Sintering and Microstructure of Ceramics

Sintering allows the consolidation - the non-cohesive granular medium becomes a cohesive material - whilst organizing the microstracture (size and shape of the grains, rate and nature of the porosity, etc ). However, the inicrostracture determines to a large extent the performances of the material all the more reason why sintering 

Chapter written by Philippe BOCH and Anne LERICHE. 

Sintering is the basic technique for the processing of ceramics, but other materials can also use it metals, carbides bound by a metalhc phase and other cermets, as well as natural materials, primarily snow and ice. 

Among the reference works on sintering, we recommend above all [BER 93] and [GER 96] the latter refers to more than 6,000 articles and deals with both ceramics and metals. We also recommend [LEE 94], which discusses ceramic microstmctures and [RIN 96], which focuses on powders. 

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Y. Zhao, L.J. Wang, et al., Preparation and Microstructure of a ZrB2-SiC Composite Fabricated by the Spark Plasma Sintering-reactive Synthesis (SPS-RS) Method, J. Am. Ceram. Soc., 90(121, 4040-4042 (2007). 

With an increase of the sintering temperature a monotonous variation of phase composition of samples takes place. The lowering of monoclinic phase amount in the samples is accompanied by the non-monotonous alteration of density and microhardness. The maximal values reach 5.73 g/cm3 and 12.6 0.5 GPa. In this case, microstructure and colour of ceramics samples are also changed with the temperature increase. 

Reijnen, P. J. L. (1967). Sintering behaviour and microstructure of aluminates and ferrites with spinel structure with regard to deviations from stoichiometry. Science of Ceramics, 4, 169-88. 

Thus, this contribution is aimed at the state of the art in boride ceramics with their problems in densification, microstructural peculiarities and exceptional mechanical properties. Starting with the unique interaction of metallic, covalent and ionic types of bonding and the crystal structures of technically important compounds, phase diagrams will be presented as far as they are of technical interest. The major part consists of the description of the synthesis and properties of ceramics and cermets, reflecting the development of suitable sintering procedures and the consequent improvement of the thermal and mechanical properties. 

In order to discuss the effect of Al-B-C addition to SiC on p-a phase transformation of SiC and microstructure of porous SiC ceramics, porous SiC ceramics without AI-B-C additives were fabricated at 2150 C, and their microstructure and fractions of SiC polytypes in the porous SiC ceramics were evaluated. Figure 6 exhibits SEM micrographs of the porous SiC ceramics without Al-B-C additives sintered at 2150°C. In the case of submicron-sized p-SiC powder, only a few SiC grains were grown into plate-like shape. As for the porous SiC ceramics using micron-sized and coarse p-SiC powders, plate-like SiC grains were not grown without additives. 

Solutes (also referred to as dopants), present in concentrations as low as a fraction of a mole percent, have a significant influence on sintering. They refer to compounds that are incorporated into solid solution to modify the microstructure and properties of ceramics. The term aliovalent dopant is sometimes used to describe a solute in which the cation valence is different from that of the host cation, whereas a solute in which the cation valence is the same as that of the host is sometimes called an isovalent dopant. For aliovalent dopants, when the valence of the solute cation is greater than that of the host cation, the solute is referred to as a donor dopant. On the other hand, an acceptor dopant refers to a solute in which the cation valence is smaller than that of the host cation. For example, TiOi and MgO are donor and acceptor dopants, respectively, for AI2O3 (the host). 

Tian, C. and Chan, S.W. (2000) Ionic conductivities, sintering temperatures and microstructures of bulk ceramic Ce02 doped with Y2O3. Solid State Ionics, 134 (1-2), 89-102. 

The idea to produce ceramic-like materials with a fine microstructure by controlled devitrificaton of base glasses was soon extended to procedures other than the controlled volume nucleation and crystallization of base glasses. Relatively fine-grained glass ceramics can also be obtained by sintering and crystallization of glass powders to dense bodies. 

Kara F, Sener O. Improvement of sintering and microstructural homogeneity of a diphasic muUite. J. Eur. Ceram. Soc. 2001 21 901-905... 

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