Reactions and phase transformation in SiO2-ZrO2 sol-gel coated alumina power

TEOS and zirconium-n-propoxide have been hydrolysed in a two-step process in the presence of Al2O3 powder in order to coat the alumina grains with two layers of SiO2 and ZrO2. The reactions and phase transformations during sintering of the coated powders to a mullite-zirconia composite were investigated by means of differential thermal analyses, X-ray diffraction, transmission electron microscopy and scanning electron microscopy. The sequence of coating influences the crystallization temperature of tetragonal zirconia from the amorphous ZrO2 layer. In the Al2O3 powder that was coated first with ZrO2 the temperature of the tetragonal zirconia crystallization is 65 K lower than in the powder coated first with SiO2. Mullite is not formed through the direct reaction of Al2O3 cores and the amorphous SiO2 layer, but only through the reaction of Al2O3 with the metastable precursor phases cristobalite and zircon (ZrSiO4). A second generation of tetragonal zirconia crystals if formed during the decomposition of zircon, simultaneously increasing the tetragonal to monoclinic ZrO2 ratio. Copyright (C) 1996 Elsevier Science Ltd