not seem to affect the particle size dependence of M_s.     

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.     

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.     

High-pressure Induced Phase Transition in ZrO2

Tetragonal ZrO₂ can be formed directly from the mono-clinic form at 25° C by applying pressures greater than 37 kbars. The transition is reversible, and the tetragonal phase cannot be retained in a metastable condition at ambient conditions.     

Phase Stability and Physical Properties of Cubic and Tetragonal ZrO2 in the System ZrO2–Y2O3–Ta2O5

The cubic ( c -ZrO₂) and tetragonal zirconia ( t -ZrO₂) phase stability regions in the system ZrO₂–Y₂O₃–Ta₂O₅ were delineated. The c -ZrO₂ solid solutions are formed with the fluorite structure. The t -ZrO₂ solid solutions having a c/a axial ratio (tetragonality) smaller than 1.0203 display high fracture toughness (5 to 14 MPa · m^1/2), and their instability/transformability to monoclinic zirconia ( m -ZrO₂) increases with increasing tetragonality. On the other hand, the t -ZrO₂ solid solutions stabilized at room temperature with tetragonality greater than 1.0203 have low toughness values (2 to 5 MPa · m^1/2), and their transformability is not related to the tetragonality.