Hot Corrosion Mechanism of Composite Alumina/Yttria-Stabilized Zirconia Coating in Molten Sulfate–Vanadate Salt

In order to improve the hot corrosion resistance of yttria-stabilized zirconia (YSZ), an Al₂O₃ overlay has been deposited on the surface of YSZ by electron-beam physical vapor deposition. Hot corrosion tests have been performed on the YSZ coatings with and without an Al₂O₃ overlay in the molten salt mixture (Na₂SO₄+0–15 wt% V₂O₅) at 950°C. The presence of V₂O₅ in the molten salt exacerbates degradation of both the monolithic YSZ coating and the composite YSZ/Al₂O₃ system. The formation of a low-melting Na₂O–V₂O₅–Al₂O₃ liquid phase is responsible for degradation of the Al₂O₃ overlay. The Al₂O₃ overlay acts as a barrier against the infiltration of the molten salt into the YSZ coating during exposure to the molten salt mixture with <5 wt% vanadate.     

Crack Bifurcation in Laminar Ceramic Composites

Crack bifurcation was observed in laminar ceramic composites when cracks entered thin Al₂O₃ layers sandwiched between thicker layers of Zr(12Ce)O₂. The Al₂O₃ layers contained a biaxial, residual, compressive stress of ∼2 GPa developed due to differential contraction upon cooling from the processing temperature. The Zr(12Ce)O₂ layers were nearly free of residual, tensile stresses because they were much thicker than the Al₂O₃ layers. The ceramic composites were fabricated by a green tape and codensification method. Different specimens were fabricated to examine the effect of the thickness of the Al₂O₃ layer on the bifurcation phenomena. Bar specimens were fractured in four-point bending. When the propagating crack encountered the Al₂O₃ layer, it bifurcated as it approached the Zr(12Ce)O₂/ Al₂O₃ interface. After the crack bifurcated, it continued to propagate close to the center line of the Al₂O₃ layer. Fracture of the laminate continued after the primary crack reinitiated to propagate through the next Zr(12Ce)O₂ layer, where it bifurcated again as it entered the next Al₂O₃ layer. If the loading was stopped during bifurcation, the specimen could be unloaded prior to complete fracture. Although the residual stresses were nearly identical in all Al₂O₃ layers, crack bifurcation was observed only when the layer thickness was greater than ∼70 μm.     

Thermal Barrier Coatings Design with Increased Reflectivity and Lower Thermal Conductivity for High-Temperature Turbine Applications

High reflectance thermal barrier coatings consisting of 7% Yittria-Stabilized Zirconia (7YSZ) and Al₂O₃ were deposited by co-evaporation using electron beam physical vapor deposition (EB-PVD). Multilayer 7YSZ and Al₂O₃ coatings with fixed layer spacing showed a 73% infrared reflectance maxima at 1.85 μm wavelength. The variable 7YSZ and Al₂O₃ multilayer coatings showed an increase in reflection spectrum from 1 to 2.75 μm. Preliminary results suggest that coating reflectance can be tailored to achieve increased reflectance over a desired wavelength range by controlling the thickness of the individual layers. In addition, microstructural enhancements were also used to produce low thermal conductive and high hemispherical reflective thermal barrier coatings (TBCs) in which the coating flux was periodically interrupted creating modulated strain fields within the TBC. TBC showed no macrostructural differences in the grain size or faceted surface morphology at low magnification as compared with standard TBC. The residual stress state was determined to be compressive in all of the TBC samples, and was found to decrease with increasing number of modulations. The average thermal conductivity was shown to decrease approximately 30% from 1.8 to 1.2 W/m-K for the 20-layer monolithic TBC after 2 h of testing at 1316°C. Monolithic modulated TBC also resulted in a 28% increase in the hemispherical reflectance, and increased with increasing total number of modulations.     

Space Charge-Mediated Ionic Transport in Yttria-Stabilized Zirconia–Alumina Composite Membranes

This paper reports ionic conductivity of yttria-stabilized zirconia (YSZ)–Al₂O₃ composite membranes. The tape cast specimens were subjected to binder burnout (500°C) and sintering (1550°C) processes to obtain 200–300 μm thick membranes. The ionic conductivity and microstructure of the membranes were characterized and are discussed in this paper. The ionic conductivity of the composite specimens was enhanced and was correlated with the number of charge carrier and their mobility. The solubility of Al₂O₃ in YSZ was minimal and nanosize Al₂O₃ of the batch sintered into microsize and existed as a distinct phase. The scanning electron microscopy micrographs revealed that YSZ and Al₂O₃ grains were strained.     

Effects of an Electroplated Platinum Interlayer on the Morphology and Phases of Chemically-Vapor-Deposited Alumina on Single-Crystal, Nickel-Based Superalloy

The feasibility of preparing a thin layer of α-Al₂O₃ on the surface of a single-crystal, Ni-based superalloy was examined using a chloride-based chemical vapor deposition (CVD) process previously developed for cutting tool applications. A coating directly deposited by this method on the alloy surface consisted of ∼1 μm α-Al₂O₃ crystals in a matrix of amorphous Al₂O₃. When the alloy surface was predeposited with an electroplated Pt layer, the coating was mostly α-Al₂O₃, but with the presence of fine microcracks on the coating surface. In comparison to the results observed for pure Pt substrate, the role of the Pt interlayer was apparently to promote the rapid formation of κ-Al₂O₃ nuclei, which subsequently transformed to α-Al₂O₃ during the CVD growth process.     

Alumina Dissolution into Silicate Slag

Dissolution of commercial white fused and tabular Al₂O₃ grains into a model silicate slag was investigated after 1 h at 1450° and 1600°C. Formation of CA₆ and hercynitic spinel layers was observed at all Al₂O₃/slag interfaces. The spinel layer was not always continuous, and so, compared with the CA₆ layer, it had a less-significant effect on the dissolution process. The CA₆ layer that formed adjacent to the tabular Al₂O₃ was incomplete at both temperatures, so that its dissolution was not a totally indirect process. These incomplete CA₆ and spinel layers meant that slag penetrated into the tabular Al₂O₃ grains, which, thus, were corroded and disintegrated by the penetrating slag. There was evidence of liquid in the CA₆ layer adjacent to the fused Al₂O₃ after 1 h at 1450°C, which also enabled direct dissolution. After 1 h at 1600°C, fused Al₂O₃ revealed a thick (∼60 μm), continuous and unpene-trated CA₆ layer, indicating fully indirect dissolution at this temperature.     

Mechanisms and Kinetics of Reaction-Bonded Aluminum Oxide Ceramics

Reaction-bonded Al₂O₃ (RBAO) ceramics were fabricated starting from mechanically alloyed Al₂O₃/Al, Al₂O₃/ Al/ZrO₂, and Al₂O₃/Al/ZrO₂/Zr mixtures. Isopressed compacts were heat-treated in air up to 1550°C. Reaction-bonding mechanisms, kinetics, and the influence of ZrO₂ and Zr additions are investigated. Independent of additive, oxidation of Al proceeds both as solid/gas and liquid/gas reaction, and the reaction kinetics follow a parabolic rate law. The reaction rate depends strongly on the particle size of Al. The activation energy of the reaction depends essentially on green density. Below the melting temperature of Al, in samples containing 45 vol% Al and 55 vol% Al₂O₃, it is 112 and 152 kJ/mol at ∼64% and ∼74% TD, respectively, while above the melting temperature, it lies in the range ∼ 26–33 kJ/mol. Zr additions reduce the activation energy to some extent. Samples with only ZrO₂ additions exhibit nearly the same activation energies as ZrO₂-free samples, though ZrO₂ has a very positive effect on the microstructural development in RBAO ceramics. Microstructure evolution and some strength data of RBAO bodies are also reported.     

Microcavity Formation in Alumina Using Ti Templates II: Mechanism and Kinetics

In part I of this work, it was found that titanium (Ti) wire encapsulated within mechanically milled alumina powder and sintered at 1350°C forms potentially useful microcavities due to the consolidation of Kirkendall porosity. Here a series of samples sintered at 1350°C in the range 0–24 h has shown the remarkable way in which these cavities form. The cavity has already started in samples quenched from the top of the heating ramp (0 min at 1350°C). It is surrounded by a diffusion zone ∼300 μm in diameter, which does not change size throughout the firing process although the contents change markedly. The diffusion zone microstructure is initially complex with phase sequence TiO₂/Al₂O₃/TiO₂+Al₂O₃/Al₂TiO₅. Microstructure evolution may be summarized as outward growth of the cavity accompanied by inward growth of the Al₂TiO₅ resulting in a ∼190-μm-diameter cavity surrounded by a 50-μm-thick layer of Al₂TiO₅. The formation of the cavity and surrounding microstructure is discussed although some features, such as the nucleation of Al₂TiO₅ in the part of the diffusion zone furthest from the Ti source and the ring of Al₂O₃, which persists in between Ti-rich parts of the diffusion zone are still poorly understood.     

Development of Surface Compressive Stresses in Zirconia–Alumina Composites by an Ion-Exchange Process

A two-step ion-exchange technique was developed for introducing compressive stresses on the surface of ZrO₂–Al₂O₃ composites. In the first step, a thin layer (∼250 μm) of Na-β"-Al₂O₃ was formed on the surface of the composite by a vapor-phase process at ∼1400°C. In the second step, Na⁺ ions were replaced by K⁺ ions by a heat treatment at ∼385°C for 2 h in a molten KNO₃ bath. Replacement of sodium by potassium led to the creation of surface compressive stresses. The flexural strength and Weibull modulus of ZrO₂–Al₂O₃ composite were ∼915 MPa and 10, respectively, for the as-sintered samples. By contrast, the flexural strength and Weibull modulus were ∼1140 MPa and 26, respectively, for the ion-exchanged samples. A residual surface compressive stress of ∼480 MPa was measured by a strain-gauge technique in K⁺-ion-exchanged samples. The presence of surface compressive stresses also was confirmed using an indentation technique. The technique developed here can be used to introduce compressive stresses on components of virtually any shape.     

Growth of Single-Crystal and Polycrystalline Thin Films of MgAl2O4 and MgFe2O4

Single-crystal and polycrystalline films of Mg-Al₂O₄ and MgFe₂O₄ were formed by two methods on cleavage surfaces of MgO single crystals. In one procedure, aluminum was deposited on MgO by vacuum evaporation. Subsequent heating in air at about 510°C formed a polycrystalline γ-Al₂O₈ film. Above 540°C, the γ-Al₂O, and MgO reacted to form a single-crystal MgAl₂O₄ film with {001} MgAl₂O₄‖{001} MgO. Above 590°C, an additional layer of MgAl₂O₄, which is polycrystalline, formed between the γ-Al₂O₃ and the single-crystal spinel. Polycrystalline Mg-Al₂O₄ formed only when diffusion of Mg²⁺ ions proceeded into the polycrystalline γ-Al₂O₃ region. Corresponding results were obtained for Mg-Fe₂O₄. MgAl₂O₄ films were also formed on cleaved MgO single-crystal substrates by direct evaporation, using an Al₂O₃ crucible as a source. Very slow deposition rates were used with source temperatures of ∼1350°C and substrate temperatures of ∼800°C. Departures from single-crystal character in the films may arise through temperature gradients in the substrate.     

Homogeneous Fabrication of Al2O3–ZrO2–SiC Whisker Composite by Surface-Induced Coating

Al₂O₃–ZrO₂–SiC whisker composites were prepared by surface-induced coating of the precursor for the ZrO₂ phase on the kinetically stable colloid particles of Al₂O₃ and SiC whisker. The fabricated composites were characterized by a uniform spatial distribution of ZrO₂ and SiC whisker phases throughout the Al₂O₃ matrix. The fracture toughness values of the Al₂O₃–15 vol% ZrO₂–20 vol% SiC whisker composites (∼12 MPa.m^1/2) are substantially greater than those of comparable Al₂O₃–SiC whisker composites, indicating that both the toughening resulting from the process zone mechanism and that caused by the reinforced SiC whiskers work simultaneously in hot-pressed composites.     

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.     

Crystallization of Pseudo-orthorhombic Anorthite on Basal Sapphire

Anorthite-glass films were grown on basal Al₂O₃ substrates using pulsed-laser deposition. The substrates were cleaned and annealed in air at 1400°C to produce crystallographically flat (0001) terraces. The films were deposited in an oxidizing environment. X-ray microanalysis confirmed the composition of the glass films to be close to that of anorthite (CaO·Al₂O₃·2SiO₂). Although anorthite usually has triclinic symmetry, subsequent crystallization of these films in air at 1200°C resulted in the formation of pseudo-orthorhombic CaAl₂Si₂O₈ ( o -anorthite), a known metastable form of the mineral. Microstructural characterization was performed using visible-light microscopy, scanning electron microscopy, and transmission electron microscopy. The films dewetted the substrate either before or after crystallization to form o -anorthite islands which had strong orientation relationships to the Al₂O₃ substrate. The epitaxy of the o -anorthite islands was accompanied by a small lattice mismatch parallel to the substrate plane. The formation of three orientational variants is consistent with the symmetry of the basal Al₂O₃ surface. The dislocation network observed at the o -anorthite/Al₂O₃ interface indicates that nucleation and growth of the anorthite occurs directly on the substrate surface without an intervening interfacial amorphous layer. The study of anorthite-glass films is important because they are present in liquid-phase-sintered Al₂O₃, and may be devitrified by postsintering heat treatments.     

Optimization of a Nanoparticle Suspension for Freeze Casting

Poly(acrylic acid) (PAA) dispersant concentration, suspension pH, and Al₂O₃ solids loading effects on PAA adsorption onto Al₂O₃ nanoparticles were studied; the stability and rheology of the Al₂O₃ nanoparticle suspensions were examined. The most desirable suspension conditions were 7.5–9.5 for pH and 2.00–2.25 wt% of Al₂O₃ for the PAA concentration. Electrical double-layer thickness and PAA adsorption layer thickness comparison showed that electrosteric stabilization was dominant. 45.0 vol% Al₂O₃ solids loading can be achieved for freeze casting. The maximum solids loading was predicted to be 50.7 vol%. The freeze-cast sample showed that pre-rest before freezing was critical for achieving desirable microstructures.     

Zirconia/Alumina Functionally Gradiented Composites by Electrophoretic Deposition Techniques

An electrophoretic deposition and sintering route was used to prepare YSZ/Al₂O₃ composites with a compositional gradient. The YSZ content was continuously decreased from the YSZ-rich surface to the Al₂O₃-rich surface, Microstructural and Vickers hardness (16–24 GPa) evidence tracked the compositional development, and the indentation fracture toughness was found to vary across the section (10–3 MPa·m^1/2).     

Effect of Alumina on Sintering Behavior and Electrical Conductivity of High-Purity Yttria-Stabilized Zirconia

The sintering behavior and electrical conductivity of high-purity 8-mol% Y₂O₃-stabilized ZrO₂ (8YSZ) with Al₂O₃ additions were investigated. The addition of 1 wt% AI₂O₃ to 8YSZ provided dense, sintered samples with 9.1% relative density at 1400°C without a holding time. Addition of 1 wt% SiO₂ enhanced the sinterability of 8YSZ. Na₂O addition of 0.1 wt% remarkably lowered it. Electrical conductivity at 1000°C in air increased slightly with increased Ai₂O₃ content up to 1 wt% and then monotonously decreased. 8YSZ with 1 wt% AI₂O₃ showed the maximum conductivity of 0.16 S/cm at 1000°C.     

Improvement of the Dispersion of Al2O3 Slurries Using EDTA-4Na

Polyacrylic acid (PAA) is known to be an effective dispersant for Al₂O₃ powder in aqueous media. However, at high solid loading (>55 vol%), the dispersion of the Al₂O₃ suspensions became difficult with only PAA as a dispersant. In this paper, ethylenediaminetetraacetic acid, tetrasodium salt, dihydrate (EDTA-4Na) was introduced to improve the dispersion of the Al₂O₃ suspensions. With the aid of EDTA-4Na, the adsorption amount of sodium polyacrylic acid (PAA-Na) increased, while the apparent viscosity of 60 vol% Al₂O₃ slurries decreased significantly. Particle size measurements showed that EDTA-4Na could help to reduce larger agglomerates, possibly by modifying the adsorbed layer thickness. The interactions between EDTA-4Na and PAA-Na were studied using Fourier-transform infrared spectroscopy analysis. Results showed that it was possible to introduce EDTA-4Na as the second dispersant to improve the dispersion of high solid content Al₂O₃ slurries.     

Solid Solution Range and Microstructures of Melt-Grown Mullite

The solid solution range of melt-grown mullite was examined by crystal-chemical methods. The maximum Al₂O₃ content as determined by EDX was ∼83.6 wt%, 75 mol%, or the nominal composition 3Al₂O₃.SiO₂. For samples of overall composition 81 to 83 wt% Al₂O₃, extra lines indicating crystallographic superstructure appeared in Guinier X-ray patterns. The corresponding TEM microstructure consisted of a mullite matrix finely twinned on (001), the twins being 0.02 to 0.10 μm wide, with oriented exsolution of α-A1₂O₃, often twinned, also being present. The analogy between mullite superstructure and that of plagioclase feldspars, as well as the relevance of these findings to the SiO₂-Al₂O₃ metastable phase equilibria are discussed.     

Aqueous Colloidal Processing of ZTA Composites

Two different zirconia-alumina composites, ZTA-30 (70 wt% Al₂O₃+30 wt% ZrO₂) and ZTA-60 (40 wt% Al₂O₃+60 wt% ZrO₂), with potential for orthopedic applications, were processed in aqueous media and consolidated by slip casting (SC), hydrolysis-assisted solidification (HAS), and gelcasting (GC) from suspensions containing 50 vol% solids loading. For comparison purposes, the same ceramic compositions were also consolidated by die pressing of freeze-dried granules (FG). In the HAS process, 5 wt% of Al₂O₃ in the precursor mixture was replaced by equivalent amounts of AlN to promote the consolidation of the suspensions. Ceramics consolidated via GC exhibited higher green (three-point bend) strengths (∼17 MPa) than those consolidated by other techniques. Further, these ceramics also exhibited superior fracture toughness and flexural strength properties after sintering for 1 h at 1600°C in comparison with those consolidated by other techniques, including conventional die pressing (FG).     

Orientation and Grain Boundary Microstructure of Alumina in Al/Al2O3 Composites Produced by Reactive Metal Penetration

The orientation and grain boundary microstructure of alumina in reactive metal penetration Al/Al₂O₃ composites are studied using orientation imaging microscopy and the results are compared with those of sintered polycrystalline Al₂O₃. The interconnected Al₂O₃ in the composite material is separated by Σ3 boundaries (twins) with a 60° rotation around the [0001] direction. A high frequency (∼100%) of Σ3 coincidence boundaries in composite alumina is remarkable since only ∼12% of boundaries in a sintered polycrystalline Al₂O₃ are of special nature. The coincidence boundaries in the in situ alumina grow in a coherent and faceted manner.