Si3N4 ceramics, applications

Soluble polysilane polymers can also be used as precursors to silicon carbide. The first such application, using (PhMeSi)n-(Me2Si)m copolymers ("Polysilastyrene"), was to strengthen silicon nitride ceramics. The Si3N4 ceramic body was soaked in polysilane and refired, leading to the formation of silicon carbide whiskers in the pore spaces and a consequent increase in strength. (U)... 

The spectrum of properties provided by dense Si3N4 ceramics has proved to be of exceptional value in numerous applications and improvements that were previously not considered feasible for lack of a suitable material. This has... 

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. 

The most common densification method is the gas-pressure sintering which guarantees better reproducibility and improved properties by only moderate increase in production costs compared to pressureless sintering. The first dense Si3N4 ceramics were produced by hot pressing [13]. Nowadays it is used for the preparation of specimens in materials development or for some applications where components of simple geometries and low quantities are required. 

As a consequence of the large diversity of microstructures which can adjusted by microstructural design, Si3N4 ceramics are a whole class of materials with an inherent large variety of properties and therefore a large variety of potential applications (Sect. 10). Different qualities depend on amount and distribution of the microstructural features (Sect. 6.1.4). Often small variations have severe... 

Especially strength and fracture toughness are essential for structural applications. Therefore, great efforts are made to optimise all preconditions for a reproducible production of reliable Si3N4 ceramics with high strength... 

In principle, the amount, composition and degree of crystallisation of the grain boundary phase are key factors which must be considered for successful development of Si3N4 ceramics for applications at elevated temperatures. It is... 

In Table 13 the interactions of Si3N4 ceramics with common metals are summarised (see also [18, 472, 473]). For application in metallurgy not only the interaction with the metal but also the interaction of oxide slags on the surface of the metals has to be taken into account. At higher temperatures most metal oxides react with the grain boundary phase. For example, V205,... 

Si3N4 ceramics cover only 1% of the total market of advanced ceramic materials, i.e., electronic and structural applications, but about 5% of the structural ceramics. They have the highest growth rates among structural ceramics [636-638]. 

The production costs are a main problem in the application of Si3N4 ceramics. The production of small series is more expensive compared to existing materials. This is connected on the one hand with the high costs of raw material and on the other hand with the production technologies. In... 

The application of Si3N4 ceramics as components for gas turbines is under development. First successful tests of those components have been carried out with 1350 °C turbine inlet temperatures. The further improvement of the turbine efficiency requires higher turbine inlet temperatures for this, additional coatings are necessary (Sect. 7.3). 

Urashima K, Ikea Y, IRAs S (1998) Features of Superior Strength Si3N4 Ceramic and its Application. In Niihara K, Hirano S, Kanzaki S, Komeya K, Miring K (eds) 6th Int Symp Ceramic Materials and Components for Engines. Jap Fine Ceramic Ass, Tokyo, p 167... 

Heinrich JG, Krtiner H (1994) Silicon Nitride Materials for Engine Applications. In Hoffmann MJ, Petzow G (eds) Tailoring of Mechanical Properties of Si3N4 Ceramics. NATO ASI Series E, Vol 276, Kluwer Academic Publishers, Dordrecht, p 19... 

Whether this is true for a given Si3N4 ceramic under application conditions depends on many parameters like a silica layer protection, diffusion coefficients, wetting behavior etc. The reader is here referred to the reference mentioned above [116]. 

Therefore, for most Si3N4 ceramics it will be necessary to have mechanical property measurements along with corrosion to evaluate their applicability. None the less there are rules of thumb, which may help in materials selection. The most important one is that the level at which oxidative corrosion becomes the prime problem, is connected to the eutectic of the system sintering additive(s) -I- Si02. 

K. Urashima, Y. Ikai and S. Iwase, Features of a Superior Strength Si3N4 Ceramic and its Applications , 6th International Symposium on Ceramic Materials end Components for Engines, 1997, 167-172. 

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