Phase Analysis in Zirconia Systems

Linear calibration curves were developed for determining the content of free ZrO₂ in partially stabilized zirconia ceramics by X-ray diffraction techniques. Two methods were studied. The matrix method, in which free ZrO₂ was considered to be distributed in a matrix (the cubic phase), gave approximately equal mass absorption coefficients for the monoclinic and cubic phases. The polymorph technique, in which the cubic phase was considered to be a polymorph of ZrO₂ and in which integrated intensities were used, gave the better results.     

Energy Crossovers in Nanocrystalline Zirconia

The synthesis of nanocrystalline powders of zirconia often produces the tetragonal phase, which for coarse-grained powders is stable only at high temperatures and transforms into the monoclinic form on cooling. This stability reversal has been suggested to be due to differences in the surface energies of the monoclinic and tetragonal polymorphs. In the present study, we have used high-temperature oxide melt solution calorimetry to test this hypothesis directly. We measured the excess enthalpies of nanocrystalline tetragonal, monoclinic, and amorphous zirconia. Monoclinic ZrO₂ was found to have the largest surface enthalpy and amorphous zirconia the smallest. Stability crossovers with increasing surface area between monoclinic, tetragonal, and amorphous zirconia were confirmed. The surface enthalpy of amorphous zirconia was estimated to be 0.5 J/m². The linear fit of excess enthalpies for nanocrystalline zirconia, as a function of area from nitrogen adsorption (BET) gave apparent surface enthalpies of 6.4 and 2.1 J/m², for the monoclinic and tetragonal polymorphs, respectively. Due to aggregation, the surface areas calculated from crystallite size are larger than those measured by BET. The fit of enthalpy versus calculated total interface/surface area gave surface enthalpies of 4.2 J/m² for the monoclinic form and 0.9 J/m² for the tetragonal polymorph. From solution calorimetry, the enthalpy of the monoclinic to tetragonal phase transition for ZrO₂ was estimated to be 10±1 kJ/mol and amorphization enthalpy to be 34±2 kJ/mol.     

Stress-Induced Transformation During Subcritical Crack Growth in Partially Stabilized Zirconia

Fracture surfaces of a commercial partially stabilized ZrO₂ that had undergone subcritical crack growth in H₂0 were characterized by scanning and transmission electron microscopy. At high stress intensities (≥4.6 MPa·m^−1/2), fracture was primarily trans granular, and coherent tetragonal ZrO₂ precipitates had undergone a martensitic transformation to monoclinic symmetry. At lower stress intensities, where power-law crack growth occurred, fracture suflaces were primarily inter-granular, and the tetragonal ZrO₂ had transformed to a new ZrO₂ polymorph with orthorhombic symmetry.     

Preparation of Ultrafine Zirconia Particles

Ultrafine ZrO₂ particles have been prepared via a new sol-gel process. This process involves the addition of excess C₂H₄O into the aqueous ZrOCl₂ solution and reacting the mixture at room temperature; a glassy ZrO(OH)₂ gel is formed moments later. An ultrafine ZrO₂ powder is obtained after the gel is dried and calcined; the powder is monoclinic. The average particle size is ∼12 nm, and its specific surface area is 55.1 m²/g. In addition, partially stabilized ZrO₂ can be prepared in the same manner, yielding a good result.     

Toughening by Monoclinic Zirconia

The toughening induced by monoclinic ZrO₂ in the absence of microcracking was investigated, using ZnO as the host material. Toughness levels K_c in excess of the host toughness K_c^M were achieved, attaining a peak toughness K_c/K_c^M ∼1.7, at monoclinic ZrO₂ volume concentrations 0.2. This toughening is attributed to crack/particle interactions, associated with the deflection and bowing of the crack by the residual strain field around the monoclinic ZrO₂ particles.     

Measurement of Phase Abundance in Magnesia-Partially-Stabilized Zirconia by Rietveld Analysis of X-ray Diffraction Data

The relative abundance of the cubic ( c ), tetragonal ( t ), monoclinic ( m ), and orthorhombic ( o ) polymorphs of ZrO₂, and the δ phase, Mg₂Zr₅O₁₂, present in samples of 3.4-wt%-magnesia-partially-stabilized zirconia have been determined by Rietveld analysis of X-ray powder diffraction data. The samples studied correspond to the as-fired (AF), and subetectoid-aged maximum-strength (MS) and thermalshock (TS) states, with their surfaces in the ground or polished condition. The polymorph abundances of the bulk and near-surface regions are discussed in relation to the type of surface treatment. Grinding produces significant quantities of both m - and o -ZrO₂ in the near-surface regions of all samples. The m content increases from about 5 wt% in the bulk, to 10, 24, and 33 wt% in AF, MS, and TS material, respectively, while the o content increases from trace amounts to about 11 wt% in all samples. The m and o phases both increase at the expense of t -ZrO₂, and the transformation is accompanied by significant lattice distortion and/or crystal size reduction. Thus, measurement of only the 'ground-surface-monoclinic' content does not give an accurate indication of the total amount of transformable t -ZrO₂ in ceramics of this kind. Polishing removes some of the ground-surface m -ZrO₂ in MS and TS, and all of the m -ZrO₂ in AF material. The o -ZrO₂ produced by grinding also declines substantially in AF and MS, but is not removed by polishing of TS. As a result, the bulk composition cannot be guaranteed, in the general case, to be accessible by X-ray analysis of polished surfaces.     

Infrared Absorption Spectroscopy in Zirconia Research

Infrared absorption spectra are shown for monoclinic ZrO₂ and for cubic stabilized ZrO₂. Nine bands are reported in monoclinic ZrO₂ in the region 800 to 200 cm^−l, whereas only one broad band is observed in cubic ZrO₂ over the same frequency range.     

Cubic-to-Tetragonal Displacive Transformation in Gd2O3–Bi2O3 Ceramics

Gd₂O₃-doped Bi₂O₃ polycrystalline ceramics containing between 2 and 7 mol% Gd₂O₃ were fabricated by pressureless sintering powder compacts. The as-sintered samples were tetragonal at room temperature. Hightemperature X-ray diffraction (XRD) traces showed that the samples were cubic at elevated temperatures and transformed into the tetragonal polymorph during cooling. On the basis of conductivity measurements as a function of temperature and differential scanning calorimetry (DSC), the cubic → tetragonal as well as tetragonal → cubic → teansition temperatures were determined as a function of Gd₂O₃ concentration. The cubic → tetragonal transformation appears to be a displacive transformation. It was observed that additions of ZrO₂ as a dopant, which is known to suppress cation interdiffusion in rare-earth oxide–Bi₂O₃ systems, did not suppress the transition, consistent with it being a displacive transition. Annealing of samples at temperatures 660°C for several hundred hours led to decomposition into a mixture of monoclinic and rhombohedral phases. This shows that the tetragonal polymorph is a metastable phase.     

Transformation of Electron-Beam Physical Vapor-Deposited 8 wt% Yttria-Stabilized Zirconia Thermal Barrier Coatings

Yttria-stabilized (8.6 mol% YO_1.5) zirconia thermal barrier coatings evolve at high temperatures from the "non-transformable," metastable tetragonal-prime phase in their as-deposited condition to a mixture of the tetragonal and cubic phases. The kinetics of the transformation at 1200° and 1425°C are reported based on X-ray diffraction measurements. Complementary Raman spectroscopy measurements indicate a sharpening of the tetragonal bands at 263 and 465 cm⁻¹ that is attributed to a systematic decrease in disorder of the Y³⁺ and oxygen vacancies with annealing. No transformation to the monoclinic form of zirconia is observed immediately after high-temperature treatment. However, partial transformation to monoclinic occurs after a prolonged time (months) at room temperature in those samples treated at 1425°C, indicating the development of isothermal martensite.     

Scanning Transmission Electron Microscope Analysis of Solute Partitioning in a Partially Stabilized Zirconia

Solute partitioning and grain-boundary chemistry in a commercial Y₂O₃-bearing partially stabilized zirconia were studied by scanning transmission electron microscopy. The ceramic contained a mixture of twinned monoclinic (minor) phase and cubic (major) phase. Fine-scale precipitation was noted within the cubic phase. The average measured YiO₃ concentrations in the two phases were near those that would be expected from the binary phase diagram, indicating that solute partitioning approaches completion.     

Ultrafine-Grained Dense Monoclinic and Tetragonal Zirconia

Nanoparticles of ZrO₂ with diameters ranging from 4 to 8 nm were synthesized by gas condensation. As-prepared n -ZrO₂ particles have a monoclinic and a high-pressure tetragonal structure depending on size. Pure ZrO₂ was sintered to full density under vacuum at 04 T _m within the monoclinic phase field. Final grain sizes in theoretically dense pellets are below 60 nm. By sintering below the monoclinic–tetragonal transition temperature, microcracking was completely avoided. Tetragonal ZrO₂ stabilized with 3 mol% Y₂O₃ was prepared by interdiffusion of nanoparticles and sintered to near-theoretical density.     

Microwave-Hydrothermal Synthesis of Nanocrystalline Zirconia Powders

Nanosized zirconium oxide (ZrO₂) powders were prepared by adding NaOH to a zirconyl chloride aqueous solution under microwave-hydrothermal conditions. The obtained results showed that the tetragonal polymorph increased with increasing NaOH concentration in the starting solution and reached the maximum value by using 1 M ZrOCl₂. The microwave-assisted hydrothermal synthesis is expected to be able to process continuously, and may lead to energy savings because of rapid heating to temperature and increased kinetics of crystallization. This method is very simple and can lead to powders with desirable characteristics such as very fine size, narrow size distribution, and good chemical homogeneity.     

Densification of Calcia-Stabilized Zirconia with Borates

Densification studies of submicrometer ZrO₂ powders stabilized with 6.5 wt% CaO (CSZ) showed borate additions (1 to 10 wt%) to be effective sintering aids. Estimated densities >99% of theoretical were obtained on sintering at 1200°Cfor 4 h with 2 wt% B₂O₃ or 5 wt% CaO·2B₂O₃ additions to the CSZ powders. Average grain sizes obtained were typically <1 μm. Partial development of a monoclinic ZrO₂ phase was observed in the sintered samples. The amount of this phase varied from ∼7 to 75 wt% and was approximately linearly dependent on the additive concentration. The effect was most marked for the B₂O₃ additions. Development of the monoclinic phase was attributed to progressive leaching of Ca from the CSZ phase by B₂O₃, in effect partially destabilizing the ZrO₂.     

Formation of Acicular Monoclinic Zirconia Particles under Hydrothermal Conditions

Acicular monoclinic ZrO₂, particles were prepared by hydrothermal treatmentat 250°C using sulfuric acid solutions containing zirconium ions. The formation process and morphology of the ZrO₂, particles were investigated. Two types of acicular monoclinic Zr0₂, particle morphologies were obtained, both elongated in the 〈001〉 direction, and the range of acicular ZrO₂, particle sizes changedfrom 0.3 to 1.3 μ m with hydrothermal conditions. Addition of MgSO₄, to the starting solution promoted the crystallization of the monoclinic ZrO₂, particles.     

High-Pressure Phase Transitions in Zirconia and Yttria-Doped Zirconia

Raman spectroscopy has been utilized to characterize the phase transformations and transition pressures in pure and doped zirconia containing 3, 4, and 5 wt% Y₂O₃. The pressure-induced transformations were investigated to over 6 GPa (at room temperature) using a diamond anvil pressure cell. Pure zirconia single-crystal samples transformed to a "new" tetragonal phase (different from the one obtained at high temperatures at atmospheric pressure) at about 4 GPa. The pressure transformation, like the temperature transition, was reversible and exhibited an approximately 0.45-GPa hysteresis at room temperature. The 3 and 4 wt% Y₂O₃ crystals underwent a monoclinic ( P 2_1/b) to tetragonal ( P 4_{2 nmc}) phase transition similar to that observed at high temperatures. This phase change was found to be irreversible on releasing the pressure. The 5 wt% Y₂O₃ at atmospheric pressure consists of a tetragonal modification in a disordered cubic matrix; a gradual, but reversible, disordering transformation of the tetragonal precipitate takes place with pressure.     

Synthesis and Characterization of Zirconia Nanorods

ZrO₂ nanorods are prepared by annealing precursor powders produced in the novel inverse microemulsion system. The length and diameter of ZrO₂ nanorods are a few micrometers and 40–100 nm, respectively. The microstructure of the resultant nanorods are studied by XRD, TEM, selected area electron diffraction, and Raman spectroscopy. The ZrO₂ nanorods are single crystalline and have monoclinic structure. The formation of ZrO₂ nanorods is discussed.     

Novel Room-Temperature Synthesis of Microcrystalline Zirconia

A novel room-temperature method is described for the preparation of microcrystalline zirconia (ZrO₂) in monoclinic form via a simple electrochemical technique. Almost-epitaxial growth is observed, with crystal sizes in the range of 5–15 μm. A tentative reaction scheme also is suggested, to elucidate the underlying process that leads to the formation of the ZrO₂ phase.     

Tetragonal-to-Monoclinic Phase Transitions in Nanocrystalline Rare-Earth-Stabilized Zirconia Prepared by a Mild Hydrothermal Method

Nanocrystalline 4-mol%-Sc₂O₃-stabilized zirconia (4ScSZ) and 4-mol%-Y₂O₃-stabilized zirconia (4YSZ) powders were prepared by a mild urea-based hydrothermal method. The as-prepared 4ScSZ and 4YSZ powders behaved with different tetragonal–monoclinic ( t – m ) transitions on calcination at temperatures between 400° and 1400°C. For the as-calcined 4ScSZ samples, the monoclinic phase fraction varies discontinuously with increasing temperature, i.e., first increases, then decreases, and finally increases again; whereas the monoclinic phase content reduces monotonously for the as-calcined 4YSZ samples, and only tetragonal phase is present over 1000°C. Such interesting results can be explained satisfactorily by considering the combined influences of crystallite size effect, microstrain, and the stabilization effect of the dopant. The microstrain relaxation is mainly responsible for the unusual phase transition in the 4ScSZ samples, while for the 4YSZ samples, the microstrain effect and crystallite size effect can be masked by the stabilization effect of the Y₂O₃ dopant due to its stronger stabilization capability.     

Twinning and Microcracking Associated with Monoclinic Zirconia in the Eutectic System Zirconia–Mullite

Crystallography and morphology of twins and microcracks in the eutectic system mullite–zirconia are discussed in view of the tetragonal-to-monoclinic transformation and associated toughening mechanisms. Specific twin relationships were observed in monoclinic ZrO₂. Highly symmetric and crystallographically well-defined microcracks were observed at the mullite–ZrO₂ interface. Microdiffraction revealed closely related crystallography of monnoclinic twins and microcracks. The number of twin variants depend on the monoclinic ZrO₂ particle size. A method to calculate twinning shear strain using microcrack morphology is suggested. This parameter is essential in several fracture-mechanics calculations.     

Water Incorporation in Tetragonal Zirconia

Samples of 3 mol% Y₂O₃-stabilized tetragonal ZrO₂ ceramics were annealed at 250°C in atmospheres of water vapor pressures of 1 bar and 26 mbar. As demonstrated by the water uptake and the lattice expansion, water molecules were incorporated into the ZrO₂ lattice during annealing, and the amount of the incorporated water is determined by the water vapor pressure. Owing to the filling of oxygen vacancies by the incorporated water molecules, part of the tetragonal ZrO₂ transformed to the monoclinic structure, and protonic defects were induced. The expected proton conduction was confirmed by the polarity of the water vapor concentration cells.