Semi-coherent zirconia inclusions in a ceramic matrix
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ocomposite ceramic materials were fabricated by conventional sintering of composite powders obtained by sol-gel coating of submicron powders. The microstructure of these MgAl2O4–ZrO2 materials was studied by transmission electron microscopy. All zirconia grains were in the tetragonal phase. In addition, the intragranular zirconia crystals exhibited heteroepitaxial orientation relationships with the surrounding spinel grains, (hkl)zirconia//(hkl)spinel. Semi-coherent interfaces along {111} planes were observed by high-resolution microscopy. The transformation toward the orthorhombic or the monoclinic phase retained the epitaxial relationships as far as possible. The presence of such heteroepitaxial intragranular crystals in sintered ceramic materials, which did not involve a melting stage, was attributed to the specificity of the material preparation process.
I. INTRODUCTION
Zirconia-toughened ceramics (ZTC) have attracted much attention during the two last decades because of their mechanical properties. The relationship between the mechanical behavior and the microstructure of the elaborated ZTC has been widely studied. It is now well established that a fine grained microstructure, typically with a mean grain size of approximately 1 m or less, results in composite materials with improved mechanical properties.1 Recently, fully dense ceramic composite materials with grain sizes in the nanometer range2 have been prepared using wet chemical routes. In particular, an important piece of work on nanocomposite materials has been devoted3 to their elaboration and mechanical properties determination. Finely dispersed intergranular nanoparticles significantly limit grain boundary mobility. For example, it is well known that the introduction of a zirconia second phase in sintered alumina leads to a decrease in the final alumina grain size of dense materials.4 Furthermore, the presence of intergranular particles inhibits the concurrent grain growth during plastic deformation at high temperatures. On the other hand, the presence of intragranular particles results in dislocation generation and pinning on cooling from the sintering temperature.3 The former role of intragranular nanodispersoids is important in oxide ceramics, such as alumina, which become ductile at high temperatures. Ohji et al.5 have shown that in alumina–silicon carbide nanocomposites, the presence of intergranular SiC particles induces transgranular crack propagation. This propagation was slowed down by intragranular silicon carbide crystals, resulting 2482
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J. Mater. Res., Vol. 15, No. 11, Nov 2000 Downloaded: 16 Mar 2015
in increased toughness. Nanodispersoids thus result in improved mechanical properties from room temperature up to high temperatures. One of the main goals of the design of nanocomposites containing intra- and intergranular particles is production of fine-grained superplastic ceramics.6 In spite of the interest in this type of ceramic, it has attracted few structural studies. In particular, the existence and t
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