The preparation of Al2O3-ZrO2 composites by inorganic sol-gel processing
Inorganic Al2O3-ZrO2 sol-gel processing was achieved using basic metal salts as precursors. Initial investigation centred on the solution chemistry of Al3+ and Zr4+ ions and on partial denitration of Al3+ and Zr4+ precursor solutions. A Partial Charge Model was applied to analyze quantitatively experimental data on the process of hydrolysis and polymerization, leading to gelation of Al3+ and Zr4+ cations in acidic inorganic sols. The kinds of polyspecies and Al3+ and Zr4+ cations, the oxolation and the role of anions in the polymerization are discussed. The mechanism of sol-gel transformation in the Al2O3-ZrO2 sols system was investigated by many techniques, which showed that the gelation resulted from the polymerization of Zr4+ at a pH conditioned by the Al3+ sol and can thus be controlled by the concentration of Al3+ , the ratio of two starting sols in the mixture, and for nitrate precursors, by the OH-/NO3- ratio. The influences of temperature, the basicity of the precursors and additives on the gelation process were studied. The mechanism obtained from studies of the Al2O3-ZrO2 sols system is a useful aid in designing new systems of sol-gel processing using readily available inorganic precursors. The rudimentary structure of the gel and its changes during ageing were discussed. The nature of the short range ordering was approximately calculated from its X-ray diffraction (XRD) pattern. Thermal behaviour of the gel was analyzed by differential thermal analysis (DTA) and thermogravimetric analysis (TGA). The structural evolution during heating was investigated by many techniques. The process of crystallization, the changes of homogeneity and phase transformation under different conditions during firing were studied in detail. The use of XRD and Fourier-transform infrared (FTIR) to trace structural evolution during heating were evaluated. The densification process of the gel during sintering was investigated. The influences of composition, preheating temperature and sintering program on the densification upon firing were discussed. Flexural strength of two typical kinds of ceramics: zirconia-toughening alumina and partially stabilized zirconia were presented. The effects of TiO2 powders used as a filler on the densification of Al2O3-ZrO2 gels during sintering were studied. The preparation of coatings using sol-gel processing in TiO2-ZrO2 and TiO2-CeO2 systems was achieved. The cracking, firing temperature and microstructure of the coatings were evaluated.