| The effect of compact structures on the phase transition and subsequent microstructure evolution during sintering of ultrafine powder compacts has been investigated. The ultrafine ceramic powders having a capillary-induced high pressure phase, n-TiO₂ powder of anatase phase and the n-Al₂O₃ powder of gamma phase, were tested. The compact density was varied by applying different compaction pressures either 65_(㎫) or 4.5_(㎬). The compacts fabricated at the low pressure, called MC, had about 60% of theoretical density (TD) and the compacts consolidated at the high pressure, called GC, had about 80% TD, irrespective of powder systems. The shrinkage behavior of compacts was affected by compaction density. The low density MC compacts exhibit two inflection temperatures, whereas the relatively high density GC compacts exhibit one inflection temperature, in the shrinkage curve obtained by uniform heating. The shrinkage curve of the MC is separated into two parts: one comes from the capillarity effect and the other comes from the effect of phase transition on the microstructure evolution. The volume contraction due. to anatase→rutile TiO₂ and γ→α-Al₂O₃ phase transition was proved not to contribute to the whole compact shrinkage. The microstructures resulting from phase transition in the loose MC compacts retarded significantly the subsequent densification from the onset of phase transition. The effect of phase transition on the subsequent densification in the sintering of n-Al₂O₃ compacts seems to be of great detriment more than that in the sintering of n-TiO₂. In spite of detrimental effect of phase transition on the subsequent densification, high density compaction is proved to be beneficial to complete densification and fine sintered microstructure evolution. |
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