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Formation of in-situ reinforced microstructure in α–sialon ceramics I: Stoichiometric oxygen-rich compositions
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: Journal of Materials Research, ISSN 0884-2914, Vol. 17, no 2, 336-342 p.Article in journal (Refereed) Published
Abstract [en]

The abnormal grain growth in α–sialon ceramics was investigated. The preparations had stoichiometric compositions on the oxygen-rich phase boundary, and they were stabilized by Y, Nd, Sm, Dy, and Yb, respectively. Specimens were prepared from α–Si3N4 as precursor powder by applying conventional hot pressing and a novel rapid consolidation process, namely spark plasma sintering (SPS). Single-phase α–sialon ceramics with in situ reinforced bimodal microstructure, i.e., large elongated grains embedded in a matrix consisting of small equiaxed grains, were obtained above 1750 °C in all systems compacted by SPS and above 1800 °C in systems stabilized by Nd and Sm but not Dy, Y, or Yb by a two-step hot-pressing procedure. It was observed that the formation of abnormally grown α–sialon grains was strongly temperature-dependent, indicating that it was encouraged by the formation of a transient liquid phase that stimulated the dissolution of any remaining nitride precursors and early formed small α–sialon grains and sequentially facilitated supersaturation by the α–sialon constituents. The presence of elongated grains improves fracture resistance in the obtained materials.

Place, publisher, year, edition, pages
2002. Vol. 17, no 2, 336-342 p.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:su:diva-22947DOI: 10.1557/JMR.2002.0047OAI: oai:DiVA.org:su-22947DiVA: diva2:189792
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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