Precipitation of Mg2Zr5O12 in MgO-Partially-Stabilized ZrO2

Decomposition of cubic, MgO-stabilized ZrO₂ solid solutions has been studied. Precipitates of the ordered compound, Mg₂Zr₅O₁₂, form, but only when a two-stage heat treatment involving nucleation at a low temperature followed by growth at a higher temperature is used. Other decomposition products are also present after such a heat treatment .     

Fracture Toughness of MgO-Partially-Stabilized ZrO2 Specimens with KR-Curve Behavior from Transformation Toughening

Transformation-toughened MgO-partially-stabilized ZrO₂ exhibits a crack-wake-induced load-point displacement in fracture samples. The equations of linear elastic fracture mechanics were self-consistently reformulated to include the residual displacement from the transformation wake. Application of these equations to double-cantilever-beam specimens of transformation-toughened ZrO₂, with the martensitic start transformation temperature close to the testing temperature, gives a decreasing K_R curve and initial toughness values near 30Mpa·m^l/2.     

Stability of ZrO2 Phases in Ultrafine ZrO2-Al2O3 Mixtures

Mixtures of ultrafine monoclinic zirconia and aluminum hydroxide were prepared by adding NH₄OH to hydrolyzed zirconia sols containing varied amounts of aluminum sulfate. The mixtures were heat-treated at 500° to 1300°C. The relative stability of monoclinic and tetragonal ZrO₂ in these ultrafine particles was studied by X-ray diffractometry. Growth of ZrO₂ crystallites at elevated temperatures was strongly inhibited by Al₂O₃ derived from aluminum hydroxide. The monoclinic-to-tetragonal phase transformation temperature was lowered to ∼500°C in the mixture containing 10 vol% Al₂O₃, and the tetragonal phase was retained on cooling to room temperature. This behavior may be explained on the basis of Garvie's hypothesis that the surface free energy of tetragonal ZrO₂ is lower than that of the monoclinic form. With increasing A1₂O₃ content, however, the transformation temperature gradually increased, although the growth of ZrO₂ particles was inhibited; this was found to be affected by water vapor formed from aluminum hydroxide on heating. The presence of atmospheric water vapor elevates the transformation temperature for ultrafine ZrO₂. The reverse tetragonal-to-monoclinic transformation is promoted by water vapor at lower temperatures. Accordingly, it was concluded that the monoclinic phase in fine ZrO₂ particles was stabilized by the presence of water vapor, which probably decreases the surface energy.     

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.     

Hot Forging Characteristics of Fine-Grained ZrO2 and Al2O3/ZrO2 Ceramics

Preliminary results indicate that large strains (∼80%) and strain rates (0.001 s⁻¹) can be obtained without tearing (or cracking) in fine-grain ZrO₂ (0.3 μm) and Al₂O₃/ZrO₂ (1 μm) ceramics. Alumina develops crystallographic and morphological texture as previously reported by Heuer et al.¹     

Solubility of TiO2 in ZrO2

The solubility of TiO₂ in tetragonal ZrO₂ is 13.8±0.3 mol% ui 1300°C, 14.9±0.2 mol% at 1400°C, and 16.1±0.2 mol% at 1500°C. These solid solutions transform to metastable monoclinic solid solutions without compositional change on cooling to room temperature.     

Kinetic Studies of Eutectoid Decomposition of CuFe5O8

The kinetics and mechanism of eutectoid decomposition of CuFe₅O₈ were studied by X-ray diffractometer, electron probe microanalyzer, and reflecting-light microscope techniques. The isothermal decomposition curves were sigmoidal and the maximum rate of decomposition occurred at ∼600°C. Sigmoidal-type curves indicated that the decomposition proceeded by a process of nucleation and growth. Kinetic data were best expressed by Avrami's equation with n≃2 and n≃4 below and above 850°C, respectively. A "site saturation" mechanism is suggested. Bainite-type and pearlite-type microstructures were observed below and above 800°C, respectively. Grain boundaries were active sites in the nucleation process.     

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.     

Nonstoichiometry of ZrO2 and Its Relation to Tetragonal-Cubic Inversion in ZrO2

The system Zr-O was studied in the composition range 50 to 66.7 at.% O by metallographic analysis, high-temperature X-ray analysis, and lattice parameter determinations at room temperature. The solubility of zirconium in zirconia was determined from 1200° to 2000°C, and the cubic-tetragonal inversion temperature of ZrO₂ was shown to be lowered to approximately 1490°C for samples containing α-Zr and oxide phase. Alpha zirconium has greater solubility in the cubic phase than in the tetragonal phase, and the inversion is revealed, in samples quenched from the cubic field, by the exsolution of α-Zr, which is metallographically characterized as striations. At room temperature, the striations are oriented with the 111 and 11° planes of monoclinic zirconia.     

Creep of CaO-Stabilized ZrO2

The compressive creep of 18 mol% CaO-stabilized ZrO₂ was studied at 1200° to 1400°C and 500 to 4000 psi. The specimens were polycrystalline with grain diameters from 7 to 29 μm. The activation energy for creep is 94 kcal/mol, and the creep rates are linearly proportional to the stress and to the inverse of the grain size. These results lead to the conclusion that creep in 18 mol% CaO-stabilized ZrO₂ may be controlled by cation diffusion associated with grain-boundary sliding.     

Synthesis of ZrO2 and Y2O3-Doped ZrO2 Thin Films Using Self-Assembled Monolayers

Undoped or Y₂O₃-doped ZrO₂ thin films were deposited on self-assembled monolayers (SAMs) with either sulfonate or methyl terminal functionalities on single-crystal silicon substrates. The undoped films were formed by enhanced hydrolysis of zirconium sulfate (Zr(SO₄)·4H₄O) solutions in the presence of HCl at 70°C. Typically, these films were a mixture of two phases: nanocrystalline tetragonal- ( t -) ZrO₂ and an amorphous basic zirconium sulfate. However, films with little or no amorphous material could be produced. The mechanism of film formation and the growth kinetics have been explained through a coagulation model involving homogeneous nucleation, particle adhesion, and aggregation onto the substrate. Annealing of these films at 500°C led to complete crystallization to t -ZrO₂. Amorphous Y₂O₃-containing ZrO₂ films were prepared from a precursor solution containing zirconium sulfate, yttrium sulfate (Y₂(SO₄)₃8·H₂O), and urea (NH₂CONH₂) at pH 2.2–3.0 at 80°C. These films also were fully crystalline after annealing at 500°C.     

Phase Relationships in the ZrO2-Sc2O3 and ZrO2-In2O3 Systems

Zirconia-rich subsolidus phase relationships in the ZrO₂–Sc₂O₃ and ZrO₂–In₂O₃ systems were investigated. Phase inconsistencies in the ZrO₂–Sc₂O₃ system resulted from a diffusionless cubic-to-tetragonal ( t' ) phase transformation not being recognized in the past. Through three different measuring techniques, along with microstructural observations, the solubility limits of the tetragonal and cubic phases were determined.     

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.     

Processing-Related Fracture Origins: III, Differential Sintering of ZrO2 Agglomerates in Al2O3/ZrO2 Composite

Large, hard ZrO₂ agglomerates remained in an Al₂O₃/ZrO₂ composite suspension after inefficient ball-milling. The ZrO₂ agglomerates shrank away from the consolidated Al₂O₃/ZrO₂ powder matrix during sintering, producing crack-like voids which were responsible for strength degradation.     

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.     

Possibility of Spinodal Decomposition in ZrO2-Y2O3 Alloys: A Theoretical Investigation

The thermodynamics and kinetics of cubic → tetragonal phase transformations in ZrO₂-Y₂O₃ alloys were investigated by using thermodynamic stability analysis and kinetic computer simulations to explore the possibility of a spinodal mechanism during decomposition. Based on a simple free energy model, it is shown that, depending on the alloy composition, a cubic phase aged within the t + c two-phase field may result in three different sequences of phase transformations: (1) direct nucleation and growth of the equilibrium t-phase from the c-phase matrix; (2) formation of a metastable t'-phase followed by nucleation and growth of the equilibrium c- and t-phases; and (3) formation of a transient t'-phase followed by its spinodal decomposition into two tetragonal phases with one of the tetragonal phases eventually transforming to the equilibrium c-phase. The temporal microstructure evolutions for different compositions were studied by using computer simulations based on the time-dependent Ginzburg-Landau (TDGL) model which incorporates the long-range elastic interactions.     

Low-Temperature Equilibria Among ZrO2, Tho2, and UO2

Studies made on low-hafnium-content ZrO₂, show that the monoclinic-tetragonal inversion temperature is 1170°C., and it is raised to approximately 1190°C. in the "natural" ZrO², which contains approximately 2% HfO₂. No explanation could be found for the knownmarked hysteresis during cooling, when the reverse polymorphic transformation takes dace at 1040°C. In the system ZrO₂-ThO₂ the monoclinic-tetragonal ZrO₂, inversion temperature is lowered to 1000°C., although the maximum solid solution extent of ZrO₂, in Thon and vice versa is approximately only 2% at this temperature. Below about 400°C. under hydrothermal conditions it was possible to prepare a continuous, although metastable series of solid solutions with the fluorite structurewith compositions varying from ThO₂, to nearly pure ZrO₂. Contrary to earlier work only 8 mole ZrO₂, dissolves in UO₂ and less than 4 mole of UO, in ZrO₂ at temperatures up to 13OO0C. A continuous series of solid solutions could be made between Th₂ and UO₂ at 13OO°C., and extensive defect fluorite solid solutions could be prepared between Tho₂ and U₃O₈; there is some evidence for exsolution into uranium-rich and thorium-rich members at low temperatures.     

Solid-State Diffusive Amorphization in TiO2/ZrO2 Bilayers

Thin films of crystalline TiO₂ were deposited on self-assembled organic monolayers from aqueous TiCl₄ solutions at 80°C; partially crystalline ZrO₂ films were deposited on top of the TiO₂ layers from Zr(SO₄)₂ solutions at 70°C. In the absence of a ZrO₂ film, the TiO₂ films had the anatase structure and underwent grain coarsening on annealing at temperatures up to 800°C; in the absence of a TiO₂ film, the ZrO₂ films crystallized to the tetragonal polymorph at 500°C. However, the TiO₂ and ZrO₂ bilayers underwent solid-state diffusive amorphization at 500°C, and ZrTiO₄ crystallization could be observed only at temperatures of 550°C or higher. This result implies that metastable amorphous ZrTiO₄ is energetically favorable compared to two-phase mixtures of crystalline TiO₂ and ZrO₂, but that crystallization of ZrTiO₄ involves a high activation barrier.