Phase Characterization of Partially Stabilized Zirconia by Raman Spectroscopy

Raman spectroscopy was used to characterize phase transformations and transition temperatures in partially stabilized zirconia containing ≤20 wt% Y₂O₃. The completeness of the martensitic transition and its thermal hysteresis was followed in samples with ≤4 wt% Y₂O₃. Between 5 and 12 wt% the spectra indicate a tetragonal modification precipitated in a disordered cubic fluorite matrix. Above 15 wt% a "density of states" spectrum prevails that is characteristic of the fully stabilized disordered cubic phase.     

High-Strength Zirconia Fibers

Fine-grained polycrystalline zirconia fibers have been formed from an acetate precursor. The fibers contained a Y₂O₃ additive, which inhibited grain growth (grain size ≤0.5 μm) and allowed the tetragonal phase to be retained at room temperature. Fibers with diameters in the range 2 to 5 μm had strengths in the range 1.5 to 2.6 GPa.     

Phase Formation and Stability in Reactively Sputter Deposited Yttria-Stabilized Zirconia Coatings

Yttria-stabilized zirconia (YSZ) coatings were produced by reactively cosputtering metallic zirconium and yttrium targets in an argon and oxygen plasma using a system with multiple magnetron sputtering sources. Coating crystal structure and phase stability, as functions of Y₂O₃ content, substrate bias, and annealing temperature, were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Results demonstrated that highly (111)-oriented tetragonal and cubic zirconia structures were formed in 2 and 4.5 mol% Y₂O₃ coatings, respectively, when the coatings were grown with an applied substrate bias. Conversely, coatings deposited with no substrate bias had random tetragonal and cubic structures. XRD analysis of annealed coatings showed that the cubic zirconia in 4.5 mol% Y₂O₃ coatings exhibited structural stability at temperatures up to 1200°C. Transformation of the tetragonal to monoclinic phase occurred in 2 mol% Y₂O₃ coating during high-temperature annealing, with the fraction of transformation dependent on bias potential and annealing temperature.     

Single-Crystal Elastic Constants of Yttria-Stabilized Zirconia in the Range 20° to 700°C

Elastic constants of single crystals of yttria-stabilized zirconia were determined through the temperature range 20° to 700°C. Crystals containing 8.1, 11.1,12.1, 15.5, and 17.9 mol% Y₂0₃were measured. The elastic constant C₁₁ was found to decrease and C₁₂ and C₄₄ to increase with increasing Y₂O₃ content; this appears to be due to decreasing coulombic interaction as Y³⁺ replaces Zr⁴⁺. Except for the 8.1 mol% Y₂O₃ crystal, the conventional elastic constants all showed normal monotonic decreases with increasing temperature. In the case of the 8.1 mol% Y₂O₃ crystal, measurements as a function of temperature were not reproducible, and it is likely that this composition at room temperature is below the composition limit of thermodynamic stability of the cubic fluorite phase.     

Subsolidus Phase Equilibria and Ordering in the System ZrO2-Y2O3

The subsolidus phase relations in the entire system ZrO₂-Y₂O₃ were established using DTA, expansion measurements, and room- and high-temperature X-ray diffraction. Three eutectoid reactions were found in the system: ( a ) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y₂O₃ and ∼490°C, ( b ) cubic zirconia solid solutiow→δ-phase Y₄Zr₃O₁₂+hexagonalphase Y₆ZrO₁₁ at 45 mol% Y₂O₃ and ∼1325°±25°C, and ( c ) yttria C -type solid solution→wcubic zirconia solid solution+ hexagonal phase Y₆ZrO₁₁ at ∼72 mol% Y₂O₃ and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y₂O₃ with ideal formula Y₄Zr₃O₁₂, and another, a new hexagonal phase, at 75 mol% Y₂O₃ with formula Y₆ZrO₁₁. They decompose at 1375° and >1750°C into cubic zirconia solid solution and yttria C -type solid solution, respectively. The extent of the cubic zirconia and yttria C -type solid solution fields was also redetermined. By incorporating the known tetragonal-cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO₂-Y₂O₃.     

Thermal Expansion Behaviro of single-Crystal Zirconia

Thermal expansion was measured for skull-melt-grown undoped single-crystal zirconia as well as for crystals containing 2.8 wt% MgO, 4 wt% CaO, and 3 to 20 wt% Y₂O₃. Samples were cycled in air from room temperature to 1500°C. The partially stabilized samples experienced a phase transformation which was strongly dependent on the composition and phase structures present; fully stabilized materials exhibited uniform behavior throughout the heating/cooling cycle.     

Sintering Kinetics at Isothermal Shrinkage: II, Effect of Y2O3 Concentration on the Initial Sintering Stage of Fine Zirconia Powder

The shrinkage behavior of fine zirconia powders containing 2.9 and 7.8 mol% Y₂O₃ was investigated to clarify the effect of Y₂O₃ concentration on the initial sintering stage. The shrinkage of powder compact was measured under both conditions of constant rates of heating (CRH) and constant temperatures. CRH measurements revealed that when the Y₂O₃ concentration of fine zirconia powder increased, the starting temperature of shrinkage shifted to a high temperature. Isothermal shrinkage measurements revealed that the increase in Y₂O₃ concentration causes the shrinkage rate to decrease. The values of activation energy ( Q ) and frequency-factor term (β₀) of diffusion at initial sintering were estimated by applying the sintering-rate equation to the isothermal shrinkage data. When the Y₂O₃ concentration increases, both Q and β₀ of diffusion increase. It is, therefore, concluded that the increase in Y₂O₃ concentration of fine zirconia powder decreases the shrinkage rate because of increasing Q of diffusion at the initial stage of sintering.     

Raman Spectroscopy of Tetragonal Zirconia Solid Solutions

Raman spectra of tetragonal zirconia were studied for the systems ZrO₂-Y₂O₃, ZrO₂-CeO₂, ZrO₂-Y₂O₃-Nb₂O₅, and ZrO₂-Y₂O₃-Ta₂O₅ There was no change in the Raman bands for tetragonal ziraconia in the ZrO₂-Y₂O₃ system as a function of Y₂O₃ content . On the other hand, in the ternary systems, the Raman lines corresponding to the stretching modes of two sets of cation-oxygen bonds shifted to greater wave numbers as either Nb₂O₅ Contents increased in Y₂O₃-stabilized tetragonal zirconia. This difference was interpreted by local bonding environments of the pentavalent cations, which were supposed to occupy tetrahedral sites in teteragonal zirconia solid solutions.     

Anisotropic Thermal Expansion Coefficients of Y2O3-Stabilized Tetragonal Zirconia

The transformability of the grains in tetragonal zirconia polycrystals is determined by thermal stresses (eigenstresses) which develop because of anisotropic expansion behavior of the tetragonal grains on cooling. Anisotropic thermal expansion coefficients based on lattice parameter determinations at room temperature and 600° and 800°C are presented for tetragonal zirconia containing 2 and 3 mol% Y₂O₃. The sources of errors in these data are discussed.     

Ferroelastic Domain Switching in Polydomain Tetragonal Zirconia Single Crystals

As-received, yttria-doped (4.2 wt% Y₂O₃) single crystals of zirconia were heated to ≥2100°C in air. Cube-shaped samples with faces perpendicular to 〈100〉 axes on the basis of the pseudocubic symmetry were cut from the crystals. X-ray and electron diffraction indicated that the crystals are polydomain with [001] axes, on the basis of the tetragonal symmetry, in three mutually orthogonal directions (perpendicular to the cube faces). The cube-shaped crystals were tested in compression at temperatures as high as 1400°C. X-ray diffraction indicated that ferroelastic domains underwent reorientation (switching) in compression. Subsequently, notched samples with the long direction of the beams along 〈100〉 on the basis of the pseudocubic symmetry were fractured in three-point bending at temperatures as high as 1000°C. X-ray diffraction from fracture surfaces showed that domain reorientation had occurred and that no monoclinic phase was observed on fracture or ground surfaces. The fracture toughness at room temperature and at 1000°C was ∼12 and ∼8 MPa · m^1/2, respectively. Preliminary experiments on polycrystalline tetragonal zirconia samples containing 5.4 wt% Y₂O₃ and sintered at ≥2100°C also showed no evidence of the monoclinic phase on fracture or ground surfaces. The toughness of the polycrystalline samples was typically 7.7 MPa · m^1/2. These results indicate that ferroelastic domain switching can occur during fracture and may contribute to toughness.     

Thermal Analysis of 3-mol%-Yttria-Stabilized Tetragonal Zirconia Powder Doped with Copper Oxide

Thermal analysis was performed upon 3-mol%-yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) which had been doped with CuO using an aqueous adsorption technique. Cyclic differential thermal analysis (DTA) scans indicated that the CuO present on the powder surfaces first transforms to Cu₂O and then melts. The molten Cu₂O then reacts with yttria at the powder surfaces to form a new phase containing Y, Cu, and O. Because Y takes time to diffuse to the particle surfaces, the apparent melting point of this new phase appears at higher temperatures in initial DTA scans than in subsequent scans. Vaporization of the molten copper-oxide-rich phase at the temperatures studied causes a gradual shift in composition from Y₂Cu₄O₅ to the less copper-rich Y₂Cu₂O₅ phase. The presence of the Y₂Cu₂O₅ phase in CuO-doped 3Y-TZP allows for previous sintering and superplasticity results to be explained.     

Thermodynamic Model of the Cubic → Tetragonal Transition in Nonstoichiometric Zirconia

Nonstoichiometric zirconia is described with a model recently developed for ZrO₂—Y₂O₃ alloys. It is thus possible to rationalize the experimental information on the cubic/tetragonal phase boundaries in zirconia.     

Qualitative X-ray Diffraction Analysis of Metastable Tetragonal (t') Zirconia

The identification and discrimination of cubic ( c ) and metastable t '-zirconia phases by conventional X-ray diffraction has proved to be a difficult problem because of the crystallographic similarities of the two phases. In this study, the c -phase and t '-phase in biphasic metastable zirconia samples have been successfully distinguished by X-ray diffraction; data were collected using a high-resolution powder diffractometer with monochromatic radiation and the peak shapes were fitted using a peak-fitting program to distinguish the different phases. From this study, the minimum temperature, T ₀, at which the t '-phase can be obtained on cooling rapidly compositions in the range 3–7 mol% Y₂O₃–ZrO₂ to room temperature was estimated as ∼1425°C. For a constant temperature above 1425°C, the lattice parameters of the t '-phase of 3–7 mol% Y₂O₃–ZrO₂ compositions within the two-phase region of the phase diagram were unaffected by the yttria content; only the amount of t '-phase formed was affected by the yttria content.     

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.     

Pressure-Temperature Phase Diagram of Zirconia

The pressure-temperature phase diagram of zirconia was determined by optical microscopy and X-ray diffraction techniques using a diamond anvil pressure cell. At room temperature, monoclinic ZrO₂ transforms to a tetragonal phase ( t II) which is related to the high-temperature tetragonal structure ( t I). The transformation pressure exhibits hysteresis and is cycle dependent. At room temperature, the initial transformation pressure for the monoclinic- t II transition on a virgin monoclinic crystal can be as high as 4.4 GPa; on subsequent cycling the transition pressure ultimately lowers to 3.29 ± 0.06 GPa. The pressure for the reverse transition is essentially constant at 2.75 ± 0.06 GPa. At pressures > 16.6 GPa, the t II form transforms to the orthorhombic cotunnite (PbCl₂) structure. With increasing temperature, the t II form transforms to the high-temperature tetragonal phase. For increasing P and T , the monoclinic- t I- t II triple point is located at T = 596°± 18°C and P = 2.26 ± 0.28 GPa, whereas for decreasing P and T , the triple point is found at T = 535°± 25°C and P = 1.7 ± 0.28 GPa.     

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.     

Anisotropic Thermal Expansion of Tetragonal Zirconia Polycrystals

Thermal expansion coefficients (α _a and α _c ) in two crystallographic axes ( a and c ) of the tetragonal phase are measured at 25°–1200°C in ZrO₂–M₂O₃ (M = Sc, In, Yb) and in ZrO₂–YTaO₄. The difference between these two thermal expansion coefficients, α _c –α _a , decreases with M₂O₃ or YTaO₄ composition even though the tetragonality ( c/a ) behaves differently in these two systems. The locus of α _c =α _a represents a maximum tetragonality for the tetragonal phase, but not the phase boundary for the cubic phase. The relationships among thermal expansion, temperature, and composition are discussed.     

Strength and Phase Stability of Yttria-Ceria-Doped Tetragonal Zirconia/Alumina Composites Sintered and Hot Isostatically Pressed in Argon–Oxygen Gas Atmosphere

Yttria-ceria-doped tetragonal zirconia (Y,Ce)-TZP)/alumina (Al₂O₃) composites were fabricated by hot isostatic pressing at 1400° to 1450°C and 196 MPa in an Ar–O₂ atmosphere using the fine powders prepared by hydrolysis of ZrOCl₂ solution. The composites consisting of 25 wt% Al₂O₃ and tetragonal zirconia with compositions 4 mol% YO_1.5–4 mol% CeO₂–ZrO₂ and 2.5 mol% YO_1.5–5.5 mol% CeO₂–ZrO₂ exhibited mean fracture strength as high as 2000 MPa and were resistant to phase transformation under saturated water vapor pressure at 180°C (1 MPa). Postsintering hot isostatic pressing of (4Y, 4Ce)-TZP/Al₂O₃ and (2.5Y, 5.5Ce)-TZP/Al₂O₃ composites was useful to enhance the phase stability under hydrothermal conditions and strength.     

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.     

Sol–Gel Processed Y-PSZ Ceramics with 5 wt% Al2O3

Y-PSZ ceramics with 5 wt% Al₂O₃ were synthesized by a sol–gel route. Experimental results show that powders of metastable tetragonal zirconia with 2.7 mol% Y₂O₃ and 5 wt% Al₂O₃ can be fabricated by decomposing the dry gel powder at 500°C. Materials sintered in an air atmosphere at 1500°C for 3 have high density (5.685 g/cm³), high content of metastable tetragonal zirconia (>96%), and high fracture toughness (8.67 MPa.m^1/2). Compared with the Y-PSZ ceramics, significant toughening was achieved by adding 5 wt% Al₂O₃.