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.