- ▸Impregnation processes are divided into metal impregnation (antimony, copper, Babbitt alloy) and resin impregnation (phenolic, furan, PTFE); metal impregnation improves strength and thermal conductivity, resin impregnation improves corrosion resistance
- ▸Antimony impregnation parameters: temperature 650-700°C, pressure 0.5-2.0 MPa, holding 2-4 hours, post-impregnation density up to 2.3-2.4 g/cm³, impregnation rate ≥85%
- ▸PVD diamond-like carbon (DLC) coating can increase graphite seal ring surface hardness to HV 3000-5000, reduce friction coefficient to 0.05-0.08, and reduce wear rate by 90%
- ▸Sulfidation treatment forms MoS₂ transfer film on graphite surface, reducing dry friction coefficient to 0.05-0.10, but temperature resistance is limited, with long-term use temperature not exceeding 300°C
Surface treatment of graphite seals is a key process for improving their performance. Proper surface treatment can significantly improve the wear resistance, corrosion resistance, thermal conductivity and self-lubrication of graphite seals. In our years of process R&D, Huahao Sealing Co., Ltd. (霍邱县华豪密封件有限公司) has established a complete surface treatment process system. This article systematically introduces the principles, parameters and applications of various surface treatment processes.
1. Impregnation Processes
1.1 Metal Impregnation
Metal impregnation involves introducing molten metal into graphite pores under vacuum or pressure, where it cools to form metal fillers that improve density, strength and thermal conductivity.
#### 1.1.1 Antimony Impregnation Process
Antimony (Sb) is the most commonly used metal impregnant for carbon graphite seals, with melting point 630°C and density 6.68 g/cm³. Process parameters:
- Preheating temperature: 400-450°C (degassing)
- Impregnation temperature: 650-700°C (antimony melting)
- Impregnation pressure: 0.5-2.0 MPa (pressure impregnation)
- Holding time: 2-4 hours
- Cooling method: furnace cooling to 200°C then air cooling
Post-impregnation performance:
- Density increases from 1.70 g/cm³ to 2.3-2.4 g/cm³
- Impregnation rate (pore filling rate) ≥85%
- Compressive strength increases from 120 MPa to 200-280 MPa
- Thermal conductivity increases from 70 W/(m·K) to 110 W/(m·K)
#### 1.1.2 Copper Impregnation Process
Copper (Cu) melting point 1083°C, density 8.96 g/cm³. Copper-impregnated graphite has higher thermal conductivity but higher cost. Process parameters:
- Impregnation temperature: 1100-1150°C
- Impregnation pressure: 1.0-3.0 MPa
- Holding time: 3-5 hours
Post-impregnation thermal conductivity reaches 140-160 W/(m·K), but copper oxidizes easily in air, limiting working temperature to below 400°C.
#### 1.1.3 Babbitt Alloy Impregnation
Babbitt alloy is tin-based or lead-based alloy with melting point 240-400°C. Babbitt-impregnated graphite has excellent conformability and embeddability, suitable for low-speed heavy-load conditions.
1.2 Resin Impregnation
Resin impregnation involves introducing liquid resin into graphite pores under vacuum, then heating to cure.
#### 1.2.1 Phenolic Resin Impregnation
Phenolic resin is the most economical resin impregnation material, curing temperature 150-180°C.
- Impregnation temperature: room temperature
- Vacuum: ≤-0.09 MPa
- Impregnation time: 30-60 min
- Curing temperature: 150-180°C × 4-6 hours
- Post-impregnation temperature resistance: 200°C
#### 1.2.2 Furan Resin Impregnation
Furan resin has better corrosion resistance than phenolic, curing temperature 160-200°C. Post-impregnation temperature resistance reaches 220°C, withstanding most organic and inorganic acids.
#### 1.2.3 PTFE Impregnation
PTFE impregnation is achieved through suspension impregnation + sintering, with sintering temperature 360-380°C. Post-impregnation has excellent chemical inertness, but thermal conductivity decreases to 50-55 W/(m·K).
2. Surface Coating Processes
2.1 PVD Diamond-Like Carbon (DLC) Coating
DLC coating forms a 1-5 μm thick amorphous carbon-hydrogen film on graphite seal ring surfaces through physical vapor deposition (PVD). Process parameters:
- Deposition temperature: 200-300°C
- Deposition time: 4-8 hours
- Coating thickness: 1-5 μm
- Coating hardness: HV 3000-5000
- Friction coefficient: 0.05-0.08 (oil lubricated)
- Wear resistance: 10 times that of uncoated
DLC coating is suitable for high PV value conditions requiring low friction, but the thin coating cannot withstand impact loads.
2.2 Plasma Sprayed Ceramic Coating
A 50-200 μm thick alumina (Al₂O₃) or chromium oxide (Cr₂O₃) coating is formed on graphite seal ring surfaces through plasma spraying. Coating hardness can reach HV 1000-1500, significantly improving wear resistance, but coating adhesion to graphite is weak with spalling risk.
2.3 Electroless Nickel Plating
Electroless nickel plating deposits a 10-30 μm thick nickel-phosphorus alloy layer on graphite seal ring surfaces, hardness HV 500-700, improving wear resistance and corrosion resistance. Process temperature 85-95°C, with no thermal effect on graphite substrate.
3. Sulfidation Treatment
3.1 Process Principle
Sulfidation treatment forms a molybdenum disulfide (MoS₂) transfer film on graphite seal ring surfaces through chemical reaction. MoS₂ has a layered structure with weak interlayer bonding, providing excellent self-lubrication.
3.2 Process Methods
1.Immerse graphite seals in molybdate solution containing sulfur
2.Heat to 150-200°C, hold for 2-4 hours
3.A 1-5 μm thick MoS₂ film forms on the surface
3.3 Performance Effects
- Dry friction coefficient decreases from 0.20 to 0.05-0.10
- Suitable for vacuum, low-temperature and other unlubricated conditions
- Long-term use temperature does not exceed 300°C (MoS₂ begins to oxidize above 300°C)
4. Laser Surface Treatment
4.1 Laser Cladding
High-energy laser beams melt metal powder (such as nickel-based, cobalt-based alloys) onto graphite surfaces, forming a 100-500 μm thick cladding layer. Cladding layer hardness can reach HV 600-1000, with stronger adhesion than plasma spraying.
4.2 Laser Texturing
Pulsed lasers form regular micro-pit arrays on seal faces (diameter 50-200 μm, depth 20-50 μm) as oil retention pockets, improving oil film retention and reducing friction coefficient by 20-30%.
5. Process Selection Guide
| Process | Performance Improvement | Applicable Scenarios | Cost |
|---------|----------------------|---------------------|------|
| Antimony impregnation | Strength + thermal conductivity | General, high temperature | Medium |
| Copper impregnation | Thermal conductivity | High-speed heavy-load | High |
| Resin impregnation | Corrosion resistance | Chemical, acid/base | Low |
| PTFE impregnation | Chemical inertness | Strong corrosion | Medium |
| DLC coating | Wear + low friction | High-speed precision | High |
| Sulfidation | Self-lubrication | Vacuum, low temperature | Low |
Conclusion
Surface treatment processes for graphite seals are important means of improving their comprehensive performance. Huahao Sealing Co., Ltd. has a complete surface treatment process system and can provide customized process solutions based on customer operating conditions. Please contact our technical team — we will provide the optimal seal surface treatment services based on professional process technology.