Translucent Al2O3/LaAl11O18 Composite

A translucent alumina composite containing 1 vol% LaAl₁₁O₁₈, prepared by the hot isostatic pressing (HIP) method, displays both high translucency and high fracture toughness. Its total forward transmission at 600 nm is 75% (thickness 1 mm), and its bending strength and fracture toughness are estimated to be 574±15 MPa and 5.9±0.46 MPa·m^0.5, respectively. Its high translucency is due to the similarity of refractive index between the additive phase (LaAl₁₁O₁₈) and the matrix (alumina).     

Solid-State Kinetics of LaAl11O18

Synthesis of LaAl₁₁O₁₈ by the solid-state reaction of La₂O₃ and Al₂O₃ occurs in two stages, i.e. LaAlO₃ forms immediately at 1450°C in air but LaAl₁₁O₁₈ formation is very slow and requires up to 141 days. First-order kinetics were observed and an activation energy of 118±4 kcal was calculated. Formation of LaAl₁₁O₁₈ depends on external O₂-gas partial pressure. In an N₂-gas atmosphere, the very slow formation of LaAl₁₁O₁₈, observed in air, is slowed even further. Possible mechanisms are also discussed.     

Slip Systems in Al2O3

Crystallographic notation for Al₂O₃ is reviewed, with particular reference to the correct basis to be used in describing slip systems. A Groves-and-Kelly calculation showed that the combination of pyramidal slip on {11¯02}<11¯01> and basal slip on (0001){112¯0} will allow homogeneous deformation of Al₂O₃ polycrystals. Furthermore, operation of either the {101¯1}<1¯011> or the {011¯2}<2¯021> slip system will also satisfy the Von Mises criterion, since each system is capable of 5 independent deformation modes. Electron microscopy of an Al₂O₃ polycrystal deformed ≅5% at 1150°C under a hydrostatic confining pressure confirmed that pyramidal slip had occurred.     

Journal. Pseudoelastic Behavior in Ce-TZP Al2O3 Ceramics

The pseudoelastic behaviour of 12 mol% Ce–TZP + 10 wt% Al₂O₃ ceramics in the loading–unloading cycles was found for the first time at room temperature, and corresponding reversible tm martensitic transformation mechanism was clearly identified the possibility of reversible ferroelastic domain switching at room temperature was also discussed.     

Dynamic Fracture Characterization of Al2O3 and SiCw/Al2O3

The dynamic stress intensity factors, which were determined with newly developed bar impact facilities and a new data reduction procedure, for an Al₂O₃ ceramic and 29 vol% SiC_w/Al₂O₃ composite were virtually identical, thus indicating that the short SiC whiskers were ineffective under dynamic fracture. SEM studies revealed five distinct fracture morphologies with increased percentage area of transgranular fracture in both materials with rapid crack propagation. Also, the high dynamic stress intensity factor caused multiple microscopic crack planes to form and then join as the crack advanced.     

Microstructure Development in Al2O3-Platelet-Reinforced Ce-ZrO2/Al2O3 Composites

Composites containing Ce-ZrO₂, Al₂O₃, and aligned Al₂O₃ platelets were produced by centrifugal consolidation and pressureless sintering, followed by heat treatments at 1600°C for varied duration. Constituents in the consolidated microstructures were either uniformly distributed throughout or segregated into gradient layers, depending critically on platelet content. Quantitative image analysis was used to examine microstructure development with heat treatment. Changes in the volume fraction, dimensional anisotropy, and gradient of pores and platelets, as well as changes in the phase gradient, were quantified. Microstructure development was strongly dependent on the initial microstructure design attained from suspension processing.     

Bimodal Sintering of Al2O3/Al2O3 Platelet Ceramic Composites

The sintering behavior of an Al₂O₃ compact containing uniformly dispersed Al₂O₃ platelets has been investigated. The results reveal a significant decrease in the sintering rate as well as the formation of voids and cracklike defects in the presence of nonsinterable platelets. The addition of a small amount (2 vol%) of tetragonal-ZrO₂ particles enhances the sintering rate, increases end-point density (∼99.5% of theoretical density) and prevents formation of sintering defects.     

Microcellular Al2O3 Ceramics from Wood for Filter Applications

Microcellular biomorphous Al₂O₃ was produced by Al-vapor infiltration in pyrolyzed rattan and pine wood-derived biocarbon preforms. At 1600°C the biocarbon preforms reacted with gaseous aluminum to form Al₄C₃. After oxidation in air at temperatures between 1550° and 1650°C, for 3 h, the biocarbon preforms were fully converted into α-Al₂O₃. Owing to the high anisotropy of biomorphous Al₂O₃, the compressive strength behavior was strongly dependent on the loading direction. The compressive strength of the specimens (0.1–11 MPa) is strongly dependent on their overall porosity and their behavior could be explained using the Gibson–Ashby model. The Darcian permeability ( k ₁), as well as the non-Darcian permeability ( k ₂), increased with an increase of the total porosity. The Darcian permeability of biomorphous Al₂O₃ was found to be in the range of 1–8 × 10⁻⁹ m², which is in the order of magnitude of gas filters, and, therefore, suitable for several technological applications.     

Surface Modification of Al2O3/TiC Cutting Ceramics

Cutting ceramics with a graded TiC-rich surface have been obtained by an in situ reaction. Al₂O₃/TiC ceramics, prepared by reaction sintering, have been subjected to various thermal posttreatments at atmospheric pressure. The temperature, dwelling time, and reaction atmosphere have been varied. It has been observed that a TiC-rich surface develops in CO-containing atmospheres. The microstructure of the bulk remains unaffected. Thermal treatment under a CO atmosphere at 1600 ° C for 1 h resulted in a dense TiC layer on the ceramic. A schematic model is proposed for this layer formation process.     

Characterization of Freeze-Dried Al2O3 and Fe2O3

Oxide crystallite formation and growth from freeze-dried sulfates were studied for the representative materials Al₂O₃ and Fe₂O₃. Transmission and scanning electron micrographs showed the formation and growth of chainlike aggregates of crystallites. Aggregation occurred as part of the nucleation and growth of the oxide, and discrete oxide particles were never present. Orientation of the chain aggregates was related to the ice structure formed during freezing. X-ray line broadening data showed that crystallite size is a function of the 1/5 to 1/7 power of time for isothermal treatments. A qualitative analysis of material transport favored the surface diffusion mechanism.     

Slow Crack Growth Behavior in Transformation-Toughened Al2O3-ZrO2(Y2O3) Ceramics

Significant increases in the critical fracture toughness (K _IC ) over that of alumina are obtained by the stress-induced phase transformation in partially stabilized ZrO₂ particles which are dispersed in alumina. More importantly, improved slow crack growth resistance is observed in the alumina ceramics containing partially stabilized ZrO₂ particles when the stress-induced phase transformation occurs. Thus, increasing the contribution of the ZrO₂ phase transformation by tailoring the Y₂O₃ stabilizer content not only increases the critical fracture toughness (K_IC) but also the K _Ia to initiate slow crack growth. For example, crack velocities ( v )≥10^–9 m/s are obtained only at K _Ia≥5 MPa.m^1/2 in transformation-toughened ( K _IC=8.5 MPa.m^1/2) composites vs K _Ia≥2.7 MPa.m^1/2 for comparable velocities in composites where the transformation does not occur ( K _IC=4.5 MPa.m^1/2). This behavior is a result of crack-tip shielding by the dissipation of strain energy in the transformation zone surrounding the crack. The stress corrosion parameter n is lower and A greater in these fine-grained composite materials than in fine-grained aluminas. This is a result of the residual tensile stresses associated with larger (≥1 μm) monoclinic ZrO₂ particles which reside along the intergranular crack path.     

Grain Growth in Two-Phase Ceramics: Al2O3 Inclusions in ZrO

The effect of Al₂O₃ inclusions with a greater average size (0.6 μm) than the average particle size of the major phase powder (<0.1 μm) on grain gowth was examined by sintering ZrO₂/Al₂O₃ composites (0,3,5,10, and 20 vol%) at 1400°C and then heat-treating at temperatures up to 1700°C. Normal grain growth was observed for all conditions. The inclusions appeared to have no effect on grain growth until the ZrO₂ grain size was ∼1.5 times the average inclusion size. Grain growth inhibition increased with volume fraction of the Al₂O₃ inclusion phase. At temperatures 1600°C, the inclusions were relatively immobile and most were located within the ZrO₂ grains for volume fractions <0.20; at higher temperatures, the inclusions could move with the grain boundary to coalesce. Grain growth was less inhilited when the inclusions could move with the boundaries, resulting in a larger increase in grain size than observed at lower temperatures. Analogies between mobile voids, entrapped within grain at lower temperature due to abnormal grain growth during the last state of sintering, and the observations concerning the mobile inclusions are made suggesting that grain-boundary movement can "sweep" voids to grain boundaries and eventually of four-grain junctions, where they are more likely to disappear by mass transport.     

High-Toughness Ce-TZP/Al2O3 Ceramics with Improved Hardness and Strength

Simulataneous additions of SrO and Al₂O₃ to ZrO₂ (12 mol% CeO₂) lead to the in situ formation of strontium aluminate (SrO · 6Al₂O₃) platelets (∼0.5 μm in width and 5 to 10 μm in length) within the Ce-TZP matrix. These platelet-containing Ce-TZP ceramics have the strength (500 to 700 MPa) and hardness (13 to 14 GPa) of Ce-TZP/Al₂O₃ while maintaining the high toughness (14 to 15 MPa ± m^1/2) of Ce-TZP. Optimum room-temperature properties are obtained at SrO/Al₂O₃ molar ratios between 0.025 and 0.1 for ZrO₂ (12 mol% CeO₂) with starting Al₂O₃ contents ranging between 15 and 60 vol%. The role of various toughening mechanisms is discussed for these composite ceramics.     

Hydration of 3CaO·Al2O3 and 3CaO·Al2O3+] Gypsum With and Without CaCl2

Paste samples of tricalcium aluminate alone, with CaCl₂, with gypsum, and with gypsum and CaCl₂ were hydrated for up to 6 months and the hydration products characterized by SEM, XRD, and DTA. Tricalcium aluminate hydrated initially to a hexagonal hydroaluminate phase which then changed to the cubic form; the transformation rate depended on the size and shape of the sample and on temperature. The addition of CaCl₂ to tricalcium aluminate resulted in the formation of 3CaO · Al₂O₃· CaCl₂·10H₂O and 4CaO · Al₂O₃· 13H₂O, or a solid solution of the two. The chloride retarded the formation of the cubic phase 3CaO · Al₂O₃· 6H₂O; the addition of gypsum resulted in the formation of monosulfoaluminate with a minor amount of ettringite. When chloride was added to tricalcium aluminate and gypsum, more ettringite was formed, although 3CaO · Al₂O₃· CaSO₄· 12H₂O and 3CaO · Al₂O₃· CaCl₂· 10H₂O were the main hydration products.     

Interreaction Between Al2O3 and a CaO-Al2O3 Melt

Poly crystalline and single-crystalline α-alumina were reacted with a eutectic CaO-Al₂O₃ melt at 1530°C. A reaction zone develops in which a strongly textured CA₆ layer, as well as a CA₂ layer, forms, with a remaining layer of unreacted CaO-AI₂O₃ melt. Silica, an impurity in the α-alumina, is rejected by the advancing CA₆ phase and accumulates as calcium alumino-silicates in channels that assist in the reaction as fast transport paths. Reaction mechanisms and welding are briefly discussed.     

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.     

Grain-Boundary Segregation in MgO-Doped Al2O3

Magnesia-doped alumina was fractured in a high-vacuum Auger electron spectrometer, and the resulting intergranular fracture surface was analyzed for surface composition. Calcium was present on the fracture surface in amounts of ∼2 to 5 at.%. There was no indication of Mg on the fracture surface. The bulk Ca concentration was 5 to 15 ppm, whereas MgO was present at the 1000-ppm level. The presence of excess Ca at the grain boundary can be explained by the fact that its ionic radius is relatively large compared to those of Al and Mg. Sputtering profiles normal to the fracture surface indicate that the segregated Ca is concentrated in a region near tha intergranular fracture surface.     

Dislocation Structures in Single-Crystal Al2O3

Chemical polishing and etching techniques were used to reveal the dislocation structures of sapphire and ruby crystals grown by the flame-fusion and flux techniques. The average density of edge dislocations lying in prism planes was 3.0 × 10⁵ per cm², which could be changed only slightly by chromium additions and annealing at 2000°C. An average basal dislocation density of 2 × 10⁵ per cm² decreased 35 to 80% on annealing. Crystal orientation (i.e., angle between the c axis and the growth axis) showed no effect on dislocation density but a pronounced effect on subboundary arrangement and density. The substructure of 0° crystals was more complex than that of 90° crystals; 60° crystals possessed a structure intermediate between 0° and 90°. Principal observations included (1) prismatic and basal slip on all as-grown crystals; (2) profuse basal slip, readily polygonized on annealing; (3) dislocation densities of flux crystals lower than those of Verneuil crystals; and (4) a rare form of basal twinning, composition plane , on all flux crystals.