Characterization of Al2O3 composites with fine Mo particulates,I. Microstructural development

Alumina based composites containing nano- or submicron-meter Mo grains in the amounts of 20 vol% or less were prepared through a dissolution of molybdenum oxide in ammonium solution, followed by spray-drying, hydrogen reduction and sintering with or without hot-pressing. The properties of alumina/molybdate solutions and the ζ-potential of alumina particles in the solution were measured. By using electron microscopic and quantified X-ray diffraction techniques, the microstructural features and the evolution of Mo paniculate in spray-dried powder and sintered bodies were analyzed. The time dependent exponent and activation energy of grain growth of Mo between 600 to 900 °C were determined. There is no glassy phase or reaction at the interfaces between Mo/Al₂O₃ of dense composites. Only one coherent interface was found, and the others are incoherent. The results reveal that submicrometric Mo grains may grow by surface diffusion in reduction stage (≤ 900 °C) and greatly retard the densification and reduce the grain size of alumina matrix in sintering stage.     

Microstructural development in the directed melt-oxidized (DIMOX) Al-Mg-Si alloys

Metal-ceramic composites produced via directed melt oxidation (DIMOX) of aluminium alloys are of recent interest. Thein situ composite forming method is based on the reaction of a molten alloy with a gaseous oxidant. In the present study, Al-Mg-Si alloys were subjected to directed melt oxidation and the progressive microstructural evolution was examined by interrupted growth experiments. In the early stages, liquid alloy oxidizes to form a duplex oxide layer (MgO+MgAl₂O₄) on the surface. The openings in these oxide layers allow the liquid alloy to wick through to form small nodules on the surface. When further wicking occurs through these nodules, a cauliflower type of colonies is formed. During the early part of the second stage, spinel growth dominates to form a multi-layered structure. In the final stage, as the magnesium reaches low levels, Al₂O₃ formation dominates the growth, and alumina crystals grow continuously for several tens of micrometres. The oxygen required for alumina formation is expected to come from two sources: (i) from the ingress of oxygen through microcracked oxide layers, and (ii) demixing of magnesium-containing oxides in the underneath layers.     

Thermal stability of aluminas by hydrothermal treatment of an alkoxide-derived gel

Alumina precursors were prepared by hydrothermal treatment of alkoxide-derived alcogels. The crystalline structure of precursor beohmites and their microstructural change during heat treatment were examined and the specific surface area of the alumina precursors after heating was measured. The alumina prepared by hydrothermal treatment at 270 °C retained high specific surface areas at high temperatures; e.g. 35.0, 8.3 and 5.4 m²g^–1 at 1200, 1400 and 1500 °C, respectively. The thermal stability of the aluminas depended on the hydrothermal temperatures. For excellent thermal stability, the following factors are necessary: (1) grain growth of beohmite as an alumina precursor, and a grain size of more than 20 nm for the (1 2 0) plane; (2) a crystallite size for the (2 0 0) plane exceeding that for the (0 0 2) plane; (3) anisotropic growth of the beohmite crystal. In the transition alumina region ( 1200 °C), the thermal stability of the alumina is caused by raising the transformation temperature, resulting from decreasing the number of grain boundaries by beohmite growth. In the -alumina region (> 1200 °C), inhibiting the three-dimensional grain growth achieves thermal stability, resulting from preservation of the anisotropic structure introduced into the beohmite.     

The effect of the composition of the intergranular film in alumina on preferential adsorption and growth

Composition of the intergranular film (IGF) found between alumina crystals has been shown to affect growth. Here, we employ molecular dynamics (MD) computer simulations to study the effect of the composition of calcium alumino-silicate IGFs on preferential adsorption and grain growth in α-Al₂O₃ at an atomistic level. The IGF is formed while in contact with two differently oriented crystals, with results showing preferential adsorption and growth along the [ ] direction of the ( ) surface in comparison to that along the surface normal on the (0001) surface for certain calcium alumino-silicate compositions. Such preferential growth is consistent with experimentally observed anisotropic grain growth in alumina, where platelets form because of faster outward growth of the prism orientations than the basal orientation. The simulations show the mechanism by which Ca ions in the IGF inhibit growth on the basal surface and the important role that the Ca/Al ratio in the IGF plays in the change from isotropic to anisotropic grain growth. At compositions with high or low Ca/Al ratios, growth along each surface normal is equivalent, indicating isotropic grain growth. The simulations provide an atomistic view of attachment onto crystal surfaces, affecting grain growth in alumina, and the importance of local chemistry of the IGF on local adsorption and growth behavior.     

The preparation of AI2O3/Ni composites by a powder coating technique

In the present study, nickel particles are coated onto the surface of alumina powder by an impregnation technique. The densification behaviour and the microstructural evolution of the nickel coated alumina powder during sintering are investigated. The strength and the toughness of the resulting Al₂O₃/Ni composites are determined. As the nickel content is less than 13 vol%, fully dense composites can be prepared by pressureless sintering. The matrix grain size decreases as nickel inclusions are added. The strength and the toughness of alumina can be increased by 23 and 42% by adding 5 and 8vol% nickel, respectively. The toughening effect is attributed to plastic deformation of ductile inclusions and crack deflection by the inclusions. The strengthening effect is attributed to microstructural refinement.     

Characterization of structural alumina ceramics used in ballistic armour and wear applications

Structural alumina ceramics used in ballistic armour and wear applications with varying alumina contents and manufactured using both slip casting and dry pressing techniques, have been investigated and characterized in terms of their hardness, elastic modulus, fracture toughness, and microstructural characteristics. For a given alumina content, fracture toughness decreases with increasing hardness. Dry pressed samples show slightly higher hardness, and lower fracture toughness for the same alumina content. The hardness, elastic modulus and fracture toughness are higher for the 98% alumina samples while the differences between the lower alumina samples (95 and 91%) are negligible. The grain sizes are bimodal with the majority 3 m and the size range narrows with decreasing alumina content. The microstructures are composed of a matrix phase, corundum (-Al₂O₃), grain boundary phases consisting of a glassy phase with varying Al₂O₃, SiO₂, and CaO contents, a crystalline phase, triclinic anorthite (CaAl₂Si₂O₈), and an additional phase, spinel (MgAl₂O₄), in the lower alumina samples. The proportion of the boundary phase increases with decreasing alumina content and no effect of fabrication method is observed.     

Effect of CeO2 on reaction-sintered mullite-ZrO2 ceramics

The effect of ceria on mullite formation and the sintering of zircon and alumina powders was investigated. Quantitative X-ray powder analysis was used to determine the formation of mullite and zirconia of both monoclinic and tetragonal forms. Scanning electron microscopy and electron-probe microanalysis were used for microstructural analysis. It was found that the addition of CeO₂ enhanced the formation of mullite and increased the fraction of tetragonal zirconia. The addition of CeO₂ caused the formation of mullite directly from reaction of zircon with alumina without decomposition of zircon into zirconia and silica. In addition to forming a liquid phase, the ceria essentially formed a solid solution with zirconia. The fracture toughness of the mullite-zirconia composites was about 5.5–6.0 MPa m^1/2.     

The effects of β-alumina on the production of Al2O3/Al composites by the directed melt oxidation process

Directed melt oxidation (DMOX) of pure aluminium has been used to produce Al/Al₂O₃ composites by growth into a particulate alumina filler in the absence of any dopants apart from a -Al₂O₃ impurity in the filler. The microstructural development and mechanisms of growth of these composites have been investigated. It is shown that the Al₂O₃ filler used in this work has both chemical and physical effects on the reaction process. The -Al₂O₃ impurity introduces sodium into the system; this increases the wettability of alumina (both filler and oxidation reaction product) by molten aluminium, and initiates DMOX reactions. In addition, the filler particle size has an effect on the directed oxidation reaction. If the particle size is too fine, no oxidation growth takes place. Filler particles limit the ingress of oxygen through the reaction front so that AIN instead of Al₂O₃ may be formed in regions behind the main reaction front. Although such AIN production is seen when magnesium is used as a dopant to initiate DMOX reactions in the Al/Al₂O₃ system, it is more marked with sodium, because the latter has a greater effect on the wettability of alumina by aluminium.     

Effect of porosity and alumina content on the high temperature mechanical properties of compocast aluminium alloy-alumina particulate composite

The effects of volume fraction of alumina and porosity on the tensile strength of Al-4 wt% Mg-alumina compocast particulate composite tested at various temperatures up to 623 K have0 been investigated with the help of a phenomenological model. The contribution of porosity on reduction of strength of composites at various levels of alumina content has been expressed as a linear function of porosity and the resulting equation contains two experimentally determined parameters, ₀, the ultimate tensile strength at zero porosity level and , a weakening factor. It is observed that decreases with an increase in volume fraction of alumine in the composite and it becomes more sensitive to alumina content of the composite with a rise in temperature. Ate given alumina contenta increases with a rise in test temperature but this effect is gradually countered by increasing alumine content of the composite. Finally, in a composite having 10.3 vol % alumina decreases with an increase in temperature. This may have occurred because the extent of particle-matrix debonding is determined by the plastic soak in the matrix and the fracture strain of a composite increases or decreases with temperature when the alumina content lies below or above 9.0 vol % of alumina respectively. At any test temperature ₀ of the composite decreases rapidly with an increase in volume fraction of alumina, but the rate of decrease of ₀ reduces at higher alumina levels. However at the elevated temperatures of 473K and 573K a sharp fall in ₀ is observed at alumina contents beyond 9.0 vol %. At a lower level of alumina content below about 8.98 vol % the fracture strain of the composites increases with an increase in temperature. However, in the case of higher alumina content beyond the level mentioned above the fracture strain of the composites decreases with the rise in temperature. At a given porosity level the fracture strain of a composite having about 9.4 vol % alumina decreases with an increase in temperature. Scanning electron microscopic observations show that the extent of the growth and linkage of voids before fracture become extensive at higher temperature. At ambient temperature the composites fail by a mixed mode of ductile and clevage fracture. At 573K a number of considerably small dimples along with the larger ones are observed in the fractured surface. At this temperature a large number of newly formed fine grains are observed in the matrix.     

Microstructural evolution during direct laser sintering in the Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> system

The microstructural evolution during direct laser sintering of LSD (Layerwise Slurry Deposition)—samples in the Al₂O₃–SiO₂ system has been investigated. Slurries with a water content of 34 wt.% and a SiO₂/Al₂O₃—ratio of about 3:1 have been used to manufacture layers which—after consecutive drying—have been sintered and laminated by laser treatment. Densified samples can be obtained with laser irradiances from 190 to 270 kW/cm² and scan velocities between 35 and 65 mm/s. Elemental mappings of the layers’ cross sections suggest an inhomogeneous phase distribution in the laser sintered LSD samples with a slight alumina concentration gradient. A lower degree of particle melting in the bottom region of the layers is plausible due to attenuation of the laser beam intensity. SEM and HRTEM micrographs show that after a few seconds of laser treatment relictic starting phase, crystalline alumina plus amorphous silica, occur together with needle like mullite, the latter formed within an amorphous aluminosilicate phase. The resulting phase assemblage reflects the non-equilibrium conditions which can be expected for short time laser treatments. Mullite nucleation within the bulk of the liquid phase rather than in the vicinity of the parent alumina phase suggests that dissolution of alumina is the rate controlling step. Subsequent thermal post treatment in air in a conventional sintering furnace causes an increase of density to about 96% and leads to additional phase reactions. Amorphous silica transforms into cristobalite and the amount of alumina is reduced by additional mullite formation. By both coalescence of individual crystals and grain growth the morphology of the newly formed mullite changes during post heat treatment.     

Phase formation and texture development in mullite/zirconia composites fabricated by templated grain growth

Mullite is a promising candidate for advanced ceramic applications but its low fracture toughness and difficulties in sintering are the main limitations for more widespread industrial applications. Therefore, mullite/zirconia composites were prepared from a reactive mixture of alumina and zircon powders. Additives, TiO₂ and MgO, were used to modify aluminosilicate glass to increase densification and <001> aluminum borate templates were incorporated to texture mullite in [001] by templated grain growth. Mullite/zirconia phase formation was complete at 1450°C in the presence of both templates and additives, as compared to 1500°C for the samples with only additives and to 1600°C for the samples with only templates. Dense mullite/zirconia composites with highly <001>-textured mullite grains (Lotgering factor ∼1) and a retention of ∼13% tetragonal ZrO₂ were fabricated after sintering at 1450°C for 2 h. A high quality of mullite texture with a degree of orientation parameter of 0.22 and a narrow distribution of elongated mullite grains within 8.8° around [001] were successfully obtained in the composites.     

Material characterisation and mechanical properties of Al2O3-Al metal matrix composites

The mechanical properties of metal matrix composites (MMCs) are critical to their potential application as structural materials. A systematic examination of the effect of particulate volume fraction on the mechanical properties of an Al₂O₃-Al MMC has been undertaken. The material used was a powder metallurgy processed AA 6061 matrix alloy reinforced with MICRAL-20, a polycrystalline microsphere reinforcement consisting of a mixture of alumina and mullite. The volume fraction of the reinforcement was varied systematically from 5 to 30% in 5% intervals. The powder metallurgy composites were extruded then heat treated to the T6 condition. Extruded liquid metallurgy processed AA 6061 was used to establish the properties of the unreinforced material.     

Mullite formation from ethyl silicate and aluminium chlorides

The formation of mullite via gels prepared from technical ethyl silicate and aluminium chlorides has been studied. Normally, gels prepared specifically with the oxide stoichiometry of mullite (3Al₂O₃·2SiO₂) do not form the mineral mullite on firing to 1200° C in the absence of a mineralizer. However, when the stoichiometric gel is homogeneous (achieved by acidic or neutral catalysts during the gel preparation) firing at 1200° C can lead to an almost quantitative yield of mullite. For a homogeneous gel, the presence of strontium or caesium salts, or an organo-tin compound such as dibutyltin diacetate or dibutyltin oxide during the gel preparation promotes almost quantitative conversion to mullite at about 1000° C. There is a threshold concentration under which conversion to mullite is incomplete, some cristobalite being formed. For the organo-tin compounds, the type of aluminium chloride is unimportant and the way in which water for the hydrolysis step is added is also unimportant. When the gel is non-homogeneous, the product obtained on firing contains cristobalite and-alumina or-alumina, with little mullite, even if strontium or caesium salts, or organ-otin compounds are present. A ceramic bond is formed from alumina and some other refractory grains during firing.     

Effect of surface roughness on grain growth and sintering of alumina

The production of ceramic bodies with less surface roughness is industrially important when one considers the aspect of final machining processes. Hence an attempt have been made to study the variation in surface roughness parameters (R _a, R _y, R _z) of alumina having three different kinds of roughness features at different sintering temperatures. Variation in surface roughness properties are also correlated with grain size. R _z shows significant difference between fine and intermediate surfaces, hence predicts small difference in their microstructural features. As a general trend, average grain size increases with increase in sintering temperature, but wide distribution of grains with enhanced non-uniform grain growth is observed when the surface is coarse. Hence, creation of fine surface in the green body is necessary for homogeneously distributed grains with controlled uniform grain growth. The final roughness and grain size of the sintered alumina depend on the initial surface roughness of the green body.     

Ageing effect on microstructural and mechanical properties of mullite-ZrO2-TiO2 composites

The microstructural and mechanical properties of mullite-zirconia composites with TiO₂ (0.25 and 1.0 mol) additions have been studied, after ageing the samples over a wide temperature range (1000 to 1500° C) for long periods of time (100 to 200 h). In the sample with 0.25 mol TiO₂ addition, changes in mullite composition and in the solid state compatibility at temperatures below 1450° C were detected. In the sample containing 1 mol TiO₂, decomposition of Al₂TiO₅ occurs atT1200° C. Both compositions exhibit no increment in zirconia average grain size during ageing and, concomitantly, there is no strength degradation until higher temperatures (>1400° C) are reached, which become more drastic when Al₂Ti₅ is present.     

Alumina-mullite-zirconia composites obtained by reaction sintering Part II. R-Curve behavior

Applying Linear Elastic Fracture Mechanics equations and sample compliance variation to quantify the instantaneous crack length, the R-curve behavior of alumina-mullite-zirconia composites obtained by reaction sintering, was evaluated as a function of zirconia and mullite content. Changes in the R-curve profile as a function of the notch geometry (Chevron and straight-through notch) was observed and discussed, based on the analysis of the y() function applied to each notch type. The influence of the y() function in the R-curve shape was observed in both the initial and the final crack propagation region where, in the latter, the R-curves presented a sharp increase. In order to suppress these effects, the R-curve values for pure alumina were deducted from those obtained for the different composites produced. The analysis of the resulting curves highlights the influence of the amount of zirconia and mullite inclusions in these composites.     

Role of magnesia and silica in alumina microstructure evolution

The effects of MgO and SiO₂ additive distributions on alumina grain morphology have been characterized using high-resolution imaging secondary ion mass spectrometry (HRI-SIMS). In alumina samples singly-doped with MgO, the concentration of Mg segregated to grain boundaries is independent of grain boundary length for a majority of grain boundaries studied. Mg segregant therefore redistributes from grain boundaries to microstructural sinks, such as pores and/or second phases, during grain coarsening. In samples singly-doped with SiO₂, abnormal grain growth develops and the concentration of Si at grain boundaries is also independent of grain boundary length. Redistribution of segregants is again necessary in this case to maintain constant grain boundary composition. Codoping with Mg/Si > 1 suppresses abnormal grain growth as a result of increased mutual solid solubility of both ions and an associated decrease in grain boundary segregation. Grain growth kinetics for doped aluminas are reconsidered in light of these observations.     

Fabrication and mechanical properties of silicon carbide–silicon nitride nanocomposites

A powder mixture of ultrafine –SiC–35 wt% –Si₃N₄ containing 6 wt% Al₂O₃ and 4 wt% Y₂O₃ as sintering additives were liquid–phase sintered at 1800°C for 30 min by hot–pressing. The hot–pressed composites were subsequently annealed at 1920°C under nitrogen–gas–pressure to enhance grain growth. The average grain–size of the sintered bodies were ranged from 96 to 251 nm for SiC and from 202 to 407 nm for Si₃N₄, which were much finer than those of ordinary sintered SiC–Si₃N₄ composites. Both strength and fracture toughness of fine–grained SiC–Si₃N₄ composites increased with increasing grain size. Such results suggested that a small amount of grain growth in the fine–grained region (250 nm for SiC and 400 nm for Si₃N₄) was beneficial for mechanical properties of the composites. The room–temperature flexural strength and fracture toughness of the 8–h annealed composites were 698 MPa and 4.7 MPa · m^1/2, respectively.     

Effect of pH on 980 °C spinel phase-mullite formation of Al2O3-SiO2 gels

Different reaction paths of mullite formation via sol-gel processing techniques are reviewed. These variations are due to differences in hydrolysis/gelation behaviours of the silica and alumina components used. Variations of pH during processing without altering other variables follow three different routes of mullite formation. In the highly acidic region(pH 1), the gel does not exhibit a 980 °C exotherm but forms -Al₂O₃. Mullite forms at high temperature by diminution of -Al₂O₃ and -cristobalite, respectively. In the pH range of 3–4.5, gels exhibit a 980 °C exotherm and develop only mullite. In the highly alkaline region (pH 14), the gel produces a Si-Al spinel phase at the 980 °C exotherm and mullite formation at the 1330 °C exotherm takes place from the intermediate Si-Al spinel phase.     

Wet erosion behaviour of low SiC alumina-SiC nanocomposites

We have investigated the erosive wear behaviour of alumina and Al₂O₃-SiC nanocomposites with SiC content between 1 and 5%. Nanocomposites (grain sizes between 3.15 and 7.16 m) and alumina (grain size 4.43 m) were eroded with SiC particles using a custom-built erosive slurry wear tester. The erosion resistance of the nanocomposites increased slightly with decreasing grain size. Nanocomposites of all grain sizes showed better wear resistance than the alumina. Erosion resistance increases with SiC content, though this effect is not strong for SiC contents greater than 2%. These results are compared with related results from the literature.