Alumina-mullite Composites Research Articles

Temperature dependence of Young’s modulus and the occurrence of an elastic anomaly in porous alumina-mullite composites prepared by starch consolidation casting

The temperature dependence of Young’s modulus has been measured by impulse excitation up to 1400 °C (heating and cooling) for alumina-mullite composites with 40–60 wt% alumina prepared by starch consolidation casting, after firing at 1570 °C, resulting in materials with high porosities (43–57 %). Using previously determined temperature master curves, both analytical predictions and numerical computations on computer-generated microstructures are compared with experimental data, resulting in excellent agreement for the alumina-mullite composites. Apart from the overall decrease of Young’s modulus during heating, an elastic anomaly starting at around 950 °C and cumulating at around 1050 °C is observed in all cases during heating (but not during cooling), and is shown to be stronger with increasing alumina content and porosity. The results are discussed in terms of a “glass phase hypothesis” and a “phase transition hypothesis”, although neither of them provides a satisfactory explanation of the observed phenomenon.

A Study on the Wear Behaviour of Monolithic Mullite Materials for Dental Applications

The aim of this study is to evaluate the potential use of the monolithic mullite samples for the dental applications. For this purpose, at first the monolithic mullite samples obtained by the powder metallurgy method have been characterized by the scanning electron microscopy (SEM) together with the EDS analysis. The pin-on-disk tribometer has been used to conduct the wear test in order to determine the volumetric loss. After the wear tests, the worn surfaces of the sintered mullite samples have been studied. The presence of the 3Al2O3.2SiO2 structure has been confirmed by the FT-IR spectroscopy through the observation of the characteristic Al-O-Si linkages at the spectra. The wear rate results derived from the volumetric losses have been measured to be around 2.42×10-6 mm3/Nm. This value has been found to be lower than those of alumina and alumina-mullite composites in the previous studies, which explains the higher wear resistance of these mullite materials. The room temperature friction coefficients of AS-1 and AS-2 samples have been determined to be 0.88 and 0.80, respectively. The wear behaviour of the samples has been correlated to the sintering temperature and the resultant relative density of the samples. The mullite samples obtained in this study have indicated better wear performance than the other materials (alumina and alumina-mullite composites) in spite of the usage of monolithic material. Therefore, this study suggests that the monolithic mullite materials have a potential to be exploited for the replacement of the previous ceramic-based dental materials. Further studies might contribute to the improvement of these materials to be utilized in the oral environment.

Composites Obtained from Alumina and Polymer Derived Ceramic

Alumina-mullite composites with low shrinkage can be made by reaction bond using mixtures of alumina, aluminum and silicon carbide. In this work, an alternative route is used to produce alumina composites with low shrinkage. Here alumina samples containing additions of 10 and 20 wt% of a preceramic polymer were warm-pressed and treated in the range of 900 -1500°C to produce alumina based composites. The obtained composites were analyzed by linear shrinkage and compared to pure alumina samples sintered at the same temperature range. It were also evaluated the density variation and crystalline phases formed during heat treatment of alumina composites. Results showed that alumina-silicon oxycarbide and alumina-mullite composites were obtained with lower shrinkage than pure alumina samples.

Preparação de compósitos alumina-mulita de baixo custo via sinterização reativa entre uma argila caulinítica da Paraíba e hidróxido de alumínio

Resumo Este estudo aborda os efeitos do processamento na microestrutura e nas propriedades de compósitos alumina-mulita via sinterização reativa de uma mistura argila caulinítica e hidróxido de alumínio. Amostras prensadas a 40 MPa foram sinterizadas entre 1300 e 1600 °C. As propriedades tecnológicas foram determinadas em função da temperatura de sinterização. A avaliação das fases cristalinas e da microestrutura foi realizada por difração de raios X e microscopia eletrônica. Os resultados indicaram a obtenção de mulita e alumina a partir de 1300 °C. Para sinterizações acima de 1450 °C foram obtidos valores de porosidade aparente inferiores a 0,8% e tensão de ruptura à flexão superior a 40 MPa. Amostras com morfologia bimodal (grãos equiaxiais e alongados) e resistência mecânica de 70 MPa foram obtidas após sinterização a 1600 °C. Este estudo demonstra a possibilidade de preparar compósitos alumina-mulita de baixo custo e com propriedades tecnológicas compatíveis com as de cerâmicas produzidas por companhias com grande atuação no mercado internacional de produtos refratários.

일축배향 기공채널과 향상된 압축강도를 갖는 다공질 알루미나/뮬라이트 층상 복합체

Three-layer porous alumina-mullite composites with a symmetric gradient porosity are prepared using a controlled freeze/gel-casting method. In this work, tertiary-butyl alcohol (TBA) and coal fly ash with an appropriate addition of Al₂O₃ were used as the freezing vehicle and the starting material, respectively. When sintered at 1300-1500℃, unidirectional macro-pore channels aligned regularly along the growth direction of solid TBA were developed. Simultaneously, the pore channels were surrounded by less porous structured walls. A high degree of solid loading resulted in low porosity and a small pore size, leading to higher compressive strength. The sintered porous layered composite exhibited improved compressive strength with a slight decrease in its porosity. After sintering at 1500℃, the layered composite consisting of outer layers with a 50 wt% solid loading showed the highest compressive strength(90.8 ± 3.7MPa) with porosity of approximately 26.4%.

Elastic Properties of Porous Alumina, Zirconia and Composite Ceramics

Micromechanical calculations and elasticity standard relations are used to predict the elastic properties of porous alumina, zirconia and kaolin-based ceramics, as well as the high-temperature Young moduli of alumina-zirconia and alumina-mullite composites. The predictions are compared with experimental results obtained via impulse excitation. It is found that the Young moduli of highly porous (cellular) alumina ceramics can be predicted via the Gibson-Ashby power-law relation, whereas for partially sintered kaolin-based ceramics our exponential relation, albeit better than the Gibson-Ashby relation, does not give a satisfactory prediction. However, once the Young moduli are known, the shear and bulk moduli can be reliably predicted in both cases, based on rough information on the Poisson ratio. The temperature dependence of the Youngs moduli of two-phase composites can be quite precisely predicted as soon as the master curves of the constituent phases and the type of porosity (convex, concave, or saddle-point) are known.

RETRACTED: Microstructure development and phase evolution of alumina–mullite nanocomposite

In this work, alumina-mullite composites (5-15 vol.%) were prepared using sol-gel derived alumina composite nanopowders. The results revealed the formation of intragranular and intergranular mullites inside and between the alumina grains, respectively. Accordingly, the intragranular mullites (average grain size, 0.3 μm) were smaller than the intergranular mullites (average grain size, 0.5 μm). Moreover, the alumina grains (average grain size, 1.0 μm) are larger than the mullites. Meanwhile, the mullites showed positive results in the prevention of the alumina grains growth and the retardation of densification. The relative density of alumina-15 vol.% mullite that was sintered at 1650°C for 2 h, was obtained as 98.7 %. After sintering at 1750°C for 2 h, the mullite was decomposed.

Microstructure development and phase evolution of alumina-mullite nanocomposite

In this work, alumina-mullite composites (5-15 vol.%) were prepared using sol-gel derived alumina composite nanopowders. The results revealed the formation of intragranular and intergranular mullites inside and between the alumina grains, respectively. Accordingly, the intragranular mullites (average grain size, 0.3 ?m) were smaller than the intergranular mullites (average grain size, 0.5 ?m). Moreover, the alumina grains (average grain size, 1.0 ?m) are larger than the mullites. Meanwhile, the mullites showed positive results in the prevention of the alumina grains growth and the retardation of densification. The relative density of alumina-15 vol.% mullite that was sintered at 1650?C for 2 h, was obtained as 98.7 %. After sintering at 1750?C for 2 h, the mullite was decomposed.

A comparative study of microstructural development in the sol–gel derived alumina–mullite nanocomposites using colloidal silica and tetraethyl orthosilicate

This paper compares the microstructure development of two alumina–15 vol% mullite composites during the sintering. The nanopowders of alumina–mullite composite were synthesized by the sol–gel method using aluminum chloride hexahydrate and two different silicon sources (colloidal silica in route 1 and tetraethyl orthosilicate in route 2). The alumina–mullite composites were prepared by pressing and sintering of the nanopowders. Although the intergranular mullites were observed in both routes, there were mullite particles in route 2 formed inside the alumina grains (intragranular mullite). Formation of the intragranular mullite is attributed to the drop in silica solubility, which occurs during the transition from metastable alumina to stable alumina. Compared to route 1, the relative density and the average grain size were increased and accelerated by route 2. The two-stage sintering is not useful for the mullite decomposition.

Stress state and fracture behavior of alumina–mullite intragranular particulate composites

We investigated the fracture behavior of intragranular particle-reinforced alumina–mullite composites in terms of the effect of inter-phase microstresses on grain and grain boundary toughness. First, we calculated the residual matrix microstresses in the composites and validated the results by experimental measurement. Then, we analyzed the influence of the stress state on crack propagating and calculated the ratios of grain boundary toughness to grain toughness and deduced the percentage of transgranular fracture (PTF) that increases with the matrix microstress increase. On basis of the above results, we established the relationship between the microstructural features, matrix microstresses, and PTF. The results agree qualitatively with the experimental observations. Finally, we interpreted the strengthening mechanism and explained the excellent wear resistance of the alumina–mullite composites in terms of fracture-mode transition.

Alumina-Mullite Composites through Interaction of Bauxite and Fly Ash

In the present investigation alumina-mullite composites with various mole ratios of Al2O3 and SiO2 were generated by interacting fly ash with bauxite. The magnitude of sintering was examined by examining the characteristic properties of the sintered product which were subsequently correlated with generated crystalline phases and microstructure. The multiplicity of possible phase relations in this particular system is complex and the associated impurities played a vital role leading to liquid phase sintering.

Fracture Toughness of Alumina-Mullite Composites Produced by Infiltration Process

Fracture Toughness of Alumina-Mullite Composites Produced by Infiltration Process

Influence of mullite additions on thermal shock resistance of dense alumina materials. Part 1: Processing studies

Homogeneous, fine grained (≈ 0·5–2 µm), and dense (≥ 97% of theoretical density) alumina–mullite composites have been prepared by a colloidal filtration route. In order to improve the uniformity of the composites, the rheological behaviour of aqueous suspensions of alumina, mullite, and alumina–mullite mixtures was studied, focusing on the optimum concentration of deflocculant, the mixing procedure, and milling time. The sintered materials were characterised (scanning electron microscopy, dynamic Young's modulus, Vickers indentation hardness, and toughness) and compared with an alumina material with similar density and microstructure. Small mullite additions (5–15 vol.-%) allow desirable values of hardness and toughness of alumina to be maintained while reducing the Young's modulus below that of alumina, so that it may be expected that the thermal shock behaviour will be improved.

Influence of mullite additions on thermal shock resistance of dense alumina materials. Part 1: Processing studies

AbstractHomogeneous, fine grained (≈ 0·5–2 µm), and dense (≥ 97% of theoretical density) alumina–mullite composites have been prepared by a colloidal filtration route. In order to improve the uniformity of the composites, the rheological behaviour of aqueous suspensions of alumina, mullite, and alumina–mullite mixtures was studied, focusing on the optimum concentration of deflocculant, the mixing procedure, and milling time. The sintered materials were characterised (scanning electron microscopy, dynamic Young's modulus, Vickers indentation hardness, and toughness) and compared with an alumina material with similar density and microstructure. Small mullite additions (5–15 vol.-%) allow desirable values of hardness and toughness of alumina to be maintained while reducing the Young's modulus below that of alumina, so that it may be expected that the thermal shock behaviour will be improved.

Residual Stress in Alumina–Mullite Composites

Lattice strains have been measured by neutron diffraction from the alumina matrix in alumina–mullite composite cylinders. Infiltration processing results in a macroscopic variation of mullite concentration and residual stress throughout the samples. At the midheight of the cylinder, the stress component parallel to the cylinder axis is in a compression of −130 ± 30 MPa with respect to the value near the top surface. At the surfaces, the stress component parallel to the surface is in a compression of −130 ± 30 MPa with respect to values near the midheight of the cylinder. An overall tensile shift of the macroscopic stress profile in a fully infiltrated specimen is interpreted as a grain interaction effect between the alumina and mullite constituents. The positional dependence of the mullite phase concentration is also determined by neutron diffraction.