Origin and Growth Kinetics of Platelike Abnormal Grains in Liquid-Phase-Sintered Alumina

The grain-growth behavior of Al₂O₃ compacts with small contents (≤10 wt%) of various liquid-forming dopants was studied. Equiaxed and/or elongated grains were observed for the following dopants: MgO, CaO, SiO₂, or CaO + TiO₂. The platelike grains, defined as the abnormal grains larger than 100 μm with an aspect ratio ≥5 and with flat boundaries along the long axis, were observed when the boundaries were wet with the liquid phase and the codoping satisfied two conditions of size and valence. These dopings were Na₂O + SiO₂, CaO + SiO₂, SrO + SiO₂, or BaO + SiO₂. However, an addition of MgO to the Al₂O₃ doped with CaO + SiO₂ resulted in the change of grain shape from platelike to equiaxial. Equiaxed grains were also observed for the MgO + SiO₂ doping, indicating that two conditions were necessary but not sufficient to develop the platelike grains. The fast growth rate of the platelike grains was explained by an increased interfacial reaction rate due to the codopants. AT the same time the codopants made the basal plane, which appeared as the flat boundaries, the lowest energy plane. The appearance of the platelike grains was favored in compacts with a small grain size and with a narrow size distribution at the onset of abnormal grain growth. Accordingly, the use of starting powders with a small particle size and narrow size distribution, smaller amounts of dopings, and high sintering temperature resulted in an increased number of the platelike grains.     

Singular Grain Boundaries in Alumina Doped with Silica

Pure Al₂O₃ powder compact sintered at 1400°C after adding 100 mol ppm of SiO₂ shows grain boundaries that are flat, even across the triple junctions. TEM observations show that these flat grain boundaries are parallel to the basal planes of the grains on one side. These flat grain boundaries must be singular. At such a low SiO₂ concentration and a low temperature, it is very unlikely that any liquid phase is present at these grain boundaries to cause such flat boundary shapes.     

Effect of Magnesium Oxide Addition on Surface Roughening of Alumina Grains in Anorthite Liquid

Alumina sintered with 5 wt% anorthite at 1620°C for 48 h has grains with flat boundaries and a size distribution representing abnormal grain growth. TEM observations of the grain triple junctions show flat grain surfaces, some of which are the (0001), ([Onemacr]012), and (1[Onemacr]01) planes. HRTEM observations confirm these surfaces to be atomically flat and hence singular. When sintered further after embedding in MgO powder, the {0001} and { 01[Onemacr]2} planes remain flat, but curved surface segments also appear. These curved surface segments are confirmed to be atomically rough by HRTEM. They are connected to the flat segments with discontinuously changing slopes. Thus, when MgO is added, the singular and rough surface phases coexist.     

Silica and Magnesia Dopant Distributions in Alumina by High-Resolution Scanning Secondary Ion Mass Spectrometry

Trace SiO₂ and MgO additive distributions in sintered alumina have been studied using high-resolution scanning secondary ion mass spectrometry (SIMS). When doped with each additive individually, evidence is seen for both strong silicon segregation to grain boundaries ( C _gb/ C _grain similar/congruent 300) in SiO₂-doped alumina and strong magnesium segregation to grain boundaries ( C _gb/ C _grain similar/congruent 400) in MgO-doped alumina. When codoped with both SiO₂ and MgO, segregation of both ions to grain boundaries is reduced by a factor of 5 or more over single doping. The additive concentrations increase proportionally in the grains, and both dopants become more uniformly distributed throughout the bulk. It is concluded that codoping with these additives increases their mutual bulk solid solubility and decreases their interfacial segregation over single doping. The beneficial effect of MgO additions in controlling microstructure development in alumina and improving corrosion resistance to aqueous HF stems from its ability to redistribute silicon ions from grain boundaries into the bulk.     

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 CaO on the Thermal Conductivity of Aluminum Nitride

The effect of CaO on the thermal conductivity of aluminum nitride pressureless sintered with 3 wt% Y₂O₃ as a sintering aid was investigated. Over the composition range of 0 to 2.0 wt% additions, CaO decreased the thermal conductivity of the sintered parts by 10%. CaO doping rendered the secondary oxide phases more wetting and thus with a greater tendency to penetrate along the grain boundaries. Furthermore, CaO segregation to the grain boundaries was observed even on those grain boundaries apparently free of secondary phases. These microstructural changes disrupted the connectivity of the high thermal conductivity AIN grains and were the main factors contributing to the decrease in the thermal conductivity of the ceramic parts. CaO additions to samples doped with SiO₂ had the opposite effect, increasing the thermal conductivity. CaO removed SiO₂ from the AIN grains and incorporated it into the oxide second phases, most likely through charge-compensating substitutions Ca²⁺+ Si⁴⁺ for Y³⁺ and/or Al³⁺. Thus, AIN samples containing both SiO₂ and CaO had higher thermal conductivity than those containing comparable amounts of SiO₂ alone.     

Conversion of Polycrystalline Al2O3 into Single-Crystal Sapphire by Abnormal Grain Growth

In a given batch more than 30%–40% of polycrystalline, MgO-doped Al₂O₃ tubes were converted into single crystals of sapphire by abnormal grain growth (AGG) in the solid state at 1880°C. Most crystals grew 4–10-cm in length in tubes with wall thicknesses of 1/2 and 3/4 mm and outer diameters of 5 and 7 mm, respectively, and had their c -axes oriented ∼ 90° and 45° to the tube axis. Initiation of AGG was associated with low values of bulk MgO concentration near 50 ppm. The unconverted tubes did not develop centimeter-size crystals but instead exhibited millimeter-size grains. The different grain structures in converted and unconverted tubes may be related to nonuniform concentration of MgO in the extruded tubes. The growth front of the migrating crystal boundary was typically nonuniformly shaped, and the interface between the single crystal and the polycrystalline matrix was composed of many "curved" boundary segments indicative of classical AGG in a single-phase material. The average velocities of many migrating crystal boundaries were quite high and reached ∼1.5 cm/h. The average grain boundary mobility at 1880°C was calculated as 2 × 10⁻¹⁰ m³/(N·s), representing the highest value reported so far in Al₂O₃ and within a factor of 2.5 of the calculated intrinsic mobility. Under similar experimental conditions sapphire crystals did not grow when a codopant of CaO, La₂O₃, or ZrO₂ was added in concentrations of several hundred ppm.     

Effect of MgO Addition on the Microstructure Development of 3 mol% Y2O3—ZrO2

MgO addition to 3 mol% Y₂O₃–ZrO₂ resulted in enhanced densification at 1350°C by a liquid-phase sintering mechanism. This liquid phase resulted from reaction of MgO with trace impurities of CaO and SiO₂ in the starting powder. The bimodal grain structure thus obtained was characterized by large cubic ZrO₂ grains with tetragonal ZrO₂ precipitates, which were surrounded by either small tetragonal grains or monoclinic grains, depending on the heat-treatment schedule.     

Faceting Behavior of Alumina in the Presence of a Glass

The faceting of alumina interfaces in the presence of a glass affects both grain growth and grain-boundary mobility during liquid-phase sintering. The geometry and movement of facets that form during this sintering process are expected to play an essential role in the development of the final microstructure, in particular, by their influence on the topology of the grain boundaries which ultimately control the properties of Al₂O₃ compacts. A new method for studying the interaction between Al₂O₃ and a glass has been developed. A thin sample of Al₂O₃ suitable for examination in a transmission electron microscope is prepared and examined and then reacted with SiO₂ and CaO via the vapor phase. This experimental approach allows the faceting behavior of glass/Al₂O₃ interfaces to be studied systematically without introducing unnecessary complications during subsequent sample preparation. Faceting occurs almost exclusively on the (0001) and {1 1 02} planes. The interaction between glass and certain structured grain boundaries in alumina has been studied using polycrystalline thin films.     

Facet–Defacet Transition of Grain Boundaries in Alumina

Grain boundaries in pure alumina powder compacts sintered at 1400°C are smoothly curved, indicating that they have atomically rough structures. When these specimens are heat-treated at temperatures between 900° and 1100°C, a small fraction of the grain boundaries develop either hill-and-valley or kinked shapes with flat segments. Some of these flat boundary segments lie on the {011[Twomacr]} plane of one of the grain pairs. These grain boundaries thus appear to become singular at these temperatures. When a corundum crystal with a basal surface is sintered in alumina powder at 1400°C, all grain boundaries formed between the corundum basal surface and small grains, as well as those between the small grains, are smoothly curved, indicating their rough structure. When heat-treated at 900°C for 3 days, about 30% of the grain boundaries between the corundum basal surface and the small grains develop kinks with flat boundary segments, and some of these flat segments lie on the basal plane of the corundum. When heat-treated again at 1400°C, all grain boundaries are curved, indicating that they become reversibly rough. These observations show that at least some of the grain boundaries in alumina undergo roughening-singular transitions at temperatures between 900° and 1100°C.     

Mechanism for the Role of Magnesia in the Sintering of Alumina Containing Small Amounts of a liquid Phase

Microstructural morphology in A1₂0₃ containing trace amounts (∼0.25 vol%) of a silicate-based amorphous phase was studied, as a finction of MgO additions, using transmission electron microscopy. Incipient abnormal grains develop during hot-pressing of undoped Al₂O₃ powder and are characterized by a large aspect ratio (>3) and long basal plane {0001} facets. The facets are completely wet by a thin (∼3 nm) amorphous phase in contrast with grain ends, which appear to be devoid of an amorphous phase. The shape of the grains is believed to result from differences in mobility between clean (i.e., unwet) grain boundaries and intergranular films which are liquid at the firing temperature. Doping with trace additions of MgO (Mg/A1=250 ppm) results in smaller, more uniform grain structures. Many grains in the MgO doped material exhibit all the features of the incipient abnormal grains in the undoped material, with the exception of the large aspect ratio. It is concluded that the role of MgO in such a system is a grain-growth inhibitor and a microstructural stabilizer. It is proposed that the additive operates by reducing the mobility difference between clean (unwet) boundaries and grain boundaries wetted by thin amorphous films. It is believed the additive accomplishes this primarily by reducing the mobility of the clean grain boundaries via a solid-solution pinning mechanism.     

Crystallization of MgO-CaO-SiO2-P2O5 Glass

The crystallization behavior of a glass with a composition of 40 wt% 3CaO · P₂O₅−60 wt% CaO · MgO · 2SiO₂ was investigated. The primary crystalline phase was apatite with a dendritic form and ellipsoidal shape. β-(3CaO · P₂O₅) and CaO · MgO · 2SiO₂ were crystallized as samples heated to 990°C, and a three-layer structure was obtained. The development and morphology of this construction were explained by both the surface crystallization of the apatite and CaO · MgO · 2SiO₂ and the bulk crystallization of apatite and the CaO · MgO · 2SiO₂-β-(3CaO · P₂O₅) composite.     

Evidence of Bilevel Solubility in the Bimodal Microstructure of TiO2-Doped Alumina

A phenomenon of bilevel solubility was observed in a TiO₂-doped (2.0 wt%) alumina with bimodal microstructure sintered in N₂. Surface contributions to dopant signals in individual grains were identified and removed, using a spatially resolved energy dispersive spectroscopy analysis. Two levels of solubility, 1.005 ± 0.166 and 0.504 ± 0.082 mol% of TiO₂ in Al₂O₃, were obtained for anisotropic and equiaxed grains, respectively. No grain size dependence of solubility was found, but segregation of SiO₂ to boundaries with the anisotropic grains was observed. This phenomenon was explained by the incorporation of Ti³⁺ into the Al₂O₃ lattice during the abnormal grain growth caused by SiO₂ liquid under a reducing atmosphere.     

Indirect Dissolution of Sapphire into Calcia-Magnesia-Alumina-Silica Melts: Electron Microprobe Analysis of the Dissolution Process

Spinel, MgAl₂O₄, has been observed to form on sapphire during sapphire dissolution into CaO-MgO-Al₂O₃-SiO₂ (CMAS) melts at 1450°. and 1550°C. Electron microprobe analysis was used to characterize the sapphire/melt interface for cases in which spinel did (indirect dissolution) or did not (direct dissolution) form on the sapphire during dissolution into CMAS melts. The concentrations of Al₂O₃, MgO, CaO, and SiO₂ were determined as a function of position within the spinel reaction product and in the adjacent melt. The rate-limiting steps for direct and indirect sapphire dissolution into CMAS melts are discussed.     

Analytical Electron Microscopic Studies of Doped Dicalcium Silicates

Dicalcium silicates having CaO/SiO₂ molar ratios of 1.8 to 2.2 were sintered at 1450°C for 90 min with or without small quantities of dopants (K₂O or Al₂O₃) and were air quenched. The microstructures of the fired samples were characterized using electron microscopy (SEM and TEM) and associated microanalytical techniques. There was no evidence for the existence of Ca_1.8SiO_3.8 or Ca_2.2SiO_4.2. Amorphous grain-boundary phases were observed between grains and as inclusions within the grains; the amounts decreased as CaO/SiO₂ ratios increased. The compositions of the amorphous phases were always rich in dopants and had a CaO/SiO₂ ratio close to that of wollastonite. High levels of Al₂O₃ were observed to enter the β-Ca₂SiO₄ grains under lime-rich conditions (CaO/SiO₂= 2.2) up to a saturation level of about 3.0 wt%. Some additional crystalline phases were observed to form depending on stoichiometry and dopant level.     

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.     

Silica Glass Segregation in 3 wt% LiF-Doped Hot-Pressed Y2Si2O7

Hot-pressed yttrium disilicate ceramics have been characterized using analytical transmission electron microscopy (TEM). The microstructure consists of large grains of the γ phase of stoichiometric γ-Y₂Si₂O₇ containing rounded glassy Y-doped SiO₂ inclusions; excess glassy SiO₂-rich material is also found at the grain boundaries. Two main reasons are found for the inhomogeneity: a slight SiO₂ excess is inferred from the composition measurements, and the LiF flux used in hot pressing would also promote glass formation. Improved high-temperature mechanical properties would only be possible if residual glass formation was minimized, strategies for doing so are discussed, and the importance of analytical TEM for studying such submicron scale inhomogeneity is underlined.     

Microstructural Characterization of Superplastic SiO2-doped TZP with a Small Amount of Oxide Addition

The microstructures of 5 wt% SiO₂-doped TZP, 5 wt% (SiO₂+ 2 wt% MgO)-doped TZP, and 5 wt% (SiO₂+ 2 wt% Al₂O₃)-doped TZP are characterized by high-resolution electron microscopy, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy. An amorphous phase is formed at multiple grain junctions but not along the grain-boundary faces in these three materials. A small addition of MgO and Al₂O₃ into the SiO₂ phase results in a marked reduction in tensile ductility of SiO₂-doped TZP. This reduction seems to correlate with segregation of magnesium or aluminum ions at grain boundaries and a resultant change in the chemical bonding state.     

Grain Growth in CaO-Stabilized ZrO2 in the Presence of a Liquid Phase

Calcia-stabilized zirconia, CSZ (7.5 wt% CaO), with impurities of A1₂O₃, SiO₂, MgO, Fe₂O₃, and TiO₂, was sintered in air at 1783 K for times (t) up to 230 h. The microstructure consisted of grains of CSZ with small amounts of pores and a CaO-Al₂O₃-SiO₂ eutectic. Grain diameters grew in proportion to t^1/3. The results are consistent with a mechanism in which grain growth is limited by diffusion of Zr⁴⁺ ions through the liquid eutectic.     

Alumina Sol-Assisted Sintering of SiC—Al2O3 Composites

The composite sol—gel (CSG) technology has been utilized to process SiC—Al₂O₃ ceramic/ceramic particulate reinforced composites with a high content of SiC (up to 50 vol%). Alumina sol, resulting from hydrolysis of aluminum isopropoxide, has been utilized as a dispersant and sintering additive. Microstructures of the composites (investigated using TEM) show the sol-originating phase present at grain boundaries, in particular at triple junctions, irrespective of the type of grain (i.e., SiC or Al₂O₃). It is hypothesized that the alumina film originating from the alumina sol reacts with SiO₂ film on the surface of SiC grains to form mullite or alumina-rich mullite-glass mixed phase. Effectively, SiC particles interconnect through this phase, facilitating formation of a dense body even at very high SiC content. Comparative sinterability studies were performed on similar SiC—Al₂O₃ compositions free of alumina sol. It appears that in these systems the large fraction of directly contacting SiC—SiC grains prevents full densification of the composite. The microhardness of SiC—Al₂O₃ sol—gel composites has been measured as a function of the content of SiC and sintering temperature. The highest microhardness of 22.9 GPa has been obtained for the composition 50 vol% SiC—50 vol% Al₂O₃, sintered at 1850°C.