Microstructural Changes During Heat Treatment of Sintered Al2O3

Samples of both high-purity and Mg-doped Al₂O₃ sintered in H₂, N₂, O₂, or vacuum were annealed at 1650° to 1850°C for times up to 64 h. In pore-free systems, grain growth is limited by the mobility of Mg-rich second-phase inclusions; in samples annealed in H₂, grain growth is limited by pore dragging with a transition toward limitation by solid-inclusion dragging at high dopant levels; in samples annealed in N₂ and O₂, grain growth is characterized by a transition from an "anchoring effect" of the pores toward a combination of pore dragging by and unpinning from the grain boundaries. Time-dependence of grain growth is insufficient to determine the mechanisms and provide an adequate foundation for model-based calculations. Observations of microstructure and its change with time, together with the rate of grain growth as a function of composition, allow elimination of alternate hypotheses and determination of the process which controls the rate of grain growth and change in pore size.     

Acoustic Emission and Microcracking in Sapphire, Sintered Al2O3, Al/Al2O3 Composite, and Aluminum

A range of materials with brittle to ductile behavior (single-crystal and polycrystalline alumina, aluminum/alumina composite, and metallic aluminum) were investigated by acoustic emission (AE) methods for microcracking during hardness indentations or cooling from elevated temperatures (800°C). During indentation, the extent of crack formation (and the AE counts) decreased in the following order: sapphire, sintered alumina, aluminum/alumina composite with no microcracking in metallic aluminum. During cooling from 800°C, polycrystalline alumina exhibited more extensive microcracking than the aluminum/alumina composite, suggesting that the metallic phase in the aluminum/alumina composite absorbs stresses more than the glassy boundary phase in sintered alumina.     

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.     

Influence of Additives on the Microstructure of Sintered Al2O3

During an investigation to determine the effect of additives in inhibiting exaggerated grain growth, it was observed that several gave strikingly similar microstructures with pore clusters at grain centers surrounded by substantially pore-free annular regions. A few very large pores were found at the grain boundaries and occasionally in the small-pore-free annular regions. The development of this structure is explained as a sequence of exaggerated grain growth followed by normal grain growth. A discontinuous grain-boundary liquid phase facilitates the final grain-growth step and takes part in the coalescence of fine pores into coarse pores.     

Load Dependence of Hardness in Sintered Submicrometer Al2O3 and ZrO2

Monolithic alumina bodies with a grain size of 0.5 μm and submicrometer tetragonal ZrO₂ (3 mol% Y₂O₃) polycrystals were produced and investigated. With decreasing indentation load, the hardness increases but may again decrease below about 5 N; relevant measurements require the application of fairly high loads. Decreasing grain sizes increase the hardness of alumina and zirconia ceramics even for very small grain sizes in the submicrometer range.     

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.     

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.     

Strength-Toughness Relations in Sintered and Isostatically Hot-Pressed ZrO2-Toughened Al2O3

The fracture toughness of fine-grained undoped ZrO₂-toughened Al₂O₃ (ZTA) was essentially unchanged by postsintering hot isostatic pressing and increased monotonically with ZrO₂ additions up to 25 wt%. The strength of ZTA with 5 to 15 wt% tetragonal ZrO₂, which depended monotonically on the amount of ZrO₂ present before hot isostatic pressing, was increased by pressing but became almost constant between 5 and 15 wt% ZrO₂ addition. The strength appeared to be controlled by pores before pressing and by surface flaws after pressing; the size of flaws after pressing increased with ZrO₂ content. The strength of ZTA containing mostly monoclinic ZrO₂ (20 to 25 wt%) remained almost constant despite the noticeable density increase upon hot isostatic pressing because the strength was controlled by preexisting microcracks whose extent did not change on postsintering pressing. These strength-toughness relations in sintered and isostatically hot-pressed ZTA are explained on the basis of R -curve behavior. The importance of the contribution of microcracks to the toughness of ZTA is emphasized.     

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.     

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.     

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.     

Thermal Conductivity of Sintered UO, and Al2O3 at High Temperatures

The thermal conductivities of sintered Al₂O₃ and UO₂ were measured in the ranges 400° to 1700°C and 300° to 2100°C respectively. The conductivity values for Al₂O₃ agreed with those reported previously at all temperatures investigated. The conductivity of UO, decreased with increasing temperature to a minimum of 0.0050 cal per cm sec °C at about 1400°C, and then increased with increasing temperature to 0.0105 cal per cm sec °C at 2100°C.     

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.     

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.     

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.     

Localization of Potassium in Al2O3

The accommodation of small amounts of intentionally added potassium in sintered and hot–pressed polycrystalline α-alumina was studied. In sintered specimens, potassium was pushed out by, and accumulated along, the grain boundaries enclosing abnormally large α-alumina grains. Potassium was also localized in hot-pressed specimens and formed large (up to 1-mm diameter), randomly dispersed precipitates of polycrystalline potassium β-alumina. Each precipitate was enclosed within a thin shell (0.5– to 1.5-mm radius) of potassium–rich material. The experimental observations of this precipitation are compared to the formation of Liesegang rings.     

Preferred Orientation in Al2O3 Substrates

The degree of preferred orientation (texture) of doctor-bladed polycrystalline 99.5% Al₂O₃ substrates from 3 sources was evaluated by qualitative optical and quantitative X-ray analyses. The substrates exhibit a fiber texture in which the basal planes lie preferentially in the plane of the substrate. The same texture is weakly evident in the unfired materials. The strength of the texture in fired substrates varies from lot to lot and among suppliers, but its character remains essentially the same. Measurements of dielectric constant, flexural strength, and thermal expansivity are consistent with the observed texture. The possible origins of the texture and its importance in substrate applications are discussed.