Tribological Characteristics of α-Alumina at Elevated Temperatures

The tribological characteristics of a high-purity α-alumina sliding on a similar material under unlubricated conditions are divided into four distinct regimes. At low temperatures, T < 200°C, tribochemical reactions between the alumina surface and water vapor in the environment control the tribological performance. The coefficient of friction in this temperature range is approximately 0.40 and the wear coefficient is less than 10⁻⁶, independent of contact load. At intermediate temperatures, 200°C < T < 800°C, the wear behavior depends on the contact load. At low loads, wear occurs by plastic flow and plowing; the coefficient of friction is approximately 0.60 and the wear coefficient is less than 10⁻⁶. At loads larger than a threshold value, severe wear occurs by intergranular fracture. The coefficient of friction increases to 0.85 and the wear coefficient increases to a value greater than 10⁻⁴. At temperatures above 800°C, formation of a silicon-rich layer on the wear track by diffusion and viscous flow of the grain-boundary phase reduces the coefficient of friction to 0.40, and the wear coefficient is reduced to a value less than 10⁻⁶. The results of the wear tests and observations of the fundamental mechanisms controlling the tribological behavior of this material are consolidated in a simple wear transition diagram.     

High-Temperature Defect Structure of Cobalt-Doped α-Alumina

Room-temperature optical absorption spectra, electron spin resonance spectra at 15° to 18°K, electrical conductivity, and emf measurements on concentration cells at 1620°C are analyzed and used to determine the defect structure of Codoped α-Al₂O₃. The crystals are mixed ionic and electronic conductors at 1620°C: ionic conduction occurs at 10^-8 atm< p O₂ < 10_-3 atm, with triply charged interstitial Al ions as the major charge carriers, and electronic conduction occurs at 10^-3 atm < p O₂ < 1 atm, with holes as the major charge carriers. A defect model based on the charge compensation of divalent Co at Al sites by triply charged Al interstitials is proposed. The mobilities and activation energy of ions and holes, the oscillator strengths of Co²⁺ and Co³⁺ absorption bands, parameters for electron spin resonance spectra, level positions of substitutional Co²⁺ and an unknown donor, and equilibrium constants for defect formation reactions are determined.     

Topotactic Growth of β-Alumina on α-Alumina

Thin foils of polycrystalline α-alumina were reacted with a potassium-rich vapor at ≤900°C. Potassium β-alumina formed along α-alumina grain boundaries and protruded from holes in the foils. Conventional transmission electron microscopy was used to analyze the α-alumina/β-alumina phase boundary for possible orientation relations.     

Size Control of α-Alumina Particles Synthesized in 1,4-Butanediol Solution by α-Alumina and α-Hematite Seeding

The effects of seed particles and shear rate on the size and shape of α-Al₂O₃ particles synthesized in glycothermal conditions are described. It is proposed that seed particles provide a low-energy, epitaxial surface in solution to lower the overall surface energy contribution to the nucleation barrier, thus increasing nucleation frequency and subsequently reducing the particle size of hexagonal α-Al₂O₃ platelets or polyhedra, depending on synthesis conditions, in 1,4-butanediol solution. Seeds have a significant effect on the size of hexagonal α-Al₂O₃ platelets in samples with high seed concentration. The particle size of α-Al₂O₃ platelets decreases from 3 to 4 µm to 100 to 200 nm by increasing the number concentration of seeds. In the case of α-Fe₂O₃ seeding, the effect of seeding on the size of α-Al₂O₃ particles closely resembles the effects obtained with α-Al₂O₃ seeding. Regardless of seed concentration, high stirring rate promotes the formation of hexagonal platelets with high aspect ratio, whereas medium and low stirring rates promote the formation of elongated platelets and polyhedra with 14 faces, respectively.     

Internal Reduction of Chromium-Doped α-Alumina

(Al,Cr)₂O₃ single crystals and polycrystals were internally reduced at 1873 K in an Al/Al₂O₃ buffer for periods of time ranging from 1 to 100 h. The growth kinetics of the reduction scale were measured. The microstructure of the reduction scale was investigated by SEM and TEM. As a result of the reduction, two types of discrete chromium precipitates developed inside the alumina matrix (inside the single crystal or the polycrystalline grains), each one being characterized by a particular morphology (needle or spheroid) and a low-energy orientation relationship with respect to the alumina matrix. In addition, larger precipitates without special orientation relationship developed along the grain boundaries and at the triple junctions of the polycrystais. In the first part of this paper, the precipitate morphology and size are described in terms of the crystallography of the interface between the two crystal structures in relation to the reduction mechanism. In the second part, the global reduction scale growth is analyzed in terms of point defect fluxes across the reduction scale.     

Seeding with γ-Alumina for Transformation and Microstructure Control in Boehmite-Derived α-Alumina

The dehydration, transformation, and densification of boehmite (γ-AlOOH) are enhanced by addition of γ-Al₂O₃ seed particles. α-Al₂O₃ microstructures with uniform 1- to 2-μm grain size and sintered densities 98% of theoretical are achieved at 1300°C Thermal analysis shows that γ-Al₂O₃ seed particles transform to α-Al₂O₃ before the matrix, thus controllably nucleating the transformation of θ-AI₂O₃ to α-Al₂O₃.     

Kinetics and Microstructural Evolution of Heterogeneous Transformation of θ-Alumina to α-Alumina

Ultrafine (<0.1 μm) high-purity θ-Al₂O₃ powder containing 3–17.5 mol%α-Al₂O₃ seeds was used to investigate the kinetics and microstructural evolution of the θ-Al₂O₃ to α-Al₂O₃ transformation. The transformation and densification of the powder that occurred in sequence from 960° to 1100°C were characterized by quantitative X-ray diffractometry, dilatometry, mercury intrusion porosimetry, and transmission and scanning electron microscopy. The relative bulk density and the fraction of α phase increased with annealing temperature and holding time, but the crystal size of the α phase remained ∼50 nm in all cases at the transformation stage (≤1020°C). The activation energy and the time exponent of the θ to α transformation were 650 ± 50 kJ/mol and 1.5, respectively. The results implied the transformation occurred at the interface via structure rearrangement caused by the diffusion of oxygen ions in the Al₂O₃ lattice. A completely transformed α matrix of uniform porosity was the result of appropriate annealing processes (1020°C for 10 h) that considerably enhanced densification and reduced grain growth in the sintering stage. The Al₂O₃ sample sintered at 1490°C for 1 h had a density of 99.4% of the theoretical density and average grain size of 1.67 μm.     

Preparation and Characterization of Macroporous α-Alumina

α-Al₂O₃ powders with three-dimensionally ordered or randomly positioned macropores were synthesized by templating with poly(methyl methacrylate) colloidal crystals. Aluminum nitrate was precipitated with ammonium hydroxide within the interstices of the template; calcination removed the polymer and converted the inorganic precursors into a macroporous skeleton of α-Al₂O₃. Subsequent calcination at higher temperatures and hot stage transmission electron microscopy experiments were performed to study sintering effects on the product morphology. These materials combine the thermal and chemical stability of corundum with a very open structure of uniform macropores that can permit facile transport of guest molecules in potential catalysis, filtration, and sensing applications.     

Ca Segregation to Basal Surfaces in α-Alumina

High-resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM) were used to investigate the structure and chemistry of (0001) α-A1₂O₃/ A1 interfaces in melt-infiltrated polycrystalline alumina composites. HRTEM revealed an interfacial region different from both Al and α-A1₂O₃, with a structural width of 0.8 ± 0.2 nm. AEM of the same interfaces revealed a Ca excess of Γ = 2.5 ± 0.5 Ca atoms per nm² (Ca/nm²). AEM of a basal twin boundary in the α-A1₂O₃ also revealed a Ca excess (Γ= 1.0 ± 0.5 Ca/nm²). Since the metal-ceramic interfaces were the free surfaces of pores before melt infiltration, it can be concluded that Ca segregates to the basal surface of alumina, as well as to basal twin boundaries. Furthermore, the Ca at the free surfaces does not reside on only one cation plane, but is spread over 4 ± 1 basal cation layers and forms an interfacial phase with a nominal composition of CaO-6A1₂O₃.     

Preparation of α-Alumina Platelets by Laser Scanning

A laser scanning with gas jet process was developed to prepare alumina platelets from an alumina powder. When the carbon-dioxide laser scanned the alumina powdery coatings prepared using an electrospraying technique, the alumina particles were heated to a melting state. The coaxial gas ejection force pushed the melting particles to obtain tabular shape grains that recrystallized into alumina platelets in the subsequent rapid-cool solidification. The phase and morphologies of powder bed were characterized by XRD and SEM. Results show that only α-alumina platelets were formed in the scanning process and the average edge length and thickness is 10 μm and 1–2 μm, respectively. Laser processing parameters such as laser energy density, scanning speed, and gas pressure were expected to play a vital role in the melting-crystallization-solidification process for obtaining platelike grains from powder beds. The preliminary experiment showed that the laser-scanning technique could be an effective means of tailoring the morphologies of particles to meet application requirements.     

Synthesis of Nanocrystalline α-Alumina Powder Using Triethanolamine

Nanocrystalline α-Al₂O₃ powders have been prepared by pyrolysis of a complex compound of aluminum with triethanolamine (TEA). The soluble metal-ion–TEA complex forms the precursor material on complete dehydration of the complex of aluminum-TEA. The single-phase α-Al₂O₃ powder has resulted after heat treatment at 1025°C. The precursors and the heat-treated final powders have been characterized by X-ray diffractometry, thermogravimetric and differential thermal analysis, and transmission electron microscopy (TEM). The average particle sizes as measured from X-ray line broadening and TEM are ∼25 nm. The powder has crystallite sizes of the same order indicates the poor agglomeration of crystallites.     

Preparation Method of Submicrometer-Sized α-Alumina by Surface Modification of γ-Alumina with Alumina Sol

A method is introduced to prepare almost-spherical submicrometer-sized α-alumina via surface modification of γ-alumina with an alumina sol. Milled γ-alumina, in the presence of 3 wt% of α-alumina with a median particle size ( d ₅₀) of 0.32 μm (AKP-30), produced irregularly shaped α-alumina with d ₅₀∼0.3 μm after heat treatment at 1100°C for 1 h. γ-alumina that had been surface-modified by milling in the presence of 3 wt% of the alumina sol resulted in almost-monosized, spherical α-alumina ∼0.3 μm in size after heat treatment at 1100°C for 1 h. Furthermore, almost-spherical α-alumina 0.1—0.2 μm in size was obtained by milling γ-alumina with 3 wt% of AKP-30 alumina in the presence of 3 wt% of the alumina sol, followed by heat treatment at 1100°C for 1 h. The alumina sol that has been introduced in this work seems to act as a dispersant, in addition to helping to form a spherical shape.     

Yttria Doping and Sintering of Submicrometer-Grained α-Alumina

The sintering behavior of α-alumina powders doped with magnesia (500 or 1500 ppm) and yttria (0, 500, or 1500 ppm) was investigated using constant-heating-rate dilato-metric experiments. The apparent activation energies for the intermediate stage of sintering were 740, 800, and 870 kJ/mol for 0, 500, and 1500 ppm yttria doping levels, respectively; these were independent of magnesia doping. Yttria-doped powder compacts exhibited systematic anomalous second peaks in the densification rate curves at certain grain sizes which were determined only by yttria doping levels. Before the anomalous peak, with lower yttrium contents at grain boundaries, yttrium in an atomic state delays densification and raises the apparent activation energy. Beyond the peak, with higher yttrium contents at grain boundaries, yttria-rich precipitation delays the densification. Within the peak, yttrium segregation near the saturation level enhances densification.     

Grain Boundaries in Titania-Doped α-Alumina with Anisotropic Microstructure

The microstructure of sol-gel-derived alpha-alumina (Al₂O₃) doped with 0.6 wt% titania, sintered at 1450°C for 1 h, consisted of thin platelets with (0001) faces in a matrix of equiaxed grains. Short facets at the edges of the platelets developed primarily parallel to the {10     2} planes, while some were parallel to the {11     3} planes; other edges showed irregular, curved boundaries. The basal surfaces of the platelets were coated with thin layers (0.5-6 nm) of an amorphous titanium-containing aluminosilicate phase, which also was present at triple points. No amorphous phase was found on the short faceted boundaries, on curved boundaries at platelet edges, or at grain boundaries of equiaxed, matrix grains. However, titanium enrichment was observed at all examined boundaries, suggesting that titanium segregation alone did not account for the development of anisotropic microstructure. Curved incursions on basal facets were associated with occasional particles of aluminum titanate (Al₂TiO₅).     

Identification of α-Alumina Surface Structures by Electron Diffraction

Single-crystal thin foils of a-alumina were annealed i:i air and examined by transmission electron microscopy. Weak reflections, which are kinematically forbidden for the a-alumina unit cell, were obscrxed in selected-area diffraction patterns obtained from foils prepared parallel to (0001), (1012), and (1120) planes. These reflections are interpreted as giving evidence for surface periodicities which differ from those expected for the simple termination of the bulk crystal.     

Enhanced Densification of Boehrmte Sol-Gels by α-Alumina Seeding

Boehmite sol-gels were seeded by adding <2.0 wt%α-alumina powder to a 20% boehmite hydrosol at pH =3. The seeded gel sintered to 98% of theoretical density after 100 min at 1200°C, whereas the unseeded gel had to be sintered at 1600°C to reach 94% of theoretical density. This difference results primarily from nucleation by the α-alumina seeds and enhanced transformation of the boehmite to α-alumina, such that a uniform, well-ordered, fine-grained α-alumina micro-structure forms prior to densification.     

Texture Development by Templated Grain Growth in Liquid-Phase-Sintered α-Alumina

The distribution and orientation of platelet-shaped particles of α-alumina in a fine-grained alumina matrix is shown to template texture development via anisotropic grain growth. The textured microstructure ranges from 4 wt% oriented platelet particles in calcined samples to nearly 100% oriented α-Al₂O₃ grains after sintering at 1400°C. A CaO + SiO₂ liquid phase creates favorable thermodynamic and kinetic conditions for anisotropic grain growth and grain reorientation during sintering. Important criteria for templated grain growth include (1) anisotropic crystal structure and growth, (2) high thermodynamic driving force for template grain growth, and (3) modification of diffusion in the system to continuously provide material to the anisotropically growing template grains.     

Sintering of Mixtures of Seeded Boehmite and Ultrafine α-Alumina

α-Alumina was fabricated by dry pressing mixtures of seeded boehmite and fine α-alumina (i.e., 0.2 and 0.3 μm diameter) to reduce the large shrinkage of boehmite-derived α-alumina. The maximum green density was obtained with mixtures containing ∼70%α-alumina for both alumina powders. The ∼15% linear shrinkage and microstructures of these samples were comparable to 100% alumina powder samples. Samples with 0.2 μm alumina sintered to densities >95% at 1300°C whereas 1400°C was needed for samples with 0.3 μm alumina. These results indicate that boehmite can be used as a substitute for relatively expensive ultrafine α-alumina powders.