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Formation of tough interlocking microstructures in silicon nitride ceramics by dynamic ripening.
Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
2002 (English)In: Nature, ISSN 0028-0836, Vol. 417, no 6886, 266-269 p.Article in journal (Refereed) Published
Abstract [en]

Ceramics based on Si3N4 have been comprehensively studied and are widely used in structural applications1, 2. The development of an interlocking microstructure of elongated grains is vital to ensure that this family of ceramics have good damage tolerance3, 4. Until now this has been accomplished by heating the appropriate powder compacts to temperatures above 1,700 °C for extended periods. This procedure involves a necessary step of controlling the size and population of seeds—added ex situ or formed in situ—to ensure selective grain growth5, 6. Here we report the very fast (within minutes) in situ formation of a tough interlocking microstructure in Si3N4-based ceramics. The microstructures are obtained by a dynamic ripening mechanism, an anisotropic Ostwald ripening process that results from the rapid heating rate. The resulting microstructures are uniform and reproducible in terms of grain size distribution and mechanical properties, and are easily tailored by manipulating the kinetics. This process is very efficient and opens up new possibilities to optimize mechanical properties and cost-effectively manufacture ceramics.

Place, publisher, year, edition, pages
2002. Vol. 417, no 6886, 266-269 p.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:su:diva-22950DOI: 10.1038/417266aOAI: oai:DiVA.org:su-22950DiVA: diva2:189795
Available from: 2004-05-05 Created: 2004-05-05 Last updated: 2011-05-31Bibliographically approved
In thesis
1. Spark Plasma Sintering of Si3N4-based Ceramics: Sintering mechanism-Tailoring microstructure-Evaluationg properties
Open this publication in new window or tab >>Spark Plasma Sintering of Si3N4-based Ceramics: Sintering mechanism-Tailoring microstructure-Evaluationg properties
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Spark Plasma Sintering (SPS) is a promising rapid consolidation technique that allows a better understanding and manipulating of sintering kinetics and therefore makes it possible to obtain Si3N4-based ceramics with tailored microstructures, consisting of grains with either equiaxed or elongated morphology.

The presence of an extra liquid phase is necessary for forming tough interlocking microstructures in Yb/Y-stabilised α-sialon by HP. The liquid is introduced by a new method, namely by increasing the O/N ratio in the general formula RExSi12-(3x+n)Al3x+nOnN16-n while keeping the cation ratios of RE, Si and Al constant.

Monophasic α-sialon ceramics with tailored microstructures, consisting of either fine equiaxed or elongated grains, have been obtained by using SPS, whether or not such an extra liquid phase is involved. The three processes, namely densification, phase transformation and grain growth, which usually occur simultaneously during conventional HP consolidation of Si3N4-based ceramics, have been precisely followed and separately investigated in the SPS process.

The enhanced densification is attributed to the non-equilibrium nature of the liquid phase formed during heating. The dominating mechanism during densification is the enhanced grain boundary sliding accompanied by diffusion- and/or reaction-controlled processes. The rapid grain growth is ascribed to a dynamic ripening mechanism based on the formation of a liquid phase that is grossly out of equilibrium, which in turn generates an extra chemical driving force for mass transfer. Monophasic α-sialon ceramics with interlocking microstructures exhibit improved damage tolerance. Y/Yb- stabilised monophasic α-sialon ceramics containing approximately 3 vol% liquid with refined interlocking microstructures have excellent thermal-shock resistance, comparable to the best β-sialon ceramics with 20 vol% additional liquid phase prepared by HP.

The obtained sialon ceramics with fine-grained microstructure show formidably improved superplasticity in the presence of an electric field. The compressive strain rate reaches the order of 10-2 s-1 at temperatures above 1500oC, that is, two orders of magnitude higher than that has been realised so far by any other conventional approaches. The high deformation rate recorded in this work opens up possibilities for making ceramic components with complex shapes through super-plastic forming.

Place, publisher, year, edition, pages
Stockholm: Institutionen för fysikalisk kemi, oorganisk kemi och strukturkemi, 2004. 92 p.
Keyword
spark plasma sintering, silicon nitride ceramics, grain growth kinetic, superplasiticity, liqiud phase sintering
National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-129 (URN)91-7265-834-7 (ISBN)
Public defence
2004-05-26, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 12 A, Stockholm, 10:00
Opponent
Supervisors
Available from: 2004-05-05 Created: 2004-05-05Bibliographically approved

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