Zirconia-mullite ceramics made from composite particles coated with amorphous phase: Part II. Effects of boria additions
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Mullite and zirconia-mullite ceramics have been prepared by coating alumina/zirconia particles with an amorphous silica layer. The effect of composition change of the amorphous silica layer by adding B2O3 was investigated. For the zirconia-free compositions, the addition of B 2 O 3 remarkably accelerates the kinetics of the crystallization of the amorphous coating layer, the viscous sintering, and the mullitization. For the zirconia-containing ceramics, the addition of B 2 O 3 enhanced the viscous sintering kinetics and delayed the decomposition of the transient zircon phase and the subsequent t- to m-ZrO2 transition, thus resulting in a higher ratio of t- to m-ZrO 2 . The B2O3-containing zirconia-mullite composites exhibit the same level of fracture toughness (X/c) as the B2O3-free zirconia-mullite composites.
I. INTRODUCTION Mullite (3A12O3 • 2SiO 2 ) shows excellent mechanical strength and high creep resistance at elevated temperatures. 13 It also has low thermal expansion, low dielectric constant, and good chemical stability. The above characteristics make it a promising candidate for structural and electronic applications. Mullite-based ceramics have been prepared by several routes, including reaction sintering of alumina and silica powders,4 sol-gel methods,5 and sintering of mullite powders.6 Recently, ceramics and ceramic composites prepared by composite powders consisting of rigid inclusion particles coated with a second phase have attracted increasing attention,7"17 since the coating process can improve the dispersion of the inclusion particles and the uniform incorporation of additives. Particularly, using this process, mullite-based ceramics can be densified via transient viscous flow of the amorphous silica layer, which is coated on the surface of the alumina particles. Then the alumina and silica phases react to form mullite at higher temperatures.1213-17 In our previous work,18 the effect of ZrO 2 on mullitebased ceramics, prepared by a process similar to that described above, has been investigated. It was found that the samples can be densified through viscous flow of the amorphous silica layer in a typical temperature range of 1100-1310 °C. It is thus reasonable to expect that the viscosity of the amorphous silica layer plays an important role in this process. Therefore, in the present paper, the effects of modifying the composition of the amorphous silica layer by adding B2O3 on the 2592
J. Mater. Res., Vol. 10, No. 10, Oct 1995
http://journals.cambridge.org
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densification, phase transformation, microstructure, and fracture toughness (K,c) are studied. II. EXPERIMENTAL PROCEDURE A. Sample preparation Four compositions (M, MB, MZ, and MZB) were studied. Stoichiometric mullite, 72 wt. % A12O3 + 28 wt. % SiO2, was used as the base composition, M. Compositions MB and MZ were prepared by adding 3 wt. % B 2 O 3 and 20 vol% ZrO 2 , respectively, to the composition M. Composition MZB was prepared by adding 3 wt. % B 2 O 3 to MZ. The starting inclusion particles were a-alumina (
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