Doctor of Philosophy
Department of Materials Engineering
Nightingale, S A., Sintering of yttria-doped zirconia ceramics in a microwave field, Doctor of Philosophy thesis, Department of Materials Engineering, University of Wollongong, 1995. http://ro.uow.edu.au/theses/1518
hybrid heating process which can sinter yttria zirconia ceramics to nearly 100% of their theoretical density in a short time has been developed. Following optimisation of process, a detailed comparison of the properties and microstructures of conventionally sintered and microwave sintered samples of 3 mol% and 8 mol% yttria zirconia was performed. Identical thermal profiles were used for both types of heating. For both materials, microwave heating was found to enhance the densification processes which occur during constant rate heating
3 mol% yttria zirconia material exhibited a shift in the grain size / density relationship which favours densification, resulting in higher density samples with smaller grain sizes at densities below 96% of theoretical density. At higher densities, significant grain growth occurs. The grain size / density relationship remained unchanged for the 8 mol% yttria zirconia. Differences in the response of the two materials are attributed to differences in the activation energy for grain growth, and in grain boundary mobility. Modulus of rupture and toughness of both microwave and conventionally sintered samples were similar.
Following isothermal heating at 1300°C, microwave heated samples were found to be significantly more dense than conventionally heated samples. The lower temperature also restricted grain growth once densification was approaching completion
When aged at 1500°C, grain growth in the 3 mol% yttria-zirconia was also found to be accelerated in the electromagnetic field, with exaggerated grain growth being observed. The larger grain size was reflected in an increase in transformability of the tetragonal phase. Grain growth in 8 mol% yttria zirconia was not significantly affected by microwave heating over the time period studied.
These findings have significant implications for the commercial application of microwave sintering. The best way to take advantage of the benefits of microwave sintering for yttria zirconia ceramics would appear to be incorporation of dwell periods temperatures in the 1200 - 1350°C range in the sintering program.