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Synthesis and Characterization of Nitrogen-rich Calcium α-Sialon Ceramics
Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, a synthesis concept has been developed, which uses nitrogen-rich liquid phases for sintering of Ca-α-sialon ceramics. First, keeping the Si/Al ratios constant, the effects of N/O ratio on the properties and microstructure were investigated through a liquid phase sintering process. Second, nitrogen-rich Ca-α-sialon ceramics, with nominal compositions: CaxSi12-2xAl2xN16, x < 2.0, was synthesized and characterized. Third, mechanical and thermal properties of nitrogen-rich Ca-α-sialons were investigated in terms of high temperature deformation resistance,reaction mechanism, phase stability and oxidation resistance, and further correlated to their phase assemblage and microstructure observation.

It has been found that increasing the N/O and Ca/Al ratio simultaneously in the materials could result in development of a microstructure with well shaped, high-aspect-ratio Ca-α-sialon grains, and an improvement in both toughness and hardness.

For the nitrogen-rich Ca-α-sialon, mono-phasic α-sialon ceramics were obtained for 0.51 ≤ x ≤ 1.32. The obtained Ca-α-sialon ceramics with elongated-grain microstructures show a combination of high hardness and high fracture toughness. Compared with the oxygen-rich Ca-α-sialons, the nitrogen-rich Ca-α-sialons exhibited approximately 150 oC higher deformation onset temperatures and decent properties even after the deformation.

The α-sialon phase was first observed at 1400 oC, however the phase pure Ca-α-sialon ceramics couldn’t be obtained until 1800 oC. The nitrogen-rich Ca-α-sialons were thermal stable, no phase transformation observed in the temperatures range1400-1600 oC. In general, mixed α/β-sialon showed better oxidation resistance than pure α-sialon in the low temperature range (1250-1325 oC), while α-sialons with compositions located at α/β-sialon border-line showed significant weight gains over the entire temperature range (1250-1400 oC).

Place, publisher, year, edition, pages
Stockholm: Institutionen för fysikalisk kemi, oorganisk kemi och strukturkemi , 2009. , 57 p.
Keyword [en]
Nitrogen-rich, SiAlON, Sintering, Microstructure, Properties
National Category
Inorganic Chemistry
Research subject
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-8568ISBN: 978-91-7155-823-7 (print)OAI: oai:DiVA.org:su-8568DiVA: diva2:200496
Public defence
2009-03-20, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 12 A, Stockholm, 13:00
Opponent
Supervisors
Available from: 2009-02-26 Created: 2009-02-17Bibliographically approved
List of papers
1. Self-Reinforced Nitrogen-Rich Calcium-α-SiAlON Ceramics
Open this publication in new window or tab >>Self-Reinforced Nitrogen-Rich Calcium-α-SiAlON Ceramics
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2007 In: Journal of the American Ceramic Society, ISSN 0002-7820, Vol. 90, no 2, 608-613 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-25766 (URN)000243754700041 ()
Note
Part of urn:nbn:se:su:diva-8568Available from: 2009-02-26 Created: 2009-02-17Bibliographically approved
2. Sialon Ceramics Prepared by Using CaH2 as a Sintering Additive
Open this publication in new window or tab >>Sialon Ceramics Prepared by Using CaH2 as a Sintering Additive
2008 (English)In: Journal of the American Ceramic Society, ISSN 0002-7820, Vol. 91, no 9, 2997-3004 p.Article in journal (Refereed) Published
Abstract [en]

In developing elongated a-sialon grains, a nitrogen-rich liquidphase sintering method was introduced by using CaH2 as asintering aid, so as to vary the N/O ratio of the liquid phaseformed in the sintering process while keeping the Si/Al ratiosconstant. With increasing addition the phase contents changedfrom single b-sialon to dual b/a-sialons and to single Ca-a-sialon.At low N/O ratios the microstructures contained mainlyequiaxed b-sialon grains, and at high N/O ratios well facetedelongated Ca-a-sialon grains. The improved toughness(KIC57.8 MPa .m1/2) and hardness (HV10517.5 GPa) propertiescan be attributed to the formation of interlocked microstructures.High-temperature compressive deformation testsindicated that the deformation onset temperature is determinedmainly by the Si/Al and N/O ratios, whereas the deformationrate is affected by the microstructure, i.e., the morphology andamounts of elongated a-sialon grains and residual glass phase,especially for the sialons with low N/O ratios.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-25767 (URN)10.1111/j.1551-2916.2008.02549.x (DOI)000259151000035 ()
Note
Part of urn:nbn:se:su:diva-8568Available from: 2009-02-26 Created: 2009-02-17 Last updated: 2012-03-23Bibliographically approved
3. Superplastic deformation of nitrogen-rich Ca-α-sialon ceramics
Open this publication in new window or tab >>Superplastic deformation of nitrogen-rich Ca-α-sialon ceramics
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2008 In: Materials Science and Engineering: A, ISSN 09215093, Vol. 475, no 1-2, 81-86 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-25768 (URN)
Note
Part of urn:nbn:se:su:diva-8568Available from: 2009-02-26 Created: 2009-02-17Bibliographically approved
4. Thermal Properties of Nitrogen-Rich Ca-α-Sialons
Open this publication in new window or tab >>Thermal Properties of Nitrogen-Rich Ca-α-Sialons
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2009 (English)In: Journal of the European Ceramic Society, ISSN 0955-2219, Vol. 29, no 16, 3409-3417 p.Article in journal (Refereed) Published
Abstract [en]

Nitrogen-rich Ca-α-Sialon (CaxSi12−2xAl2xN16 with x = 0.2, 0.4, and 0.8, 1.2 and 1.6) ceramics were prepared from the mixtures of Si3N4, AlN and CaH2 powders in a hot press at 1800 °C using a pressure of 35 MPa and a holding time of 4 h, and then were investigated with respect to reaction mechanism, phase stability and oxidation resistance. In addition the sample with x = 1.6 was prepared in the temperature range 600–1800 °C using a pressure of 35 MPa and a holding time of 2 h. The α-Sialon phase was first observed at 1400 °C but the α-Si3N4 and AlN phases were still present at 1700 °C. Phase pure Ca-α-Sialon ceramics could not be obtained until the sintering temperature reached 1800 °C. The phase pure nitrogen-rich Ca-α-Sialon exhibited no phase transformation in the temperature range 1400–1600 °C. In general, mixed α/β-Sialon showed better oxidation resistance than pure α-Sialon in the low temperature range (1250–1325 °C), while α-Sialons with compositions located at α/β-Sialon border-line showed significant weight gains over the entire temperature range tested (1250–1400 °C). The phases formed upon oxidation were characterized by X-ray, SEM and TEM studies.

Identifiers
urn:nbn:se:su:diva-25769 (URN)2122/10.1016/j.jeurceramsoc.2009.07.015 (DOI)000270694400014 ()
Note
Part of urn:nbn:se:su:diva-8568Available from: 2009-02-26 Created: 2009-02-17 Last updated: 2010-01-26Bibliographically approved
5. Nitrogen-Rich α-Sialons Stabilized by Y, Yb, and Nb Cations
Open this publication in new window or tab >>Nitrogen-Rich α-Sialons Stabilized by Y, Yb, and Nb Cations
Manuscript (Other academic)
Identifiers
urn:nbn:se:su:diva-25770 (URN)
Note
Part of urn:nbn:se:su:diva-8568Available from: 2009-02-26 Created: 2009-02-17 Last updated: 2010-01-13Bibliographically approved
6. Spark Plasma Sintering of B4C Ceramics: The Effects of Milling Medium and TiB2 Addition
Open this publication in new window or tab >>Spark Plasma Sintering of B4C Ceramics: The Effects of Milling Medium and TiB2 Addition
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In: Journal of Marerials Research, ISSN 0884-2914Article in journal (Refereed) Submitted
Identifiers
urn:nbn:se:su:diva-25771 (URN)
Note
Part of urn:nbn:se:su:diva-8568Available from: 2009-02-26 Created: 2009-02-17Bibliographically approved

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