Key Points
- Carbon graphite and silicon carbide are not substitutes but the classic "soft + hard" face pairing for mechanical seals
- Silicon carbide hardness HV 2000-2800 (carbon graphite only HS 40-90), far exceeding carbon graphite in wear resistance but more brittle
- Silicon carbide thermal conductivity 80-150 W/(m·K), comparable to carbon graphite and far superior to alumina ceramic
- Silicon carbide resists nearly all acids and alkalis (except HF), better chemical resistance than carbon graphite, but costs 3-8x impregnated carbon graphite
- Mainstream pairing: carbon graphite (soft face, self-lubricating sacrificial) + reaction-bonded silicon carbide (hard face, maintains flatness)
1. Positioning of the Two Materials
Mechanical seal face pairing typically follows the "one soft, one hard" principle: the soft face provides self-lubrication and sacrificial wear, while the hard face maintains flatness and withstands high-speed friction. Carbon graphite and silicon carbide are the classic combination:
- Carbon graphite: as the soft face (either rotating or stationary ring), leveraging layered structure self-lubrication to reduce friction, and protecting the hard face through preferential wear
- Silicon carbide (SiC): as the hard face, maintaining face flatness through extreme hardness and wear resistance under high-speed friction
They are not competitors but complementary pairing materials. A single seal assembly typically uses both.
2. Physical Property Comparison
| Property | Carbon graphite (impregnated) | Reaction-bonded SiC | Pressureless sintered SiC |
|---------|-------------------------------|---------------------|---------------------------|
| Density (g/cm³) | 1.80-2.20 | 3.05-3.10 | 3.10-3.21 |
| Hardness | HS 40-90 | HV 2000-2800 | HV 2500-2800 |
| Compressive (MPa) | 150-340 | 1500-2500 | 2000-3000 |
| Flexural (MPa) | 40-100 | 350-500 | 400-550 |
| Thermal cond. (W/m·K) | 70-150 | 80-150 | 80-120 |
| CTE (×10⁻⁶/°C) | 2.5-5.0 | 4.0-4.5 | 4.0-4.5 |
| Max temp (°C) | 600 (non-oxidizing) | 1400 | 1600 |
Key conclusions:
1.SiC hardness is 30-50x carbon graphite, far exceeding in wear resistance
2.Both have comparable thermal conductivity, far superior to alumina ceramic (25-35 W/m·K)
3.Carbon graphite has better toughness and resists brittle fracture; SiC is brittle and prone to thermal shock cracking
3. Chemical Resistance Comparison
| Media | Carbon graphite (phenolic impregnated) | Silicon carbide |
|-------|----------------------------------------|-----------------|
| Hydrochloric acid (30%) | Good | Excellent |
| Sulfuric acid (50%) | Good | Excellent |
| Nitric acid (30%) | Fair | Excellent |
| Hydrofluoric acid | Poor (needs PTFE impregnation) | Poor (SiC reacts with HF) |
| Sodium hydroxide (30%) | Good | Excellent |
| Acetone, alcohols | Excellent | Excellent |
Key conclusions:
1.SiC resists nearly all acids and alkalis (except HF), with better chemical stability than carbon graphite
2.Carbon graphite resistance depends on impregnation type; strongly oxidizing media (e.g., concentrated nitric acid) cause degradation
3.Neither is suitable for hydrofluoric acid; PTFE or filled PTFE pairings are needed
4. Cost and Machinability Comparison
- Carbon graphite blank: ~30-80 yuan/kg, good machinability, can be turned and ground
- Reaction-bonded SiC blank: ~250-600 yuan/kg, requires diamond wheels, high machining cost
- Pressureless sintered SiC blank: ~400-900 yuan/kg, harder and more difficult to machine
SiC unit price is 3-8x carbon graphite, with higher machining cost. However, SiC wear resistance far exceeds carbon graphite, providing longer service life. In media with solids or high-speed/high-pressure service, SiC total life cost may be lower.
5. Typical Application Scenarios
5.1 Mainstream Service: clean water, mildly corrosive media, low-to-medium pressure
- Pairing: impregnated carbon graphite + reaction-bonded SiC
- Service: centrifugal pumps, circulation pumps, cooling water pumps, media pressure <2 MPa, temperature <150°C
- Advantage: cost-effective, stable fluid film, 8000-12000 hour life
5.2 Particulate Media: sewage, slurry, pulp
- Pairing: pressureless sintered SiC vs pressureless sintered SiC (hard vs hard)
- Service: abrasive media with solids
- Note: particles rapidly wear the carbon graphite soft face, so SiC vs SiC is used, but at higher cost and less stable fluid film
5.3 Strongly Corrosive Chemical: HCl, H₂SO₄, caustic
- Pairing: furan-impregnated carbon graphite + reaction-bonded SiC
- Service: chemical pumps, reactors, strong acid/alkali media
- Advantage: balances corrosion resistance and cost-effectiveness
5.4 High-Temperature Hot Oil: thermal oil, asphalt
- Pairing: antimony-impregnated carbon graphite + reaction-bonded SiC
- Service: 200-400°C, hot oil pumps
- Advantage: balances high-temperature resistance and load capacity
6. Why "Carbon Graphite + SiC" is the Mainstream Pairing
1.Complementary strengths: carbon graphite self-lubrication reduces friction, SiC maintains face flatness
2.Film stability: carbon graphite soft face preferentially wears, forming micro valleys that retain media and stabilize the film
3.Cost balance: cheap carbon graphite is periodically replaced, wear-resistant SiC is reused long-term
4.Thermal coordination: comparable thermal conductivity allows bidirectional heat dissipation, avoiding local overheating
5.Fault tolerance: carbon graphite serves as the sacrificial wear face, wearing first during abnormal conditions to alert and protect the expensive SiC hard face
These combined advantages make "carbon graphite + SiC" the default pairing for over 90% of industrial mechanical seals. Only special conditions (particulate media, HF) require alternative pairings.