Forming And Heat Treatment Of Silicon Nitride

Forming and Heat Treatment of Silicon Nitride The most common method to form and subsequently sinter silicon nitride is hot pressing with MgO addition; however, thanks to current advancements, pressureless sintering is achievable. In order to densify the silicon nitride compact without introducing high pressure, yttrium oxide (Y2O3) and aluminum oxide (Al2O3) must be added.

The compacts can then be sintered at a temperature of 1750°C without increasing pressure; however, the maximum densification rate occurs between temperatures of 1450-1500°C. The total shrinkage and temperature at which densification begins depends on the production route of the silicon nitride powder used.

Since Si3N4 is sensitive to impurities in the atmosphere, heat treatment of Si3N4 powder requires N2 atmosphere or vacuum. Contamination of the atmosphere with H2O, O2 or H2 has a significant effect on the product being heat treated.

Removal Of Impurity Gases

The removal of impurity gases, especially oxygen, is difficult in commercial production. If the metallic silicon is still present (which has not yet reacted to form Si3N4) and oxygen or water vapor is present, the metallic silicon will oxidize to SiO2, resulting in the formation of a glassy phase upon cooling.

The formation of silicon oxide r O2 due to the presence of H2O and /O can be detected visually by the “glaze” on the surface of the Si3N4 product. 5 The presence of oxygen and/or water vapour can also adversely affect Si3N4 already formed, possibly leading to oxidation of silicon nitride (forming Si2N2O) or to dissociation of silicon nitride (the former reaction is more thermodynamically stable):

2Si3N4 + 1.5O2 –> 3Si2N2O + N2, ΔG1227°C = -1063 kJ/mole

Si3N4 + 3O2 –> SiO2 + 2N2, ΔG1227°C = -802 kJ/mole

In the temperature range of 1000-1400°C, the activation energy (Q) for the oxidation of silicon nitride is about 486 kJ/mol, while for the oxidation of silicon metal, Q = 112 kJ/mol for the same temperature range. In contrast, the presence of a small amount (0.2%) of H2 gas during nitridation has been shown to result in increased weight gain, implying a more complete nitridation of silicon particles. 5

Bonding Stage

The Si3N4 bonded phase can be formed in situ during a proprietary thermal process using all three methods of silicon nitride formation. These processes produce very large (up to 900 lb) industrial Si3N4 bonded silicon carbide parts with very tight tolerances, typically ± 0.04 inches.

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