Phase Evolution,Microstructure, and Mechanical Properties of Alumina–Mullite–Zirconia Composites Prepared by Iranian Andalusite

This paper investigates the effects of Iranian andalusite and short milling times on alumina–mullite–zirconia composites. Andalusite powder was added at 0, 2.5, 5, and 10 wt% to an alumina–zircon mixture and the raw materials were milled for 1 or 3 h. The sintering of samples was carried out at the temperatures of 1550°C, 1600°C, and 1650°C for 3 h. Microstructural changes, phase composition, physical properties, and mechanical strength of the sintered composites were characterized by scanning electron microscopy, X‐ray diffraction, density, and strength measurement tests. Results show that andalusite promoted the decomposition of zircon and accelerated the reaction sintering of alumina–zircon, which leads to the formation of much more mullite phase and improvements to the composites’ thermal shock resistance up to about 50%.     

Transient liquid phase sintering of high-purity mullite for high-temperature structural ceramics

High-purity mullite ceramics, promising engineering ceramics for high-temperature applications, were fabricated using transient liquid phase sintering to improve their high-temperature mechanical properties. Small amounts of ultrafine alumina or silica powders were uniformly mixed with the mullite precursor depending on the silica-alumina ratio of the resulting ceramics to allow for the formation of a transient liquid phase during sintering, thus, enhancing densification at the early stage of sintering and mullite formation by the reaction between additional alumina and the residual glassy phase (mullitization) at the final stage of sintering. The addition of alumina powder to the silica-rich mullite precursor resulted in a reaction between the glassy silica and alumina phases during sintering, thereby forming a mullite phase without inhibiting densification. The addition of fine silica powder to the mullite single-phase precursor led to densification with an abnormal grain growth of mullite, whereas some of the added silica remained as a glassy phase after sintering. The resulting mullite ceramics prepared using different powder compositions showed different sintering behaviors, depending on the amount of alumina added. Upon selecting an optimum process and the amount of alumina to be added, the pure mullite ceramics obtained via transient liquid phase sintering exhibited high density (approximately 99%) and excellent high-temperature flexural strength (approximately 320 MPa) at 1500 °C in air. These results clearly demonstrate that pure mullite ceramics fabricated via transient liquid phase sintering with compositions close to those of stoichiometric mullite could be a promising process for the fabrication of high-temperature structural ceramics used in an ambient atmosphere. The transient liquid phase sintering process proposed in this study could be a powerful processing tool that allows for the preparation of superior high-temperature structural ceramics used in the ambient processing atmosphere.     

Preparation and properties of hierarchical structural alumina/mullite composites

Hierarchical structural alumina/mullite composites constructed by alumina platelets inter-locked porous matrices and mullite whiskers secondary structures had been designed and prepared based on the fluorine-catalyzed gas-phase process. In the composites, mullite whiskers grew on the alumina platelets of the matrices to form cactus-like structures, before that, topaz rods as transitional secondary structures formed at lower temperature. The fluorine-etching effects on secondary structures’ nucleation were discussed. The alumina/mullite composites (1300 °C) had low bulk density (1.10 g/cm³), high porosity (71.4%), and proper compression strength (～14.2 MPa), meanwhile, the average filtration efficiencies of PM2.5 and PM10 during the filtration tests were 78% and 76%, respectively. The introduced mullite whiskers with length of ～1 µm had not induced obvious changes on the structural parameters, hence, the alumina/mullite composites (1300 °C) possessed similar particle filtration performances compared with the alumina porous matrices, and both of the two species materials could be applied for hot gas filtration.     

Microstructure and mechanical properties of in-situ grown mullite toughened 3Y-TZP zirconia ceramics fabricated by gelcasting

In-situ grown mullite toughened zirconia ceramics (mullite-zirconia ceramics) with excellent mechanical properties for potential applications in dental materials were fabricated by gelcasting combined with pressureless sintering. The effect of sintering temperature on the microstructure and mechanical properties of mullite-zirconia ceramics was investigated. The results indicated that the columnar mullite produced by reaction was evenly distributed in the zirconia matrix and the content and size of that increased with the increase of sintering temperature. Mullite-zirconia ceramics sintered at 1500 °C had the optimum content and size of the columnar mullite phase, generating the excellent mechanical properties (the bend strength of 890.4 MPa, the fracture toughness of 10.2 MPa.m^1/2, the Vickers hardness of 13.2 GPa and the highest densification). On the other hand, zirconia particles were evenly distributed inside the columnar mullite, which improved the mechanical properties of columnar mullite because of pinning effect. All of this clearly confirmed that zirconia grains strengthened columnar mullite, and thus the columnar mullite was more effective in enhancing the zirconia-based ceramics. Simultaneously, the residual alumina after reaction was distributed evenly in the form of particle, which improved the mechanical properties of the sample because of pinning effect. Overall, the synergistic effect of zirconia phase transformation toughening with mullite and alumina secondary toughening improved the mechanical properties of zirconia ceramics.     

Microstructure, Microchemistry, and Flexural Strength of Mullite Ceramics

The microstructure of mullite ceramics hot-pressed and sintered at different temperatures was studied using transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) with EDS, and electron probe microanalysis (EPMA). The specimens, consisting of stoichiometric mullite grains without glassy phase, are obtained by hot-pressing stoichiometric mullite powder at 1575°C for 1 h. Silica-rich glassy phases are observed using TEM at three-grain junctions of mullite grains in specimens heated at and above 1600°C. However, high-resolution transmission electron micrographs show no glassy phase at two-grain boundaries in all specimens. SEM with EDS analyses show that the average value of Al₂O₃ contents of mullite grains increases slightly with increasing temperature. These results are consistent with a published Al₂O₃–SiO₂ phase diagram. The flexural strength of mullite specimens at room temperature depends on their microstructure, such as the grain size and grain size distribution of mullite grains. The strength is high at room temperature and up to 1200°C, and it decreases at and above 1350°C, irrespective of the presence of the glassy phase.     

Wear behavior and microstructure of alumina-mullite-zirconia composites prepared by a novel method: Coating of zircon powder by aluminum alkoxide

The formation of the mullite phase is the main challenge in the preparation of alumina-mullite-zirconia (AMZ) composites. To overcome this limitation, a novel method based on the coating of zircon powder with aluminum alkoxide was proposed in this study. Reaction sintering of alumina and coated zircon was carried out at 1630 °C for 3 h. The microstructural, physical, mechanical, and tribological properties of samples were compared with the conventional AMZ composites prepared by common mixing of alumina and zircon. The microstructural analysis indicated the higher alumina phase of the sample prepared with the conventional method. On contrary, the samples prepared with the proposed method included higher percentages of mullite phase. In terms of mechanical properties, the conventional AMZ samples performed better. However, due to the beneficial effect of the mullite phase in tribological applications, the samples prepared with this new method show superior wear resistance. Especially, the samples prepared from 30 wt% aluminum alkoxide exhibited the best wear resistance. The delamination and adhesive wear mechanisms govern the wear process.     

The Effect of La2O3 Addition on Sol–Gel Derived Mullitization

Monophasic gel with stoichiometric 3Al₂O₃·2SiO₂ composition and gels with 0.99, 1.96, and 2.91 mol% La₂O₃ added were sol–gel derived. The crystallization path, structure evolution, microstructure, and morphology of calcined premullite powders and sintered ceramic bodies have been investigated as a function of La₂O₃ content and sintering temperature. In addition to mullite, spinel phase at about 980°C, and α‐alumina at above 1000°C were determined; however, neither La₂O₃ nor La‐related compounds had crystallized. The La₂O₃ predominately incorporated into the glassy phase, enhanced with La₂O₃ level, which affected both mullite structure and composition, as confirmed by electron microscopy, Rietveld structure refinement, determination of unit cell parameters, electron microscopy, and achieved density of the sintered bodies. Increased thermal treatment changes the alumina/silica ratio in mullite (towards 3:2 below 1200°C, and toward 2:1 above), and decreases the mullite/amorphous ratio. Sintered dense ceramic bodies revealed a positive densification effect and increased sinterability as a result of the lanthanum‐induced increase in glassy phase.     

High-toughness mullite ceramics fabricated by hot-press sintering of pyrophyllite and AlOOH at low temperatures

High-toughness mullite ceramics were fabricated through hot-press sintering (HPS) of pyrophyllite and AlOOH, which were wet-milled and well mixed using a planetary ball mill. The impacts of sintering temperatures and contents of AlOOH on mullite phase formation, densification, microstructure and mechanical properties in ceramic materials were investigated through XRD, SEM and mechanical properties determination. The results indicated that high-toughness mullite ceramics could be successfully prepared by HPS at temperatures higher than 1200°C for 120 min. Increasing the sintering temperature from 1000 to 1300°C significantly enhanced the flexural strength and fracture toughness of samples. The highest flexural strength of 297.97±25.32 MPa and fracture toughness of 4.64±0.11 MPa⋅m^1/2 were obtained for samples sintered at 1300°C. Further increase of temperature to 1400°C resulted in slight decrease of flexural strength and fracture toughness. Compared with the mullite ceramics prepared only using pyrophyllite as raw material, incorporation of AlOOH into raw material significantly increased the mechanical properties of final mullite ceramics. And stoichiometric AlOOH and pyrophyllite as starting material gave the best performance in fracture toughness. The high-toughness of mullite ceramics were ascribed to the high mullite phase content, fine mullite whiskers and in situ formed, intertwined three-dimensional network structure obtained through HPS at a low temperature of 1300°C.     

Porous mullite blocks with compositions containing kaolin and alumina waste

In the production of alumina by the Bayer process, the calcination step generates a waste containing ~90% aluminum oxide (Al₂O₃). Due to the high content of this oxide, this waste can be used as a source of alumina in porcelain formulations, especially those used in the synthesis of mullite. The purpose of this study was to produce porous mullite blocks using compositions containing kaolin and alumina waste. The compositions were formulated based on a mullite stoichiometry of 3:2. Heat treatments were carried out in a conventional furnace at temperatures of 1450 to 1500 °C, applying a heating rate of 5 °C/min and a 1-h hold time at the firing temperature threshold. The powders were characterized by means of X-ray fluorescence (XRF); X-ray diffraction (XRD); thermal analysis (TGA-DTA); scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The physic mechanical properties of the test specimens: water absorption, apparent porosity, linear shrinkage and flexural strength were also evaluated. The XRD results revealed the formation of mullite as the major phase. The morphological analysis by SEM revealed typical mullite needles originating from clay minerals. The size of the mullite needles was calculated based on the TEM analysis, which indicated diameters smaller than 400 nm, confirming the nanometric dimensions of the needles. The flexural strength test of the specimens indicated that this parameter ​​tends to increase as the temperature is raised.     

Damage assessment of alumina fibre-reinforced mullite ceramic matrix composites subjected to cyclic fatigue at ambient and elevated temperatures

The damage evaluation behaviour of alumina fibre-reinforced mullite ceramic matrix composites subjected to cyclic fatigue was investigated by means of acoustic emission (AE) monitoring and forced resonance techniques. AE technique provided sufficient information about the damage initiation and progression in real time whilst the forced resonance (FR) technique allowed the detection of changes in elastic modulus (E) and internal friction (Q⁻¹) that occurred with increasing number of cyclic fatigue at room temperature. From the two non-destructive detection techniques results combined with microstructural observations, it is concluded that the composite cyclic fatigue damage evolution begins with multiple crack formation within the matrix and is followed by delamination (interfacial failure). Final failure of the composite is caused by fibre fracture and extensive cyclic sliding along the fibre/matrix interface. The strong bonding between mullite matrix and alumina fibre caused by the glassy phase within the mullite matrix determined the fatigue performance of the composite at 1350°C. Regions with glassy phase failed catastrophically as a result of early fibre fracture.     

On the physico-mechanical,electrical and dielectric properties of mullite-glass composites

Mullite-glass composites were obtained by solid-state reactive sintering of kaolinite clay and kaolin waste mixtures with waste additions up to 100 wt%. The structural and microstructural analysis of starting powders and sintered samples were evaluated by X-ray diffractometry (XRD) and field-emission scanning electron microscopy (FESEM). The mechanical properties were evaluated by measuring the flexural strength of sintered bodies. Electrical properties of the composites were assessed by impedance spectroscopy (at 30 °C and from 400 to 700 °C) in air. A viscous flux mechanism resulting from the glassy phase filled up the open porosity and increased the mechanical strength. Electrical conductivity, dielectric constant and dielectric loss were strongly dependent on the microstructural features, namely glassy phase and porosity. The activation energies (0.89–0.99 eV) for electrical conduction were lower than typical literature values of mullite-based materials. The results indicated that the herein synthesized mullite-glass composites with up to 53.6 wt% mullite are promising low-cost materials for electronics-related applications.     

Improvement of the mechanical properties of SiC reticulated porous ceramics with optimized three-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with three-layered struts were fabricated by polymer replica method, followed by infiltrating alumina slurries containing silicon (slurry-Si) and andalusite (slurry-An), respectively. The effects of composition of infiltration slurries on the strut structure, mechanical properties and thermal shock resistance of SiC RPCs were investigated. The results showed that the SiC RPCs infiltrated with slurry-Si and slurry-An exhibited better mechanical properties and thermal shock resistance in comparison with those of alumina slurry infiltration, even obtained the considerable strength at 1300 °C. In slurry-Si, silicon was oxidized into SiO₂ in the temperature range from 1300 °C to 1400 °C and it reacted with Al₂O₃ into mullite phase at 1450 °C. Meantime, the addition of silicon in slurry-Si could reduce SiC oxidation of SiC RPCs during firing process in contrast with alumina slurry. With regard to slurry-An, andalusite started to transform into mullite phase at 1300 °C and the secondary mullitization occurred at 1450 °C. The enhanced mechanical properties and thermal shock resistance of SiC RPCs infiltrated alumina slurries containing silicon and andalusite were attributed to the optimized microstructure and the triangular zone (inner layer of strut) with mullite bonded corundum via reaction sintering. In addition, the generation of residual compressive stress together with better interlocked needle-like mullite led to the crack-deflection in SiC skeleton, thus improving the thermal shock resistance of obtained SiC RPCs.     

Enhanced thermal properties of silica-based ceramic cores prepared by coating alumina/mullite on the surface of fused silica powders

In this study, an alumina/mullite coating was synthesized on the surface of fused silica powders to form an alumina/mullite-silica core-shell structure. The effects of the alumina/mullite coating on the cristobalite crystallization, thermal properties, and leachability of the silica-based ceramic cores were investigated using the simulated casting process. The X-ray diffraction results indicated that the crystallization of cristobalite was significant at the simulated casting temperature of approximately 1400 °C. An increase in the cristobalite content during this stage resulted in a large thermal expansion because of its higher coefficient of thermal expansion compared with that for fused silica. The addition of optimum amounts of the alumina/mullite powders resulted in an increase in the initial shrinkage temperature and a decrease in the shrinkage of the specimens. When the coating powders were added at 43 wt%, the initial shrinkage temperature increased from 1092 °C to 1200 °C and the shrinkage decreased sharply. Leaching tests showed that the silica-based ceramic cores were removed in the form of stripped layers. The washing and shaking process accelerated the disintegration of the ceramic core and improved its leachability.     

Microstructure evolution and mechanical behavior of a mullite fiber heat-treated from 1100 to 1300°C

In this paper, the effect of phase transformation on microstructure evolution and mechanical behaviors of mullite fibers was well investigated from 1100 to 1300°C. In such a narrow temperature range, the microstructure and mechanical properties showed great changes, which were significant to be studied. The temperature of the alumina phase transformation started at below 1100°C. The main phases in fibers were γ-Al₂O₃ and δ-Al₂O₃ with amorphous SiO₂ at 1150°C. The stable α-Al₂O₃ formed at 1200°C. Then the mullite phase reaction occurred. As the alumina phase reaction took place, the tensile strength increased with the increasing temperature. In particular, the filaments achieved the highest strength at 1150°C with 1.98 ± 0.17 GPa, and the Young's modulus was 163.08 ± 4.69 GPa, showing excellent mechanical performance. After 1200°C, the mullite phase reaction went on with the crystallization of orthorhombic mullite. The density of surface defects increased rapidly due to thermal grooving, which led to mechanical properties degrade sharply. The strength at 1200°C was 1.01 ± 0.15 GPa with a strength retention of 63.13%, and the Young's modulus was 184.14 ± 10.36 GPa. While at 1300°C, the tensile strength was 0.64 ± 0.14 GPa with a strength retention of only 40.00%.     

Sintering and microstructural study of mullite prepared from kaolinite and reactive alumina: Effect of MgO and TiO2

Mullite ceramic was prepared using kaolinite and synthesized alumina (combustion route) by solid-state interaction process. The influence of TiO₂ and MgO additives in phase formation, microstructural evolution, densification, and mechanical strengthening was evaluated in this work. TiO₂ and MgO were used as sintering additives. According to the stoichiometric composition of mullite (3Al₂O₃·2SiO₂), the raw materials, ie kaolinite, synthesized alumina, and different wt% of additives were wet mixed, dried, and uniaxially pressed followed by sintering at different temperature. 1600°C sintered samples from each batch exhibit enhanced properties. The 1 wt% TiO₂ addition shows bulk density up to 2.96 g/cm³ with a maximum strength of 156.3 MPa. The addition of MgO up to 1 wt% favored the growth of mullite by obtaining a density and strength matching with the batch containing 1 wt% TiO₂. These additives have shown a positive effect on mullite phase formation by reducing the temperature for complete mullitization by 100°C. Both additives promote sintering by liquid phase formation. However, the grain growth, compact microstructure, and larger elongated mullite crystals in MgO containing batch enhance its hardness properties.     

Porosity features and gas permeability analysis of bi-modal porous alumina and mullite for filtration applications

Bimodal porous structures were prepared by combining conventional sacrificial template and partial sintering methods. These porous structures were analysed by comparing pore characteristics and gas permeation properties of alumina/mullite specimens sintered at different temperatures. The pore characteristics were investigated by SEM, mercury porosimetry, and capillary flow porosimetry. A bimodal pore structure was observed. One type of pore was induced by starch, which acted as a sacrificial template. The other pore type was due to partial sintering. The pores produced by starch were between 2 and 10 µm whereas those produced by partial sintering exhibited pore size of 0.1–0.5 µm. The effects of sintering temperature on porosity, gas permeability, and mullite phase formation were studied. The formation of the mullite phase was confirmed by XRD. Compressive strengths of 37.9 MPa and 12.4 MPa with porosities of 65.3% and 70% were achieved in alumina and mullite specimens sintered at 1600 °C.     

Thermal exposure effects on the long‐term behavior of a mullite fiber at high temperature

The behavior of an oxide fiber at elevated temperatures was analyzed before and after thermal exposures. The material studied was a mullite fiber developed for high‐temperature applications, CeraFib 75. Heat treatments were performed at temperatures ranging from 1200°C to 1400°C for 25 hours. Quantitative high‐temperature X‐ray analysis and creep tests at 1200°C were carried out to analyze the effect of previous heat treatment on the thermal stability of the fibers. The as‐received fibers presented a metastable microstructure of mullite grains with traces of alumina. Starting at 1200°C, grain growth and phase transformations occurred, including the initial formation of mullite, followed by the dissociation of the previous alumina‐rich mullite phase. The observed transformations are continuous and occur until the mullite phase reaches a state near the stoichiometric 3/2 mullite. Only the fibers previously heat treated at 1400°C did not show further changes when exposed again to 1200°C. Overall, the heat treatments increased the fiber stability and creep resistance but reduced the tensile strength. Changes observed in the creep strain vs. time curves of the fibers were related to the observed microstructural transformations. Based on these results, the chemical composition of the stable mullite fiber is suggested.     

Development of AMZ composites via mutual attraction of particles in wet processing

In this paper, three approaches were investigated for preparing alumina-mullite-zirconia (AMZ) composites. The weight ratio of alumina to zircon was selected to be 85/15. In the first approach, common well-known reaction sintering or solid-state mixing of alumina and zircon were used. In the second approach, suspensions of the raw materials were prepared to examine the effect of wet processing method. The third approach was based on the use of aluminum alkoxide coated zircon and alumina. The sintering of samples was carried out at the temperature of 1630°C for 3 hours. Phase composition and Rietveld refinement method, microstructural observation, EDS analysis, as well as physical and mechanical properties were used and determined to characterize the sintered samples. The results showed that aluminum alkoxide coating can develop the mentioned reaction sintering and mullite formation. However, the desired mechanical properties were not obtained. Wet processing approach resulted in more interesting data about the formation of mullite and could improve the microstructure homogeneity of final composites. Higher amount of tetragonal zirconia, good densification, high hardness, and fine microstructure were obtained by the wet approach. These interesting results were attributed to the fine discrete particles provided by mutual attraction in the wet preparation method.     

High temperature mullite dissolution in ceramic bodies derived from Al-rich sludge

Mullite-based refractory ceramic materials were produced from an industrial Al-rich sludge derived from wastewater treatment of aluminium anodising or surface coating industrial processes, and other low cost raw materials (ball clay, kaolin and diatomite). Cylindrical samples processed by uniaxial dry pressing (32 MPa) were sintered at various temperatures (1250–1650 °C) to study the mullitisation process. The performance of the materials at high temperature (1650 °C) was evaluated through different techniques (XRD, SEM/EDS, optical microscopy, and impedance spectroscopy) to access the microstructural changes occurring under prolonged tests (dwell times up to 100 h). For dwell times <80 h, a preferential dissolution of the smaller mullite grains in the glassy phase and its partial re-precipitation onto the coarser ones, leading to an overall coarsening of the mullite crystals. For dwell times >80 h, coarse α-alumina and Cr-doped alumina developed at the surface of the specimens, being accompanied by the formation of pores in the vicinity of alumina grains. Near alumina grains, additional relevant features include the increase of surface roughness, the appearance of concentration gradients within the glassy phase, which became almost depleted in Al and enriched in alkalines. The continuity of the glassy phase and its enrichment in alkaline species enhanced the electrical conductivity of the material, enabling the use of impedance spectroscopy to access the microstructural changes occurring during prolonged heat treatment.     

Fabrication and characterisation of porous mullite ceramics from high voltage insulator waste

High voltage insulator waste (HVIW) eliminated from high voltage transmission lines was used in the preparation of porous mullite ceramics. The fabrication and characterisation of porous mullite ceramics are described. The relationship between phase transitions, sintering temperature and physicochemical properties of the porous mullite ceramics are analysed and compared. The results show that the complex properties are the best when the mass ratio of HVIW/clay is 4∶1, the amount of pore former is 500 mL kg^?1 and the sintering temperature is 1500°C, the cold crushing strength reaches 4·8 MPa. Increasing the sintering temperature could strengthen the specimens. The average pore diameter of the specimens enlarged with the increasing sintering temperature. Meanwhile, the crystalline and morphology of mullite grains advanced and the maximum grain diameter reached to 1·5 μm. This improvement was attributed to the fact that complex properties of specimens could be optimised by new formation of new mullite phase.