Aluminium titanate-mullite composites

Abstract

Aluminium titanate precursor powders containing 2.5wt% MgO additive were prepared by two powder processing methods, namely the hydrolysis of alkoxides and co-precipitation of inorganic salts. The hydrolysis-derived precursor powder showed a higher chemical homogeneity and powder reactivity than the coprecipitation-derived precursor powder. Different phase transformation paths of TiO2 and Al2O3 were observed in the two powders. The presence of the MgO additive lowered the formation temperature of aluminium titanate to 1220°C in both powders. The higher powder reactivity and chemical homogeneity of the hydrolysis-derived precursor powder however led to incomplete reaction and a non-uniform microstructure due to the rapid grain growth of Al2 TiO5 during its formation process. 75vol% aluminium titanate-25vol% mullite composites were prepared by reaction sintering of the hydrolysis-derived powders. In the composite powder the formation reactions of aluminium titanate and mullite were retarded. The composite powder exhibited a high chemical homogeneity, but showed a poor sinterability. The addition of excess SiO2 (1wt%, 3wt% and 5wt%, relative to Al2TiO5) or MgO (2.5wt%) could markedly enhance densification of the composites by reducing the constraining effect caused by the formation of Al2TiO5 and mullite by providing liquid phases at the firing temperature. The presence of liquid phases during sintering promoted grain growth of Al2TiO5 and also the formation of elongated mullite and aluminium titanate grains. Microcracks were formed during cooling from the firing temperature. A strong interfacial bonding between aluminium titanate and mullite was observed in the MgO-containing sample. The addition of excess SiO2 or MgO improved the mechanical strength of the 75vol% aluminium titanate-25vol% mullite composite, due to the higher sintering density and to the more homogeneous microstructure. The thermal expansion of the composite samples with excess SiO2 or MgO was relatively low due to the extensive microcracking. After annealing the sintered samples at 1100°C for 100 hours, Al2TiO5 in the present composite system showed a good thermal stability due to the solid solution effect of SiO2 or MgO. The effect of the mullite content on engineering properties of the composites prepared by a gel coating powder processing method was investigated. The formation of mullite at ~980°C was enhanced in the composite precursors due to the epitactic nucleation effect of the aluminium titanate particles. The densification of the composite samples was increased due to the formation of a liquid phase during sintering. The grain growth of Al2TiO5 was progressively suppressed as the mullite content increased and thus the degree of microcracking decreased. For the sample containing 50vol% mullite only a few, small microcracks were observed. The mechanical strength increased with the increase of mullite content. The fracture mode progressively changes from predominantly intergranular for the single-phase Al2TiO5 sample to more transgranular for the sample containing 50vol% mullite. A strong dependence of the mechanical strength on the grain size of aluminium titanate, Ơf α GAT-3, was determined for all the composite samples plus the single-phase Al2TiO5 sample. The same grain size exponent of - 3 was also found for the single-phase Al2TiO5 system, using the bibliographic data. It was then concluded that the main effect of the mullite second phase on the mechanical strength was to inhibit the grain growth of Al2TiO5. The thermal expansion coefficient increased with the increase of the mullite amount as a result of the reduction in the degree of microcracking. The thermal shock resistance tended to decline with increasing the mullite amount. The introduction of mullite significantly improved the thermal stability of aluminium titanate. The microcracking temperature and crack volume were determined from the cooling curves obtained during sintering. The temperature for microcrack initiation of the aluminium titanate-mullite composites was found to follow the grain size relationship initially developed for the single-phase pesudobrookite oxides. The microcracking in the present composite system was determined by the aluminium titanate constituent. Although a good correlation between the mechanical strength and the Al2TiO5 grain size was found, the fitting of the data was limited to samples obtained by the same processing method. For samples with different processing conditions a better correlation was found between the crack volume and the mechanical strength, this being Ơf α V crak -1.2. Therefore the crack volume was the most important variable determining the mechanical strength of the aluminium titanate or aluminium titanate-based composites. Five models of the thermal expansion coefficient of composites were used to evaluate the flaw-free thermal expansion of the present composite system. It was found that the experimental data fitted quite well with the Thomas model particularly when the mullite volume percentage was lower than 35vol%. The normal-stress model was used to analyze the microcracking-dependent thermal expansion coefficient of the present aluminium titanate-mullite composites. The experimental data were in good agreement with the theoretical curve developed for the single-phase aluminium titanate, particularly when the volume fraction of mullite was low. For a given crack volume the addition of mullite lowered the thermal expansion of aluminium titanate, slightly improving the engineering properties of aluminium titanate.