Aluminum Infiltration into Molybdenum Silicide Preforms

The presence of Mo₅Si₃ in MoSi₂ preforms hinders the reactive infiltration of aluminum. To understand the role of Mo₅Si₃, the kinetics of aluminum infiltration into pure Mo₅Si₃ is studied. Irrespective of the initial composition (MoSi₂ or Mo₅Si₃) of the preform, the final product always contains Mo(Al,Si)₂. However, the aluminum content in the two cases is different: when the preform is MoSi₂, the aluminum content is 14–18 at.%, and, when the preform is Mo₅Si₃, the aluminum content is 25–27 at.%. The activation energy for the reactive infiltration of aluminum into the Mo₅Si₃ preform is ∼26 kJ/mol.     

Reactive Hot-Pressed Alumina–Boron Nitride Composites with Y2O3 Sintering Additive

The effect of Y₂O₃ addition (0–5 wt%) on the densification and properties of reactive hot-pressed alumina (Al₂O₃)–boron nitride composites based on the reaction between aluminum borate (2Al₂O₃·B₂O₃) and aluminum nitride (AlN) was investigated. The densification process was very sensitive to the amount of Y₂O₃. Compared with a low relative density of 79.3 theoretical density (TD)% for material with no Y₂O₃ addition, the material density reached 98.6 TD% with 0.25% Y₂O₃ addition. High Y₂O₃ additions resulted in the formation of a new phase Al₅Y₃O₁₂. The grain growth of the Al₂O₃ matrix was promoted by the Y₂O₃ addition. Owing to the high density and the small Al₂O₃ particle size the sample with 0.25% Y₂O₃ addition demonstrated the highest bending strength of 540 MPa.     

Aluminum Nitride Prepared by Nitridation of Aluminum Oxide Precursors

Aluminum nitride (AlN) powders were prepared from the oxide precursors aluminum nitrate, aluminum hydroxide, aluminum 2-ethyl-hexanoate, and aluminum isopropoxide (i.e., Al(NO₃)₃, Al(OH)₃, Al(OH)(O₂CCH(C₂H₅)(C₄H₉))₂, and Al(OCH(CH₃)₂)₃). Pyrolyses were performed in flowing dry NH₃ and N₂ at 1000°–1500°C. For comparison, the nitride precursors aluminum dimethylamide (Al(N(CH₃)₂)₃) and aluminum trimethylamino alane (AlH₃·N(CH₃)₃) were exposed to the same nitridation conditions. Products were investigated using XRD, TEM, EDX, SEM, and elemental analysis. The results showed that nitridation was primarily controlled by the water:ammonia ratio in the atmosphere. Single-phase AlN powders were obtained from all oxide precursors. Complete nitridation was not obtained using pure N₂, even for the non-oxide precursors.     

TEM Investigation of Impurity Phases and the Penetration of Liquid Aluminum in Hot Isostatically Pressed TiB2 Compacts

The microstructure of materials compacted from commercially produced TiB₂ powders was investigated using transmission electron microscopy. A number of impurity phases that are introduced during the various processing stages were identified. After exposure to liquid aluminum, grain boundaries and triple junctions of TiB₂ were found to be penetrated by aluminum. In the penetrated regions pure aluminum, two aluminum oxides, and an (Al₂OC)₁- _x (AlN) _x . phase were identified. A SiO₂ glass phase, introduced during hot isostatic pressing, is believed to be responsible for the formation of alumina. None of the other impurity phases were found to react with aluminum.     

Preparation of Sinterable Cubic Aluminum Oxynitride by the Carbothermal Nitridation of Aluminum Oxide

Sinterable cubic aluminum oxy nitride (ALON) has been prepared by carbon reduction of aluminum oxide inflowing nitrogen. Three different sources of Al₂O₃ (A1₂O₃ from clay, commercial A1₂O₃, and A1₂O₃ derived from AlCl₃.6H₂O) and two different sources of carbon (carbon black and starch) were used. Pressed pellets of ALON powder were sintered in N₂ at 1950°C greater than 95% of theoretical density.     

Pressureless Sintering of Dense Si3N4 and Si3N4/SiC Composites with Nitrate Additives

The effect of aluminum and yttrium nitrate additives on the densification of monolithic Si₃N₄ and a Si₃N₄/SiC composite by pressureless sintering was compared with that of oxide additives. The surfaces of Si₃N₄ particles milled with aluminum and yttrium nitrates, which were added as methanol solutions, were coated with a different layer containing Al and Y from that of Si₃N₄ particles milled with oxide additives. Monolithic Si₃N₄ could be sintered to 94% of theoretical density (TD) at 1500°C with nitrate additives. The sintering temperature was about 100°C lower than the case with oxide additives. After pressureless sintering at 1750°C for 2 h in N₂, the bulk density of a Si₃N₄/20 wt% SiC composite reached 95% TD with nitrate additives.     

Iron as an Oxygen Tracer at the Aluminum-Alumina Interface

During wetting experiments of pure aluminum on the (0001) plane of sapphire, an aluminum sample was unintentionally contaminated with 31 atoms/parts per million of iron. Transmission electron microscopy investigations of the sample after cooling showed the formation of FeAlO₃ at the aluminum-Al₂O₃ interface. Formation of FeAlO₃ confirms the existence of a high oxygen activity at the liquid aluminum-sapphire interface, which is considered to be the reason for the strong adhesion between aluminum and Al₂O₃. A low indexed orientation relationship between FeAlO₃ and Al₂O₃ was determined, where [22¯1]_FeAlO3‖ [11¯00]_Al2O₃ and (110)_FeAlO3‖ (0001)_Al2O₃.     

Effects of Aluminum Concentration on the Oxidation Behaviors of Reactively Sputtered TiAlN Films

Polycrystalline TiAlN films were deposited on a substrate of high-speed steel via a radio-frequency-bias reactive-sputtering process. The effects of aluminum concentration (0–60 at.%) on the high-temperature oxidation behavior of TiAlN films were explored by using in situ thermogravimetric analysis and high-temperature X-ray diffractometry. The composition and distribution of the oxidizing layers over TiAlN films were investigated. Results indicated that the oxidation resistance increased as the aluminum concentration increased. The type and location of oxidizing phases also were dependent on the aluminum concentration. Three major oxides-i.e., Al₂O₃, TiO₂, and TiO-were observed. The thickness of the Al₂O₃ layer increased and the TiO₂ gradually changed to TiO as the aluminum content increased. Thermodynamic calculations were compared to experimental observations, and they showed good agreement.     

Synthesis of Novel Niobium Aluminide-Based Composites

A reactive sintering process has been used to produce almost fully dense composites with interpenetrating networks of NbAl₃ and Al₂O₃. The process involves the reaction synthesis of niobium aluminides and Al₂O₃ from compacts of intensively milled aluminum and Nb₂O₅ powder mixtures. During carefully controlled heating under an inert atmosphere, the oxide reduction by aluminum to form niobium aluminides and Al₂O₃ proceeds at temperatures below the melting point of aluminum. At temperatures of >1000°C, the reaction-formed niobium aluminides and Al₂O₃ sinter. The present paper discusses processing parameters, such as attrition milling, the heating cycle, and the metal:ceramic ratio in the starting mixture, that control microstructure development and mechanical properties.     

Low-Temperature Synthesis of BaAl2O4/Aluminum-Bearing Composites by the Oxidation of Solid Metal-Bearing Precursors

BaAl₂O₄/aluminum-bearing composites have been synthesized via the low-temperature oxidation of Ba-Al precursors. Ba-Al powder mixtures that were prepared via high-energy vibratory milling were uniaxially pressed into bar-shaped specimens that were then exposed to a series of heat treatments in pure, flowing oxygen at temperatures up to 640°C. Oxidation at a temperature of 300°C resulted in the formation of barium peroxide (BaO₂). Additional heat treatment at a temperature of 550°C resulted in the consumption of BaO₂ and some aluminum to yield BaAl₂O₄ and Al₄Ba. The oxidation of Al₄Ba at a temperature of 640°C yielded additional BaAl₂O₄. Microstructural analyses revealed that a well-dispersed, co-continuous mixture of Al₂O₃-excess BaAl₂O₄ and 99.5% pure aluminum was produced.     

Molecular Dynamics Simulations of Calcium Aluminosilicate Intergranular Films on (0001) Al2O3 Facets

Molecular dynamics simulations of intergranular films (IGF) containing SiO₂, Al₂O₃, and CaO in contact with two surface terminations of the basal plane of Al₂O₃ were performed to model faceted grain boundaries in sintered Al₂O₃. In both the aluminum-terminated and the oxygen-terminated crystal surfaces, cage structures were observed in the intergranular film at the interface. Complete epitaxy of aluminum and silicon cations from the IGF was observed on the oxygen termination of the crystal surface. Calcium segregated to specific sites at the interface in all systems studied. Segregation of aluminum ions to the interface was observed from IGFs with high Al₂O₃ content. High-SiO₂ IGFs impeded the growth of the first of the two aluminum layers parallel to the basal plane, whereas CaO promoted the growth of this layer. However, CaO impeded the growth of the second aluminum layer parallel to the basal plane.     

Phase Relations Applicable to the Garnet Phase Y3Fe4AlO12

Phase relations of the system Fe₂O₃-Y₂O₃-Al₂O₃ were studied at 1500° and 1525°C in air and in oxygen at 1 atm. Isothermal-isobaric sections indicate that the liquids phase field at 1500°C is larger in oxygen than in air. In either atmosphere, at this temperature, the composition of the garnet phase in equilibrium with a liquid is enriched in aluminum relative to the liquid. In the same manner, yttrium orthoferrite is enriched in aluminum relative to garnet in equilibrium between these two phases. The limit of solid solubility of excess iron-aluminum and/or yttrium in the garnet phase Y₃Fe₄AlO₁₂ was determined by X-ray diffraction techniques to be 0.2 ± 0.05 mole % Y₂.O_3.     

Physical Properties and Structure of rf-Sputtered Amorphous Films in the System Al2O3–Y2O3

Amorphous films in the system Al₂O₃–Y₂O₃ were prepared by the rf sputtering method in the range of 0–76 mol% Y₂O₃, and their density, refractive index, and elastic constants were measured. All of the physical properties of the amorphous Al₂O₃–Y₂O₃ films had a similar compositional dependence; that is, they increased continuously, but not linearly with increasing Y₂O₃ content. To confirm the coordination states of aluminum and yttrium ions in the amorphous Al₂O₃–Y₂O₃ films, the Al K α X-ray emission spectra and the X-ray absorption near edge structures (XANES) were measured. The average coordination number of aluminum ions in the amorphous films containing up to about 40 mol% Y₂O₃ content was 5, that is a mixture of 4-fold- and 6-fold-coordinated states. In the region of more than about 50 mol% Y₂O₃, the fraction of the 6-fold-coordinated aluminum ions increased with increasing Y₂O₃ content, while the results led to the conclusion that the coordination number of yttrium ions was always 6, regardless of composition. These results indicate that, in amorphous films in the system Al₂O₃–Y₂O₃, the change of the coordination state of aluminum ions has an important effect on physical properties.     

Stability of ZrO2 Phases in Ultrafine ZrO2-Al2O3 Mixtures

Mixtures of ultrafine monoclinic zirconia and aluminum hydroxide were prepared by adding NH₄OH to hydrolyzed zirconia sols containing varied amounts of aluminum sulfate. The mixtures were heat-treated at 500° to 1300°C. The relative stability of monoclinic and tetragonal ZrO₂ in these ultrafine particles was studied by X-ray diffractometry. Growth of ZrO₂ crystallites at elevated temperatures was strongly inhibited by Al₂O₃ derived from aluminum hydroxide. The monoclinic-to-tetragonal phase transformation temperature was lowered to ∼500°C in the mixture containing 10 vol% Al₂O₃, and the tetragonal phase was retained on cooling to room temperature. This behavior may be explained on the basis of Garvie's hypothesis that the surface free energy of tetragonal ZrO₂ is lower than that of the monoclinic form. With increasing A1₂O₃ content, however, the transformation temperature gradually increased, although the growth of ZrO₂ particles was inhibited; this was found to be affected by water vapor formed from aluminum hydroxide on heating. The presence of atmospheric water vapor elevates the transformation temperature for ultrafine ZrO₂. The reverse tetragonal-to-monoclinic transformation is promoted by water vapor at lower temperatures. Accordingly, it was concluded that the monoclinic phase in fine ZrO₂ particles was stabilized by the presence of water vapor, which probably decreases the surface energy.     

Synthesis of Al4SiC4

The conditions necessary for synthesizing Al₄SiC₄ from mixtures of aluminum, silicon, and carbon and kaolin, aluminum, and carbon, as starting materials, were examined in the present study. The standard Gibbs energy of formation for the thermodynamic reaction SiC( s ) + Al₄C₃( s ) = Al₄SiC₄( s ) changed from positive to negative at 1106°C. SiC and Al₄C₃ formed as intermediate products when the mixture of aluminum, silicon, and carbon was heated in argon gas, and Al₄SiC₄ then formed by reaction of the SiC and Al₄C₃ at >1200°C. Al₄C₃, SiO₂, Al₂O₃, SiC, and Al₄O₄C formed as intermediate products when the mixture of kaolin, aluminum, and carbon was heated under vacuum, and Al₄SiC₄ formed from a reaction of those intermediate products at >1600°C.     

Mullite–Aluminum Phosphate Laminated Composite Fabricated by Tape Casting

Mullite–aluminum phosphate (3Al₂O₃·2SiO₂/AlPO₄) laminated composites were fabricated by tape casting. AlPO₄ had a density of 1.56 g/cm³, which corresponds to 61% of theoretical density, and a bending strength of 1.5 MPa after sintering at 1600°C for 10 h. The aluminum phosphate functioned as a porous, weak, and chemically stable interphase which was able to deflect cracks in a laminated composite. To increase the strength of the weak interphase material, 10 and 30 vol% of mullite were added.     

Microstructure and Properties of Co2+: ZnAl2O4/SiO2 Nanocomposite Glasses Prepared by Sol–Gel Method

Transparent bulk Co²⁺: ZnAl₂O₄/SiO₂ nanocomposites containing nanocrystalline Co²⁺: ZnAl₂O₄ dispersed in silica glass matrix were obtained by the sol–gel method. The gels of composition 89SiO₂–6Al₂O₃–5ZnO− x CoO ( x =0.2, 0.4, 0.6, 0.8, 1.0) (mol%) were prepared at room temperature by using two different aluminum salts, aluminum nitrate and aluminum alkoxide (aluminum-iso-propoxide, Al(OPrⁱ)₃), as starting materials. The transparent gels were converted to the crystalline phase of gahnite by heating above 900°C. The microstructural evolution of gels was characterized. The effect of Co²⁺ concentration on spectroscopic properties was also discussed. Co²⁺: ZnAl₂O₄ nanocrystals dispersed in the SiO₂-based glass are formed at lower heat-treatment temperature and shorter heating time by using Al(OPrⁱ)₃ as raw material.     

27Al Nuclear Magnetic Resonance of Glassy and Crystalline Zr(1−x)AlxO(2−x/2) Materials Prepared from Solution Precursors

The local environment of the aluminum atoms in a series of metastable Zr_{(1− x )}Al _x O_{(2− x/ 2)} crystalline materials (0.08 ≤ x ≤ 0.57), prepared by diffusion-limited crystallization of amorphous precursors, has been determined by ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR). Results show the existence of aluminum in 4-, 5-, and 6-fold coordination in both the amorphous and crystalline states. Although the relative amounts of each type of coordination show no compositional dependence in the amorphous state, the results for the crystalline materials show a systematic decrease in the average aluminum coordination number with increasing aluminum content. Comparisons of MAS NMR results between pure Al₂O₃ precursors and Zr _{(1- x )}Al _x O_{(2- x /2)} crystalline materials processed under similar conditions show a profound effect of ZrO₂ on the coordination environment of the aluminum atom. Both a random distribution model and a model that assumes small-scale clustering of aluminum ions are considered to explain the trends in the type of aluminum coordination as a function of composition.     

Alumina/Epoxy Interpenetrating Phase Composite Coatings: I, Processing and Microstructural Development

Interpenetrating phase composite (IPC) coatings consisting of continuously connected Al₂O₃ and epoxy phases were fabricated. The ceramic phase was prepared by depositing an aqueous dispersion of Al₂O₃ (0.3 μm) containing orthophosphoric acid, H₃PO₄, (1–9.6 wt%, solid basis) and heating to create phosphate bonds between particles. The resulting ceramic coating was porous, which allowed the infiltration and curing of a second-phase epoxy resin. The effect of dispersion composition and thermal processing conditions on the phosphate bonding and ceramic microstructure was investigated. Reaction between Al₂O₃ and H₃PO₄ generated an aluminum phosphate layer on particle surfaces and between particles; this bonding phase was initially amorphous, but partially crystallized upon heating to 500°C. Flexural modulus measurements verified the formation of bonds between particles. The coating porosity (and hence epoxy content in the final IPC coating) decreased from ∼50% to 30% with increased H₃PO₄ loading. The addition of aluminum chloride, AlCl₃, enhanced bonding at low temperatures but did not change the porosity. Diffuse reflectance FTIR showed that a combination of UV and thermal curing steps was necessary for complete curing of the infiltrated epoxy phase. Al₂O₃/epoxy IPC coatings prepared by this method can range in thickness from 1 to 100 μm and have potential applications in wear resistance.     

Reactions in Silica-Alumina Mixtures

Examination of mixtures of extremely pure silica and alumina shows that the greatest reactivity is not encountered with stoichiometric ratios to form mullite with the formula 3Al₂O₃. 2SiO₂ but rather with the formula 2A1₂O₃-SiO₂, and that reactivity also depends on the crystalline modification of alumina. A sharp exothermic differential thermal peak at 980°C. is attributed to three simultaneous reactions dependent on the silica-alumina ratio of the mixture: (1) the crystallization of gamma alumina, (2) the crystallization of a hydrogen aluminum spinel (HAl₅O₅), and (3) the reaction of silica with the hydrogen aluminum spinel to form mullite.