Silicon nitride (Si₃N₄) is a naturally formed material created during the development of the earth due to the ammonia-rich atmosphere, silicon-rich crust and high

temperatures.Silicon nitride was first developed synthetically in 1859 by Deville and Wöhler, resulting in a patent filed in Germany.¹

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.

Silicon nitride has two different phases: the low-temperature (α) phase and the high-temperature (β) phase.⁴ Both phases are of the hexagonal crystal structure.⁵ The phase transition from α to β occurs between the temperatures of 1400 and 1600°C, depending on impurity (alumina) concentration.^5,6 It has been observed that additions of alumina reduce the activation energy of the phase transformation.⁶

In addition to the α and β phases, a fiber or whisker phase has also been detected.^1,5 These whiskers are a volatile phase that generally forms on the specimens or furnace wall. The formation is due to poor gas flow rate and/or impurities in the nitriding atmosphere.⁵

Decomposition or oxidation of Si₃N₄ can occur at a variety of different temperatures depending on atmosphere chemistry.^5,7 It has been demonstrated that at 1700°C, Si₃N₄ will start to decompose to Si_(l,g) and N_2(g); at 1880°C, the pressure of dissociated N_2(g) reaches 1 bar.^1,7

Three common commercial routes can be used to produce Si₃N₄:^1,6,7

• Direct nitridation^1-5,7,8

• Diimide decomposition^1,5,7,9

• Carbothermal reduction^1,7

Direct Nitridation

Direct nitridation is the most well understood Si₃N₄ synthesis process. The reaction is as follows:^1,2,7

3Si + 2N₂ --> Si₃N₄ + Heat (at ~1400°C)          (1)

The highly exothermic (~ 725 kJ/mole at 1400°C) nature of direct nitridation, coupled with the melting point of silicon (1414°C), makes temperature control during the direct nitridation process important.^1,2,5,10 If the temperature reaches the melting point of silicon before an adjacent layer of Si₃N₄ has completely enveloped the silicon particle surface, the silicon particles will fuse together and the liquid silicon will not wet the partial coating of silicon nitride.^1,2,5 The liquid silicon will therefore coalesce into globules larger than the original silicon particles and reduce the effective surface area for the direct nitridation reaction.⁵

Direct nitridation results in a theoretical weight gain of 66.67% of the weight of silicon metal; however, empirical evidence shows the weight gain of a fully nitrided part to be approximately 60% due to volitization of silicon during the process.²

The kinetics of the direct nitridation reaction can be improved through the use of catalysts. The most substantial impurity in silicon is iron, which is introduced in the milling process.^1-3,8 It has been demonstrated that the iron impurity acts to catalyze the nitriding reaction by removal of the SiO₂ film formed on each silicon particle. It is thought that a eutectic melt is formed with the approximate chemistry of ferrosilite (FeSiO₃).¹

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).

Diimide Decomposition

The starting reactants for diimide decomposition are silicon tetracholoride (SiCl₄) and ammonia (typically gas or liquid state).^1,5,9 Combining these two components results in a reaction that will occur at temperatures greater than or equal to 0°C:

SiCl₄(l) + 6NH₃ --> Si(NH)_2(s) + 4NH₄Cl_(s) + Heat (at 0°C)   (2)

The initial reaction for diimide decomposition is an extremely exothermic reaction that needs to be carefully monitored for safety reasons.⁷ When ammonia is used in excess, the additional compound formed will contain excess amide (NH) groups, which upon heating will reduce the compound to a less complex, more stable form (i.e., the product described in Equation 2).⁹ To form extremely pure Si(NH)₂, a sol-gel route is used:⁹

• SiCl₄ is treated with ammonia

• The resulting particles are dissolved in water

• A fluorine containing salt is added, causing the solution to gel

• Supernatant is filtered from the gel

• The gel is dissolved in a mineral acid

• The resulting solution is filtered

• NaOH_(aq) is added to re-precipitate the gel in a pure form

• The gel is removed and washed in water

The produced gel is then heated to approximately 1000°C in a N₂ atmosphere for approximately 30 minutes, resulting in a decomposition to amorphous Si₃N₄:^1,5,9

Si(NH)₂ --> Si₃N₄(amorphous) (at 1000°C)        (3)

The amorphous phase developed can be converted to the alpha phase via further heating to ~1450°C (in N₂).^1,5

Carbothermal Reduction

Carbothermal reduction was first patented in Germany in 1896. During the rediscovery of Si₃N₄ in the 1950s, it was again patented by the Carborundum Co. in 1955.¹ Carbothermal reduction has been described as the most commercially viable method for making Si₃N₄.^1,7 Carbothermal reduction is also the most commonly used method due to the safety of the process; all reactants and products are non-toxic and environmentally safe, and the reaction is endothermic in nature:⁷

3SiO₂ + 6C + 2N₂ + Heat --> α-Si₃N₄ + 6CO (at 1450-1500°C) (4)

The carbothermal reduction process can be divided into three steps:⁷

• Preparation of the raw material

• Reaction

• Carbon removal

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 reaction is observed to occur at a temperature between 1450-1500°C. Below 1450°C, the reaction kinetics are extremely slow, and above 1500°C another reaction resulting in the formation of SiC is thermodynamically more stable.⁷

Similar to the other methods of silicon nitride production, the partial pressure of oxygen is an important parameter; in addition, for carbothermal reduction, the partial pressure of CO is important. If the partial pressure of oxygen is too great, the oxygen in the atmosphere, rather than the supplied SiO₂, will react with the carbon reactant to form carbon monoxide. Hoffman suggests the partial pressure of oxygen must be kept below 2 x 10^-20 to ensure the carbothermal reduction.⁷ A 98.5% conversion of SiO₂ to Si₃N₄ has been demonstrated with the carbothermal reduction reaction (see Equation 4) by heat treating in N₂ at 1470°C for six hours.⁷

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.

Forming and Heat Treatment

The most common method of forming and subsequently sintering silicon nitride is hot pressing with an addition of MgO; however, due to current advances, pressure-less sintering is achievable.¹ To densify a silicon nitride compact without introducing high pressures, additions of yttria (Y₂O₃) and alumina (Al₂O₃) must be included.^6,7 The compacts can then be sintered without an increase in pressure at a temperature of 1750°C; however, the maximum rate of densification occurs between the temperatures of 1450-1500°C. The total shrinkage and temperature at which densification begins depends on the production route for the silicon nitride powders used.⁷

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 formation of silicon oxide due to the presence of H₂O and/or O₂ can be visually detected by a “glaze” on the surface of the Si₃N₄ products.⁵ The presence of oxygen and/or water vapor will also have an adverse effect on the already formed Si₃N₄, possibly causing the silicon nitride to oxidize (forming Si₂N₂O) or result in the dissociation of silicon nitride (with the former reaction more thermodynamically stable):¹

2Si₃N₄ + 1.5O₂ --> 3Si₂N₂O + N2, ΔG_1227°C = -1063 kJ/mole    (5)

Si₃N₄ + 3O₂ --> SiO₂ + 2N₂, ΔG_1227°C = -802 kJ/mole      (6)

The activation energy (Q) for the oxidation of silicon nitride (see Equation 5) at temperature in the range of 1000-1400°C is approximately 486 kJ/mole, whereas for the oxidation of silicon metal, Q = 112 kJ/mole in the same temperature range. In contrast, it has been demonstrated that a small amount (0.2%) of H₂ gas present during nitriding leads to an increase in weight gain, implying more complete nitridation of the silicon particles.⁵

Bonding Phase

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. 

For more information, contact the author at (518) 436-1263 or kdecarlo@blaschceramics.com, or visit www.blaschceramics.com.

Visit www.ceramicindustry.com/pods to listen to our podcast with the author!

References

1. Riley, F.L., “Silicon Nitride and Related Materials,” J. Am. Ceram. Soc., 83 [2] 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.