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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, pages
Stockholm: Institutionen för fysikalisk kemi, oorganisk kemi och strukturkemi, 2004. 92 p.
Keyword [en]
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)oai:DiVA.org:su-129 (OAI)diva2:189799 (DiVA)
Public defence
2004-05-26, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 12 A, Stockholm, 10:00
Opponent
Supervisors
Available from2004-05-05 Created:2004-05-05Bibliographically approved
List of papers
1. Formation of in-situ reinforced microstructure in α–sialon ceramics I
Open this publication in new window or tab >>Formation of in-situ reinforced microstructure in α–sialon ceramics I : Stoichiometric oxygen-rich compositions
2002 (English)In: Journal of Materials Research, ISSN 0884-2914, Vol. 17, no 2, 336-342Article 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.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-22947 (URN)10.1557/JMR.2002.0047 (DOI)
Available from2004-05-05 Created:2004-05-05 Last updated:2011-05-31Bibliographically approved
2. Formation of in situ Reinforced Microstructures in α-sialon Ceramics
Open this publication in new window or tab >>Formation of in situ Reinforced Microstructures in α-sialon Ceramics : Part II. In the Presence of a Liquid Phase.
2002 (English)In: Journal of Materials Research, ISSN 0884-2914, Vol. 17, no 5, 1136-1142Article in journal (Refereed) Published
Abstract [en]

In situ reinforced microstructures with well-dispersed elongated grains, up to 10 μm in length, embedded in matrices consisting of submicron equiaxed grains, were developed by hot pressing Y-, Yb-, and (Y + Yb)-doped a-sialon ceramics containing approximately 3 vol% extra liquid phase at a comparatively low sintering temperature, 1800 °C. The liquid phase, thermodynamically compatible with a-sialon, was introduced by raising the oxygen content of an already oxygen-rich α-sialon composition, e.g., by increasing the O/N ratio in RExSi12-(3x+n) Al3x+nOnN16−n. Two different α–Si3N4 precursor powders, one fine-grained and one coarse, and one coarse β–Si3N4 powder were used, and the influence of particle size and crystalline modification of the precursor Si3N4 powder on the formation of elongated a-sialon grains was investigated. The formation of elongated α-sialon grains was promoted by introducing an extra liquid phase and by using a fine-grained α–Si3N4 powder, whereas the coarse β–Si3N4 powder did not yield any elongated grains at all. The obtained in situ reinforced α-sialon ceramics were both hard and tough, with a Vickers hardness and a fracture toughness of 21 GPa and approximately 5 MPa m1/2, respectively.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-22948 (URN)10.1557/JMR.2002.0168 (DOI)
Available from2004-05-05 Created:2004-05-05 Last updated:2011-05-31Bibliographically approved
3. Formation of in situ reinforced microstructures in α-sialon ceramics
Open this publication in new window or tab >>Formation of in situ reinforced microstructures in α-sialon ceramics : Part III. Static and dynamic ripening
2004 (English)In: Journal of Materials Research, ISSN 0884-2914, Vol. 19, no 8, 2402-2409Article in journal (Refereed) Published
Abstract [en]

Dual cation (Yb + Y)-stabilized a-sialon ceramics with either stoichiometric composition or nonstoichiometric composition that yield less than 3 vol% of an additional intergranular liquid/glass phase were consolidated by spark plasma sintering (SPS). This process allows very fast heating and cooling, thus providing a unique possibility to monitor and manipulate the kinetics of phase transformation and grain growth during sintering. Below a temperature threshold, full densification and complete a-sialon formation are accompanied by very limited grain growth. The grain growth kinetics were investigated both by post heat-treatment of SPS pre-consolidated monophasic a-sialon bodies consisting of sub-micron sized equiaxed grains in a conventional graphite furnace using extended holding times (hours) and directly rapid annealing in the SPS apparatus above the temperature threshold (within minutes). Post heat treatment in the graphite furnace yielded in situ reinforced microstructures consisting of interlocking elongated grains only in the presence of an additional intergranular liquid/glass phase. Direct annealing by SPS process yielded in situ reinforced microstructures whether or not an additional liquid/glass was involved. The former microstructures are formed via the static Ostwald ripening mechanism whereas the latter ones are generated via a dynamic ripening mechanism. This demonstrates that the dynamic ripening provides an efficient means of developing in situ reinforced microstructure in a-sialon ceramics with improved mechanical properties.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-22949 (URN)10.1557/JMR.2004.0291 (DOI)
Available from2004-05-05 Created:2004-05-05 Last updated:2011-05-31Bibliographically approved
4. Formation of tough interlocking microstructures in silicon nitride ceramics by dynamic ripening.
Open this publication in new window or tab >>Formation of tough interlocking microstructures in silicon nitride ceramics by dynamic ripening.
2002 (English)In: Nature, ISSN 0028-0836, Vol. 417, no 6886, 266-269Article 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.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-22950 (URN)10.1038/417266a (DOI)
Available from2004-05-05 Created:2004-05-05 Last updated:2011-05-31Bibliographically approved
5. Reaction sequences occurring in dense Li-doped sialon ceramics
Open this publication in new window or tab >>Reaction sequences occurring in dense Li-doped sialon ceramics : influence of temperature and holding time
2003 (English)In: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 13, no 9, 2285-2289Article in journal (Refereed) Published
Abstract [en]

Spark Plasma Sintering (SPS) has been used to consolidate a lithium-doped duplex α–β sialon with the overall composition Li0.5Si9.5Al2.5O2.0N14. The kinetics of densification has been studied, and the phase transformation, reactions and grain growth occurring in the dense compacts upon further heat treatment have been investigated. Two sources of Si3N4 powders were used, namely α- and β-Si3N4. Green bodies heated at a rate of 100 °C min−1 yielded fully dense compacts at 1450 °C (α-Si3N4) and 1500 °C (β-Si3N4) without holding, and these compacts consisted mainly of a locally formed liquid and precursor Si3N4 particles. Upon further heating it was observed that α-sialon is formed initially, irrespectively of whether α-Si3N4 or β-Si3N4 powder is used as Si3N4 source; and when α-Si3N4 is used as starting powder, almost monophasic α-sialon compacts are formed before any transformation to β-sialon takes place on further heating. When β-Si3N4 is used as starting powder the formation of β-sialon is kinetically promoted, and compacts containing α-sialon, β-sialon and β-Si3N4 are obtained before the equilibrium phase assemblage is reached, i.e. a lithium-doped duplex α–β sialon ceramic. These observations can be interpreted in terms of the Ostwald step rule. Grain growth does not occur until the equilibrium phase assemblage has been established. The separation of grain growth from densification and phase transformation has implications for preparing Si3N4-based nano-ceramics and provides possibility for further studies of the kinetics of grain growth in Si3N4-based ceramics.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-22951 (URN)10.1039/B304899C (DOI)
Available from2004-05-05 Created:2004-05-05 Last updated:2011-05-31Bibliographically approved
6. Rapid formation and deformation of Li-doped sialon ceramics
Open this publication in new window or tab >>Rapid formation and deformation of Li-doped sialon ceramics
2004 (English)In: Journal of The American Ceramic Society, ISSN 0002-7820, E-ISSN 1551-2916, Vol. 87, no 4, 727-729Article in journal (Refereed) Published
Abstract [en]

Two lithium-doped sialon ceramics were densified and superplastically deformed by spark plasma sintering (SPS). Rapid densification with linear shrinkage rates of approximately 5 × 10−3 s−1 were observed for samples heated at a rate of 100°C/min up to ∼1400°C under a uniaxial pressure of 40 MPa. Isothermal deformation by SPS-preprepared, fully densified ceramics performed at T≥ 1450°C yielded strain rates in the order of 10−2 s−2. It is suggested that a high heating rate promotes material transport via formation of a nonequilibrated oxygen-rich liquid of low viscosity. This finding most likely holds true for other liquid-phase sintered ceramics as well and has implications for cost-effective manufacturing of ceramic components.

Keyword
sialon, lithium, densification
National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-22952 (URN)10.1111/j.1551-2916.2004.00727.x (DOI)
Available from2004-05-05 Created:2004-05-05 Last updated:2011-05-31Bibliographically approved
7. Formidable increase of superplasticity of ceramics in presence of an electric field
Open this publication in new window or tab >>Formidable increase of superplasticity of ceramics in presence of an electric field
2003 (English)In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 15, no 12, 1006-1009Article in journal (Refereed) Published
Abstract [en]

The ductility of silicon nitride-based ceramics is dramatically enhanced in the presence of a pulsed electric field/current that induces movement of the charged species present in the grain boundary glassy/liquid phase, and thereby promotes grain sliding along the grain boundaries. The Figure shows an α-sialon component before (right) and after (left) deformation.

Keyword
Ceramics, Microstructure, Silicon nitrides, Superplasticity
National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-22953 (URN)10.1002/adma.200304863 (DOI)
Available from2004-05-05 Created:2004-05-05 Last updated:2011-06-17Bibliographically approved

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