Molten Glass Corrosion Resistance of Immersed Combustion-Heating Tube Materials in Soda-Lime-Silicate Glass

The corrosion resistance of molybdenum, molybdenum disilicide, and a SiC_(p)/Al₂O₃ composite to molten soda-lime-silicate glass was studied. The ASTM-C621–84 corrosion test method was modified because of inherent inaccuracies in the method and Si attack of platinum crucibles. Specimen-glass interfacial regions were characterized using XRD, SEM, and EDS. After 48 h of exposure at 1565°C, the half-down corrosion recessions of Mo, MoSi₂, and SiC_(P)/Al₂O₃ were 0.11, 0.316, and 0.26 mm, respectively. Mo oxidized to form a MoO₂ surface scale which cracked, allowing glass seepage and further oxidation. Silicon was leached out of MoSi₂ into the glass, leaving a Mo₅Si₃ interface and particles of Mo near the interface. For the SiC_(P)/Al₂O₃ composite, bubbles observed at the interfacial regions formed from oxidation of SiC to form CO. Thermodynamic modeling corroborated these experimental observations.     

Growth of Tabular α-Al2O3 Grains on Porous Alumina Substrate

Gradient, porous alumina ceramics were prepared with the characteristics of microsized tabular α-Al₂O₃ grains grown on a surface with a fine interlocking feature. The samples were formed by spin-coating diphasic aluminosilicate sol on porous alumina substrates. The sol consisted of nano-sized pseudo-boehmite (AlOOH) and hydrolyzed tetraethyl orthosilicate [Si(OC₂H₅)₄]. After drying and sintering at 1150°–1450°C, the crystallographic and chemical properties of the porous structures were investigated by analytical electron microscopy. The results show that the formation of tabular α-Al₂O₃ grains is controlled by the dissolution of fine Al₂O₃ in the diphasic material at the interface. The nucleation and growth of tabular α-Al₂O₃ grains proceeds heterogeneously at the Al₂O₃/glass interface by ripening nano-sized Al₂O₃ particles.     

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.     

Strengthening of Porous Alumina by Pulse Electric Current Sintering and Nanocomposite Processing

Porous Al₂O₃ and SiC–dispersed-Al₂O₃ (Al₂O₃/SiC) nanocomposites with improved mechanical properties were fabricated using pulse electric current sintering (PECS). Microstructures with fine grains and enhanced neck growth, as well as high fracture strength, could be achieved via PECS of Al₂O₃. The incorporation of fine SiC particles into an Al₂O₃ matrix significantly increased the fracture strength of porous Al₂O₃. Based on microstructural observations, it was revealed that the refinement of Al₂O₃ grains and neck growth occurred by PECS and nanocomposite processing.     

Equilibrium Amorphous Silicon–Calcium–Oxygen Films at Interfaces in Copper–Alumina Composites Prepared by Melt Infiltration

The microstructure of copper–alumina (Cu-Al₂O₃) composites that have been prepared via the melt infiltration of liquid copper into porous alumina preforms was studied in detail, using various transmission electron microscopy (TEM) techniques. Two different samples—with open pore diameters of 0.2 and 0.8 μm—were investigated. For both specimens, a single crystalline copper network that extended throughout the open porosity of the alumina preform was observed. An amorphous glass phase that contained silicon and calcium was observed at the Al₂O₃/Cu/Al₂O₃ triple junctions. The diameters of these amorphous pockets, which were strongly faceted along the Al₂O₃ grains, were up to 20 and 100 nm for the initial pore sizes of 0.2 and 0.8 μm, respectively. A glass phase that contained silicon and calcium also was present at the Cu/Al₂O₃ interfaces, whereas the Al₂O₃ boundaries remained dry. Detailed high-resolution transmission electron microscopy investigations have shown that the interfacial glass phase at the Cu/Al₂O₃ interfaces exhibited a uniform equilibrium film thickness along the interface region. However, the interfacial film thickness was dependent on the orientation of the Al₂O₃ grain, and its value varied from 0.4 nm for Al₂O₃ rhombohedral-plane termination ((1¯012)) up to 1 nm for Al₂O₃ basal-plane termination ((0001)).     

Stability of Mullite as Derived from Equilibria in the System CoO-Al2O3-SiO2

The free energy of reaction for the formation of mullite from its oxide components was derived from equilibrium studies in the system CoO-Al₂O₃-SiO₂. Within this system there appears, at solidus temperature in a certain composition area, the phase assemblage mullite + silica + spinel (= cobalt aluminate) + liquid. Determination of the oxygen pressure of a gas phase at which metallic cobalt precipitates from this phase assemblage and from the phase assemblage spinel (= cobalt aluminate) + corundum in the system CoO-Al₂O₃ permits calculation of ΔG° for the reaction 3Al₂O₃+ 2SiO₂= Al₆Si₂O₁₃. The value obtained at 1422°C is -5.8 kcal.     

Microstructural Changes in β-Silicon Nitride Grains upon Crystallizing the Grain-Boundary Glass

Crystallizing the grain-boundary glass of a liquid-phase-sintered Si₃N₄ ceramic for 2 h or less at 1500° led to formation of δ-Y₂Si₂O₇. After 5 h at 1500°, the δ-Y₂Si₂O₇ had transformed to β-Y₂Si₂O₇ with a concurrent dramatic increase in dislocation density within β-Si₃N₄ grains. Reasons for the increased dislocation density are discussed. Annealing for 20 h at 1500° reduced dislocation densities to the levels found in as-sintered material.     

Forming and Sintering of in Situ Alumina Composite with Hydraulic Inorganic Binder

Two types of barium aluminate binders were prepared by heat treatment of barium aluminate precursors, synthesized by using solution processes, at low temperatures ( T < 500°C). One was barium monoaluminate (BaAl₂O₄), and the other was barium aluminate binder (BAH binder) composed of amorphous phase, barium aluminate monohydrate (BaAl₂O₄·H₂O), and BaAl₂O₄. The setting time of the BAH binder was controlled by adjusting the heat-treatment temperature of the BAH binder precursor. The addition of the synthesized BaAl₂O₄ powders to Al₂O₃ powders improved the bending strength of Al₂O₃ matrix green bodies. The synthesized BaAl₂O₄ powders led to the in situ forming of barium hexaaluminate (BaO· x Al₂O₃, x = 6.9: BA6) platelets in the matrix by reacting with Al₂O₃ during sintering. The formed BA6 platelets inhibited the grain growth of the matrix Al₂O₃ grains.     

Microstructures and Properties of Three Composites of Alumina, Mullite, and Monoclinic SrAl2Si2O8

Three composites that were 96% alumina were mixed and uniaxially dry-pressed into bars and pellets; all had monoclinic SrAl₂Si₂O₈ as an intergranular phase. The diffraction patterns, microstructure, density, dielectric properties, strength, and toughness were measured. The first composition, which contained crystalline SrCO₃, Al₂O₃, and SiO₂, in a 1:1:2 molar ratio, as the 4% component, densified but was generally inferior to the second and third compositions, which contained strontium aluminosilicate (SrAl _x Si _y O _z , SAS) glass as the 4% component, in terms of mechanical properties, defects, and monoclinic SrAl₂Si₂O₈ transformation. The second composition, which lacked B₂O₃, was very tough and was comparable to commercial alumina, in terms of the dielectric constant. The third, which contained 0.068% of B₂O₃ that was dissolved in the SAS glass as a sintering aid, had high strength and toughness and no macroscopically visible defects. Mullite formed, in addition to monoclinic SrAl₂Si₂O₈ in all three composites. Alumina–monoclinic SrAl₂Si₂O₈ composites of the third composition had room-temperature properties that were comparable to commercial aluminas that contained 96% alumina and also had potential for mechanical and refractory applications.     

Crystallization of MgO-Al2O3-SiO2-ZrO2 Glasses

The crystallization of MgO-Al₂O₃-SiO₂-ZrO₂ glasses at 1000°C was studied. Isothermal heat treatments of a cordierite-based glass (2MgO.2Al₂O₃.5SiO₂= Mg₂Al₄Si₅O₁₈) with 7 wt% ZrO₂ produced surface crystallization of α-cordierite and tetragonal ZrO₂ ( t -ZrO₂). These phases advanced into the glass by cocrystallization of t -ZrO₂ rods in an α-cordierite matrix with a well-defined orientation relation. The t -ZrO₂ rods were unstable with respect to diffusional breakup (a Rayleigh instability) and decomposed into rows of aligned ellipsoidal and spheroidal particles. The t -ZrO₂ was very resistant to transformation to monoclinic symmetry. With a similar glass containing 15 wt% ZrO₂, surface crystallization of α-cordierite and t -ZrO₂ was accompanied by internal crystallization of t -ZrO₂ dendrites. Transformation of the dendrites to mono-clinic symmetry was observed under some conditions.     

Intragranular Particle Residual Stress Strengthening of Al2O3–SiC Nanocomposites

Al₂O₃/SiC ceramic nanocomposites were fabricated from nanocrystalline Al₂O₃ (10 nm in diameter) and SiC (15 nm in diameter) powders, and a theoretical model of intragranular particle residual stress strengthening was investigated. The SiC nanoparticles in the Al₂O₃ grains create a normal compressive stress at the grain boundaries and a tangential tensile stress in the Al₂O₃ grains, resulting in the "strengthening" of the grain boundaries and "weakening" of the grains. The model gives a good explanation of the experimental results of the authors and others which are difficult to be explained by the existing strengthening models, i.e. the maximum strength is normally achieved at about 5 vol% of SiC particles in the Al₂O₃–SiC ceramic nanocomposites. According to the model, there exists an optimum amount of SiC for strengthening, below which the grain boundaries are not fully "strengthened" and the fracture is mainly intergranular, above which the grains are "weakened" too much and the fracture is mainly transgranular, and at which the fracture is a mixture of intergranular and transgranular.     

Indentation of Silicate-Glass Films on Al2O3 Substrates

The deformation of thin layers of glass on crystalline materials has been examined using newly developed experimental methods for nanomechanical testing. Continuous films of anorthite (CaAl₂Si₂O₈) were deposited onto Al₂O₃ surfaces by pulsed-laser deposition. Mechanical properties such as Young's modulus and hardness were probed with a high-resolution depth-sensing indentation instrument. Nanomechanical testing, combined with AFM in situ imaging of the deformed regions, allowed force-displacement measurements and imaging of the same regions of the specimen before and immediately after indentation. This new technique eliminates any uncertainty in locating the indentation after unloading. Emphasis has been placed on examining how the Al₂O₃ substrate crystallographic orientation will affect mechanical composite response of silicate-glass film/Al₂O₃ system.     

Conversion from β-Ce2O3·11Al2O3 to α-Al2O3 in Tetragonal ZrO2 Matrix

Composites of β-Ce₂O₃·11Al₂O₃ and tetragonal ZrO₂ were fabricated by a reductive atmosphere sintering of mixed powders of CeO₂, ZrO₂ (2 mol% Y₂O₃), and Al₂O₃. The composites had microstructures composed of elongated grains of β-Ce₂O₃·11Al₂O₃ in a Y-TZP matrix. The β-Ce₂O₃·11Al₂O₃ decomposed to α-Al₂O₃ and CeO₂ by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce₂O₃·11Al₂O₃ divided into several grains of α-Al₂O₃ and ZrO₂ doped with Y₂O₃ and CeO₂. High-temperature bending strength of the oxygen-annealed α-Al₂O₃ composite was comparable to the β-Ce₂O₃·11Al₂O₃ composite before annealing.     

Nucleation and Growth of β'-SiAlON

Elongated β'-SiAlON grains grown from several finegrained Y_m/3Si_12(m+n)Al_m+nO_nN_16–r compositions with α-Si₃N₄, AlN, Al₂O₃, and Y₂O₃ starting materials have been examined. These grains have large aspect ratios and are oriented along the [0001] axis. TEM structural and chemical analysis suggests that they are nucleated from various seed crystals, which can be α-Si₃N₄, β-Si₃N₄, or other β'-SiAlON. The β'-SiAlON seed and the initial precipitation on β-Si₃N₄ show a higher content of Al and O, indicating that a large transient supersaturation of Al and O in the liquid is instrumental for β'-SiAlON formation, whereas subsequent growth proceeds under a much lower driving force. The misfit between phases is accommodated by interfacial dislocations ( c -type and a -type). Fully grown β'-SiAlON grains usually contain several variants independently nucleated from the same seed. In particular, the two alternative α/β phase-matching possibilities result in two [0001] growth habits separated by a twin boundary.     

Effects of Impurities and Manufacturing Methods on the Devitrification of Silica Fibers

Devitrification resistance is one of the most important properties of silica fibers. The devitrification behavior of silica fibers made by the acid leaching method, the sol-gel method, and the high-purity silica fusing method, and the effects of small amounts of Al₂O₃ and B₂O₃, were studied by using X-ray diffraction. The results indicated that the devitrification tendency of silica fibers made by the high-purity silica fusing method is apparently greater than that of the fibers made by the other two methods. A small amount of B₂O₃ has the effect of retarding devitrification, while Al₂O₃ promotes devitrification of the fibers. The combined effect of Al₂O₃ and B₂O₃ is to strongly enhance the devitrification of silica fibers. The results are discussed in the context of structural analysis.     

Fabrication of Fine YAG-Particulate-Dispersed Alumina Fiber

This paper involves novel fabrication processes for polycrystalline α-Al₂O₃-matrix composite fibers that contain nanosized yttrium aluminum garnet (YAG) particles. Dense α-Al₂O₃/YAG nanocomposite fibers with a fine and homogeneous microstructure can be successfully fabricated via a modified sol-gel process and α-Al₂O₃ seed-particle addition. YAG nanoparticles have been homogeneously dispersed within Al₂O₃-matrix grains as well as at grain boundaries. Effects of α-Al₂O₃ seed particles and YAG nanodispersions on crystallization and microstructure development of nanocomposite fibers are discussed.     

Formation of a Highly Protective Amorphous Aluminum Silicate Layer on Steel during Granite–Dry Steam Interaction

Formation of an amorphous Al₂Si₂O₅ layer on a steel pipe surface, after granite–dry steam interaction in the presence of copper, has been determined by XRD analysis and SEM (with energy dispersive X-ray microanalysis). Kinetic data indicate high-protective properties of the Al₂Si₂O₅ layer, which formed a three-dimension substrate ∼5 μm thick, after 100 h exposure.     

Hercynite-Free Ceramic Liner for Composite Steel Pipe Made Using a Self-Propagating High-Temperature Synthesis Gravitational-Thermite Process

A ceramic-lined steel pipe was fabricated using a self-propagating high-temperature synthesis gravitational-thermite process. Some rare-earth oxides and glass powders were added to the aluminothermite. The experimental results showed that the ceramic liner consisted of three phases: corundum (Al₂O₃), glass, and iron particles. Neither hercynite (FeO·Al₂O₃) nor iron ions existed in the liner of the composite pipe.     

γ-Y2Si2O7, a Machinable Silicate Ceramic: Mechanical Properties and Machinability

In this paper, the mechanical properties of bulk single-phase γ-Y₂Si₂O₇ ceramic are reported. γ-Y₂Si₂O₇ exhibits low shear modulus, excellent damage tolerance, and thus has a good machinability ready for metal working tools. To understand the underlying mechanism of machinability, drilling test, Hertzian contact test, and density functional theory (DFT) calculation are employed. Hertzian contact test demonstrates that γ-Y₂Si₂O₇ is a "quasi-plastic" ceramic and the intrinsically weak interfaces contribute to its machinability. Crystal structure characteristics and DFT calculations of γ-Y₂Si₂O₇ suggest that some weakly bonded planes, which involve Y–O bonds that can be easily broken, are the sources of the low shear deformation resistance and good machinability.     

Alumina Sol-Assisted Sintering of SiC—Al2O3 Composites

The composite sol—gel (CSG) technology has been utilized to process SiC—Al₂O₃ ceramic/ceramic particulate reinforced composites with a high content of SiC (up to 50 vol%). Alumina sol, resulting from hydrolysis of aluminum isopropoxide, has been utilized as a dispersant and sintering additive. Microstructures of the composites (investigated using TEM) show the sol-originating phase present at grain boundaries, in particular at triple junctions, irrespective of the type of grain (i.e., SiC or Al₂O₃). It is hypothesized that the alumina film originating from the alumina sol reacts with SiO₂ film on the surface of SiC grains to form mullite or alumina-rich mullite-glass mixed phase. Effectively, SiC particles interconnect through this phase, facilitating formation of a dense body even at very high SiC content. Comparative sinterability studies were performed on similar SiC—Al₂O₃ compositions free of alumina sol. It appears that in these systems the large fraction of directly contacting SiC—SiC grains prevents full densification of the composite. The microhardness of SiC—Al₂O₃ sol—gel composites has been measured as a function of the content of SiC and sintering temperature. The highest microhardness of 22.9 GPa has been obtained for the composition 50 vol% SiC—50 vol% Al₂O₃, sintered at 1850°C.