Relations Between Coarsening and Densification and Mass Transport Path in Solid-state Sintering of Ceramics: Model Analy
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Relations between coarsening and densification and mass transport path in solid-state sintering of ceramics: Model analysis J. L. Shi State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding-Xi Road, Shanghai, 200050, People’s Republic of China (Received 15 September 1997; accepted 19 October 1998)
The correlations between coarsening (including grain and pore growth) and densification, and the effects of mass transport on particle coarsening and densification were discussed based on the simple particle array models and for the real particle compacts. Grain boundary motion could cause particle coarsening only under a certain particle size distribution but not densification; mass transport is reasoned to contribute to both grain growth (particle coarsening) and shrinkage for one-dimensional particle arrays. Under a certain limitation for the change of the particle size aspect ratio during sintering, very limited effects of grain grown by itself on the shrinkage of particle arrays through reinitiating the sintering could be found. For a real powder compact system, mass transport between the particles, which surround a pore, contributes to the particle coarsening and densification when the pore is thermodynamically unstable and only to particle coarsening when the pore is thermodynamically stable. The mass transport mechanism for both particle coarsening and densification would be the same, which cannot exclude, at least on thermodynamics, the contribution from surface diffusion in the intermediate stage of sintering.
I. INTRODUCTION
The theories for the solid-state sintering of ceramics include the sintering thermodynamics and the densification kinetics.1 However, both the pore stability and the densification rate were associated with mass transport path and interfered by the coarsening (including grain and pore growth) process. So to understand the sintering process, the following two questions should be answered: (i) by what means, or by what mass transport path, the pores can shrink and the compacts can densify; (ii) the correlation among pore growth and shrinkage, grain growth, and densification. These two questions have been discussed in literature, but were not thoroughly solved.2 A brief presentation is given below about the current knowledge regarding the two questions. A. Correlations between grain growth and densification
According to various sintering models from the two sphere ones for the initial stage of sintering to the microstructural model for the intermediate stage of sintering, smaller particle size was reasoned to contribute to faster densification. Therefore, grain growth was found to affect densification only by negative ways; i.e., stronger grain growth was always to be unfavorable for densification. However, the attempt to achieve den1378
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J. Mater. Res., Vol. 14, No. 4, Apr 1999
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