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.

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.

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 .