Showing papers in "Journal of the American Ceramic Society in 1994"

Effect of Dopants on Zirconia Stabilization—An X‐ray Absorption Study: I, Trivalent Dopants

Abstract: Local atomic structures of Zr and dopant cations in zirconia solid solutions with Fe2O3, Ga2O3, Y2O3, and Gd2O3 have been determined. The Zr ions in both partially stabilized tetragonal and fully stabilized cubic zirconia have their own characteristic structures which are dopant-independent. The dopant cations substitute for Zr ions despite severe local distortions necessitated by the large difference in dopant–O distance ana Zr─O distance. Dopant ionic size determines the preferred locations of oxygen vacancies. Vacancies introduced by oversized dopants (Y and Gd) are located as nearest neighbors to Zr atoms, leaving 8-fold oxygen coordination to dopant cations. Undersized dopants (Fe and Ga) compete with Zr ions for the oxygen vacancies in zirconia, resulting in 6-fold oxygen coordination and a large disturbance to the surrounding next nearest neighbors. Since oxygen vacancies associated with Zr can provide stability for tetragonal and cubic zirconia, these results suggest an explanation for the observation that oversized trivalent dopants are more effective than undersized trivalent dopants in stabilizing cubic and tetragonal phases.

In situ-toughened silicon carbide

Abstract: A new processing strategy based on atmospheric pressure sintering is presented for obtaining dense SiC-based materials with microstructures consisting of (1) uniformly distributed elongate-shaped [alpha]-SiC grains and (2) relatively high amounts (20 vol%) of second-phase yttrium aluminum garnet (YAG). This strategy entails the sintering of [beta]-SiC powder doped with [alpha]-SiC, Al[sub 2]O[sub 3], and Y[sub 2]O[sub 3]. The Al[sub 2]O[sub 3] and Y[sub 2]O[sub 3] aid in the liquid-phase sintering of SiC and form in situ YAG, which has a significant thermal expansion mismatch with SiC. During a subsequent grain-growth heat treatment, it is postulated that the [alpha]-SiC seeds'' assist in controlling in situ growth of the elongated [alpha]-SiC grains. The fracture pattern in the in situ-toughened SiC is intergranular with evidence of copious crack-wake bridging, akin to toughened Si[sub 3]N[sub 4] ceramics. The elongate nature of the [alpha]-SiC grains, together with the high thermal-residual stresses in the microstructure, enhance the observed crack-wake bridging. This bridging accounts for a measured twofold increase in the indentation toughness of this new class of in situ-toughened SiC relative to a commercial SiC.

Raman Scattering Study of Cubic–Tetragonal Phase Transition in Zr1−xCexO2 Solid Solution

Abstract: A structural phase transition between the cubic (space group, Fm3m) and tetragonal (space group, P42/nmc) phases in a zirconia–ceria solid solution (Zr1−xCexO2) has been observed by Raman spectroscopy. The cubic–tetragonal (c–t″) phase boundary in compositionally homogeneous samples exists at a composition X0 (0.8 < X0 < 0.9) at room temperature, where t″ is defined as a tetragonal phase whose axial ratio c/a equals unity. The axial ratio c/a decreases with an increase of ceria concentration and becomes 1 at a composition X′0 (0.65 < X′0 < 0.7) at room temperature. The sample with a composition between X0 and X′0 is t″ ZrO2. By Raman scattering measurements at high temperatures, the tetragonal (t″) → cubic and cubic → tetragonal phase transitions occur above 400°C in Zr0.2 Ce0.8O2 solid solution.

Critical Appraisal of Limiting Strain Rates for Compression Testing of Ceramics in a Split Hopkinson Pressure Bar

Abstract: The split Hopkinson pressure bar (SHPB) is being widely used to determine the dynamic compressive strength of ceramics and ceramic composites. However, extreme caution needs to be exercised while testing these high-strength ceramics at high strain rates. The highest strain rate at which ceramics can be tested using an SHPB without violating the underlying assumptions is found to be in the range of 2500-3000/s. It is also shown that at very high loading rates, dispersion in the transmitted pulse can lead to discrepancies in measuring the dynamic failure strength of ceramics.

Impedance Spectroscopy of Hydrating Cement‐Based Materials: Measurement, Interpretation, and Application

Abstract: This work concerns the state of the art for use of impedance spectroscopy for studying the evolving microstructure of cement-based materials during hydration. Features of the spectra are discussed and related to components of the microstructure with the assistance of pixel-based computer modeling techniques. It is proposed that the enormously high relative dielectric constants (∼105) observed just after set are the result of dielectric amplification and are related to the distribution of pore sizes and the thickness of product C─S─H layers separating the pores. The conductivity is related to the volume fraction of porosity, the conductivity of the pore solution, and the interconnectivity of the porosity. The conductivity, when normalized by that of the pore solution, i.e., inverse formation factor, is a measure of this interconnectivity and can be used to predict such engineering properties as ionic diffusivity and water permeability. Composite mixing laws are employed to aid in explaining the behavior of the conductivity and to obtain a qualitative measure of the pore shape with hydration. Procedures for predicting the conductivity of the pore solution and for subtracting out electrode lead effects at high frequency are discussed.

Effect of Glass Additions on BaO–TiO2–WO3 Microwave Ceramics

Abstract: The effect of glass addition on the properties of BaO–TiO2-WO3 microwave dielectric material N-35, which has Q= 5900 and K= 35 at 7.2 GHz for samples sintered at 1360°C, was investigated. Several glasses including B2O3, SiO2, 5ZnO–2B2O3, and nine other commercial glasses were selected for this study. Among these glasses, one with a 5 wt% addition of B2O3 to N-35, when sintered at 1200°C, had the best dielectric properties: Q= 8300 and K= 34 at 8.5 GHz. Both Q and K increased with firing temperature as well as with density. The Q of N-35, when sintered with a ZnO–B2O3 glass system, showed a sudden drop in the sintering temperature to about 1000°C. The results of XRD, thermal analysis, and scanning electron microscopy indicated that the chemical reaction between the dielectric ceramics and glass had a greater effect on Q than on the density. The effects of the glass content and the mixing process on the densification and microwave dielectric properties are also presented. Ball milling improved the densification and dielectric properties of the N-35 sintered with ZnO–B2O3.

Bonelike Hydroxyapatite Induction by a Gel‐Derived Titania on a Titanium Substrate

Abstract: Gel-derived titania coating on commercial pure (c.p.) titanium induces hydroxyapatite formation onto its surface from a simulated body fluid (SBF, a metastable calcium phosphate solution). The induced apatite is similar to bone apatite in that it is poorly crystallized, calcium-deficient, and carbonate-containing. Furthermore, the carbonate (CO2–3) groups go into the apatite lattice and lie at the positions of PO3–4 and OH– to replace these ionic groups, resembling the (CO2–3) groups of bone apatite. Therefore, the apatite induced by the gel-derived titania is said to be bonelike. A chemical stimulation, stemming from abundant hydroxyl groups and negative charges at the surface of the titania gel, is believed to be responsible for the bonelike apatite induction. The potential of bone-bonding is predicted for the gel-derived titania, for it is an efficient bonelike apatite inducer in the SBF.

Apatite Coating on Organic Polymers by a Biomimetic Process

Abstract: Dense and uniform layers of a biologically active carbonate-containing hydroxyapatite can be formed on various kinds of organic polymers by the following biomimetic method. First, a substrate is set in contact with particles of CaO–SiO2-based glass soaked in a simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma for forming the apatite nuclei on the substrate. Second, the substrate is soaked in another solution highly supersaturated with respect to the apatite, e.g, with ion concentrations 1.5 times those of SBF (1.5SBF) for making the apatite nuclei grow on the substrate in situ. The induction period for the apatite nucleation, which is defined as the time of the first treatment required for forming enough of the apatite nuclei to make the continuous layer after the second treatment, was almost 24 h for most of the examined polymers. The adhesive strength of the formed apatite layer to the polymers was as high as 3 to 4 M Pa for poly(ethylene terephthalate), poly-ether sulfone, and poly (vinyl alcohol) hydrogel. This type of apatite–organic polymer composite is expected to be useful for repairing not only living hard tissues but also soft ones.

Hardness-grain-size relations in ceramics

Abstract: Both Vickers and Knoop hardness (H), measured at two or more loads in the range of 100–2000 g (most commonly 100 and 500 g) for a variety of dense oxide and non-oxide materials, covering a range of grain sizes (G), including single crystals where possible, were shown to generally be consistent with (often more limited) literature data. Apparently, conflicting trends of H (1) showing either no G dependence, (2) decreasing from single-crystal or large G values with decreasing G, or (3) having the generally accepted increase with decreasing G are shown to be due to the combination of the limited extent of data and H generally heing determined by two basic trends. These two trends are (a) the normal inverse G (i.e., H–G−1/2) dependence at finer G, (b) a variable G minimum at intermediate G, and (c) H increasing with increasing G at larger G (to. single-crystal values). The H minimum is due to local cracking around the indent (mostly along grain boundaries), generally reaching a maximum effect, e.g., minimum in H, when the indent and grain sizes are similar, and tends to be greater for Vickers vs Knoop indents, higher loads and probably greater grain boundary Impurity, additive contents, and stresses.

Toughness Properties of a Silicon Carbide with an in Situ Induced Heterogeneous Grain Structure

Abstract: Toughness characteristics of a heterogeneous silicon carbide with a coarsened and elongated grain structure and an intergranular second phase are evaluated relative to a homogeneous, fine-grain control using indentation–strength data. The heterogeneous material exhibits a distinctive flaw tolerance, indicative of a pronounced toughness curve. Quantitative evaluation of the data reveals an enhanced toughness in the long-crack region, with the implication of degraded toughness in the short-crack region. The enhanced long-crack toughness is identified with crack-interface bridging. The degraded short-crack toughness is attributed to weakened grain or interface boundaries and to internal residual stresses from thermal expansion mismatch. A profound manifestation of the toughness-curve behavior is a transition in the nature of mechanical damage in Hertzian contacts, from classical single-crack cone fracture in the homogeneous control to distributed subsurface damage in the heterogeneous material.

Electric-field-induced fatigue crack growth in piezoelectrics

Abstract: When subjected to large alternating electric fields, ferroelectric ceramics may experience cracking and mechanical degradation. This article describes an experimental procedure for characterizing crack extension from preexisting flaws in such materials subject to high-amplitude, alternating electric fields. A new mode of electric-field-induced fatigue crack growth is identified. Fracture mechanics concepts are applied to interpret the observed cracking.

Effect of Grain Size on Hertzian Contact Damage in Alumina

Abstract: The role of microstructural scale on deformation-microfracture damage induced by contact with spheres is investigated in monophase alumina ceramics over a range 3--48 [mu]m in grain size. Measurement of a universal indentation stress-strain curve indicates a critical contact pressure [approx] 5 GPa, above which irreversible deformation occurs in alumina. A novel sectioning technique identifies the deformation elements as intragrain shear faults, predominantly crystallographic twins, within a confining subsurface zone of intense compression-shear stress. The twins concentrate the shear stresses at the grain boundaries and, above a threshold grain size, initiate tensile intergranular microcracks. Below this threshold size, classical Hertzian cone fractures initiate outside the contact circle. Above the threshold, the density and scale of subsurface-zone microcracks increase dramatically with increasing grain size, ultimately dominating the cone fractures. The damage process is stochastic, highlighting the microstructural discreteness of the initial deformation field; those grains which lie in the upper tail of the grain-size distribution and which have favorable crystallographic orientation relative to local shear stresses in the contact field are preferentially activated. Initial flaw state is not an important factor, because the contact process creates its own flaw population. These and other generic features of the damage process will be discussedmore » in relation to microstructural design of polycrystalline ceramics.« less

Microstructure Control of Silicon Nitride by Seeding with Rodlike β‐Silicon Nitride Particles

Abstract: The effect of seeding on microstructure development and mechanical properties of silicon nitride was investigated by the use of morphologically regulated rodlike β-Si3N4 singlecrystal particles with a diameter of 1 μm and a length of 4 μm as seed crystals. Silicon nitride with a bimodal microstructure was fabricated under a relatively low nitrogen gas pressure of 0.9 MPa owing to the epitaxial growth of β-silicon nitride from the seed particles. Grain growth from seeds followed the empirical equation Dn–D0n=kt, with growth exponents of 3 and 5 for the c-axis direction and the a-axis direction, respectively, being analogous to the kinetics of matrix grain growth. By seeding morphologically regulated particles, fracture toughness of silicon nitride was improved from 6.3 to 8.4–8.7 MPa·m1/2, retaining high strength levels of about 1 GPa.

Effects of Glass Additions on (Zr,Sn)TiO4 for Microwave Applications

Abstract: The effects of glass additions on the properties of (Zr,Sn)TiO4 as a microwave dielectric material were investigated. The (Zr,Sn)TiO4 ceramics with no glass addition sintered at 1360°C gave Q= 4900 and K= 37 at 7.9 GHz. Several glasses, including SiO2, B2O3, 5ZnO–2B2O3, and nine commercial glasses, were tested during this study. Among these glasses, (Zr,Sn)TiO4 sintered with ZnO-B2O3–SiO2 (Corning 7574) showed more than 20% higher density than that of pure (Zr,Sn)TiO4 sintered at the same temperature. A 5-wt% addition of SiO2, to (Zr,Sn)TiO4, when sintered at 1200°C, gave the best Q: Q= 2700 at 9 GHz. Results of XRD analysis and scanning electron microscopy and the effect of glass content are also presented.

Temperature–Time Texture Transition of Pb(Zr1−xTix)O3 Thin Films: I, Role of Pb-rich Intermediate Phases

Abstract: Lead-rich intermediate phases, in particular Pt,,Pb and PbO, are found to form on Pt(ll1) atop Ti-buffered Si substrates prior to formation of Pb(Zr, -*TiX)O3 (PZT) thin films. Pt,_,Pb is a [ill] textured transient intermetallic phase that nucleates PZT[111] texture. PbO is a [OOl] textured layer compound that nucleates PZT[100] texture. The formation conditions and lattice matching of these intermediate phases have been examined. The presence of other possible intermediate phases, such as pyrochlore and Zr and Ti-rich phases, has also been investigated but found unrelated to the texture selection of perovskite PZT.

Grain Boundary Defect Chemistry of Acceptor‐Doped Titanates: Space Charge Layer Width

Abstract: The grain boundary space charge depletion layers in acceptor-doped SrTiO3 and BaTiO3 ceramics were investigated by impedance spectroscopy in the time and frequency domain. Based on the layer width and its dependence on the acceptor concentration, the temperature, and the oxygen partial pressure during annealing, a suggestion for a refined Schottky model is proposed. The local distribution of the donor-type grain boundary states causing the depletion layer and the resulting band bending are discussed.

Effect of Particle Size on the Room‐Temperature Crystal Structure of Barium Titanate

Abstract: The room-temperature tetragonal-to-cubic transformation in BaTiO3 powders with decreasing particle size has been carefully studied, using materials prepared mainly by hydrothermal methods. Hydrothermal BaTiO3 powders exhibited a more uniform particle size distribution than oxalate-route powders, with X-ray diffraction and electron microscopy indicating that powders 0.19 μm in size were fully cubic while powders 0.27 μ were completely tetragonal (within a 5% detection limit for cubic material) at room temperature. The tetragonal-to-cubic transformation temperature was also found to lie in the range of 121°± 3°C for BaTiO3 powders with room-temperature (c/a) values > 1.008. No transformation could be detected using differential scanning calorimetry for BaTiO3 particles with a (c/a) > 1.008 at room temperature. BaTiO3 powder with a particle size just too small (0.19 μm) to be tetragonal at room temperature remained cubic down to 80 K. Different models for the cubic-to-tetragonal room-temperature transformation are discussed. Hydroxyl ions do not appear to greatly affect the cubic-to-tetragonal transformation, which appears to be essentially dependent on particle size. It is concluded that a model based on surface free energy, as previously discussed for the monoclinic-to-tetragonal transformation at room temperature of fine ZrO2 particles, is consistent with the experimental data.

Oxidation Mechanisms and Kinetics of 1D-SiC/C/SiC Composite Materials: I, An Experimental Approach

Abstract: The oxidation of unidirectional SiC/C/SiC model composites has been investigated through thermogravimetric analysis, optical/electron microscopy, and electrical measurements. The influence of temperature and carbon interphase thickness on the oxidation of the composites is discussed. The oxidation involves three phenomena: (i) reaction of oxygen with the carbon interphase resulting in pores around the fibers, (ii) diffusion of oxygen and carbon oxides along the pores, and (iii) reaction of oxygen with the pore walls leading to the growth of silica layers on both the fibers and matrix. In composites with a thin carbon interphase (e.g., 0.1 μm) treated at T > 1000°C the pores are rapidly scaled by silica. Under such conditions, the oxidation damages are limited to the vicinity of the external surface and the materials exhibit a self-healing character. Conversely, long exposures (300 h) at 900°C give rise to the formation of microcracks in the matrix related to mechanical stresses arising from the in situ SiC/SiO2 conversion, fly, the self-healing character is not observed in composites with a thick interphase (e.g., 1 μm) since carbon is totally consumed before silica can seal the pores.

Piezospectroscopic Determination of Residual Stresses in Polycrystalline Alumina

Abstract: The residual stresses created in polycrystalline aluminum oxide as a result of its constrained anisotropic thermal contraction are measured with the technique of piezospectroscopy using the fluorescence from trace Cr[sup 3+] impurities. The average residual stresses in the crystallographic a and c directions are determined as a function of grain size for a high-purity alumina, as is the width of the stress distribution (assuming it to be Gaussian). Over the range of grain sizes investigated, from 2 to 16 [mu]m, the residual stresses exhibit a dependence on grain size consistent with the prediction of the Evans-Clarke model of thermal stress relaxation by grain boundary diffusion.

Slip and Twinning in Sapphire (α-Al2O3)

Abstract: The plastic deformation of sapphire (α-Al2O3) has been studied under hydrostatic confining pressure at temperatures below the ambient pressure brittle-to-ductile transition temperature. Samples oriented for prism plane slip (Type I samples) were deformed via dislocation slip at temperatures as low as 200°C. Samples oriented for basal slip (Type II samples) could be plastically deformed at temperatures as low as 400°C but showed more complicated deformation behavior, inasmuch as the sample orientation also allowed for the activation of basal twinning and two of the three rhombohedral twin systems. The temperature dependence of the critical resolved shear stress (τcrss), ln τcrss= ln τ0– M·T for basal slip was significantly greater than that for prism plane slip (Bbasal > Bprism plane), causing the latter system to be the easy slip system below ∼600°C (basal slip is the easy slip system at elevated temperatures). Type II samples deformed primarily by basal twinning in preference to both rhombohedral twinning and basal slip. The different temperature dependence of τcrss for basal and prism plane slip is attributed to details of the dislocation core structure; prism plane dislocations, having a large Burgers vector (∣bPP∣= 0.822 nm), can dissociate into three collinear par-tials (∣bP∣= 0.274 nm) separated by relatively low-energy stacking faults, whereas the comparable dissociation of basal dislocations (∣bB∣= 0.476 nm) produces two ncncol-linear partials separated by a relatively high energy stacking fault. Thus, dissociation of basal dislocations is most likely restricted to the dislocation core, which is manifested in a higher Peierls stress at low temperatures for basal slip compared to prism plane slip.

Creep of duplex microstructures

Abstract: The tensile creep behavior of two ceramic composite systems exhibiting duplex microstructures was studied relative to their single-phase constituents in the temperature and stress ranges of 1,100--1,350 C and 35--75 MPa. The equi-volumetric compositions in the Al[sub 2]O[sub 3]:c-ZrO[sub 2] (8 mol% Y[sub 2]O[sub 3]) and Al[sub 2]O[sub 3]:Y[sub 3]Al[sub 5]O[sub 12] systems both exhibit lower creep rates than either of their single-phase constituents. This effect is attributed to Y[sup 3+] (and possibly Zr[sup 4+]) present in the Al[sub 2]O[sub 3] as a segregant which lowers the creep rate by [approximately]2 orders of magnitude. It is believed that the segregation of Y[sup 3+] to the Al[sub 2]O[sub 3] grain boundaries hinders the interface reaction believed to control the creep. If one of the single-phase constituents is taken to be the Y[sup 3+]-doped Al[sub 2]O[sub 3], the creep of the duplex microstructures can be modeled using standard composite theory applied to flow.

Oxidation Kinetics of Chemically Vapor‐Deposited Silicon Carbide in Wet Oxygen

Abstract: The oxidation kinetics of chemically vapor-deposited SiC in dry oxygen and wet oxygen (P(sub H2O) = 0.1 atm) at temperatures between 1200 C and 1400 C were monitored using thermogravimetric analysis. It was found that in a clean environment, 10% water vapor enhanced the oxidation kinetics of SiC only very slightly compared to rates found in dry oxygen. Oxidation kinetics were examined in terms of the Deal and Grove model for oxidation of silicon. It was found that in an environment containing even small amounts of impurities, such as high-purity Al2O3 reaction tubes containing 200 ppm Na, water vapor enhanced the transport of these impurities to the oxidation sample. Oxidation rates increased under these conditions presumably because of the formation of less protective sodium alumino-silicate scales.

Structure and Energetics of Alumina Surfaces Calculated from First Principles

Abstract: First-principles calculations based on density–functional theory and pseudopotentials are used to calculate the energies and relaxed structures of five low-index surfaces of α-alumina. Evidence for the reliability of the theoretical methods is provided by calculations of the lattice parameters and internal coordinates of the perfect crystal, which agree very closely with experimental data. It is shown that, for some of the surfaces, relaxation effects reduce the surface energy by over a factor of two and lead to major changes of surface structure, with particularly large effects being found for the (0001) and (1010) surfaces. Results are also presented for the distribution of valence electron charge in the surface region. These results suggest that α-alumina is highly ionic even for ions at the surface.

Phase Development and Luminescence in Chromium-Doped Yttrium Aluminum Garnet (YAG:Cr) Phosphors

Abstract: The phase development and luminescence of chromium-doped yttrium aluminum garnet (Y3Al5O12:Cr or YAG:Cr) phosphors, prepared by both a chemical precipitation technique and a solid-state reaction, were studied. The YAG structure formed at a much lower temperature and by a different phase development sequence when the chemical method was employed. The light output of the chemically synthesized powders, measured by laser excitation, was discovered to increase with increasing heat treatment temperature and was found to be brightest when the YAG:Cr phosphor had excess aluminum.

Calcium Concentration Dependence of the Intergranular Film Thickness in Silicon Nitride

Abstract: High-resolution electron microscopy and nano-beam analytical electron microscopy have been used to characterize both the intergranular silicate film thickness and its local composition in a series of high-purity Si3N4 ceramics doped with 0–450 at. ppm Ca. Calcium was detected at both two-grain junctions and triple junctions, even in the 80-ppm-Ca-doped specimen. The thickness of the intergranular film at two-grain junctions was found to depend sensitively on Ca content. In undoped material, the thickness was 1.0 ± 0.1 nm. With increasing Ca content, the thickness decreased in the dilute region (80 ppm Ca), but then increased. The variation in film thickness can be qualitatively understood in terms of the balance of three long-range forces acting normal to the film, namely the van der Waals dispersion force, a structural “steric” force, and an electrical-double-layer force. By comparing the measured thicknesses to those predicted, estimates for the structural correlation length and the inverse Debye length can be made. These estimates have values of ∼ 0.22 nm and approximately 0.3–0.5 nm, respectively, for the calcia-free and 80 ppm calcia materials.

Model for the Developing Microstructure in Portland Cement Pastes

Abstract: A method is proposed for quantitatively predicting the volume of the major phases in hydrated cement pastes as a function of (1) the composition of the cement, (2) the degree of reaction, and (3) the initial water: cement ratio. This procedure is then used to develop a quantitative model for the surface area and volume of porosity that is accessible to nitrogen in calcium silicate hydrate (C-S-H). Published values for surface areas and volume of pores are compared with the predictions made by the model. An implication of the model is that there are two types of C-S-H, or perhaps regions within the C-S-H: one that nitrogen can penetrate and one that it cannot.

Effect of Dopants on Zirconia Stabilization—An X‐ray Absorption Study: III, Charge‐Compensating Dopants

Abstract: X-ray absorption spectra at the Zr-, Y-, and Nb-K edges of ZrO[sub 2]-YNbO[sub 4] solid solutions have been measured at 10 K to determine the local atomic structures. Both Y and Nb cations substitute for Zr in the cation network but maintain rather different local oxygen coordination from Zr. In tetragonal zirconia solid solutions, Y adopts a YO[sub 8] structure with a bond length of 2.32 [angstrom]. This is the same as the structure found in ZrO[sub 2]-Y[sub 2]O[sub 3] solid solutions, confirming the previous conclusion that Y is not associated with oxygen vacancies. Nb has an NbO[sub 4] structure with a bond length of 1.90 [angstrom]. This is shorter than the Zr-O[sub I] distance of 2.10 [angstrom]. The strong Nb-O coordination increases the bonding disparity between Zr-O layers, thus increasing tetragonality. This is similar to the trend previously established for ZrO[sub 2]-GeO[sub 2] solid solutions. Severe distortion of neighboring cations around the undersized Nb, similar to that previously found for undersized Fe[sup 3+] and Ga[sup 3+], is also observed. At higher temperatures, local Y-Nb cation ordering occurs at a concentration below the solubility limit, similar to the Zr-Ge ordering reported previously. This cation ordering mechanism allows the charge-compensating Y-Nbmore » pair to stabilize the tetragonal structure but increase tetragonality.« less

Effect of Notch‐Root Radius on the Fracture Toughness of a Fine‐Grained Alumina

Abstract: The dependence of K IC on the notch-root radius has been examined for a notch radius as small as a few micrometers in a dense, fine-grained, polycrystalline alumina ceramic. The notch radius can be systematically varied by using a semimanual procedure in a special jig which polishes out rather than cuts the specimen. K IC is independent of the notch sharpness for notch-root radii <10 μm. The results are critically compared with those obtained by other standard techniques and discussed in terms of residual compressive stresses introduced during the notching procedure

Oxidation Mechanisms and Kinetics of 1D-SiC/C/SiC Composite Materials: II, Modeling

Abstract: A model, based on a simple axisymmetrical fiber/interphase/matrix assembly, is derived to depict the oxidation behavior of 1D-SiC/C/SiC composites within the temperature range 900-1300°C and for 10

Kinetics and Mechanisms of Oxidation of 2D Woven C/SiC Composites: I, Experimental Approach

Abstract: The oxidation behavior of a 2D woven C/SiC composite partly protected with a SiC seal coating and heat-treated (stabilized) at 1,600 C in inert gas has been investigated through an experimental approach based on thermogravimetric analyses and optical/electron microscopy. Results of the tests, performed under flowing oxygen, have shown that the oxidation behavior of the composite material in terms of oxidation kinetics and morphological evolutions is related to the presence of thermal microcracks in the seal coating as well as in the matrix. Three different temperature domains exist. At low temperatures ( 1,100 C), such diffusion mechanisms are limited by sealing of the microcracks by silica; therefore, the degradation of the composite remains superficial. The study of the oxidation behavior of (i) the heat-treated composite in a lower oxygen content environment (dry air) and (ii) themore » as-processed (unstabilized) composite in dry oxygen confirms the different mechanisms proposed to explain the oxidation behavior of the composite material.« less