Silicon nitride ceramics, properties

Keywords Silicon nitride ceramics, Phase relations, Processing, Microstructure, Properties... 

Silicon nitride has the composition Si3N4 and its chemical bonding is predominantly covalent. Si3N4 represents the backbone of silicon nitride ceramics, a class of ceramic materials which, because of their exceptional profile of properties, are gaining increasing acceptance in engineering applications. 

Table 10 summarises all methods for the densification of Si3N4 used at present. The resulting Si3N4 ceramics classified according to the densification routes are also listed together with several remarks on manufacturing characteristics, properties and applications. For comparison with the sintered qualities, information on reaction bonded silicon nitride ceramics are also included but will be treated in more detail in Sect. 8. 

Silicon nitride ceramics are not merely only one material but several classes of materials. All of them are multiphased, i.e., they exhibit a heterogeneous microstructure which has formed during sintering (Sect. 6). Therefore in all classes a large variety of properties is predominant and as a consequence also a large variety of potential applications (Sect. 10). Often little variations in the powders and the processing parameters cause remarkable changes in the microstructure which have a pronounced effect on properties (Sects. 6 and 7). 

Woetting G, Feuer H, Gugel E (1993) The Influence of Powders and Processing Methods on Microstructure and Properties of Dense Silicon Nitride. In Chen IW, Becher PF, Mitomo M, Petzow G, Yen TS (eds) Silicon Nitride Ceramics, Mat Res Soc Symp Proc 287. Mat Res Soc, Pittsburgh, p 133... 

Becher PF, Hwang SL, Lin HT, Ticgs TN (1994) Microstructural contributions to the fracture resistance of silicon nitride ceramics. In Hoffmann MJ, Petzow G (eds) Tailoring of Mechanical Properties of Si3N4 Ceramics, NATO ASI Ser E Vol. 276, Kluwer Academic Publishers, Dordrecht, p 87... 

Y NMR has also been reported extensively from yttrium sialon ceramics. Yttria is an important sintering aid for densifying silicon nitride. The properties of the sialon produced are determined by the nature of the grain boundary phase, which is usually a crystalline or glassy yttrium sialon. The often disordered nature of such phases can mean that broader 89Y resonances are... 

Properties. Properties of structural silicon nitride ceramics are given in Table 2. These values represent available, well-tested materials. However, test methodology7 and the quality of the specimens, particularly7 their surface finish, can affect the measured values. Another important material property is tensile strength. Values obtained on Norton s NT154 material are 750 MPa at RT, 500 MPa at 1200°C, and 350 MPa (50,000 psi) at 1400°C (62). 

The thermodynamics of the above-elucidated SiC/C and SijN Si composites are determined by the decomposition of silicon carbide and silicon nitride, respectively, into their elements. The chemistry of ternary Si-C-N composites is more complex. If producing Si-C-N ceramics for applications at elevated temperature, reactions between carbon and silicon nitride have to be considered. Figure 18.2, which exhibits a ternary phase diagram valid up to 1484°C (1 bar N2) displays the situation. The only stable crystalline phases under these conditions are silicon carbide and silicon nitride. Ceramics with compositions in the three-phase field SiC/Si3N4/N are unknown (this is a consequence of the thermal instability of C-N bonds). Although composites within the three-phase field SiC/Si3N4/Si are thermodynamically stable even above 1500°C, such materials are rare. The reasons are difficulties in the synthesis of the required precursors and silicon melting above 1414°C. The latter aspect is of relevance, since liquid silicon dramatically worsens the mechanical properties of the derived ceramics. 

C. A composition in this two-phase field should have superior high-temperature mechanical properties. Greskovich [21] has synthesized ceramics in this phase field, and high-temperature stress rupture tests showed that they are the most stable silicon nitride ceramics among all of the other systems studied. 

Silicon nitride ceramics should have high flexural strength and fracture toughness at room temperature and good creep resistance at elevated temperatures. Because these properties are determined by the microstructure of these ceramics, further improvement of these ceramics is possible through alloy design. The following sections will be devoted to micro-... 

Silicon nitride ceramics are one of the most promising materials for structural applications. The properties of the silicon nitride ceramics depend on the phases present and their microstructures. This paper describes how to approach microstructural design of these materials by using the phase equilibrium diagrams of silicon nitride-metal oxide systems. 

OXYNITRIDE GLASSES EFFECTS OF COMPOSITION ON GLASS FORMATION AND PROPERTIES WITH IMPLICATIONS FOR HIGH TEMPERATURE BEHAVIOUR OF SILICON NITRIDE CERAMICS... 

H. Miyazaki, Y. Yoshizawa, K. Hirao. Effect of Crystallization of Intergranular Glassy Phases on the Dielectric Properties of Silicon Nitride Ceramics. Mater. Sci. Eng., 148.257-60 (2008). 

The protective properties of rutile are fairly low compared to those of alumina or silica. Thus, the presence of TiN or TiC limits the high-temperature applications of the above composites. At small amounts and particle size of TiN in silicon nitride ceramics, a continuous silicate film can be formed, covering TiN particles and protecting them from further oxidation [178]. 

Silicon nitride ceramic materials have been intensively studied for many years because of their great potential for use in structural applications at room and high temperatures. This is due to their excellent mechanical properties in combination with good corrosion and thermal shock resistance. 

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