Silicon nitride offers solutions to problems faced in industries that require high-temperature, high-strength materials and very small allowable tolerances.
Development lay dormant until 1953, however, when the synthesis process was rediscovered (and again patented) by the Carborundum Co. in Niagara Falls.1-3 This rediscovery of silicon nitride yielded solutions to problems faced in industries that required high-temperature, high-strength materials and very small allowable tolerances.
Another additive introduced to aid in the nitridation process is fluorine, typically added as CaF2 or BaF2.1-3,8 The optimal fluorine addition is currently unknown; 1 wt% (relative to silicon) is typically added. It has been demonstrated, however, that the concentration should not exceed 5 wt% (relative to silicon).
The raw material fabrication generally requires all particles to have a high surface area for an increased reaction rate. Typically, a fumed silica (amorphous) powder is used and mixed with carbon in a specific ratio. Increasing the ratio of C:SiO2 generally increases the rate of Si3N4 formation up to a maximum point (as dictated by Equation 4).
The carbon removal step removes excess carbon and oxygen from the prepared silicon nitride to increase the purity. This is carried out by leaking in atmospheric air during the cooling range of 600-850°C. The cooling rate is slowed during this process to have the temperature hold at 600-850°C for one to eight hours.
Heat treatment of Si3N4 powders requires an N2 atmosphere or vacuum due to the sensitivity of the Si3N4 to impurities in the atmosphere.3,5,8 Contamination of the atmosphere by H2O, O2 or H2 has significant effects on the products being heat treated.1 Removal of impurity gases, specifically oxygen, is difficult in commercial production. If the silicon metal is still present (not yet reacted to form Si3N4) with oxygen or water vapor present, the silicon metal will oxidize to SiO2, resulting in the formation of a glassy phase upon cooling.
The Si3N4 bonding phase may be formed in-situ during proprietary heat treatment processes using all three methods of forming silicon nitride. Through these processes, very large (weighing up to 900 lbs) industrial Si3N4-bonded silicon carbide parts have been produced with tight tolerances, typically ± 0.04 in.
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1. Riley, F.L., “Silicon Nitride and Related Materials,” J. Am. Ceram. Soc., 83  245-65, 2000.
2. Nicholson, K.C., “Silicon Nitride-Bonded Refractory Oxide Bodies and Method of Making,” U.S. Pat. 2636828, April 28, 1953.
3. Nicholson, K.C., “Manufacture of Silicon Nitride-Bonded Articles,” U.S. Pat. 2618565, November 18, 1953.
4. Thompson, D.S., and Pratt, P.L., “The Structure of Silicon Nitride,” Science of Ceramics, Vol. 3, edited by Stewart, G.H., Academic Press, New York, 1967, pp. 33-51.
5. Popper, P., and Ruddlesden, A.C., “The Preparation, Properties and Structure of Silicon Nitride,” Trans. Br. Ceram. Soc., 60 603-26 (1961).
6. van Dijen, F.K., Kerber, A., Vogt, U., Pfeifer, W., and Schulze, M., “A Comparative Study of Three Silicon Nitride Powders, Obtained by Three Different Syntheses,” Silicon Nitride ’93, Vol. 89-91, Edited by Hoffmann, M.J., Becher, P.F., and Petzow, G., 1994, pp. 19-28.
7. Hofmann, H., Vogt, U., Kerber, A., and v. Dijen, F.K., “Silicon Nitride Powder from Carbothermal Reaction,” Silicon Nitride Ceramics: Scientific and Technological Advances, Edited by Chen, I.W., Becher, P.F., Mitomo, M., Petzow, G., and Yen, T.S., Materials Research Society, Pittsburgh, 1993, pp. 105-20.
8. Atkinson, A., Moulson, A.J., and Roberts, E.W., “Nitridation of High-Purity Silicon,” J. Am. Ceram. Soc., 59 [7-8] 285-9, 1976.
9. Sowa, F.J., “Production of Metallic Nitrides,” U.S. Pat. 2606815, August 12, 1952.
10. CRC Handbook of Chemistry and Physics, 82nd ed., Edited by Lide, D.R., CRC Press, New York, NY, 2002.
K.J. DeCarlo, Ph.D., is the Research and Development Manager at Blasch Precision Ceramics.