Zirconia Crystals Research Articles

Improving the mechanical properties of zirconia in instrument bearings

The article presents the issues of replacing leucosapphires in jeweled bearings of the axes of precision instruments with nanostructured crystals of partially stabilized zirconia. The statement is substantiated that doping with rare earth elements provides an improvement in the performance properties of precision instruments by improving the mechanical properties of bearing materials. The efficiency of doping of zirconia crystals with cerium and neodymium oxides is studied. It was found that doped crystals have increased plasticity, which provides an increase in the crack resistance of crystals. Special attention is paid to the issues of increasing the survivability of high-speed rotor bearings by replacing the thrust bearing of leucosapphire with nanostructured crystals of partially stabilized zirconia doped with cerium and neodymium. The efficiency of improving the mechanical properties is confirmed by the X-ray phase analysis of crystals. The phase composition is studied by Raman scattering and the lattice parameters are determined. The increased crack resistance of the thrust bearing is confirmed by tests performed using the kinetic microhardness method.

Surface Characteristics and Bioactivity of Zirconia (Y-TZP) with Different Surface Treatments.

ABSTRACTBackground:Zirconia being a bio-inert material needs to be surface treated to render it more bioactive and enhance its osseointegration potential. However, bioactivity studies focusing on the ability of sandblasting and ultraviolet photofunctionalization (UVP) surface treatments in inducing apatite precipitation using simulated body fluid (SBF) are lacking.Aim:The aim of the study was to comparatively evaluate the effect of two different surface treatments—sandblasting with 50 µm alumina and UVP with ultraviolet C (UVC) light on the bioactivity of zirconia.Materials and Methods:A total of 33 discs with dimensions 10 mm × 2 mm were obtained from zirconia blanks (Amann Girrbach, Koblach, Austria) and randomly divided into three groups (n = 11), namely Group I (untreated), Group II (sandblasted), and Group III (UVP). Surface characteristics of representative test samples were analyzed using X-ray diffraction (XRD), atomic force microscopy (AFM), contact angle goniometry, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX), to assess type of crystal phase of zirconia, surface roughness, wettability, surface topography, and elemental composition, respectively. SBF was prepared and calcium content in SBF (Ca-SBF) was determined using inductively coupled plasma mass spectrometry (ICP-MS).Results:Data were analyzed by one-way analysis of variance (ANOVA), post hoc Tukey honestly significant difference (HSD), and Student’s t test for statistical significance (P < 0.05, significant; P < 0.01, highly significant). Surface characteristics analyses revealed that XRD showed predominant tetragonal (t) zirconia crystal phase for all test groups. Mean surface roughness (Sa) of Group I was 41.83 nm, and it was significantly lesser than that of Group II (115.65 nm) and Group III (102.43 nm). Mean contact angles were 98.26°, 86.77°, and 68.03° for Groups I, II, and III, respectively, and these differences were highly significant. Mean pre-immersion Ca content in SBF was found to be 159 mg/L. Mean post-immersion Ca content was 70.10, 60.80, and 56.20 mg/L for Groups I, II, and III, respectively. Significant differences were found between Group I as compared to both Groups II and III. Bioactivity of Group III was marginally, but insignificantly higher with respect to Group II. Groups II and III were insignificant with respect to each other. Post-immersion XRD revealed predominant “t” phase, and SEM-EDX revealed well-formed, abundant calcium apatite layer on the treated samples as compared to that on untreated sample and an increasing Ca/P ratio from 1.15, 1.79 to 2.08, respectively from Group I to Group III.Conclusion:Within the limitations of this study, both sandblasting and UVP significantly and similarly improved bioactivity of zirconia as compared to the untreated samples, which was corroborated by the SEM-EDX results.

Comparison of mechanical properties of zirconia crystals partially stabilized with yttria and gadolinia

(ZrO2) 0.97(Y2O3)0.03 and (ZrO 2)0.97(Gd2O3)0.03 solid solution crystals have been grown using directional melt crystallization in cold container. The structure and phase composition of the crystals have been studied using X-ray diffraction and transmission electron microscopy. The mechanical properties of the crystals e.g. microhardness and fracture toughness have been studied with microindentation. We show that the main hardening mechanism in the 3GdSZ and 3YSZ crystals is transformation hardening. No microhardness anisotropy has been observed in the crystals. The highest fracture toughness among the crystals has been obtained for the 3GdSZ crystals in the {100} plane.

Measurement and modeling of micro residual stresses in zirconium crystals in three dimension

The performance of the zirconium alloys used in nuclear reactors can be affected by the state of the residual stresses that develop during manufacturing of the reactor core components. In this paper, residual stresses within individual grains of a textured α-zirconium polycrystal are studied. For this purpose, three-dimensional synchrotron X-ray diffraction is used to measure elastic strain tensor, center-of-mass (COM), orientation, and stress tensor of more than 11,000 grains in a zirconium sample. The grain measured COMs and orientations are used to reconstruct the 3D microstructure of the sample using the weighted Voronoi tessellation technique. The microstructure is subsequently imported into Abaqus to simulate the experiment using a crystal plasticity finite element model. The state of the thermal residual stresses that develop during slow cooling from 700 °C, and those that develop after unloading from 1.2% applied tensile strain are discussed. It is shown that both thermal and mechanical micro residual stresses, and their variations within a grain, are correlated with grain size. Also, due to strong anisotropy of the single crystal, residual stresses are significantly affected by the configuration of local grain neighborhood.

Improving the microstructural and mechanical properties of in-situ zirconia-mullite composites by optimizing the simultaneous effect of mechanical activation and additives

The current study aims to investigate the microstructural properties, diametral tensile strength, Weibull modulus, and surface roughness (Ra) of the zirconia-mullite composites. The simultaneous effect of duration of the mechanical activation (MA) process of starting materials, as well as TiO2 and ZnO additions on the mentioned properties, were studied. X-ray diffraction analysis showed that mullite was the main crystalline phase in 6–24 h mechanically activated-TiO2 containing samples which were subsequently heat treated at 1450 °C. Prolonging the duration of the MA process to 72 h led to the crystallization of the tetragonal zirconia as one of the main crystalline phase in the mentioned samples. As the heat treatment temperature increased to 1550 °C, the dependence of the type of the crystalline phases on the milling time eliminated and mullite became the main crystalline phase in 6–72 h MA processed samples. The transformation of starting materials to mullite and t-zirconia was not progressed effectively in ZnO containing samples which were heat treated at 1450 °C. However, by increasing the temperature of the heat treatment process to 1550 °C, both mullite and zirconia were efficiently crystallized in 6–72 h MA processed ZnO containing ones. Scanning electron microscopy results revealed that prolonging the duration of the milling process of the additive containing mixtures from 6 to 72 h could eliminate the acicular morphology of the mullite phase. 72 h mechanically activated ZnO containing sample which was later heat-treated at 1550 °C had the highest noticeable diametral tensile strength of 270 ± 15 MPa. The Weibull modulus of composite samples increased from 10.07 to 12.37 and 20.95 by the addition of TiO2 and ZnO to the starting mixture, respectively. Finally, it was found that prolonging the milling time increased the Weibull modulus remarkably and decreased the Ra values as a result of the homogeneity development in the composite samples.

Structural, Mechanical, and Transport Properties of Scandia and Yttria Partially Stabilized Zirconia Crystals

Crystals of (ZrO2)1 –x – y(Sc2O3)x(Y2O3)y (x = 0.003–0.045, y = 0.005–0.03) solid solutions have been grown by directional solidification in a cold crucible. All of the crystals consist of a mixture of two tetragonal phases of zirconia, t and t', differing in the degree of tetragonality (c/ $$\sqrt 2 a$$ ): 1.014–1.015 and 1.004–1.005 for the t- and t'-phases, respectively. All of the crystals have high microhardness (13.5–15.0 GPa) and high fracture toughness (on the order of 6–7 MPa m1/2). Their fracture toughness decreases with an increase in the total content of the stabilizing oxides, which is well consistent with the associated changes in phase composition, namely, with the increase in the percentage of the (transformable) t-phase. All of the crystals are similar in electrical conductivity: on the order of 0.04 S/cm at a temperature of 1173 K.

Effects of Selective Laser Modification and Al Deposition on the Hot Corrosion Resistance of Ceria and Yttria-Stabilized Zirconia Thermal Barrier Coatings

The air-plasma-sprayed ceria and yttria-stabilized zirconia (CYSZ) coating was modified by selective laser remelting and Al deposition to enhance hot corrosion resistance. The dotted coating was obtained after selective laser remelting. Magnetron sputtering was used to deposit an Al film on the dotted coating, and a vacuum heat treatment was subsequently performed to produce a dense α-Al2O3 overlay. Hot corrosion behavior of the following three types of coatings was investigated: plasma-sprayed, dotted, and dotted coatings combined with Al deposition (DA). Hot corrosion behaviors were evaluated in a mixture of 55 wt % V2O5 and 45 wt % Na2SO4 molten salts at 1000 °C for 30 h. The hot corrosion reaction between molten salts and zirconia stabilizers (Y2O3 and CeO2) led to the generation of monoclinic zirconia, YVO4, and CeVO4 plate-shaped crystals, and the mineralization of CeO2. The results indicated that the hot corrosion resistance of the DA coating was the best, and the dotted coating had superior hot corrosion resistance in comparison with the plasma-sprayed coating. The minimal surface roughness and dense dotted units improved the hot corrosion resistance of the dotted coating. The dense α-Al2O3 overlay with chemical inertness effectively inhibited the infiltration of molten salts, which led to the optimal hot corrosion resistance of the DA coating.

Anisotropic Lattice Vibration, Motion of Atoms, and Local Structure in Pure Zirconia Crystals with Vacancies

In this study, we performed classical molecular dynamics computer simulations at a constant pressure and temperature for a pure zirconia (ZrO2) crystal containing zirconium vacancies and oxygen vac...

Mechanical properties and transformation hardening mechanism in yttria, ceria, neodymia and ytterbia co-doped zirconia based solid solutions

The local phase composition was studied in the vicinity of indentations for partially stabilized zirconia crystals doped with rare-earth ions with constant and variable valence. Our aim was to study the effect of co-doping rare-earth ion type (Сe3+, Сe4+, Nd3+ and Yb3+) on the intensity of the tetragonal to monoclinic transition which determines the transformation hardening mechanism in the crystals. We demonstrated the effect of co-doping ion type on the degree of the tetragonal to monoclinic transition. The phase transition is the most intense in ceria ion doped crystals, this being one of the factors contributing to their high fracture toughness.

Scintillation Properties of Yttrium-stabilized Zirconia Crystals Synthesized by the Floating-zone Method

Scintillation Properties of Yttrium-stabilized Zirconia Crystals Synthesized by the Floating-zone Method

Anisotropy of the mechanical properties and features of the tetragonal to monoclinic transition in partially stabilized zirconia crystals

We have studied the structure, phase composition and mechanical properties, e.g. microherdness and impact toughness of partially stabilized zirconia crystals in the 2–4 mol.% stabilizing yttria concentration range. We have shown that with increasing concentration of stabilizing yttria the microhardness increases while the impact fracture toughness varies nonmonotonically and reaches the maximum for the partially stabilized zirconia crystal with 3 mol.% Y2O3. The anisotropy of the microhardness and impact fracture toughness was studied for wafers perpendicular to the 〈100〉, 〈110〉 and 〈111〉 directions with different indenter diagonal orientations. We have shown that the microhardness of the partially stabilized zirconia crystals depends but slightly on crystallographic orientation whereas the impact fracture toughness differs between crystallographic faces of the crystals. The highest impact toughness was obtained for the {100} plane and the indenter diagonal aligned parallel to the 〈100〉 direction, while the lowest impact toughness was obtained for the {110} plane and the indenter diagonal aligned parallel to the 〈110〉 direction. Local Raman study of the monoclinic phase distribution in the vicinity of the indentation shows that the maximum quantity of the monoclinic phase forms along the 〈110〉 direction regardless of indenter diagonal orientation. We have shown that the strain anisotropy during the phase transition has variable effect on crack impedance. The effect of transformation hardening mechanism depends on crack plane orientation and transition induced stress.

Facile in situ synthesis of highly dispersed chromium oxide incorporated into mesoporous ZrO2 for the dehydrogenation of propane with CO2

Highly dispersed chromium oxide-modified mesoporous ZrO2 (xCrZrO2) was prepared via an acid–base pair pathway and evaporation-induced self-assembly method for the dehydrogenation of propane reaction with CO2. The resulting materials exhibit mesoporous structure and different zirconia crystals with doped chromium species. Catalytic performance results revealed that the 7CrZrO2 catalyst with abundant dispersed oligomeric Cr6+ species and isolated Cr6+ species showed the highest catalytic activity among the catalysts studied; initial conversion 68% and yield 40.5% were obtained. The following conclusions were drawn by utilizing the in situ/operando Raman spectroscopy and discrete Fourier transform calculations to investigate the structure of chromium species, the redox reaction path, and the formation of coke under real reaction conditions. The oligomeric and isolated Cr3+ species with activated oxygen are denoted as active oxidative chromium species, which are the main active sites of redox reaction. The foremost effect of CO2 is inhibiting rather than eliminating coke. Moreover, a possible new coke formation route is proposed.

Structure and transport properties of zirconia crystals co-doped by scandia, ceria and yttria

This work is a study of the effect of co-doping (ZrO2)0.9(Sc2O3)0.1 solid solution with yttria and/or ceria on the phase composition, local structure and transport properties of the crystals. The solid solution crystals were grown using directional melt crystallization in cold crucible. We show that ceria co-doping of the crystals does not stabilize the high-temperature cubic phase in the entire crystal bulk, unlike yttria co-doping. Ceria co-doping of the (ZrO2)0.9(Sc2O3)0.1 crystals increases their conductivity, whereas the addition of 1 mol.% yttria tangibly reduces the conductivity. Equimolar co-doping of the (ZrO2)0.9(Sc2O3)0.1 crystals with ceria and yttria changes the conductivity but slightly. Optical spectroscopy of the local structure of the crystals identified different types of optical centers. We found that the fraction of the trivalent cations having a vacancy in the first coordination sphere in the ceria co-doped crystals is smaller compared with that in the yttria co-doped crystals.

Characterization of terbium containing cubic zirconia crystal for high power laser applications

A magneto-active terbium containing cubic zirconia crystal is an attractive material for creating Faraday isolators thanks to a high value of its Verdet constant and low absorption coefficient. The optical anisotropy parameter of this crystal which shows the distribution of orientations of the axes of thermally induced birefringence in a thermally loaded optical element has negative value. It means that there exists a dedicated orientation of crystallographic axes, at which the axes of thermally induced birefringence are arranged in the one direction and thermally induced depolarization may be reduced substantially. For assessing potential of this crystal for high-power applications we have investigated its temperature dependence of the Verdet constant that proved to correspond to the paramagnetic behavior. Thermo-optical characteristics are also of principal importance when operating with high-power laser radiation. We present results of measurements of thermo-optical characteristics Q and P which determines values of polarization and phase distortions of passing laser radiation. Measured values can be used to determine the characteristics of Faraday isolators based terbium containing cubic zirconia crystals in various implementations of their optical schemes for various parameters of laser radiation.

Structural and bio-mineralization features of alumina zirconia composite influenced by the combined Ca2+ and PO43− additions

The structural and bioactivity features of alumina zirconia composite (AZC) due to Ca2+ and PO43− additions are demonstrated. An in situ synthetic approach, starting from the solution precursors is devised for the powder synthesis in which the assorted range of Ca2+ and PO43− additions were done to the equimolar concentrations of Al3+ and Zr4+ precursors. The results witnessed the unique crystallization of tetragonal zirconia (t-ZrO2) at 1100 °C while Ca2+, PO43− and Al2O3 remained in their amorphous state in the system. On further heat treatment, α-Al2O3 crystallized at 1200 °C, which enforced t- → m-ZrO2 transformation while Ca2+ and PO43− still retained their amorphous state. The immersion tests in simulated body fluid (SBF) solution validated the enhanced bio-mineralization activity of AZC due to Ca2+ and PO43− additions. The results from the indentation tests demonstrated good uniformity in the elastic modulus and hardness data of the investigated specimens. Further, in vitro cell culture tests ascertained the bioactivity of all the AZC compositions.

Temperature effect on the phonon dispersion stability of zirconium by machine learning driven atomistic simulations

It is well known that conventional harmonic lattice dynamics cannot be applied to energetically unstable crystals at 0 K, such as high temperature body centered cubic (BCC) phase of crystalline Zr. Predicting phonon spectra at finite temperature requires the calculation of force constants to the third, fourth and even higher orders, however, it remains challenging to determine to which order the Taylor expansion of the potential energy surface for different materials should be cut off. Molecular dynamics, on the other hand, intrinsically includes arbitrary orders of phonon anharmonicity, however, its accuracy is severely limited by the empirical potential field used. Using machine learning method, we developed an inter-atomic potential for Zirconium crystals for both the hexagonal closed-packed (HCP) phase and the body centered cubic phase. The developed potential field accurately captures energy-volume relationship, elastic constants and phonon dispersions. The instability of BCC structure is found to originate from the double-well shape of the potential energy surface where the local maxima is located in an unstable equilibrium position. The stabilization of the BCC phase at high temperature is due to the dynamical average of the low-symmetry minima of the double well due to atomic vibrations. Molecular dynamics simulations are then performed to stochastically sample the potential energy surface and to calculate the phonon dispersion at elevated temperature. The phonon renormalization in BCC-Zr is successfully captured by the molecular dynamics simulation at 1188 K.

Progressive modelling and experimentation of hydrogen diffusion and precipitation in anisotropic polycrystals

Hydrogen embrittlement of zirconium alloys is a major concern in nuclear industry. It is known that diffusion of hydrogen atoms in zirconium lattice depends on the localized state of stresses, the patterning of which is significantly affected by the elastic and plastic anisotropy of zirconium crystals. This study focuses on developing a model to understand the effects of such localized stress fields on hydrogen diffusion and precipitation. For this purpose, a crystal plasticity finite element code is updated and coupled with a series of newly developed subroutines for simulating stress-assisted-diffusion of hydrogen atoms in zirconium polycrystals. An electron backscatter diffraction experiment is also conducted on a hydrided Zircaloy-2 sample to compare the measured distribution of the hydride phase with the calculated one. It is shown that even in the absence of macroscopic loads, thermal residual stresses within zirconium grains can disturb the uniform distribution of hydrogen atoms by inducing large hydrostatic stresses at the vicinity of grain boundaries. Such stress concentrations can further affect the sequence of hydrides nucleation. Also, it is shown that by alternating local stress fields, hydride induced transformation strain can change the distribution of the hydrogen atoms in zirconium matrix.

Preparation and optical properties of Ho3+-doped YSZ single crystals

Single crystals of yttria-stabilized zirconia (YSZ) doped with different contents of holmium were prepared by the optical floating zone method, and their structures and phase composition were characterized by XRD and Raman spectroscopy. These showed that the metastable tetragonal phase was stabilized by the substitutions. Transmittance of each of the Ho3+-doped YSZ single crystals was larger than 85% in the visible region. Measurements of fluorescence spectra indicated a strong emission at a wavelength of 553.5 nm, and two weak emission peaks at 671 nm and 759 nm when the crystals were excited by light with a wavelength of 448 nm. The YSZ doped with 0.75 mol% of Ho2O3 had a relatively higher fluorescence intensity and possessed high color purity (99.7%) in the green region, indicating that it can be used to produce efficient green emission.

Atomistic simulations to study crack tip behaviour in single crystal of bcc niobium and hcp zirconium

In this article, molecular dynamics based simulations were carried out to study the fracture toughness of single crystals of niobium (Nb) and zirconium (Zr). Separate set of simulations were performed with different orientations of crack plane in Nb and Zr. In each case, efforts were made to align the crack front with principal planes of corresponding crystal structure, that was bcc for Nb and hcp for Zr. Deformation in single crystal of Nb and Zr was governed either by twinning or emergence of dislocations from the crack tip and surfaces. The deformation mechanism in single crystal of Nb and Zr also helps in governing the overall toughness of the material, while deforming via twinning leads to higher change in the crack length, whereas dislocations emerging from the crack tip blunts the opening and improves the toughness.

Multiscale approach for determining hydrogen diffusivity in zirconium

The current research presents an approach which is used to determine the diffusivity of hydrogen in the hexagonal close packed (hcp) zirconium crystal, using a combination of first principles calculations and kinetic Monte Carlo (KMC) simulations. Rate constants found through the energy landscapes of hydrogen motion between different interstitial sites in the zirconium lattice were used in KMC to determine the values of bulk diffusivity. We simulated a stress-free environment to eliminate the effect of stress. It is hypothesized that stress could act as a driving forces for diffusion. We found that hydrogen diffusivity in hcp Zr is closely isotropic, with a slightly higher diffusivity in the axis direction. The individual diffusion jumps were closely investigated to identify the reasons for the isotropic nature of the diffusivity in the anisotropic hcp Zr lattice. We also use this study to validate the modeling approach followed to extend it to other diffusion studies of similar nature, which comprises of clearly understood diffusion steps.