High Performance Non-Oxide Ceramics II
The nitrides and carbides of boron and silicon are proving to be an excellent choice when selecting materials for the design of devices that are to be employed under particularly demanding environmental and thermal con- tions. The high degree of cross-lin
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Max-Planck-Institut fur Metallforschung, Heisenbergstr. 5, 70569 Stuttgart, Germany e-mail: [email protected] Fraunhofer Institut fur Keramische Technologien und Sinterwerkstoffe, Winterbergstrasse 28, 01277 Dresden, Germany e-mail: [email protected]
Silicon nitride ceramics is a generic term for a variety of alloys of Si3N4 with additional compounds necessary for a complete densification of the Si3N4 starting powder. They are heterogeneous, multicomponent materials characterised by the inherent properties of the crystalline modifications a and /? of Si3N4 and the significant influence of the densification additives. With a view to ability of the a and ft modification to form solid solutions a-Si3N4 (ass) and /?-Si3N4 (/?ss) solid solutions can be distinguished. Each group contains engineered materials with interesting properties for special applications. Phase relations and microstructures determine the properties decisively. Composition of the phases, the distribution of the grains, their aspect ratio and the grain boundary phase are pronounced microstructural features. The formation of the microstructure strongly depends on the one hand on the quality of the Si3N4 starting powders, which closely is related to the chemistry of the production process, and on the other on the liquid phase sintering as the most important step in the densification route. The interrelation between pure Si3N4, the densification of the powder including the role of sintering additives, microstructural engineering, physicochemical properties of the sintered Si3N4 ceramics (SSN, GPSN, HPSN, HIP-SSN, HIP-SN) are described in more detail and compared to reaction bonded Si3N4 ceramics (RBSN), which are produced by nitridation of silicon powders. Keywords: Silicon nitride ceramics, Phase relations, Processing, Microstructure, Properties
1
Introduction
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2
Crystalline Modifications
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2.1 2.2
Crystal Structures and Inherent Properties a/)8 Relations
53 56
3
Phase Diagrams
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3.1 3.1.1 3.1.2 3.2 3.2.1 3.2.2 3.3
Thermodynamics and Phase Diagrams Si-N-0 System Si-N-O-Additives Systems /?-Si3N4 Solid Solutions (/?ss) Extended jS-Si3N4 Solid Solutions Low /3-Si3N4 Solid Solutions CC-S13N4 Solid Solutions (ass)
60 60 61 63 64 65 68
Structure and Bonding, Vol. 102 © Springer-Verlag Berlin Heidelberg 2002
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G. Petzow • M. Herrmann
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Si3N4 Powders
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4.1 4.2
Powder Synthesis Powder Characterisation
72 76
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Consolidation of Si3N4 Powders
78
5.1 5.1.1 5.1.2 5.2 5.3 5.3.1 5.3.2 5.3.3 5.3.3.1 5.3.3.2 5.3.3.3 5.3.4 5.4 5.5
Processing Processing Additives Sintering Additives Shaping Densification Densification Methods Densification Mechanism Influence of Densification Parameters Decomposition Influence of Carbon Influence of Powder Bed Densified Materials Finishing Precursor-Derived Si3N4 Ceramics
79 79 80 82 82 83 83 87 88 89 89 90 90 91
6
Microstructures
91
6.1 6.1.1 6.1.2 6.1.3 6.1.4 6.1.4.1 6.1.4.2 6.1.4.3 6.2 6.3 6.4 6.5
Development of Microstructures in /?ss Ceramics Influence of Starting
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