Synthesis of functionally gradient aluminium titanate/alumina composites

Abstract

By virtue of its very low thermal expansion coefficient and low thermal conductivity, aluminium titanate (A12TiOs) is highly suitable for applications where thermal insulation and thermal shock resistance are required. However, AlzTiO5 does present two important problems, which have restricted its wider industrial use. The first relates to the extensive microcracking, which results in low thermal expansion but also in low strength. In view of this, composite materials like AlzTiOs-mullite [1], AI2TiOs-ZrO 2 [2], AlzTiO5-TiZrO4-ZrO 2 [3] and A12TiOs-mullite-ZrO2 [4] have been developed. Thermal decomposition of the material at temperatures below 1280 °C is the second problem [5]. On the other hand, alumina is a very versatile engineering material with relatively high strength, high hardness, and excellent oxidation and chemical resistance. However, like most ceramics, it has an intrinsically poor thermal shock resistance and low fracture toughness, which restricts its use in structural applications. It follows that the marriage of A12TiO5 and alumina should lead to a layered composite that will display all of the advantages of both materials. This concept has been successfully exploited by Marple and Green [6-8] and Low and colleagues [9-11] in synthesizing, respectively, layered mullite/alumina and mullite/ZTA with improved mechanical properties such as strength and fracture toughness. The method involved encasing an alumina or ZTA host body with a mullite (3A1203"2SiO2) protective layer [6, 9]. This was done by infiltrating the host body with a silica-rich solution (e.g. ethyl-silicate), followed by firing the composite at an appropriate temperature. Due to thermal expansion mismatch between mullite and the host body, desirable macroscopic compressive stresses can be induced in the casing. The protective layer inhibits crack growth, thereby improving the effective fracture toughness and strength. In this letter, we report the preliminary results of functionally-gradient A12TiOs-alumina composites by encasing an alumina host with an A12TiO5 layer using an infiltration technique. The microstructureproperties relationship is discussed. The procedure for sample preparation was based on an infiltration method adopted by Marple and Green [6] and Low et al. [9] in their processing of layered mullite-ZTA composites. High purity alumina (A1000G, Alcoa, USA) powder was used to make the ceramic preform for infiltration. The powder was