Effect of fluoride and oxide additives on the phase transformation in alumina/clay ceramics

Abstract

The effect of various types of additives such as fluoride (calcium fluoride (CaF2), sodium fluoride (NaF) and potassium fluoride (KF)) and oxide (titanium dioxide (Ti02) and ferric oxide (Fe203)) on the thermal phase transformation in alumina/clay ceramics was investigated. Two different types of kaolin clays (A12Si205(OH)4), i.e., kaolinite and halloysite, were utilised, though the clays mixed with the fluoride additives were dehydroxylated in advance. The powder samples with oxide and fluoride additives were pressed and heated to 1550C and 1600C, respectively, prior to physical and mechanical testing. The sintered samples were quantitatively analysed by X-ray diffraction (XRD) with an internal standard using the Rietveld method. In general, additives enhanced densification by liquid phase sintering, but higher porosities were obtained with some types of additives. The major phase was mullite after sintering, and the cations from additives seem to incorporate into the mullite structure. The flexural strength and Vickers hardness will also be presented.