Morphology of Platelike Abnormal Grains in Liquid-Phase-Sintered Alumina

The microstructural features of platelike grains in liquid-phase-sintered Al₂O₃ were investigated. The flat boundaries of platelike grains wetted with a liquid phase were basal planes. After impingement of platelike grains, flat boundaries progressively changed to curved boundaries which consisted of both basal and rhombohedral planes as microscopic facets. Second-phase particles were observed for most doped samples. Small-angle subgrain boundaries inside platelike grains were also observed.     

Impurity-Dependent Morphology and Grain Growth in Liquid-Phase-Sintered Alumina

The alumina grains in liquid-phase-sintered (LPS) materials prepared from different commercial sources have a predominantly platelet morphology. Generally, the MgO:(CaO + BaO + Na₂O + K₂O) ratio in the chemical composition controls the morphology in LPS alumina that is 91–94 wt% pure. Within a given range of SiO₂ content (i.e., 4.3–5.2 wt% in the chemical composition), a low MgO:(CaO + BaO + Na₂O + K₂O) ratio (i.e., <1.0) in the LPS compositions favors the formation of elongated grains, whereas ratios of >1.0 result in equiaxed grains. SiO₂ contents outside the 4.3–5.2 wt% range favor the formation of elongated grains. A tendency to form platelike grains is observed for LPS alumina with a purity of 91–94 wt% when both the MgO:(CaO + BaO + Na₂O + K₂O) ratio and the SiO₂ content are relatively low. The sintered density generally increases as the SiO₂ content in the chemical composition decreases.     

Effect of the Liquid-Forming Additive Content on the Kinetics of Abnormal Grain Growth in Alumina

Alumina specimens with various amounts of CaO and SiO₂ (1:2 ratio) were prepared, and their abnormal grain growth (AGG) kinetics were investigated. A plot of the area fraction covered by abnormal grains versus log (sintering time) had a sigmoidal shape with an apparent incubation period before the onset of AGG. The overall kinetics of AGG was similar to that of a phase transformation controlled by nucleation and growth. The incubation time and the end point of AGG were strongly dependent on the amount of liquid-forming additives. Correspondingly, the final microstructure was affected by the liquid content: a large grain size and a high aspect ratio at low liquid content and a small grain size and a low aspect ratio at high liquid content.     

Abnormal Grain Growth of Alumina: CaO Effect

The critical concentration of Ca required for the onset of abnormal grain growth in alumina was determined by controlled doping of Ca in ultrapure alumina (>99.999%), by sintering under clean contamination-free conditions, and by microstructural characterization. As in the case of Si, the excess concentration of Ca beyond its solubility limit was inversely related to the average grain size at the moment of first appearance of abnormal grains, which corresponds to the moment of sufficient enrichment of Ca in grain boundaries to form stable intergranular liquid films. However, the critical concentration of Ca was found to be in the range of only a few tens of ppm, which is lower than that of Si by almost 2 orders of magnitude. The equivalent silica concentration to form such a stable intergranular calcium aluminate glass film and its minimum thickness were estimated from the inverse relationship with the assumption that the glass composition is close to calcium hexaluminate.     

Abnormal Grain Growth in Alumina: Synergistic Effects of Yttria and Silica

Abnormal grain growth without strong anisotropy or faceting of the grains has been observed in high-purity yttria-doped alumina specimens, often starting at the surface and spreading right through the bulk at higher sintering temperatures. This appears to occur because of an interaction between Si contamination from sintering and the yttria doping; no such effect is seen for undoped samples. Similar microstructures were observed after deliberate Y/Si codoping. Analytical STEM showed that some grain boundaries bordering on large grains contained more Si than Y. HRTEM and diffuse dark-field imaging revealed thin (0.5–0.9 nm) disordered layers at some boundaries bordering large grains. It appears that Si impurities are accumulating at some boundaries and together with the Y inducing a grain boundary structural transformation that accounts for the dramatically increased mobility of these boundaries.     

Effect of Liquid Content on the Abnormal Grain Growth of Alumina

Alumina specimens with small amounts of CaO and TiO₂ were prepared and their microstructural evolution during sintering was investigated. Because of the appearance of a liquid phase during sintering, a duplex microstructure of a few abnormal grains and fine matrix grains was obtained when the CaO + TiO₂ content was small (≤0.04 wt%). When the CaO + TiO₂ content was relatively high (≥0.1 wt%), many grains grew and impinged upon each other. As a result, a rather uniform and homogeneous microstructure was observed.     

Abnormal Grain Growth in Alumina with Anorthite Liquid and the Effect of MgO Addition

Abnormal grain growth (AGG) in alumina with anorthite liquid has been observed with varying anorthite and MgO contents, at 1620°C. When only anorthite is added to form a liquid matrix, the grain–liquid interfaces have either flat or hill-and-valley shapes indicating atomically flat (singular) structures. The large grains grow at accelerated rates to produce AGG structures with large grains elongated along their basal planes. This is consistent with the slow growth at low driving forces and accelerated growth above a critical driving force predicted by the two-dimensional nucleation theory of surface steps. With increasing temperature, the AGG rate increases. The number density of the abnormally large grains increases with increasing anorthite content. The addition of MgO causes some grain–liquid interfaces to become curved and hence atomically rough. The grains also become nearly equiaxed. With increasing MgO content the number density of the abnormally large grains increases until the grain growth resembles normal growth. This result is qualitatively consistent with the decreasing surface step free energy associated with partial interface roughening transition.     

Transparent Alumina with Submicrometer Grains by Float Packing and Sintering

Commercially available α-alumina powder with high-purity, submicrometer particle size and narrow particle-size distribution is used as starting material to prepare dense ceramic parts with transparent properties. The powder is dispersed and stabilized in a water-based suspension. Controlled consolidation and drying by float packing leads to homogeneous green compacts, which can be densified without additives by sintering in air at 1275°C to transparency, while the mean grain size remains at 0.4 μm. The in-line transmittance for wavelengths of 300–450 nm is comparable to commercial polycrystalline alumina tubes for lighting technologies, whose grain sizes are larger by a factor of 40.     

Abnormal Grain Growth at the Interface of Centrifugally Cast Alumina Bilayer during Sintering

Extensive abnormal grain growth occurs locally at the interface of centrifugally cast bilayers of MgO-doped (1000 ppm) Al₂O₃ (99.8% purity) during sintering. Results of systematic experiments indicate that the abnormal grain growth is attributable to particle aggregates segregated at the interface during centrifugal casting. Apparently, the particle aggregates contain a higher concentration of impurities than do the fine primary particles, causing local abnormal grain growth at the interface.     

Effect of Coarse-Powder Portion on Abnormal Grain Growth during Hot Pressing of Commercial-Purity Alumina Powder

By classification, two powder portions, one consisting of coarse particles and the other consisting of fine particles, were separated from a MgO-doped (1000 ppm) commercial-purity Al₂O₃ powder. Examinations of microstructure evolution during hot pressing showed that extensive abnormal grain growth occurred for the coarse portion. For the fine portion, although there was an indication that grain-size distribution deviated from normal distribution on prolonged hot pressing, such extensive abnormal grain growth did not occur. Extensive abnormal grain growth also occurred when the coarse portion was mixed into a high-purity powder that exhibited no abnormal grain growth alone. Chemical analyses revealed that the coarse portion contained the higher concentration of impurities but lower concentration of magnesium than the fine portion. It was discussed that particle aggregates in the coarse portion might have been responsible for the higher concentration of impurities but lower concentration of magnesium and, thus, for the extensive abnormal grain growth.     

Micromechanics of Deformation in Porous Liquid-Phase-Sintered Alumina under Hertzian Contact

A series of fine-grained porous alumina samples, with and without a liquid phase, were fabricated in compositions matched closely to commercially available alumina used as microelectronic substrates. Hertzian indentation on monolithic specimens of the glass-containing samples produced a greater quasi-ductile stress–strain response compared with that observed in the pure alumina. Maximum residual indentation depths, determined from surface profilometry, correlated with the stress–strain results. Moreover, microstructural observations from bonded interface specimens revealed significantly more damage in the form of microcracking and under extreme loading, pore collapse, in the glass-containing specimens. The absence of the typical twin faulting mechanism observed for larger-grained alumina suggests that the damage mechanism for quasi-ductility in these fine-grained porous aluminas was derived from the pores acting as a stress concentrator and the grain boundary glass phase providing a weak path for short crack propagation.     

Dihedral Angles in Magnesia and Alumina: Distributions from Surface Thermal Grooves

Dihedral angles, Ψ_s, from surface thermal grooves were measured using a metal reference line technique for polycrystalline MgO, undoped Al₂O₃, and MgO-doped Al₂O₃. The values of Ψ_s span the following ranges: 89° to 116° for MgO at 1520 K, 76° to 166° for undoped Al₂O₃ at 1870 K, and 90° to 139° for MgO-doped Al₂O₃ at 1870 K. For all three systems, the median Ψ_s values are 105° to 113°, implying that the median γ_gb/γ_s is 1.1 to 1.2, in contrast to metals where γ_gb/γ_s ranges typically from 1/4 to 1/2. The widths in Ψ_s distributions were different for the three materials with the width increasing in the following order: MgO, MgO-doped Al₂O₃, undoped Al₂O₃. For all three materials, the grainboundary grooves and their corresponding Ψ_s were not symmetrical with respect to the surface normal. The asymmetry for MgO was due to the pinning of the grain boundaries by the surface thermal grooves. The range of inclination angles of the grain boundary to the surface was a function of Ψ_s, with the maximum inclination angles of ∼13°, in quantitative agreement with theory.     

Kinked Grain Boundaries in Alumina Doped with Y2O3

Polycrystalline alumina specimens with and without MgO doping show smoothly curved grain boundaries after heat treatment at 1400°C indicating their rough structure. When heat-treated at 1400° and 1500°C for 24 h after packing in an alumina–YAG powder mixture, many grain boundaries (without any liquid phase) develop kinks of large and small scales as observed by scanning electron microscopy and transmission electron microscopy. The addition of Y₂O₃ at concentrations close to the solubility limit is thus shown to induce the grain boundary transition to singular structures.     

Exuding Liquid from Grain Boundaries in Alumina

Bicrystals of alumina with anorthite glass in the boundary were prepared by hot pressing. Annealing of the bicrystals leads to the migration of the intergranular liquid to the free surface of the sample. It is proposed that the migration is driven by the difference in the wetting behavior of the free surface and the boundary.     

Effect of Titania and Lithia Doping on the Boundary Migration of Alumina under an Electric Field

Boundary migration under an electric field was investigated for pure, TiO₂-doped, and Li₂O-doped Al₂O₃ specimens. Boundary migration rates in TiO₂-doped and Li₂O-doped Al₂O₃ specimens were much faster compared with that of pure Al₂O₃. In all specimens, the migration rate was observed to depend on the applied bias direction. Compared with pure Al₂O₃, the dependence of boundary migration on bias direction became more pronounced in TiO₂-doped Al₂O₃ but less pronounced in Li₂O-doped Al₂O₃. The results were explained in terms of the variation of grain sizes, mobility, and electrostatic potential of boundaries because of doping.     

Effects of Magnesium Oxide on Grain-Boundary Segregation of Calcium During Sintering of Alumina

The advantage of certain amounts of MgO addition in alumina sintering has been realized, and it is common practice. In an attempt to understand the role of MgO in the presence of CaO in commercial-grade alumina, grain-boundary segregation of Ca was investigated by scanning Auger electron microprobe (SAM) using an ultrapure alumina after controlled doping of CaO and/or MgO. The commercial-grade alumina, which usually contains a small amount of CaO, is difficult to sinter to high density. The pure alumina composition (<99.999%) gives "clean" boundaries when it is sintered under "clean" conditions. As the powder was doped with 100 ppm of CaO and sintered at 1300° to 1500°C, all of the grain boundaries were enriched by Ca as observed by others. However, it was also discovered that some of the grain boundaries are enriched by an exceptionally high concentration of Ca. Such a large variation of Ca contents depending on the grain-boundary facets disappeared when samples were codoped with small amounts of MgO. The results suggest that MgO plays a beneficial role in controlling the anisotropic segregation of Ca to various interfaces including grain boundaries and pore surfaces during sintering of alumina. MgO thus enhances chemical homogeneity of commercial-grade alumina.     

Effect of Titania and Lithia Doping on the Boundary Migration of Alumina under an Electric Field

Boundary migration under an electric field was investigated for pure, TiO₂-doped, and Li₂O-doped Al₂O₃ specimens. Boundary migration rates in TiO₂-doped and Li₂O-doped Al₂O₃ specimens were much faster compared with that of pure Al₂O₃. In all specimens, the migration rate was observed to depend on the applied bias direction. Compared with pure Al₂O₃, the dependence of boundary migration on bias direction became more pronounced in TiO₂-doped Al₂O₃ but less pronounced in Li₂O-doped Al₂O₃. The results were explained in terms of the variation of grain sizes, mobility, and electrostatic potential of boundaries because of doping.     

Anisotropy of Grain Growth in Alumina

Grain growth in theoretically dense undoped and MgO-doped polycrystalline Al₂O₃ was studied, and average grain-boundary migration rates were compared with those of a -plane and c -plane sapphire during migration into the same undoped and MgO-doped materials. The results are discussed in terms of a grain-size-dependent grain-boundary mobility-grain-boundary energy product, M _bγ_b. The grainsize dependencies of the M _bγ_b products for seed and matrix grains differ. Seed orientation appears to affect the nature of solute-boundary interactions. The importance of grain-boundary structure on migration characteristics is also indicated by a demonstration of twin-formation-enhanced grain growth.     

Texture Development and Microstructure Evolution in Liquid-Phase-Sintered α-Alumina Ceramics Prepared by Templated Grain Growth

Texture development in alumina that contains calcia and silica and has been templated with platelet-shaped α-Al₂O₃ particles has been evaluated. The texture fraction is shown to be related directly to template growth. Texture quality is controlled by the template concentration, decreasing at template concentrations of >10%, as a result of template–template interactions during tape casting. Almost fully textured alumina has been obtained at template concentrations of ≥20%. The growth of template grains is much more rapid in the radial direction and is shown to be inversely related to the thickness of the grain-boundary liquid. The activation energy for growth (376 kJ/mol) and the inverse relation with the grain-boundary thickness indicate that template growth in the radial direction is controlled by Al³⁺ diffusion.     

Anisotropic Abnormal Grain Growth in TiO2/SiO2-Doped Alumina

The effect of TiO₂/SiO₂ addition on the grain growth of alumina was reinvestigated. TiO₂ promoted the grain growth, but there was no abnormal grain growth. However, codoping of TiO₂ and SiO₂ resulted in a duplex microstructure consisting of large platelike grains, ∼800 μm long and ∼100 μm thick, and fine matrix grains. The observed anisotropic abnormal grain growth was explained in terms of liquid formation during heat treatment.