ZIRCONIA-MULLITE CERAMICS MADE FROM COMPOSITE-PARTICLES COATED WITH AMORPHOUS PHASE .1. EFFECT OF ZIRCONIA ADDITION

Mullite-based ceramics added with 0-20 vol% stabilized zirconia have b een prepared by alumina/zirconia particles coated with an amorphous si lica layer. All samples can be densified through the viscous flow of t he amorphous silica layer in the typical temperature range of 1100-131 0-degrees-C. For the ZrO2-free mullite ceramics, the viscous densifica tion kinetics is inhibited by increasing the content of the alumina in clusion particles and by crystallization of the amorphous silica layer . However, for the zirconia-mullite ceramics, the addition of the zirc onia inclusion particles accelerates the viscous densification kinetic s. Mullitization kinetics is also enhanced by the addition of zirconia . As the sintering temperature is high, a porous, duplex microstructur e is observed in samples with or without zirconia. Zirconia addition e nhances the development of this microstructure. As the sintering tempe rature and/or zirconia content is increased, ZrO2 particles tend to co arsen, resulting in a decreased tetragonal to monoclinic ratio. Fractu re toughness K(IC) increases with the zirconia content. Mullite-20 vol % ZrO2 composite sintered at 1600-degrees-C has a K(IC) of 3.8 MPa. m 1/2.