Temperature-Dependent Optical Reflectivity of Tetragonal-Prime Yttria-Stabilized Zirconia

The optical reflectance of dense, metastable, tetragonal-prime zirconia plates, made by densifying electron beam physical vapor-deposited powder, is reported as a function of temperature up to 1673 K (1400°C) over the range of 400–1500 cm⁻¹ (6.67–25 μm). Curve fitting of the reflectance as a function of temperature was performed using two different damped oscillator models, each with three infrared (IR)-active modes. Oscillator parameters were then used to calculate the values of the indices of refraction and absorption as a function of temperature using the classical dispersion theory. The reflectance data of tetragonal-prime yttria-stabilized zirconia at room temperature are qualitatively similar to that reported for the equilibrium tetragonal phase in that it can be fit with three IR-active modes.     

Determination of Scattering and Absorption Coefficients for Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier Coatings

Prediction of radiative transport through translucent thermal barrier coatings (TBCs) can only be performed if the scattering and absorption coefficients and index of refraction of the TBC are known. To date, very limited information on these coefficients, which depend on both the coating composition and the microstructure, has been available for the very commonly utilized plasma-sprayed 8 wt% yttria-stabilized zirconia (8YSZ) TBCs. In this work, the scattering and absorption coefficients of freestanding plasma-sprayed 8YSZ coatings were determined from room-temperature normal-incidence directional-hemispherical reflectance and transmittance spectra over the wavelength range from 0.8 to 7.5 μm. Spectra were collected over a wide range of coating thickness from 60 to almost 900 μm. From the reflectance and transmittance spectra, the scattering and absorption coefficients as a function of wavelength were obtained by fitting the reflectance and transmittance values predicted by a four flux model to the experimentally measured values at all measured 8YSZ thicknesses. While the combined effects of absorption and scattering were shown in general to exhibit a nonexponential dependence of transmittance on specimen thickness, it was shown that for sufficiently high absorption and optical thickness, an exponential dependence becomes a good approximation. In addition, the implications of the wavelength dependence of the plasma-sprayed 8YSZ scattering and absorption coefficients on (1) obtaining accurate surface-temperature pyrometer measurements and on (2) applying mid-infrared reflectance to monitor TBC delamination are discussed.     

Effects of Yttria-Stabilized Zirconia on Plasma-Sprayed Hydroxyapatite/Yttria-Stabilized Zirconia Composite Coatings

The low bonding strength between hydroxyapatite (HA) and the metal substrate interface of plasma-sprayed HA coating has been a point of potential weakness in its application as a biomedical prosthesis. In the present study, yttria-stabilized (8 wt%) zirconia (YSZ) has been used to enhance the mechanical properties of HA coatings. The effects of YSZ additions (in the range 10–50 wt%) on the phase composition, microstructure, bond strength, elastic modulus, and fracture toughness of plasma-sprayed HA/YSZ composite coatings have been studied. The results indicated that decomposition of HA during plasma spraying was reduced significantly with the addition of zirconia. The higher the zirconia content, the lower the amount of calcium oxide, tricalcium phosphate, and tetracalcium phosphate formed in the coatings. In addition, there was a trace of calcium zirconate formed when less than 30 wt% zirconia was present. A solid solution of HA mixed with YSZ formed during plasma spraying; however, the amount of unmelted particles increased as the zirconia increased. The mechanical properties of the HA/YSZ composite coatings, such as bond strength, elastic modulus, and fracture toughness, increased significantly as the contents of zirconia increased.     

EXAFS Study of Yttria-Stabilized Zirconia

The local structural environments of Y³⁺ and Zr⁴⁺ in 18 wt% Y₂O₃-stabilized zirconia were studied using extended X-ray absorption fine structure spectroscopy over the temperature range –120° to 770°C. The measured cation-oxygen distances reflect those of the parent oxides, with the mean Zr-O distance 0.017 nm shorter than the mean Y-O distance. The spread in the Zr–nearest-neighbor and Zr–next-nearest-neighbor distances is considerably larger than observed for Y³⁺. This result is attributed to the anion vacancies being preferentially sited adjacent to the smaller Zr⁴⁺ cation which, with ensuing relaxations, permits a closer contact between Zr⁴⁺ and its oxygen neighbors. Thus, the structural environment of these Zr⁴⁺ ions resembles that of the 7-coordinated Zr⁴⁺ in monoclinic zirconia. Increasing the temperature of the sample results in the local structural environments of the two cations becoming more alike, suggesting that increased anionic mobility leads to an increasingly random distribution of anion vacancies.     

Subeutectoid Degradation of Yttria-Stabilized Tetragonal Zirconia Polycrystal and Ceria-Doped Yttria-Stabilized Tetragonal Zirconia Polycrystal Ceramics

Yttria-doped tetragonal zirconia containing 2 and 3 mol% Y₂O₃ (Y-TZP), and CeO₂-doped Y-TZP containing 0 to 12 mol% CeO₂ were sintered at 1350°C in a tetragonal single-phase field for 2 h in air, and the degradation behavior at low temperature in air and in hot water was observed. X-ray photoelectron spectroscopy studies on the surface of hydrothermally treated samples show evidence for the formation of a YO(OH) species, along with the simultaneous formation of purely tetragonal zirconia nuclei that retain their coherence in the Y-TZP matrix. Above a critical size, the tetragonal nuclei spontaneously transform to a monoclinic structure, giving rise to macro- and microcracking. The strong tetragonal grains degrade to produce a spalling phenomenon that facilitates further degradation. Y-TZP ceramics alloyed with adequate amounts of CeO₂ show no tetragonal-to-monoclinic transformation after hydrothermal treatment.     

In Vitro Aging of Yttria-Stabilized Zirconia

Yttria-stabilized zirconia was aged in distilled water, saline, and Ringer's solution for 140, 304, and 453 d. The significant decreases in strength (13% to 22%) is attributed to the phase transformation of zirconia from tetragonal to monoclinic. The effect of aging stabilized zirconia in physiological media is still unresolved.     

Determination of Scattering and Absorption Coefficients for Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier Coatings at Elevated Temperatures

The temperature dependence of the scattering and absorption coefficients for a set of freestanding plasma-sprayed 8 wt% yttria-stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) was determined at temperatures up to 1360°C in a wavelength range from 1.2 μm up to the 8YSZ absorption edge. The scattering and absorption coefficients were determined by fitting the directional-hemispherical reflectance and transmittance values calculated by a four-flux Kubelka–Munk method to the experimentally measured hemispherical-directional reflectance and transmittance values obtained for five 8YSZ thicknesses. The scattering coefficient exhibited a continuous decrease with increasing wavelength and showed no significant temperature dependence. The scattering is primarily attributed to the relatively temperature-insensitive refractive index mismatch between the 8YSZ and its internal voids. The absorption coefficient was very low (<1 cm⁻¹) at wavelengths between 2 μm and the absorption edge and showed a definite temperature dependence that consisted of a shift of the absorption edge to shorter wavelengths and an increase in the weak absorption below the absorption edge with increasing temperature. The shift in the absorption edge with temperature is attributed to strongly temperature-dependent multiphonon absorption. While TBC hemispherical transmittance beyond the absorption edge can be predicted by a simple exponential decrease with thickness, below the absorption edge, typical TBC thicknesses are well below the thickness range where a simple exponential decrease in hemispherical transmittance with TBC thickness is expected. [Correction added after online publication August 11, 2009: "edge to a shorter wavelengths" has been updated as "edge to shorter wavelengths."]     

Strength Analysis of Yttria-Stabilized Tetragonal Zirconia Polycrystals

Tensile strength of Y₂O₃-stabilized ZrO₂ polycrystals (Y-TZP) was measured by a newly developed tensile testing method with a rectangular bar. The tensile strength of Y-TZP was lower than that of the three-point bend strength, and the shape of the tensile strength distribution was quite different from that of the three-point bend strength distribution. It was difficult to predict the distribution curve of the tensile strength using the data of the three-point bend strength by one-modal Weibull distribution. The distribution of the tensile strength was analyzed by two- or three-model Weibull distribution coupled with an analysis of fracture origins. The distribution curve of the three-point bend strength which was estimated by multimodal Weibull distribution agreed favorably with that of the measured three-point bend strength values. A two-modal Weibull distribution function was formulated approximately from the distributions of the tensile and three-point bend strengths, and the estimated two-modal Weibull distribution function for the four-point bend strength agreed well with the measured four-point bend strength.     

Tensile Strength of Yttria-Stabilized Tetragonal Zirconia Polycrystals

Tensile strengths of 2.0 to 5.0 mol% Y₂O₃-stabilized ZrO₂ polycrystals were described using the newly developed tensile testing method. The tensile test was conducted by attaching three strain gauges on both sides of a rectangular bar that was 10 mm by 1 mm by 200 mm. The tensile strength of tetragonal ZrO₂ polycrystals (TZP) containing 2.0 mol% Y₂O₃ showed 745 MPa, whereas the bend strength of this material was 1630 MPa. Inelastic behavior of the stress-strain curve was observed at critical stresses and strains of 500 to 700 MPa and 0.25% to 0.35%, respectively. Although deviation from proportionality was observed to be small, it increased with the increase of temperature from −100° to 200°C.     

Slip Casting of Yttria-Stabilized Tetragonal Zirconia Polycrystals

The rheological and casting parameters of 3-mol%-yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) powder obtained by a wet-chemical coprecipitation route have been studied. Colloidal stability has been studied through zeta potential measurements. An organic surfactant has been used as deflocculant. Viscosity, casting rate, and green densities have been determined for suspensions with 28.2, 33.6, and 40.3 vol% solids content. Relative density, grain size, and t -ZrO₂ evolution versus temperature and soaking time are also reported.     

Diffusion of Water Species in Yttria-Stabilized Zirconia

The diffusion of moisture-related species into both tetragonal and cubic single crystals at temperatures characteristic of moisture-enhanced low-temperature degradation of tetragonal zirconia is reported using secondary ion mass spectrometry (SIMS). The crystals were immersed in D and O¹⁸ isotopically labeled water and in O¹⁸ gas environments over the temperature range of 100°–220°C and SIMS profiles of H, D, O¹⁸, and O¹⁸D determined. The oxygen diffusivities in both tetragonal and cubic crystals were the same, irrespective of whether the crystals were exposed to gas or water. Furthermore, the oxygen diffusivities were consistent with extrapolations of the oxygen diffusivities in cubic crystals reported at higher temperatures using the reported activation energy for oxygen vacancy diffusion. Both H and D diffuse into the tetragonal crystal when immersed in water but despite having a higher concentration of vacancies no diffusion of H or D was detectable in the cubic single crystals exposed under identical conditions. Quantification of the H and D diffusion into the tetragonal crystal proved unreliable due to charging effects even though the SIMS profiling was performed at liquid nitrogen temperatures. Nevertheless, it was concluded that D can diffuse in tetragonal zirconia at room temperatures over a period of several months. Finally, although a species of mass 20 amu, which is the same as the isotopic labeled OH (O¹⁸D) ion, was detected in the tetragonal single crystal zirconia exposed to water, it was concluded that this was a SIMS artifact and that moisture does not diffuse in zirconia as either an OH or H₂O ion.     

High-Temperature Creep of Yttria-Stabilized Zirconia Single Crystals

Creep of 9.4-mol%-Y₂O₃-stabilized cubic ZrO₂ has been studied between 1300° and 1550°C. Conventional power-law creep (stress exponent n ∼ 4.5) is found at the higher temperatures, with an activation energy (∼6 eV) corresponding to cation diffusion. Transition to a different creep mechanism occurs at the lower temperatures, as indicated by higher values of the stress exponent ( n ∼ 7) and an activation energy (∼7.5 eV) higher than that for cation self-diffusion. The lower-temperature behavior is caused by a competition between cross-slip-controlled and recovery-controlled creep. Consideration of all the creep and diffusion data now available suggests that the rate-controlling high-temperature mass transport in Y₂O₃-stabilized ZrO₂ can be described by D = 10⁻³ exp(-5.0 eV/ kT ) m²·s⁻¹.     

Strength and Toughness of Tape-Cast Yttria-Stabilized Zirconia

The biaxial flexural strength and fracture toughness of tape-cast yttria-stabilized zirconia, for application as the electrolyte in solid oxide fuel cells, have been measured at room temperature and at a typical operating temperature of 900°C. The flexural strength was measured in ring-on-ring loading and decreased from 416 MPa at room temperature to 265 MPa at 900°C. The fracture toughness was measured using two different techniques: indentation fracture and double-torsion loading. The latter was more reliable and gave a fracture toughness of 1.61 ± 0.12 MPa·m^1/2 at room temperature and 1.02 ± 0.05 MPa·m^1/2 at 900°C. The flexural strength and fracture toughness were quantitatively consistent with fracture being initiated at the observed surface defects. The lower fracture toughness at 900°C is partly due to a reduction in elastic modulus and partly due to a reduction in the work of fracture.     

Solvothermal Synthesis and Processing of Yttria-Stabilized Zirconia Nanopowder

A solvothermal method has been used to produce nanoparticles of cubic/tetragonal yttria-stabilized zirconia (YSZ) with uniform particle size, homogenous composition, and high crystallinity. The effects of reaction time, reactant concentration, processing temperature, and solvent composition on prime particle size and YSZ phase proportion have been investigated. The effect of solvent, a mixture of ethanol/2-propanol, on crystallite size and agglomerate size was studied and discussed according to Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and surface chemistry. The prime particle diameter was 2–4 nm and secondary particle size was 140–180 nm. After compaction and sintering at 1000°C, YSZ with a grain size of approximately 50 nm in diameter was obtained with a typical relative density of 94%.     

Mechanism of Thermal Transport in Zirconia and Yttria-Stabilized Zirconia by Molecular-Dynamics Simulation

We present results of molecular-dynamics simulations of the thermal conductivity, κ, of ZrO₂ and Y₂O₃-stabilized ZrO₂ (YSZ). For both pure ZrO₂ and YSZ with low concentrations of Y₂O₃, we find that the high-temperature κ is typical of a crystalline solid, with the dominant mechanism being phonon-phonon scattering. With increasing Y₂O₃ concentration, however, the mechanism changes to one more typical of an amorphous system. In particular, phononlike vibrational modes with well-defined wave vectors appear only at very low frequencies. As in amorphous materials, the vast majority of vibrational modes, while delocalized, do not propagate like ordinary phonon modes but transport energy in a diffusive manner. We also find that the few highest frequency modes are localized and do not contribute to κ.     

Zirconium Nitride Precipitation in Nominally Pure Yttria-Stabilized Zirconia

Nominally pure yttria-stabilized zirconia alloys are shown to contain unexpectedly large amounts of dissolved nitrogen. Its presence in the lattice was detected through the observation of large precipitates in alloys with three different concentrations of yttria deformed in compression in argon in the temperature range 1600°–1800°C. Electron diffraction, EDS and PEELS analyses, and Moiré imaging were used to identify the precipitates as ZrN. The possible origin of the nitrogen, its likely effects on properties, and the role of annealing atmosphere are briefly discussed.     

Stress Relief Mechanisms in Magnesia- and Yttria-Stabilized Zirconia

Stabilized zirconias exhibit pronounced photoelastic effects when viewed with transmitted polarized light. Thin sections reveal stress concentrations around pores and inclusions and at grain boundaries. The yield point of these materials, at some high temperature, is exceeded and plastic deformation tends to relieve these stress concentrations. The microhardness measurements of magnesia-stabilized zirconia indicate both solution hardening and solute segregation and help to explain the physical behavior of this system as the amount of MgO is varied.     

High-Temperature Optical Absorption Measurement of Single-Crystal, Yttria-Stabilized Zirconia

High-temperature optical absorption measurements were performed on single-crystal yttria-stabilized zirconia. The optical band gap (E_g) can be represented in the temperature range 15° to 1010°C as E_g(T)=3.40-5.16x10⁻⁴T(K)(eV).     

Diffusional Creep and Kinetic Demixing in Yttria-Stabilized Zirconia

The creep behavior of fine-grained yttria-stabilized zirconia with 25 mol% Y₂O₃ has been characterized as part of an investigation of kinetic demixing in solid-solution oxides which are subjected to a nonhydrostatic state of stress. At temperatures between 1400° and 1600°C, the steady-state strain rate of (Zr_0.6Y_0.4)O_1.8 samples with average grain sizes between 2.5 and 14.5 μm can be summarized by the flow law ɛ= 6.5 × 10⁻⁷σ^1.2 exp[−550 (kJ/mol)/ RT ] d ^−2.2 (s⁻¹) for stresses in the range 8 to 60 MPa, where σ is in pascals and d is in meters. This flow law indicates that deformation occurs by a Nabarro-Herring creep mechanism in which the creep rate is limited by cation lattice diffusion. Kinetic demixing was not observed in deformed polycrystalline samples even though diffusional creep was rate limited by cation lattice diffusion. This result can be explained if the cation diffusivities are approximately equal or if extensive grain rotation occurs during diffusional creep.