Structure of Incoherent ZrO2/Al2O3 Interfaces

High-resolution electron microscopy was used to image incoherent ZrO₂/Al₂O₃ interfaces in ZrO₂-toughened Al₂O₃ containing intragranular ZrO₂. These particles are generally spherical but are sometimes faceted. High-resolution electron micrographs provide atomic-level information on the interfacial structure. For spherical particles, both ledgelike images and misfit dislocation-like images accommodated the lattice misfit, depending on the orientation of the interface, while faceted particles imply at least one low-energy ZrO₂/Al₂O₃ interface.     

TIOx/Al2O3 Multilayer Ceramic Thin Films

Titanium oxide/aluminum oxide films have been deposited using molecular beam epitaxy methods and characterized by reflection high-energy electron diffraction and transmission electron microscopy techniques. Growth on silicon substrates below 973 K resulted in primarily amorphous multilayers. At 1323 K, the deposition of titanium in an oxygen atmosphere on (0001) Al₂O₃ substrates resulted in films of Ti₂O₃. These films consisted of small domains, up to 60 nm, slightly misoriented from a [1120] ∥ [1120] orientation relationship. Two variants of Ti₂O₃ were observed due to multiple positioning during growth. Closing the titanium shutter during growth resulted in an oriented TiO₂ film.     

Exaggerated Grain Growth in ZrO2-Toughened Al2O3

Annealing of ZrO₂-toughened Al₂O₃ (ZTA) at elevated temperatures causes growth of both the intergranular ZrO₂ particles and the Al₂O₃"matrix" grains. Exaggerated ("breakaway") grain growth occurs in some, but not all, specimens. Analytical electron microscopy of two ZTA's, both of which contained a continuous amorphous (glassy) grain-boundary phase, but only one of which showed breakaway grain growth, revealed that the occurrence of breakaway grain growth could be correlated with the chemistry of the ubiquitous glassy grain-boundary phase.     

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.     

Influence of the Y2O3 Content and Temperature on the Mechanical Properties of Melt-Grown Al2O3–ZrO2 Eutectics

The effect of Y₂O₃ content on the flexure strength of melt-grown Al₂O₃–ZrO₂ eutectics was studied in a temperature range of 25°–1427°C. The processing conditions were carefully controlled to obtain a constant microstructure independent of Y₂O₃ content. The rod microstructure was made up of alternating bands of fine and coarse dispersions of irregular ZrO₂ platelets oriented along the growth axis and embedded in the continuous Al₂O₃ matrix. The highest flexure strength at ambient temperature was found in the material with 3 mol% Y₂O₃ in relation to ZrO₂(Y₂O₃). Higher Y₂O₃ content did not substantially modify the mechanical response; however, materials with 0.5 mol% presented a significant degradation in the flexure strength because of the presence of large defects. They were nucleated at the Al₂O₃–ZrO₂ interface during the martensitic transformation of ZrO₂ on cooling and propagated into the Al₂O₃ matrix driven by the tensile residual stresses generated by the transformation. The material with 3 mol% Y₂O₃ retained 80% of the flexure strength at 1427°C, whereas the mechanical properties of the eutectic with 0.5 mol% Y₂O₃ dropped rapidly with temperature as a result of extensive microcracking.     

Wet Milling of Fe/Al/Al2O3 and Fe2O3/Al/Al2O3 Powder Mixtures

Wet milling of Al₂O₃-aluminide alloy (3A) precursor powders in acetone has been investigated by milling Fe/Al/Al₂O₃ and Fe₂O₃/Al/Al₂O₃ powder mixtures. The influence of the milling process on the physical and chemical properties of the milled powders has been studied. Particle refinement and homogenization were found not to play a dominant role, whereas plastic deformation of the metal particles leads to the formation of dislocations and a highly disarranged polycrystalline structure. Although no chemical reactions among the powder components in Fe₂O₃/Al/Al₂O₃ powder mixtures were observed, the formation of a nanocrystalline, ordered intermetallic FeAl phase in Fe/Al/Al₂O₃ powder mixtures caused by mechanical alloying was detected. Chemical reactions of Fe and Al particle surfaces with the atmosphere and the milling media lead to the formation of highly porous hydroxides on the particle surfaces. Hence the specific surface area of the powders increases, while the powder density decreases during milling. The fraction of Fe oxidized during milling was determined to be 0.13. The fraction of Al oxidized during milling strongly depends on the metal content of the powder mixture. It ranges between 0.4 and 0.8.     

Field-Assisted Densification of Superhard B6O Materials with Y2O3/Al2O3 Addition

B₆O is a possible candidate of superhard materials with a hardness of 45 GPa measured on single crystals. Up to now, densification of these materials was only possible at high pressure. However, recently it was found that Al₂O₃ can be utilized as an effective sintering additive, similar to the addition of Y₂O₃/Al₂O₃ that was used in this work. The densification behavior of the material as a function of applied pressure, its microstructure evolution, and the resulting mechanical properties were investigated. A strong dependence of the densification with increasing pressure was found. The material revealed characteristic triple junctions filled with amorphous residue composed of B₂O₃, Al₂O₃, and Y₂O₃, while no amorphous grain-boundary films were observed along internal interfaces. Mechanical testing revealed on average a hardness of 33 GPa, a fracture toughness of 4 MPa·m^1/2, and a strength value of 520 MPa.     

Eutectic Composition in the Pseudobinary of Y4Al2O9 and Y2O3

The eutectic composition between Y₄Al₂O₉ and Y₂O₃ was determined using electron probe microanalysis (EPMA) on directionally solidified specimens with hypo- and hypereutectic compositions. The microstructures of the specimens as a function of composition differ considerably with small deviation from the eutectic composition (70.5 mol% Y₂O₃ and 29.5 mol% Al₂O₃). Based on the current results and other published data, the pseudobinary system between Al₂O₃ and Y₂O₃ is revised.     

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.     

Aluminum Titanate Formation by Solid-State Reaction of Coarse Al2O3 and TiO2 Powders

Reaction kinetics in a coarse equimolar powder mixture were slow enough to allow for the different stages to be identified, notably in the lower and higher temperature ranges, respectively. In the former ( T ≤ 1600 K), Al₂TiO₅ nucleation was hindered by the strain energy contribution to the overall driving force. The setting up of metastable layer sequences Al₂TiO₅/TiO₂/Al₂O₃ was found to occur generally during subsequent growth. The high Al mobility in the TiO₂ provided a rapid aluminum transport from the metastable Al₂O₃/TiO₂ interface to the TiO₂/Al₂TiO₅ front. At temperatures above ∼1700 K the Al₂O₃/TiO₂ interface was very rapidly sealed off by Al₂TiO₅ formation. Reactant transport across the Al₂TiO₅ was slow because of the low mobilities in the product phase. Therefore, much lower product growth velocities were observed at higher temperatures than at lower temperatures.     

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.     

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.     

Uniformly Porous Al2O3/LaPO4 and Al2O3/CePO4 Composites with Narrow Pore-Size Distribution

Porous Al₂O₃/20 vol% LaPO₄ and Al₂O₃/20 vol% CePO₄ composites with very narrow pore-size distribution at around 200 nm have been successfully synthesized by reactive sintering at 1100°C for 2 h from RE₂(CO₃)₃· x H₂O (RE = La or Ce), Al(H₂PO₄)₃ and Al₂O₃ with LiF additive. Similar to the previously reported UPC-3Ds (uniformly porous composites with a three-dimensional network structure, e.g. CaZrO₃/MgO system), decomposed gases in the starting materials formed a homogeneous open porous structure with a porosity of ∼40%. X-ray diffraction, ³¹P magic-angle spinning nuclear magnetic resonance, scanning electron microscopy, and mercury porosimetry revealed the structure of the porous composites.     

Discontinuous Dissolution of Iron Aluminate Spinel in the Al2O3–Fe2O3 System

When sintered 85Al₂O₃–15Fe₂O₃ (in wt%) specimens consisting of corundum grains and spinel particles were annealed at temperature where only a corundum phase was stable, phase transformation of spinel into metastable FeAIO₃ and subsequently complete dissolution of the metastable phase occurred together with the migration of grain boundaries at the surface of the specimens. Since the grain boundary migration was induced by grain boundary diffusion of Fe₂O₃ from the transforming and dissolving particles, the boundary migration by temperature decrease corresponds to a discontinuous dissolution of the spinel particles and a chemically induced grain boundary migration by temperature change. Inside the specimens, however, the transformation—dissolution and the grain boundary migration were suppressed because of unavailable accommodation of the volume expansion due to the transformation.     

Fabrication and Microstructure Characterization of Continuous Porous TiO2/Al2O3 Composite Membrane

Using a multipass extrusion process, continuous porous Al₂O₃ body (∼41% porosity) was produced and used as a substrate to fabricate continuous porous TiO₂/Al₂O₃ composite membrane. The diameter of the continuous pores of the porous Al₂O₃ body was about 150 μm. The TiO₂ nanopowders dip coated on the continuous pore-surface Al₂O₃ body existed as rutile and anatase phases after calcination at 520°C in air. However, after aging of the fabricated continuous porous TiO₂/Al₂O₃ composite membrane in 20% NaOH at 60°C for 24 h, a large number of TiO₂ fibers frequently observed on the pore surface. The diameter of the TiO₂ fibers was about 150 nm having a high specific surface area. However, after 48-h aging period, the diameter of the TiO₂ fibers increased, which was about 3 μm. Most of the TiO₂ fibers had polycrystalline structure having nanosized rutile and anatase crystals of about 20 nm.     

Coupled Grain Growth Effects in Al2O3/10 vol% ZrO2

Coupled crystallization has been observed in the Al₃O₃/10%-ZrO₂ system by heating an amorphous precursor Al /Zr copolymerized alkoxide network structure. A finely divided two-phase material results which stabilizes tetragonal ZrO₂ to 1700°C and exhibits an unprecedented microstructure. During crystallization, the grain growth of ZrO₂ is coupled to the γ→α phase transformation of Al₂O₃.     

Solubility of NiO in Al2O3

Solubility of NiO in Al₂O₃ was determined by electron probe microanalysisy A diffusion couple method was used by coupling an NiO-doped Al₂O₃ polycrystal to a pure single crystal of Al₂O₃. The solubility of NiO in Al₂O₃ in air was 230 wt ppm (157 at. ppm of cations) and 170 wt ppm (116 at. ppm) at 2073 and 1973 K, respectively. The solubility of NiO in Al₂O₃ obtained in this work was compared with our previous work of the solubility of MgO in Al₂o₃.     

Tape Casting of Al2O3/ZrO2 Laminated Composites

Fracture toughness of ZrO₂-toughened alumina could he increased by macroscopic interfaces, such as those existing in laminated composites. In this work, tape casting was used to produce A/A or A/B laminates, where A and B can be Al₂O₃, Al₂O₃/5 vol% ZrO₂, and Al₂O₃/l0 vol% ZrO₂. An increase of toughness is observed, even in the Al₂O₃/Al₂O₃ laminates.     

Strength and Fracture Toughness of Isostatically Hot-Pressed Composites of Al2O3 and Y2O3-Partially-Stabilized ZrO2

Composites of Al₂O₃ and Y₂O₃ partially-stabilized ZrO₂ were isostatically hot-pressed using submicrometer powders as the starting material. The addition of Al₂O₃ resulted in a large increase in bending strength. The average bending strength for a composite containing 20 wt% Al₂O₃ was 2400 MPa, and its fracture toughness was 17 MN·w^−3/2     

The System Al2O3-Cr2O3-Sio2

Liquidus phase equilibrium data are presented for the system Al₂O₃-Cr₂O₃-SiO₂. The liquidus diagram is dominated by a large, high-temperature, two-liquid region overlying the primary phase field of corundum solid solution. Other important features are a narrow field for mullite solid solution, a very small cristobalite field, and a ternary eutectic at 1580°C. The eutectic liquid (6Al₂O₃-ICr₂O₃-93SiO₂) coexists with a mullite solid solution (61Al₂O₃-10Cr₂O₃-29SiO₂), a corundum solid solution (19Al₂O₃-81Cr₂O₃), and cristobalite (SO₂). Diagrams are presented to show courses of fractional crystallization, courses of equilibrium crystallization, and phase relations on isothermal planes at 1800°, 1700°, and 1575°C. Tie lines were sketched to indicate the composition of coexisting mullite and corundum solid solution phases.