At-Temperature Neutron Diffraction Investigation of the Aging Process in Magnesia-Partially-Stabilized Zirconia

A time-resolved neutron powder diffraction technique has been used to follow the changes occurring during the aging of magnesia-partially-stabilized zirconia at 1100°C. Through quantitative phase analyses it has been possible to follow the development with aging time of tetragonal zirconia and of the δ-phase (Mg₂Zr₅O₁₂, related to cubic, but with ordered anion vacancies), both at the expense of cubic zirconia. Changes in lattice parameters have been attributed to the expulsion of MgO stabilizer from the tetragonal zirconia precipitates as the aging proceeds. The broadening of peaks in the neutron diffraction pattern suggests there is considerable strain in the tetragonal precipitates in the c -direction, which is the short dimension in these lenticular precipitates. On cooling, there is some transformation of tetragonal zirconia to the monoclinic and orthorhombic phases.     

Neutron Diffraction Studies of Phase Transformations between Tetragonal and Orthorhombic Zirconia in Magnesia-Partially-Stabilized Zirconia

Neutron powder diffraction and conventional dilatometry have been used to investigate the tetragonal-to-orthorhombic phase transformation and the orthorhombic-to-tetragonal reversion in a high-toughness magnesia-partially-stabilized zirconia. For this material, the onset temperature on cooling for the tetragonal-to-orthorhombic transformation (determined by dilatometry) was 192 K, and the reversion on subsequent heating occurred between 500 and 620 K. Neutron diffraction patterns were recorded at temperatures down to 19 K then up to 664 K, and analyzed by the multiphase Rietveld method to determine the amounts of different phases as well as their lattice parameters and unit-cell volumes. It is notable that, at its maximum, the orthorhombic phase amounted to 45% of the sample by weight. Length changes were measured, using pushrod dilatometers, in the temperature range 80 to 700 K. Length changes calculated from the neutron diffraction determinations of the proportions and unit-cell volumes of the different phases are in very good agreement with the directly measured values.     

Structural and Mechanical Property Changes in Toughened Magnesia-Partially-Stabilized Zirconia at Low Temperatures

The mechanical properties of high-toughness magnesia-partially-stabilized zirconia were found to be dramatically altered by a single cooling cycle between room temperature and − 196°C. Raman spectroscopy and X-ray diffraction were used to correlate the changes in mechanical properties with structural changes that occur at temperatures below ∼− 100°C. Most of the tetragonal precipitates that are responsible for toughening transformed to an orthorhombic phase with unit-cell volume intermediate between those of the tetragonal and monoclinic phases. The orthorhombic phase was stable with heating to 300°C, but it transformed back to the tetragonal structure when heated to 400°C. Surprisingly, the orthorhombic phase was not readily transformable by stress, with the consequence that, after the cooling cycle, most of the high-toughness properties of the original tetragonal-containing material were lost.     

'Polymorph Method' Determination of Monoclinic Zirconia in Partially Stabilized Zirconia Ceramics

The polymorph method, which provides phase analysis from a small number of integrated intensities in a powder diffraction scan, is adapted for the determination of monoclinic zirconia in a mixture with cubic, tetragonal. and orthorhombic zirconias and the γ-phase (Mg₂Zr₅O₁₂). Such a mixture is representative of Mg-PSZ after subeutectoid aging. The quantitative determination of the monoclinic depends in principle on a knowledge of the relative amounts of the other phases present in the mixture. It is demonstrated, however, that without this knowledge, even in complex mixtures, the traditional polymorph method analysis gives an acceptable estimate of the monoclinic fraction in the sample.     

Crystal Structure of Metastable Tetragonal Zirconia up to 1473 K

The crystal structure of metastable tetragonal zirconia prepared via the alkoxide method has been investigated at temperatures up to 1473 K, to clarify the similarity between this metastable phase and the tetragonal phase at high temperature. The lattice constants, tetragonality, oxygen shift parameter, and equivalent isotropic thermal parameter of the metastable tetragonal phase are proportional to the temperature. These parameters, when extrapolated to the high-temperature range, are very similar to those of the high-temperature tetragonal phase. Present results indicate that the structure of the metastable tetragonal phase is the same as that of the high-temperature tetragonal phase.     

Raman Investigation of the Nitridation of Yttria-Stabilized Tetragonal Zirconia

Raman spectroscopy has been used to obtain Raman spectra of yttria-stabilized tetragonal zirconia subject to surface nitridation induced by contact with zirconium nitride. Raman spectra recorded from regions at increasing distance from the source of nitridation have been used to obtain diffusion profiles from samples treated at different times and temperatures. The coupling of X-ray diffraction data previously taken and of the Raman spectra shows that in the samples there is a two-phase region (tetragonal + cubic) near the nitrided surface and that, at larger distance inside the samples, there is only one phase (tetragonal). Fitting of the diffusive profiles in the single-phase tetragonal region with an appropriate diffusion function allows the determination of the diffusion coefficient of nitrogen in tetragonal zirconia which is expressed in terms of the preexponential factor, D ₀= (3.98 ± 0.5) × 10⁻³ cm²/s, and the activation energy, Q = 170 ± 10 kJ/mol.     

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.     

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.     

Crystallite and Grain-Size-Dependent Phase Transformations in Yttria-Doped Zirconia

In pure zirconia, ultrafine powders are often observed to take on the high-temperature tetragonal phase instead of the "equilibrium" monoclinic phase. The present experiments and analysis show that this observation is one manifestation of a much more general phenomenon in which phase transformation temperatures shift with crystallite/grain size. In the present study, the effect of crystallite (for powders) and grain (for solids) size on the tetragonal → monoclinic phase transformation is examined more broadly across the yttria–zirconia system. Using dilatometry and high-temperature differential scanning calorimetry on zirconia samples with varying crystallite/grain sizes and yttria content, we are able to show that the tetragonal → monoclinic phase transformation temperature varies linearly with inverse crystallite/grain size. This experimental behavior is consistent with thermodynamic predictions that incorporate a surface energy difference term in the calculation of free-energy equilibrium between two phases.     

Preparation and Structural Characterization of Ultrafine Zirconia Powders

Factors concerning the stabilization of the tetragonal and cubic phases, metastable at room temperature, with respect to the monoclinic stable phase in ultrafine zirconia powders are studied. The importance of Na⁺ ions in the initial zirconia amorphous gel in obtaining a cubic phase has been confirmed. By an X-ray diffraction study using a new peak profile fitting procedure, the amount of the crystalline phases and their microstructural properties (crystallite size and lattice distortions) have been obtained.     

Growth of Tetragonal Zirconia Coatings by Reactive Sputter Deposition

Zirconia coatings were produced by reactive dc magnetron sputter deposition, using a system with multiple sputter sources and a biased substrate stage. Tetragonal zirconia with either a random orientation or a highly (111) preferred orientation was formed by applying a substrate bias. Coating grown with no substrate bias had the equilibrium monoclinic structure. X-ray diffraction and transmission electron microscopy analyses revealed that bias sputtering could effectively decrease crystalline size in the as-deposited coating, which resulted in room-temperature stabilization of the tetragonal phase. The fraction of tetragonal phase, the desired phase for transformation-toughening behavior, was strongly dependent on the substrate bias and post-deposition annealing temperature.     

Structural Transformation in Cubic Zirconia

The structure of CaO-stabilized cubic zirconia has been investigated by X-ray diffraction at high temperatures and pressures up to 1000°C and ∼35 GPa, using a diamond-anvil cell interfaced with aYAG laser heating system. At ∼1000°C and 15 GPa, the cubic phase transforms directly into an orthorhombic phase with the PbCl₂ structure. This high-pressure phase is quenchable. The zero-pressure lattice parameters are a=0.3327±0.0004 nm, b=0.5566±0.0003 nm, and c =0.6487±0.0005 nm, the volume change being 10.5%.     

Structure and Ionic Mobility of Zirconia at High Temperature

The high-temperature structure of zirconia was studied by powder neutron diffraction up to 2400°C. The boundaries of the domain of the nonstoichiometric tetragonal form are defined. They are consistent with a tetragonal-cubic transition at 2350°C for stoichiometric zirconia. The changes in the structural parameters of the tetragonal form (unit-cell, positional parameters and thermal B factors) with temperature give evidence of medium zirconium and oxygen mobilities. The oxygen ions are, however, always more mobile than the zirconium ions. An enhancement with temperature of the structural an-isotropy tends to weaken the weaker of the two distinct Zr-0 bonds of the tetragonal zirconia. It results in the transformation into the cubic form which is accompanied by a change in unit-cell volume; this change becomes sharper as the composition tends toward stoichiometry. This transition is probably followed by an increase of the ionic mobility.     

Structural Evolution of Thermal-Sprayed Yttria-Stabilized ZrO2 Thermal Barrier Coatings with Annealing—A Neutron Diffraction Study

We studied the influence of the annealing temperature on the atomic structure of yttria-stabilized tetragonal zirconia (YSZ) which was deposited as a 300 μm thick thermal barrier coating (TBC) on nickel superalloy substrates by plasma spraying. To obtain neutron powder diffraction patterns of the barrier coatings we used an experimental technique where the sample is randomly rotated in the neutron beam. The time-averaged neutron diffraction pattern was then analyzed using the Rietveld refinement technique without any need for corrections. This allowed the comparison of the average crystals structures from bulk tetragonal samples obtained via common ceramic routes and those of micrometer thick films deposited on substrates using plasma spray or other nonequilibrium techniques.     

Crystal Structures of Two Orthorhombic Zirconias

The crystal structures of orthorhombic zirconias formed by cooling magnesia-partially-stabilized zirconia (Mg-PSZ) (space group Pbc 2₁) and by quenching zirconia powder from 600°C and 6 GPa (space group Pbca ) are compared and contrasted. It is demonstrated that the two structures are easily distinguished by the neutron powder diffraction techniques used to establish them. The occurrence of two distinct phases is hence proved. Structural relationships between these two phases and also with the in situ high-pressure structure proposed from X-ray diffraction (XRD) are discussed. The three structures are virtually indistinguishable by XRD and so the structure of the high-pressure form "in situ" is considered to remain unknown.     

Crystal Structure of Zirconia Prepared with Alumina by Coprecipitation

Zirconia was prepared by firing the coprecipitate from ZrOCl₂and AlCl₃mixed aqueous solution with ammonia. When fired above 600°C, the products were fine crystalline tetragonal zirconia of crystallite size <10 nm. In previous studies, the tetragonal phase had been assumed to be a (Zr_{1− x} ⁴⁺Al _x ³⁺)O_{2− x /2}solid solution, where x ≤ 0.25. However, X-ray diffraction pattern simulation and Al K -edge XANES spectroscopy confirmed the present product to be a mixture of t -ZrO₂fine powder with a small amount of δ-Al₂O₃of very low crystallinity, even below the expected compositional range of x ≤ 0.25 in the (Zr_{1− x} ⁴⁺Al _x ³⁺)O_{2− x /2}solid solution.     

Nitrogen-Containing Tetragonal Zirconia

The present paper gives evidence for the replacement of oxygen by nitrogen when partially stabilized tetragonal zirconia is reacted with nitrogen above 1400°C to give a tetragonal product but in the t ' (nontransformable) form. No transformation toughening is therefore expected from this nitrogencontaining tetragonal phase. These results may explain why less-effective toughening is obtained in nitrogen-containing ceramic-based composites than in equivalent zirconia-alumina and other oxide-based systems.     

Fundamental Optical Absorption Edge of Sputter-Deposited Zirconia and Yttria

Zirconia and yttria films were sputter deposited onto unheated fused silica substrates using a metal target and rare gas-oxygen discharges. Double-beam spectrophotometry was used to measure the transmission and reflection as a function of incident photon energy, E , from which the absorption coefficient, α( E ), was calculated. An indirect interband transition at E _i= 4.70 eV and two direct interband transitions at E _g1= 5.17 eV and E _g2= 5.93 eV occur in monoclinic zirconia. Two direct interband transitions at E _g1= 5.07 eV and E _g2= 5.73 eV occur in cubic yttria. The absorption edge structure is modified when unusual phases, such as tetragonal zirconia, and zirconia and yttria with no longrange crystallographic order, are present.     

Crystal Structure of Metastable Tetragonal Zirconia by Neutron Powder Diffraction Study

The crystal structure of metastable tetragonal zirconia prepared by the alkoxide method without any dopant has been examined by neutron diffraction at room temperature. The lattice parameters of this sample were a = 0.3591 ± 0.0001 nm and c = 0.5169 ± 0.0001 nm. The oxygen ions are shifted from their lattice sites in the 〈001〉 direction with a magnitude of Δ/ c = 0.046 ± 0.004.     

Elastic Constants of Tetragonal Zirconia Measured by a New Powder Diffraction Technique

Elastic constants for 12 mol% Ce-doped tetragonal zirconia have been determined from peak shifts in neutron diffraction patterns recorded under applied uniaxial stress. When these diffraction data are combined with a measured value of Young's modulus, a complete set of elastic constants is obtained. The values are c ₁₁= 327, c ₁₂= 100, c ₁₃= 62, c ₃₃= 264, c ₄₄= 59, and c ₆₆= 64 (units: GPa). These are the first reported results using a new technique for the measurement of elastic constants for anisotropic materials via neutron diffraction measurements on polycrystalline samples.