Silicon nitrides (Si3N4) are a range of advanced engineering ceramics characterized by high strength, toughness and hardness and excellent chemical and thermal stability.
The different types of silicon nitride, RBSN, HPSN, SRBSN and SSN, result from their methods of fabrication, which governs their resulting properties and applications.
Reaction-Bonded Silicon Nitride (RBSN)
RBSN is made by carefully nitriding a silicon (Si) ‘dough’ or compact at about 1450°C. During the reaction, the Si3N4 grows in the porosity in the compact. This results in little dimensional change accompanied by an increase in density. This feature is the biggest benefit of RBSN, as it often allows parts to be made to near net shape without the need for subsequent expensive grinding operations.
A consequence of this reaction is that the final product contains porosity and as such its mechanical properties are inferior to those of the other types of silicon nitride, as shown in the table below. However, the material is relatively cheap to produce and has found application particularly as kiln furniture.
Hot Pressed Silicon Nitride (HPSN)
These HPSNs have found application as cutting tools for example. Applications tend to be for simple shapes as the hot pressed ‘billet’ must be diamond ground to obtain the required profile. As a result HPSN parts tend to be expensive and have largely been superseded by SSN.
Sintered Reaction-Bonded Silicon Nitride (SRBSN)
More recent developments of these materials have shown improvements to toughness and impact resistance as a result of the growth of crack deflecting silicon nitride needle-like grains.
Sintered Silicon Nitride
SSN covers silicon nitride materials which are densified by pressureless sintering in a nitrogen atmosphere at around 1750°C. . In order to aid densification, various combinations of sintering additives such as yttrium oxide, magnesium oxide and aluminum oxide are used. SSN and sialon generally offer the best mechanical properties available for a silicon nitride and are the most widely used in industrial applications such as molten metal handling, industrial wear, metal forming, the oil and gas industries and the chemical and process industries.