Microstructural Studies in Alkoxide-Derived Mullite/Zirconia/Silicon Carbide-Whisker Composites

Mullite composites toughened with ZrO₂ (with or without a MgO or Y₂O₃ stabilizer) and/or SiC whiskers (SiC( w )) were fabricated by hot-pressing powders prepared from Al, Si, Zr, and Mg(Y) alkoxide precursors by a sol–gel process. Micro-structures were studied by using XRD. SEM, and analytical STEM. Pure mullite samples contained prismatic, preferentially oriented mullite grains. However, the addition of ZrO₂, as well as the hot-pressing temperature, affected the morphology and grain size in the composites; a fine, uniform, equiaxed microstructure was obtained. The effect of SiC( W ) was less pronounced than that of ZrO₂. Glassy phases were present in mullite and mullite/SiC( W ) composites, but were rarely observed in Al₂O₃-rich or ZrO₂-containing samples. The formation of zircon due to the reaction between ZrO₂ and SiO₂ and the considerable solid solution of SiO₂ in ZrO₂ prevented the formation of the glassy phase, whereas the reaction between Al₂O₃ and MgO in MgO-containing samples formed a spinel phase and also deprived the ZrO₂ phase of the stabilizer. Intergranular ZrO₂ particles were either monoclinic or tetragonal, depending on size and stabilizer content; small intragranular ZrO₂ inclusions were usually tetragonal in structure.     

Reactive Coating of Zircon on Mullite and Mullite—Alumina Substrates

A stable zirconia coating of 20–30 pm thickness on a mullite substrate with a different alumina/silica ratio was obtained by reactive coating of zircon. It is shown that the Al₂O₃/ SiO₂ ratio of the mullite substrate drastically affects the morphology of the zirconia coating. The results are explained on the basis of the Al₂O₃–SiO₂–ZrO₂ phase equilibria.     

Effect of Sol-Gel Mixing on Mullite Microstructure and Phase Equilibria in the α-Al2O3-SiO2 System

Starting powders containing 72 wt% Al₂O³ and 28 wt% SiO₂, were prepared by sol-gel methods classified as colloidal and polymeric. Compacts fired at 1700°C showed significant differences in microstructure. The specimens formed with the colloidal powder had mullite grains of prismatic shape and a liquid phase; with polymeric powder, mullite grains were granular with no liquid phase present. It is shown that the mullite grains in the first case are higher in AI₂O₃ content, resulting in an excess of SiO₂ which is the base for the liquid phase. In the second case, the mullite grains have the same Al₂O₃ content as the starting powders. The presence of a liquid phase in the first case is considered to be metastable, resulting from the nature of the starting materials and processing conditions employed.     

Fabrication of Mullite Ceramics With Ultrahigh Porosity by Gel Freeze Drying

Porous mullite (3Al₂O₃·2SiO₂) ceramics with an open porosity up to 92.9% were fabricated by a gel freeze-drying process. An alumina (Al₂O₃) gel mixed with ultrafine silica (SiO₂) was frozen and sublimation of ice crystals was carried out by drying the frozen body under a low pressure. Porous mullite ceramics were prepared in air at 1400°–1600°C due to the mullitization between Al₂O₃ and SiO₂. A complex and porous microstructure was formed, where large dentritic pores with a pore size of ∼100 μm contained small cellular pores of 1–10 μm on their internal walls. Owing to the complete mullitization, a relatively high-compressive strength of 1.52 MPa was obtained at an open porosity of 88.6%.     

Novel Low-Temperature Processing Route of Dense Mullite Ceramics by Reaction Sintering of Amorphous SiO2-Coated γ-Al2O3 Particle Nanocomposites

Dense mullite ceramics were successfully produced at temperatures below 1300°C from amorphous SiO₂-coated gamma-Al₂O₃ particle nanocomposites (AS-gammaA). This method reduces processing temperatures by similar/congruent300°C or more with respect to amorphous SiO₂-coated alpha-Al₂O₃ particle microcomposites (AS-alphaA) and to other Al₂O₃-SiO₂ reaction couples. The good densification behavior and the relatively low mullite formation temperature make AS-gammaA nanocomposites an excellent matrix raw material for polycrystalline aluminosilicate fiber-reinforced mullite composites.     

Corrosion of Mullite by Molten Salts

The interaction of molten salts of different Na₂O activities and mullite is examined with furnace and burner tests. The more-acidic molten salts form small amounts of Al₂O₃; the more-basic molten salts form various Na₂O–Al₂O₃–SiO₂ compounds. The results are interpreted using the Na₂O–Al₂O₃–SiO₂ ternary phase diagram, and some possible diffusion paths are discussed. The generally higher melting points of Na₂O–Al₂O₃–SiO₂ compounds lead to better behavior of mullite in molten salts, as compared to SiO₂-protected ceramics such as SiC. Mullite-coated SiC is discussed, and the corrosion behavior is evaluated.     

Effect of Oxygen Vacancies on the Hot Hardness of Mullite

The structure of mullite, which has a composition ranging from 3Al₂O₃·2SiO₂ to Al₂O₃·2SiO₂, contains ordered oxygen vacancies. Sillimanite, Al₂O₃·SiO₂, has a similar structure but with no vacancies. The indentation hardness of polycrystalline mullite (3Al₂O₃·2SiO₂) was measured from room temperature up to 1400°C and compared with that of single-crystal sillimanite (Al₂O₃·SiO₂) up to 1300°C. It was found that both materials show the same variation in hardness with temperature, suggesting that the structures have a similar resistance to plastic deformation, and therefore that the oxygen vacancies in the mullite structure are not the primary cause of mullite's resistance to high-temperature deformation.     

Novel Synthesis of Mullite Powder with High Surface Area

Boehmite (AlO(OH)) solid-solution gel, which yields stoichiometric mullite (3Al₂O₃2SiO₂) at high temperatures, has been prepared by the hydrazine method. The formation process leading to 3Al₂O₃2SiO₂ is discussed. The as-prepared powder and powders heated below 1200°C consist of very fine particles showing needlelike morphology, whereas the particles of mullite powder show thin prismatic morphology. The mullite powder after heating at 1300°C has a high surface area (87 m²/g).     

Mechanisms and Kinetics of Reaction-Bonded Aluminum Oxide Ceramics

Reaction-bonded Al₂O₃ (RBAO) ceramics were fabricated starting from mechanically alloyed Al₂O₃/Al, Al₂O₃/ Al/ZrO₂, and Al₂O₃/Al/ZrO₂/Zr mixtures. Isopressed compacts were heat-treated in air up to 1550°C. Reaction-bonding mechanisms, kinetics, and the influence of ZrO₂ and Zr additions are investigated. Independent of additive, oxidation of Al proceeds both as solid/gas and liquid/gas reaction, and the reaction kinetics follow a parabolic rate law. The reaction rate depends strongly on the particle size of Al. The activation energy of the reaction depends essentially on green density. Below the melting temperature of Al, in samples containing 45 vol% Al and 55 vol% Al₂O₃, it is 112 and 152 kJ/mol at ∼64% and ∼74% TD, respectively, while above the melting temperature, it lies in the range ∼ 26–33 kJ/mol. Zr additions reduce the activation energy to some extent. Samples with only ZrO₂ additions exhibit nearly the same activation energies as ZrO₂-free samples, though ZrO₂ has a very positive effect on the microstructural development in RBAO ceramics. Microstructure evolution and some strength data of RBAO bodies are also reported.     

Sintering and Crystallization of CaO–Al2O3–ZrO2–SiO2 Glasses Containing Different Amount of Al2O3

In this work several complementary techniques have been employed to carefully characterize the sintering and crystallization behavior of CaO–Al₂O₃–ZrO₂–SiO₂ glass powder compacts after different heat treatments. The research started from a new base glass 33.69 CaO–1.00 Al₂O₃–7.68 ZrO₂–55.43SiO₂ (mol%) to which 5 and 10 mol% Al₂O₃ were added. The glasses with higher amounts of alumina sintered at higher temperatures (953°C [lower amount] vs. 987°C [higher amount]). A combination of the linear shrinkage and viscosity data allowed to easily find the viscosity values corresponding to the beginning and the end of the sintering process. Anorthite and wollastonite crystals formed in the sintered samples, especially at lower temperatures. At higher temperatures, a new crystalline phase containing ZrO₂ (2CaO·4SiO₂·ZrO₂) appeared in all studied specimens.     

Fabrication and Properties of Low-Shrinkage Reaction-Bonded Mullite

Mullite ceramics were fabricated according to the recently developed reaction-bonded Al₂O₃ (RBAO) technology. Green compacts consisting of mechanically alloyed Al, SiC, and Al₂O₃ were heat-treated in two steps. During the first hold at 1200°C, Al and SiC were oxidized to form Al₂O₃ and SiO₂. On further heating, mullite was formed which then sintered during the second hold at 1550°C. All reactions involved in the process were associated with volume expansions that almost compensated for the shrinkage on sintering. Processing details and microstructure development are discussed. Reaction-bonded mullite ceramics exhibit high fracture strength, e.g., 290 MPa at a density of 97% of theoretical density.     

Reaction Bonding and Mechanical Properties of Mullite/Silicon Carbide Composites

Based on the RBAO technology, low-shrinkage mullite/SiC/ Al₂O₃/ZrO₂ composites were fabricated. A powder mixture of 40 vol% Al, 30 vol% A1₂O₃ and 30 vol% SiC was attrition milled in acetone with TZP balls which introduced a substantial ZrO₂ wear debris into the mixture. The precursor powder was isopressed at 300–900 MPa and heattreated in air by two different cycles resulting in various phase ratios in the final products. During heating, Al oxidizes to Al₂O₃ completely, while SiC oxidizes to SiO₂ only on its surface. Fast densification (at >1300°C) and mullite formation (at 1400°C) prevent further oxidation of the SiC particles. Because of the volume expansion associated with the oxidation of Al (28%), SiC (108%), and the mullitization (4.2%), sintering shrinkage is effectively compensated. The reaction-bonded composites exhibit low linear shrinkages and high strengths: shrinkages of 7.2%, 4.8%, and 3%, and strengths of 610, 580, and 490 MPa, corresponding to compaction pressure of 300, 600, and 900 MPa, respectively, were achieved in samples containing 49–55 vol% mullite. HIPing improved significantly the mechanical properties: a fracture strength of 490 MPa and a toughness of 4.1 MPa^.m^1/2 increased to 890 MPa and 6 MPa^.m^1/2, respectively.     

Phase Transformation of Diphasic Aluminosilicate Gels

Aluminosilicate gels with compositions Al₂O₂/SiO₂ and 2 were prepared by gelling a mixture of colloidal pseudo-boehmite and a silica sol prepared from acid-hydrolyzed Si(OC₂H₅)₄. Upon heating the pseudo-boehmite transforms to γ-Al₂O₃ around 400°C, then to δ-Al₂O₃ at 1050°C, and at 1200°C reacts with amorphous SiO₂ to form mullite. Some twinned θ-Al₂O₃ forms before mullite. Nonstoichiometric specimens have a similar transformation sequence, but form mullite grains with inclusions of either Al₂O₃ or cristobalite, often associated with dislocation networks or micropores. Mullite grains are formed by nucleation and growth and have equiaxed shape.     

Processing of Microcellular Mullite

A new processing route for manufacturing partially interconnected open-cell, microcellular mullite ceramics has been developed. The strategy adopted for making microcellular mullite ceramics entailed the following steps: (i) fabricating a formed body from combining polysiloxane, Al₂O₃ (a reactive filler), polymer microbeads (used as sacrificial templates), and Y₂O₃ (a sintering additive); (ii) cross-linking the polysiloxane in the formed body; (iii) transforming the polysiloxane by pyrolysis into SiO₂; and (iv) synthesizing mullite by reacting SiO₂ and Al₂O₃. By controlling the sintering temperature and the microbead and additive contents, it was possible to adjust the porosity so that it ranged from 38% to 85%.
The compressive strengths of the microcellular ceramics with ∼40% and ∼70% porosities were ∼90 and ∼10 MPa, respectively. The superior compressive strengths were attributed to the homogeneous distribution of small (≤20 μm), spherical cells with dense struts in the microcellular ceramics.     

Thermal Evolution and Sintering Behavior of a 2:1 Mullite Gel

The thermal evolution of a mullite gel of composition 2Al₂O₃·SiO₂ has been investigated. The gel crystallized at 1300°C into an alumina-rich mullite and corundum, instead of single-phase 2Al₂O₃·SiO₂ mullite. The amount of Al₂O₃ that dissolved in the mullite structure has been determined in the 1300–1780°C temperature range by measuring the mullite lattice parameters. A maximum limit for the amount of Al₂O₃ in solid solution has been observed. Densification of the gel powders has been analyzed up to temperatures of 1780°C. The microstructure of dense materials always showed the presence of residual Al₂O₃ particles.     

Oxygen Grain-Boundary Diffusion in Polycrystalline Mullite Ceramics

Oxygen tracer diffusivities of low- and high-alumina mullite ceramics (72 wt% Al₂O₃, 28 wt% SiO₂ and 78 wt% Al₂O₃, 22 wt% SiO₂, respectively) were determined. Gas/solid exchange experiments were conducted in an atmosphere enriched in the rare stable isotope ¹⁸O, and the resulting ¹⁸O isotope depth distributions were analyzed using SIMS depth profiling. The investigation showed that grain-boundary diffusivities for both mullite ceramics were several orders of magnitude higher than mullite volume diffusivity. Activation enthalpies of oxygen diffusion were 363 ± 25 kJ/mol for the low-alumina and 548 ± 46 kJ/mol for the high-alumina materials. Because the glassy grain-boundary films were not identified, the differences between the low- and high-alumina materials might be explained by different impurity concentrations in the grain boundaries of the two materials.     

Characterization and Sintering Behavior of Alkoxide-Derived Aluminosilicate Powders

Submicrometer SiO₂-Al₂O₃ powders with compositions of 46.5 to 76.6 wt% Al₂O₃ were prepared by hydrolysis of mixed alkoxides. Phase change, mullite composition, and particle size of powders with heating were analyzed by DTA, XRD, IR, BET, and TEM. As-produced amorphous powders partially transformed to mullite and Al-Si spinel at around 980°C. The compositions of mullite produced at 1400° and 1550°C were richer in Al₂O₃ than the compositions of stable mullite solid solutions predicted from the phase diagram of the SiO₂-Al₂O₃ system. Particle size decreased with increasing Al₂O₃ content. The sintered densities depended upon the amount of SiO₂-rich glassy phase formed during sintering and the green density expressed as a function of particle size.     

Decomposition of Mullite

Free surfaces of 2:1 mullite (2Al₂O₃·SiO₂) specimens decomposed with the evolution of SiO and O₂ when they were heated at high temperatures under low partial pressures of O₂; this reaction was analyzed thermodynamically. In addition, bubbles were observed at internal interfaces between mullite and fused-SiO₂ diffusion couples. These bubbles, when formed at 1 atm ambient pressure between 1650° and 1800°C, resulted from reaction of Si particles and residual SiO₂-rich glass in the fused cast mullite.     

Effects of Zirconium Oxide on the Reaction Bonding of Aluminum Oxide

In the reaction-bonded aluminum oxide (RBAO) process, attrition-milled Al/Al₂O₃ powder compacts are reacted in air to form Al₂O₃-based ceramics. Typically, ZrO₂ powder is added to control the final microstructure and properties, resulting in fine-grained, high-strength materials. In this study, the effects of ZrO₂ on the reactivity of RBAO samples were examined using thermogravimetric analysis, X-ray diffractometry, and other techniques. Previously, it was shown that a self-propagating reaction could occur during firing of RBAO in air. This reaction was attributed solely to the fast and highly exothermic oxidation of the aluminum on the surface of the sample, but the results of the current study show that it is related to a reaction between aluminum and ZrO₂, forming Al₂O₃ and Al₃Zr. As a consequence, a new reaction sequence for RBAO consistent with these observations has been proposed.     

Fabrication of Silicon Carbide-Mullite Composite by Melt Infiltration

The melt infiltration method was used to fabricate a SiC-mullite composite at high temperature. Mullite was successfully obtained from a SiO₂ and Al₂O₃ powder mixture by melting above 1830°C in a BN crucible with a lid. When infiltrated into a porous SiC preform, the mullite significantly reacted with SiC to form gaseous SiO and CO, even at the lowest investigated temperature of 1830°C, consuming SiO₂ and leaving Al₂O₃ and silicon phases in the sample. The relevant reactions were studied in detail. A closed system was adopted to suppress the reaction, and a dense composite was successfully obtained.