Zirconia-mullite ceramics made from composite particles coated with amorphous phase: I. Effect of zirconia addition
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Mullite-based ceramics added with 0-20 vol % stabilized zirconia have been prepared by alumina/zirconia particles coated with an amorphous silica layer. All samples can be densified through the viscous flow of the amorphous silica layer in the typical temperature range of 1100-1310 °C. For the ZrO2-free mullite ceramics, the viscous densification kinetics is inhibited by increasing the content of the alumina inclusion particles and by crystallization of the amorphous silica layer. However, for the zirconia-mullite ceramics, the addition of the zirconia inclusion particles accelerates the viscous densification kinetics. Mullitization kinetics is also enhanced by the addition of zirconia. As the sintering temperature is high, a porous, duplex microstructure is observed in samples with or without zirconia. Zirconia addition enhances the development of this microstructure. As the sintering temperature and/or zirconia content is increased, ZrO 2 particles tend to coarsen, resulting in a decreased tetragonal to monoclinic ratio. Fracture toughness KIC increases with the zirconia content. Mullite-20 vol% ZrO 2 composite sintered at 1600 °C has a KIC of 3.8 MPa • m1/2.
I. INTRODUCTION Mullite (3A12O3 • 2SiO2) shows excellent mechanical strength and high creep resistance at elevated temperatures.1"3 It also has low thermal expansion, low dielectric constant, and good chemical stability. These characteristics make it a promising candidate for electronic and structural applications. To improve its low fracture toughness at room temperature, reinforcing agents such as zirconia particles can be incorporated into the mullite matrix.4"7 Zirconia-mullite composites have been prepared by several routes, including reaction sintering of alumina and zircon powders,6 reaction sintering of alumina and silica powders with the addition of zirconia,7'8 solgel routes,9-10 and sintering of mullite and zirconia powders.11 Recently, ceramics and ceramic composites prepared by composite powders consisting of rigid inclusion particles coated with a second phase have attracted increasing attention.12"22 The advantages of the coating process are improvement of dispersion of the inclusion particles and uniform incorporation of additives (e.g., ZrO 2 ). Using this process, mullite and mullite matrix composites can be densified via transient viscous-flow of the amorphous silica, which is coated on the surface of the alumina/zirconia particles, then the alumina and silica phase react to form mullite at higher temperatures.17-18'22 In the present paper, mullite-based ceramic materials were prepared by alumina/zirconia particles coated with an amorphous silica layer. Formation of the zirconia-free J. Mater. Res., Vol. 10, No. 1, Jan 1995
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mullite ceramics will be discussed first. Then the influences of addition of yttria-stabilized zirconia particles on the densification, microstructure, phase transformation, and fracture toughness (KJC) will be discussed. II. EXPERIMENTAL PROCEDURE A. Sample prep
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