In the early stages of its development, silicon nitride, Si3N4, powder compacts could not be sintered without the application of high mechanical pressure. If Si3N4 is compacted into a green shape and heated in air, it does not sinter and form a dense, strong material. Rather, it decomposes to silicon and nitrogen and fails to give the improved properties.
A major step in developing high density silicon nitride was the accidental discovery in the mid 1970’s that Si3N4 can be sintered if the additives developed for hot-pressed silicon nitride are included in the compact and precautions are taken to limit the loss of silicon nitride. The most commonly used additives are magnesium oxide (MgO) and yttrium oxide (Y2O3), which can be used separately, in combination or with aluminium oxide (Al2O3). The additives promote liquid phase sintering.
Precautions must also be taken to limit the decomposition of silicon nitride or loss of the additives. The compact is sintered in a bed of silicon nitride powder and/or under a high pressure nitrogen atmosphere (typically 1 to 8 MPa). The protective atmosphere suppresses the evaporation of silicon, nitrogen and additives, and enables the sintering reaction to take place. Higher temperatures (1825 - 2080°C) and longer times (up to 5 hours) are needed than used to produce hot-pressed silicon nitride, and strengths approaching those of hot-pressed silicon nitride can be achieved.