Ferrosilicon Nitride (FeSi₃N₄)
Chemical Composition: Produced by high-temperature nitridation of ferro silicon alloy (typically containing 65%-75% Si) in a nitrogen atmosphere. The main phase is Si₃N₄ (accounting for 70%-85%), with small amounts of free Fe (10%-15%) and unreacted silicon.
Physical Form: Grayish-white to dark gray powder or granules, with a density of approximately 3.2-3.4g/cm³ and a hardness of HV 1400-1800.
Crystal Structure: Dominated by α-Si₃N₄ with a small amount of β phase. Iron elements are dispersed in the matrix in the form of fine particles.
Silicon Nitride (Si₃N₄)
Chemical Composition: A pure-phase ceramic material with an atomic ratio of Si:N being 3:4 and a theoretical density of 3.18g/cm³.
Physical Form: White or light gray powder, which forms a highly dense ceramic body after sintering, with a hardness of HV 1800-2200 (for sintered bodies).
Crystal Structure: Mainly exists in two forms: α phase (low-temperature stable type) and β phase (high-temperature stable type). Industrial products adjust the proportion of the two phases by controlling the sintering process.
Comparison of Key Properties
| Comparison Dimension | Ferrosilicon Nitride, FeSi₃N₄ | Silicon Nitride, Si₃N₄ | Core Impact |
|---|---|---|---|
| Core Components and Purity | Si 65%-75%, N 18%-22%, Fe 10%-15%, composite phase structure | Si₃N₄ purity ≥99% (industrial grade), ≥99.9% (high-end grade), pure phase ceramic | Purity determines the upper limit of performance; silicon nitride iron balances functionality and cost, while silicon nitride focuses on ultimate performance. |
| Key Physical Properties | Thermal conductivity 15-30 W/(m・K), flexural strength 300-600 MPa, hardness HV 1400-1800 | Thermal conductivity 40-170 W/(m・K) (β phase up to 200), flexural strength 700-1500 MPa, hardness HV 1800-2200 | Silicon nitride outperforms silicon nitride iron in all aspects, especially in high temperature and mechanical strength. |
| Chemical Stability | Oxidation at 1300-1400℃ forms a SiO₂ protective film, resistant to acid and alkali corrosion (except strong oxidants) | Stable at 1600-1700℃, resistant to corrosion by most chemical media, pure phase structure with no impurity precipitation | Silicon nitride is suitable for higher temperature and more severe corrosion environments. |
| Difficulty of Manufacturing Process | High-temperature nitriding of ferrosilicon (1350-1450℃, 8-12 hours), a mature process. | Reaction sintering / hot pressing sintering (1700-1850℃, requires sintering aids), complex process | Silicon nitride iron has a large production capacity (1.5 million tons/year globally, with China accounting for 65%), ensuring high supply stability. |
Differences in Preparation Processes
Preparation of Ferrosilicon Nitride
Global production capacity: approximately 1.5 million tons/year, with China accounting for 65%.
Raw Material Preparation:
Select ferrosilicon alloy (65%-75% Si) and crush it to a size of less than 1mm.
Nitridation Reaction:
Introduce high-purity nitrogen (>99.99%) into a vertical resistance furnace, heat to 1350-1450℃, and react for 8-12 hours to form a composite phase where iron particles are wrapped in Si₃N₄.
Post-treatment:
After cooling, crush and screen the product, and remove free iron through magnetic separation to control the Fe content within 10%-15%.
Preparation of Silicon Nitride
Reaction Sintering Method:
Press silicon powder into a compact, which then reacts in nitrogen at 1350-1450℃ to synthesize α-Si₃N₄. Secondary sintering is required for densification.
Hot Pressing Sintering Method:
Add sintering aids such as MgO and Y₂O₃, and sinter at 1700-1850℃ under a pressure of 20-30MPa to obtain high-density β-Si₃N₄.
Gas Pressure Sintering Method:
Sinter in high-pressure nitrogen (>1MPa) to inhibit the decomposition of Si₃N₄ and produce high-purity ceramic components.
Comparison of Core Application Fields
Applications of Ferrosilicon Nitride
Refractories:
Used in taphole clay of large blast furnaces (e.g., Baosteel's 4966m³ blast furnace) to improve erosion resistance and thermal shock stability, reducing the fluctuation of taphole depth by 30%.
Iron and Steel Metallurgy:
Serves as a substitute for part of FeSi and FeN as a deoxidizer, reducing alloy costs by 15%-20% in the production of HRB400 rebars.
Wear-Resistant Coatings:
Thermal sprayed FeSi₃N₄ coatings are applied to mining machinery, with a wear rate 50% lower than that of traditional carbon steel.
Applications of Silicon Nitride
High-Temperature Structural Parts:
Used in aero-engine turbine blades (the GE9X engine adopts Si₃N₄ ceramic bearings), which can withstand a high temperature of 1300℃ and reduce weight by 30%.
Electronic Field:
Silicon nitride substrates for 5G base stations have a thermal conductivity of 170W/(m·K), with heat dissipation efficiency twice that of Al₂O₃.
Cutting Tools:
Si₃N₄-based ceramic tools for processing nickel-based alloys can achieve a cutting speed of 300m/min, with a service life 5 times that of cemented carbide.
Selection Guide and Industry Recommendations
Material Selection Criteria
For low-cost deoxidation or refractory materials, ferrosilicon nitride is preferred (its cost is only 1/5-1/10 of silicon nitride).
For applications requiring high-temperature strength or insulation performance, silicon nitride must be used (such as in semiconductor packaging and high-temperature bearings).
Industry Trends
Ferrosilicon Nitride:
Developing towards low silicon (60% Si) and high nitrogen (N 20%+) to meet the smelting requirements of ultra-low carbon steel.
Silicon Nitride:
The thermal conductivity is being improved to over 200W/(m·K) through nanocrystalline technology (e.g., nano β-Si₃N₄ developed by the Shanghai Institute of Ceramics, Chinese Academy of Sciences).
