Silicon nitride (Si3N4) is a non-oxide structural ceramic material that is usually black or dark grey in colour, and often polished to give a smooth and strikingly reflective surface appearance. Popular for its high shock and thermal resistance, its typical applications include metal forming, industrial wear situations and molten metal handling .
Properties of a silicon nitride
Some silicon nitrides are highly porous and therefore have low density and oxidation resistance. When treated properly, silicon nitride offers good hardness at extremely high temperatures, good creep resistance, high wear resistance, low coefficient of thermal expansion, chemical resistance, oxidation resistance, and increased mechanical strength [2,3,4]:
|Silicon Nitride (Si3N4)|
|Mechanical Properties||Thermal Properties|
|Compressive / Crushing Strength||600 to 2950 MPa||Maximum Temperature||1000 to 1330°C|
|Elastic Modulus||140 to 310 GPa||Maximum Thermal Shock||290 to 750°C|
|Flexural Strength||130 to 810 MPa||Specific Heat Capacity||720 to 800 J/kg-K|
|Fracture Strength||3.1 to 6.2 MPa-m1/2||Thermal Conductivity||12 to 31 W/m-K|
|Poisson’s Ratio||0.24 - 0.27||Thermal Expansion||2.5 - 3.2 µm/m-K|
|Density||2.2 to 3.4 g/cm3||Dielectric Constant||8.0 - 10.0|
|Dielectric Strength||18 kV/mm|
Production of silicon nitride
Silicon nitride is synthesised typically by the chemical reaction of metallic silicon and gaseous nitrogen. Parts are carefully pressed and sintered by well-developed methods that give rise to particular properties and therefore define the end applications.
Methods of formation vary depending on the type of silicon nitride required. Each adds specific strengths to the resultant material.
Reaction-bonded silicon nitride (RBSN)
Processing RBSN starts by forming a silicon “dough” or compact which undergoes a surface hardening process called nitriding at temperatures of about 1450°C. RBSN is more porous (about 25% porosity) and inferior compared to other types of silicon nitride in terms of mechanical properties, but is relatively cost-effective and used in applications such as kiln furniture .
Hot-pressed silicon nitride (HPSN)
HPSN is made from a mix of fine Si3N4 powder and a flux of magnesia in a graphite die, subjected to high temperature and pressure (usually to 1800°C and 40MPa). It is highly dense compared to RBSN and has far better mechanical properties .
Sintered reaction-bonded silicon nitride (SRBSN)
SRBSN is an upgraded version of RBSN. To reduce the porosity of the final product, sintering additives are supplemented to the initial powdered mix, which then gives it the capability to be sintered (under temperatures greater than 1800°C and atmospheric pressure) after the reaction-bonding stage [1,5].
Sintered silicon nitride (SSN)
To make an SSN, the silicon nitride undergoes pressureless sintering at around 1750°C with a combination of sintering additives such as magnesium oxide, yttrium oxide and aluminum oxide to promote the densification process. Density amounts to 98% with a strength range of 600 - 700 MPa [1,5].
However, shrinkage of up to 20% occurs during the densification process. To overcome this issue, fully sintered formations are re-machined by a precise diamond tool treatment to gradually abrade excess material until the desired shape and size are achieved. Yet, since these components are inherently tough and hard, this process becomes labour-intensive and costly .
Applications of silicon nitride
As it is able to withstand the rigours of severe thermal, mechanical and wear situations, silicon nitride produces high-quality components for use in extremely demanding applications.
The biggest application of silicon nitride is found in the automotive industry, particularly the various wear and combustion parts in reciprocating engines where stresses at high temperatures are prevalent. In diesel engines, silicon nitrides are used in glow plugs (for quicker start-up time), precombustion chambers (to reduce emissions or act as a muffler), and turbochargers (to reduce engine lag). Silicon nitride is also used in cam followers, tappet shims, precision shafts and axles.
Silicon nitride ceramic ball and roller bearings are preferred over steel for high-temperature applications. This prolongs the bearing life at higher speeds as it would possess better corrosion resistance. Its higher modulus of elasticity compared to steel indicates that it is more rigid, thus creating a smooth motion and less vibration with the contacting surfaces .
Silicon nitride is also utilised in metal cutting tools for cast iron, hard steel, and nickel-based alloys due to its hardness, fracture toughness, and thermal shock resistance.
Another application is handling molten metal – for which metallic components are often unsuitable. Weld jigs/positioners, spray nozzles, cylinder wall coatings, turbine rotor, blades, pipes for rockets, and missiles employ silicon nitride for its strength, electrical properties and shock resistance. Applications in the aluminium casting industry, as well, benefit from silicon nitride’s good corrosion resistance, thermal shock resistance, and thermal conductivity. These include heating tubes, pump liners, riser tubes, and thermocouple protection sheaths .