Silicon nitride, like Silicon Carbide, is also available in two main types, reaction bonded and sintered.
Silicon nitride ceramics are electrical insulators and are resistant to attack by many molten metals. With low thermal conductivity and excellent thermal shock resistance, silicon nitride ceramics are used in many RF heating applications where the material is in contact with hot metal parts.
The high strength of sintered silicon nitride ceramics has found many applications in the automotive and machine tool industries for bearing and wear parts which run in very arduous abrasive environments.
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There are six main types of silicon nitride which can be produced, reflecting the different processes used:
Reaction-Bonded Silicon Nitride (RBSN)
Here the shaped powder is nitrided in molecular nitrogen, to produce a product consisting of two different forms (alpha and beta silicon nitride). The original dimensions of the silicon compact stay relatively unchanged, it is therefore possible to partially nitride a complex shaped component adding strength. This will allow the piece to be machined with standard tools before completing the process.
The next five types are all produced by sintering silicon nitride with oxide additives.
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Sintered Reaction-Bonded Silicon Nitride (SRBSN)
This material is processed in the same initial manner as RBSN except some sintering aids are added to the starting powder. Once the parts have been green machined and nitrided they are subjected to a second sintering cycle which allows the product to densify (This causes 15% linear shrinkage).
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Hot Pressed Silicon Nitride (HPSN)
HPSN is formed through the application of heat and pressure through a graphite die. The problem however is that only simple shaped billets can be produced, with components being machined using relatively expensive grinding.
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Pressureless Sintered Silicon Nitride (SSN)
Here more complex components can be fabricated, due to the cost-effective method of production. Components are fired under a nitrogen atmosphere, with no application of pressure. As the reduction in surface area becomes the main driving force for sintering, powders with a high surface area are needed. This can effect the quality of the liquid phase due to the high oxygen content of the powder, which in turn can alter the overall composition of the second phase.
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Partial Pressure Sintered Silicon Nitride (PSSN)
This process is identical to Pressureless Sintering except a slight over pressure is used during the sintering process to aid densification. This reduces sintering flaws and gives a slight improvement to the mechanical properties. This provides a very cost effective solution vs. Hipping as the component is not required to be encapsulated to avoid degradation during the HIP process.
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Hot Isostatically Pressed Silicon Nitride (HIPSN)
Glass encapsulated parts are placed in an “autoclave”, with heat and pressure applied. The result is a slight decrease in strength but a substantial improvement in reliability.
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