Microstructure of liquid phase sintered superplastic silicon carbide ceramics
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Microstructure of liquid phase sintered superplastic silicon carbide ceramics Chong-Min Wang,a) Mamoru Mitomo, and Hideyuki Emoto National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba-shi, Ibaraki, 305, Japan (Received 9 October 1996; accepted 19 March 1997)
Superplastic silicon carbide ceramics were fabricated at low temperatures by a liquid phase sintering very fine b –SiC powder. The microstructural features of this material, of both before and after the superplastic deformation, have been investigated by transmission electron microscopy. Evaluated from the point of view of phase transformation, dislocation motion, and dynamic grain growth, the materials show very stable microstructures, indicating that the superplastic deformation process was dominated by the liquid phase assisted grain boundary sliding. In addition, the material is also characterized by the formation of clusters of fine SiC particles (5 –20 nm) encapsulated in layered graphitized carbon.
I. INTRODUCTION
Of the covalently bonded ceramics, silicon carbide (SiC) and silicon nitride (Si3 N4 ) show several aspects of similarity, at least from the point of view of ceramic processing: (i) Bulk material respectively based on these two compounds can be fabricated by a high temperature sintering of a powder compact.1–3 (ii) Particle surfaces of the powders are inevitably covered with an oxide layer.4 (iii) At high temperatures, the two compounds similarly undergo a phase transformation.5,6 It has been found that, with the addition of carbon and boron, SiC can be densified by solid state sintering with which the microstructure of the materials can be empirically optimized, but is still far from the sense of microstructural designing.1,2 However, Si3 N4 -based materials are now generally fabricated by liquid phase sintering during which it takes full advantage of both the surface silica as an in situ component of the liquid phase and the phase transformation, broadly on the line of seeding,7 for controlling the microstructure and tailored properties,8,9 and this is not matched by the processing of SiC based ceramics. The similarities between SiC and Si3 N4 as mentioned above thus suggest a general interest in liquid phase sintering of SiC ceramics for controlled microstructure.10–15 In parallel with the successful fabrication of superplastic ceramics based on fine-grained b –Si3 N4 ,16 Mitomo et al. have obtained fine-grained superplastic SiC ceramics by liquid phase sintering of a b –SiC powder at relatively low temperature,17 and we report in this paper the microstructural features of this material both before and after the superplastic deformation, with one particular
a)
Present address: Department of Materials Science and Engineering, Whitaker Laboratory, Lehigh University, Bethlehem, Pennsylvania 18015.
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http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 12, Dec 1997
Downloaded: 13 Mar 2015
point being the observation of carbon nanoparticles encapsulating SiC par
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