Wilson, Paul James (2011)
Eng.D. thesis, University of Birmingham.
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An investigation has been performed into the formulation of silica-based ceramic core materials for investment casting. The formulation of 3 materials that are currently used by Ross Ceramics was investigated in detail and the data from this used to determine the elements of the formulation that should be investigated in more detail. The techniques used included a variety of mechanical tests at both room and high temperatures, dilatometry, phase analysis by XRD and chemical analysis by XRF, pore analysis by simple Archimedes tests and mercury porosimetry and both scanning electron and transmitted light microscopy. A large number of materials were created with different formulations and the various characterisation techniques employed were used to determine the function of the different constituent.
It was determined that the function of zircon was to prevent grain boundary movement at elevated temperatures via the Zener effect, by acting as a non-reactive secondary phase. The zircon also had an additional consequence from the inherent contamination, with alumina, from the ball-mill procedure performed by the supplier. This had the effect of significantly affecting several high temperature properties. The effect of several dopant materials: TiO2, Al2O3, MgO, cristobalite seed and Molochite addition was also investigated. The latter two were determined to function mainly by their inherent contamination. The other additives affected the materials in different ways depending on the amount used and the presence of any phase eutectics that had detrimental effects on the high temperature material properties. The particle size distribution of the materials was also investigated. It was determined from the experiments that the D20 was an important factor for deterring final material properties and that most materials obeyed a modified Griffith’s crack theory relation, with the exception of materials with a larger zircon particle size than that of the main silica constituent.
From this work new materials could be created using the knowledge gained and material properties could be optimised to meet specific requirements by changing zircon quantity, dopant levels and particle size.
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