Imaging Secondary-Ion Mass Spectroscopy Observation of the Scavenging of Siliceous Film from 8-mol%-Yttria-Stabilized Zirconia by the Addition of Alumina

The scavenging of a resistive siliceous phase via the addition of Al₂O₃ was studied, using imaging secondary-ion mass spectroscopy (SIMS), given the improved grain-boundary conductivity in 8-mol%-yttria-stabilized zirconia (8YSZ). The grain-boundary resistivity in 8YSZ decreased noticeably with the addition of 1 mol% of Al₂O₃. Strong SiO₂ segregation at the grain boundaries was observed in a SIMS map of pure 8YSZ that contained 120 ppm of SiO₂ (by weight). The addition of 1 mol% of Al₂O₃ caused the SiO₂ to gather around the Al₂O₃ particles. The present observations provided direct and visual evidence of SiO₂ segregation at the grain boundaries (which had a deleterious effect on grain-boundary conductivity) and the scavenging of SiO₂ via Al₂O₃ addition.     

Structure and Chemistry of Yttria-Stabilized Cubic-Zirconia Symmetric Tilt Grain Boundaries

The atomic structures of two symmetric [001] tilt grain boundaries in yttria-stabilized cubic-zirconia, Σ5 (310) and near-Σ13 (510), are studied by Z -contrast scanning transmission electron microscopy. Both boundaries are composed of periodic arrays of highly symmetric structural units, with a distinct unit for each boundary. Oxygen K -edge electron energy-loss spectra show that the oxygen coordination is similar between the bulk and grain boundary, indicating that oxygen ions within the grain boundary reside in distorted tetrahedral sites. Atomic models of the grain boundaries are proposed that are consistent with the experimental data. The core structures are different from previously studied metal or oxide grain boundaries and are unique to the fluorite structure. Yttrium segregation to the grain boundaries is also investigated by electron energy-loss spectroscopy. Yttrium is found to segregate preferentially to the Σ5 grain boundary, and the spatial distribution of the segregation layer is confined to within 1 nm of the boundary plane.     

Effect of Electric Field on the Migration of Grain Boundaries in Alumina

The effect of an external electric field on the grain-boundary migration in Al₂O₃ ceramics has been investigated. The boundary migration is dependent on the direction and magnitude of the applied bias, and the observed boundary migration behavior is attributed to the presence of an electrostatic potential that inherently forms at the grain boundaries of Al₂O₃ ceramics. The results give experimental evidence that the boundary phenomena in oxide ceramics are related to the grain-boundary potential.     

Grain Boundary Migration in Cubic Zirconia–Yttria Induced by Addition of Magnesia at Varying Concentrations

When partially sintered cubic ZrO₂–10 mol% Y₂O₃ specimens are heat-treated at 1500°C with powder mixtures of MgO and ZrO₂–10Y₂O₃ at varying ratios, the grain boundaries migrate, leaving behind new solid solutions enriched with MgO but depleted of Y₂O₃. With increasing MgO content in the solute source powder, the average migration velocity increases, and the MgO content increases and the Y₂O₃ content decreases slightly in these new solid solutions. With increasing MgO content in the solute source, the grain boundaries tend to be corrugated and faceted. Migration reversal is also observed at the corrugated boundaries. These variations of the grain boundary migration behavior with the MgO content in the solute source are consistent with the diffusional coherency strain energy as the driving force for the migration.     

Grain boundary energies in yttria-stabilized zirconia

The three-dimensional microstructure of 8% yttria-stabilized zirconia (YSZ) was measured by electron backscatter diffraction and focused ion beam serial sectioning. The relative grain boundary energies as a function of all five crystallographic grain boundary parameters were determined based on the assumption of thermodynamic equilibrium at the internal triple junctions. Grain boundaries with (100) orientations have low energies compared to boundaries of other orientations, and all [100] twist boundaries have relatively low energies. Other classes of boundaries with lower than average energies include [100] symmetric tilt boundaries with disorientations less than 40° and [111] twist boundaries with disorientations greater than 20°. At fixed misorientations, the relative areas of boundaries are inversely correlated to the relative grain boundary energy. The results suggest that texturing microstructures to increase the relative areas of [100] twist boundaries might increase the oxygen ion conductivity of YSZ ceramics.     

Dihedral Angles in Magnesia and Alumina: Distributions from Surface Thermal Grooves

Dihedral angles, Ψ_s, from surface thermal grooves were measured using a metal reference line technique for polycrystalline MgO, undoped Al₂O₃, and MgO-doped Al₂O₃. The values of Ψ_s span the following ranges: 89° to 116° for MgO at 1520 K, 76° to 166° for undoped Al₂O₃ at 1870 K, and 90° to 139° for MgO-doped Al₂O₃ at 1870 K. For all three systems, the median Ψ_s values are 105° to 113°, implying that the median γ_gb/γ_s is 1.1 to 1.2, in contrast to metals where γ_gb/γ_s ranges typically from 1/4 to 1/2. The widths in Ψ_s distributions were different for the three materials with the width increasing in the following order: MgO, MgO-doped Al₂O₃, undoped Al₂O₃. For all three materials, the grainboundary grooves and their corresponding Ψ_s were not symmetrical with respect to the surface normal. The asymmetry for MgO was due to the pinning of the grain boundaries by the surface thermal grooves. The range of inclination angles of the grain boundary to the surface was a function of Ψ_s, with the maximum inclination angles of ∼13°, in quantitative agreement with theory.     

Distribution and Energies of Grain Boundaries in Magnesia as a Function of Five Degrees of Freedom

The multiplicity of distinct grain boundary configurations in polycrystals has made it difficult to determine the relative frequency with which each configuration is adopted. As a result, the physiochemical properties of each boundary and the influence of the distribution of boundaries on macroscopic materials properties are not well understood. Using a semiautomated system, we have measured all five macroscopically observable degrees of freedom of 4.1 × 10⁶ boundary plane segments making up 5.2 × 10⁶μm² of grain boundary interface area in a magnesia polycrystal. Our observations demonstrate that not all grain boundary configurations occur with the same frequency and that the relative free energies of the different interfacial configurations influence the population distribution. Furthermore, the results indicate that relative grain boundary energies can be estimated based on the free surface energies.     

Effect of MgO Addition on the Microstructure Development of 3 mol% Y2O3—ZrO2

MgO addition to 3 mol% Y₂O₃–ZrO₂ resulted in enhanced densification at 1350°C by a liquid-phase sintering mechanism. This liquid phase resulted from reaction of MgO with trace impurities of CaO and SiO₂ in the starting powder. The bimodal grain structure thus obtained was characterized by large cubic ZrO₂ grains with tetragonal ZrO₂ precipitates, which were surrounded by either small tetragonal grains or monoclinic grains, depending on the heat-treatment schedule.     

Microstructural and Chemical Influences of Silicate Grain-Boundary Phases in Yttria-Stabilized Zirconia

The microstructure and chemistry of grain-boundary phases in silicate-doped Y₂O₃─ZrO₂ ceramics were evaluated by analytical electron microscopy. Two different silicate compositions were used: one an aluminosilicate and the other a borosilicate glass. These grain-boundary phases had a significant impact on the grain morphology, the chemical composition of the grains, and the crystallization of second phases. These results indicate that controlled additions of specific glass phases may provide a means for tailoring the microstructure and physical properties of zirconia ceramics.     

Effects of Polarity on Grain-Boundary Migration in ZnO

The migration of boundaries between single crystal and polycrystalline ZnO was investigated using single crystals with well-defined crystallographic faces. The migration rate of the basal (0001) planes through polycrystalline ZnO depended on the crystallographic polarity of the basal plane. Grain boundary migration in the [0001] direction was much faster than in the [000[Onemacr]] direction. Migration rates of boundaries in nonpolar directions were somewhat slower than that in the [0001] direction. Slow growth in the [000[Onemacr]] direction and rapid growth in nonpolar directions found in the present study help to explain the anisotropic growth of the grains that contain head-to-head inversion boundaries in ZnO varistor ceramics.     

Discontinuous Coarsening of Tetragonal Precipitates in Partially Stabilized Zirconia Induced by Diffusional Coherency Strain under Applied Stress

When partially stabilized zirconia with 6 mol% MgO and 4 mol% CaO is aged at 1450°C, intragranular precipitation occurs and concurrently the boundaries between the grains migrate, forming a Ca-enriched precipitate-free cubic phase and large tetragonal precipitates behind them. At these compositions and temperature the boundary migration is rapid and shows the characteristics of a discontinuous coarsening. A uniaxial compressive stress applied to this specimen during the aging treatment increases or decreases the migration rate of the boundaries parallel or perpendicular to the stress axis, respectively, in agreement with the prediction that a compressive coherency strain due to the diffusion of Ca atoms is produced at the surface of the retreating grains and drives the migration. The diffusional coherency strain energy is thus expected to be the dominant driving force for the discontinuous coarsening in this solid.     

Diffusion-Induced Grain-Boundary Migration in Ceria-Stabilized Tetragonal Zirconia Polycrystals

The microstructure and microchemistry of grain-boundary regions in (CeO₂+ La₂O₃)-stabilized tetragonal ZrO₂ polycrystals (Ce(La)-TZP) were studied by means of transmission electron microscopy (TEM). Evidence was found for the existence of crystalline and vitreous intergranular phases situated in small pockets at multiple grain junctions and in thin films along grain boundaries. In this ceramic system grain-boundary migration was observed in situ in the TEM in sample areas subjected to electron irradiation. Interfaces migrated away from their centers of curvature. Evidence was found for Ce de-alloying in the volume swept by the advancing boundaries. It is suggested that the coherency lattice strain brought about by a partial reduction of Ce, resulting in the diffusion of Ce³⁺ along grain boundaries to free surfaces, is the driving force for this phenomenon.     

Size-induced grain boundary energy increase may cause softening of nanocrystalline yttria-stabilized zirconia

An increase in hardness with reducing grain sizes is commonly observed in oxide ceramics in particular for grain sizes below 100 nm. The inverse behavior, meaning a decrease in hardness below a critically small grain size, may also exist consistently with observations in metal alloys, but the causing mechanisms in ceramics are still under debate. Here we report direct thermodynamic data on grain boundary energies as a function of grain size that suggest that the inverse relation is intimately related to a size-induced increase in the excess energies. Microcalorimetry combined with nano and microstructural analyses reveal an increase in grain boundary excess energy in yttria-stabilized zirconia (10YSZ) when grain sizes are below 36 nm. The onset of the energy increase coincides with the observed decrease in Vickers indentation hardness. Since grain boundary energy is an excess energy related to boundary strength/stability, the results suggest that softening is driven by the activation of grain boundary mediated processes facilitated by the relatively weakened boundaries at the ultra-fine nanoscale which ultimately induce the formation of an energy dissipating subsurface crack network during indentation.     

Structure of Alumina Grain Boundaries Prepared with and without a Thin Amorphous Intergranular Film

The presence of a thin amorphous intergranular film along grain boundaries in alumina is expected to affect the properties of the interface and hence those of the material. In the present study, two types of grain boundaries have been formed in hot-pressed alumina bicrystals. In one case, the surfaces of the sintered crystals were kept as clean as possible, while in the other a thin layer of SiO₂ was intentionally deposited onto the surface of one crystal. The distribution of SiO₂ along the resulting grain boundary was then monitored by transmission electron microscopy and compared with the morphological features of the interface. In the special cases chosen here, the glass receded into large pores which grew into the alumina itself. However, the presence of the glassy phase during the early stages of sintering clearly did influence the characteristics of the resulting grain boundaries.     

Superplastic Bulging of Fine-Grained Zirconia

A tetragonal zirconia containing 2 mol% Y₂O₃, with 0.3 mol% CuO addition as a grain-boundary phase, was superplastically stretched at 1150°C using a hemispherical punch. Mechanical analyses were performed to establish that a biaxial tensile stress/strain state was achieved in the process with a maximal strain of 0.5 in the thinned hemispherical shell. The material was damage tolerant up to a critical strain rate, approximately 10⁻³ s⁻¹.     

Exuding Liquid from Grain Boundaries in Alumina

Bicrystals of alumina with anorthite glass in the boundary were prepared by hot pressing. Annealing of the bicrystals leads to the migration of the intergranular liquid to the free surface of the sample. It is proposed that the migration is driven by the difference in the wetting behavior of the free surface and the boundary.     

Thermodynamic Calculation of the ZrO2–YO1.5–MgO System

A thermodynamic evaluation of the ZrO₂-MgO system has been developed and combined with previous assessments of the ZrO₂–YO_1.5 and YO_1.5–MgO systems to describe the ZrO₂–YO_1.5-MgO Systems to describe the ZrO₂–YO_1.5-MgO system by means of Bonnier's equation. The calculated results are shown by isothermal and vertical sections, a projection of the liquidus surfaces, and the reaction scheme. Comparisons between calculated and experimental diagrams demonstrate that the calculations satisfactorily account for most of the available experimental data.     

Effects of Magnesia Fines Addition and Spinel with Different Compositions on Thermal Expansion Behavior of Alumina Magnesia Castables

Effects of magnesia fines addition ( 4% ,6% and 8% in mass) and spinel with different compositions ( alumina-rich,magnesia-rich and stoichiometric spinel) on thermal expansion behavior of alumina magnesia castables were researched using tabular corundum, magnesia fines,spinel fines,ρ-Al2O3,Secar-71 cement and SiO2 fume as main starting materials. The results show that: ( 1) thermal expansion coefficients of specimens with 4 mass% and 6 mass% magnesia fines have the similar change tendency,increasing slightly with temperature rising; when magnesia addition is 8 mass% ,the thermal expansion coefficient increases obviously at 1 050 ℃ and reaches the peak at 1 350 ℃ ; ( 2) when MgO content is the same,the specimen with magnesia-rich spinel has the lowest thermal expansion coefficient; ( 3) for the castables specimens with the same MgO content,the specimen with magnesia has higher thermal expansion coefficient than that with presynthesized spinel,which is related with the secondary spinelization during the heating process.     

Structure of Sapphire Bicrystal Boundaries Produced by Liquid-Phase Sintering

The structure and composition of sapphire bicrystal boundaries produced by liquid-phase sintering depended on the crystallographic misorientation of the crystals across the boundary and on the orientation of the boundary. Basal twist boundaries of 15° or 30° were not wetted by glass, but contained significant amounts of Ca and Si at the boundary. For tilt boundaries of 7° or 12°, the glass wetted segments of boundaries that contained the basal plane of either crystal. Boundary segments with orientations of 40° or more from the basal plane, however, were dewetted (i.e., "dry"). Boundary segments oriented less than ∼40° from the basal orientation were partially wetted, consisting of segments of wetted and dry grain boundaries. For the 12° tilt boundary, Ca and Si could be detected on portions of the boundary that contained no glass. For bicrystal boundaries having tilts of ≤4°, dewetting occurred for all observed boundary orientations. Basal-oriented segments in these small angle tilt boundaries contained noticeable concentrations of adsorbed Ca and Si, while nonbasal segments were apparently free of Ca and Si. Most results could be explained based on a combined Wulff plot construction, which predicts partially wetted grain boundaries and "missing" angles for unwetted grain boundaries. Results that could not be explained by the construction included growth step ledges bounded by nonequilibrium facet planes.     

Cation Interdiffusion in Polycrystalline Cubic C-Type Yttria–Zirconia–Hafnia Solid Solutions

Cation inter diffusion in cubic C-type solid solutions was investigated for the polycrystalline systems Y₂O₃—Zr_xHf₁-_x-1O₂ and Y₂O₃–ZrO₂ in the small-grain, deep-penetration condition at 1615° to 1822°C. Lattice and grain-boundary diffusion parameters were calculated from cation concentration distributions by using Oishi and Ichimura's grain-boundary diffusion equation. The results indicated that Zr–Y and Zr–Hf interdiffusion coefficients decreased with an increase in Y content. The cation diffusivities were lower than anion diffusivities, and the interdiffusion parameters were lower in the C-type cubic than in fluorite-type cubic solid solutions. The results were compared with diffusivities in other C-type cubic oxides.