Abstract
ZrO 2/mullite composites (ZMC) with homogeneously dispersed ZrO 2 grains were prepared from colloidal or sol-gel processes of the precursors, which were a mixture of colloidal pseudo-boehmite (γ-AlOOH), zirconia and silicic acid gel, or prepared from dissociated zircon with alumina powder. After pressureless sintering of the ZMCs, their microstructure was examined by means of X-ray diffractometry, scanning electron microscopy and analytical transmission electron microscopy techniques. The microstructure of the ZMCs showed a difference in scale. ZrO 2 and mullite grains grown in the gel matrix were formed at temperatures as low as 1100 and 1300 °C, respectively. Experimental results indicated that heat treatment from 1300 to 1600 °C influences the growth of mullite and fine ZrO 2 grains in ZMCs, especially for the composite prepared from sol-gel methods in which the ZrO 2 grew from tens of nanometres to micrometre size. The effects of the presence of ultra-fine ZrO 2 on retarding the grain growth of mullite and the increase of metastable t-phase ZrO 2 are also discussed. The growth of fine ZrO 2 grains in a mullite matrix belongs to a mechanism of coalescence.
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