Embedding carbon nanofibers (CNFs) into metal matrices such as aluminum (Al), copper (Cu), or magnesium (Mg) enhances mechanical properties, including strength-to-weight ratios and wear resistance. Two primary methods for achieving this are powder metallurgy and infiltration. Both techniques involve distinct processes and yield composites with unique characteristics suitable for aerospace, automotive, and structural applications.
**Powder Metallurgy for CNF-Metal Composites**
Powder metallurgy is a widely used method for fabricating CNF-reinforced metal matrices. The process involves blending metal powders with CNFs, followed by compaction and sintering.
1. **Powder Preparation**:
The metal powder (Al, Cu, or Mg) is mixed with CNFs in a ball mill or ultrasonic disperser to ensure uniform distribution. A typical composition ranges from 1-10 vol% CNFs, as higher concentrations may lead to agglomeration and reduced mechanical properties.
2. **Compaction**:
The blended powder is compacted using uniaxial or isostatic pressing at pressures between 200-800 MPa. Cold compaction is common, but warm compaction (at slightly elevated temperatures) improves particle bonding without premature sintering.
3. **Sintering**:
The compacted green body is sintered in a controlled atmosphere (argon or vacuum) to prevent oxidation. Sintering temperatures depend on the metal:
- Aluminum: 550-600°C
- Copper: 800-900°C
- Magnesium: 450-550°C
Holding times vary from 30 minutes to 2 hours.
4. **Secondary Processing**:
Hot extrusion or forging may follow sintering to further densify the composite and align CNFs, enhancing anisotropic strength.
**Mechanical Properties via Powder Metallurgy**
- **Strength-to-Weight Ratio**: CNF reinforcement improves specific strength. For example, Al-CNF composites with 5 vol% CNFs exhibit a 20-30% increase in tensile strength while maintaining low density.
- **Wear Resistance**: CNFs reduce wear rates by forming a protective tribolayer. Cu-CNF composites show a 40-50% reduction in wear compared to pure Cu under dry sliding conditions.
**Challenges**:
- Poor interfacial bonding between CNFs and metals may limit load transfer. Surface treatments like Ni-P electroless coating on CNFs improve adhesion.
- Porosity in sintered parts can reduce fatigue life.
**Infiltration Methods for CNF-Metal Composites**
Infiltration involves permeating a porous CNF preform with molten metal. This technique achieves high CNF loading and excellent matrix-fiber bonding.
1. **Preform Fabrication**:
CNFs are arranged into a porous preform using techniques like vacuum filtration or binder-assisted pressing. The preform porosity typically ranges from 30-70%.
2. **Metal Infiltration**:
- **Pressureless Infiltration**: The preform is immersed in molten metal (e.g., Mg at 700°C) under inert gas. Capillary action drives infiltration, but wettability is often poor without flux (e.g., K2ZrF6 for Mg).
- **Pressure-Assisted Infiltration**: External pressure (5-15 MPa) forces molten metal into the preform. This method is effective for Al and Cu, which have higher melting points.
- **Vacuum Infiltration**: A vacuum removes trapped gases, improving penetration.
**Mechanical Properties via Infiltration**
- **Strength-to-Weight Ratio**: Mg-CNF composites produced by pressure infiltration achieve 15-25% higher specific strength than pure Mg, with CNF loadings up to 20 vol%.
- **Wear Resistance**: Al-CNF composites exhibit wear rates 3-5 times lower than unreinforced Al due to CNFs hindering abrasive particle penetration.
**Challenges**:
- High-temperature processing risks CNF degradation. For Al, temperatures exceeding 800°C damage CNFs unless protective coatings (e.g., TiC) are applied.
- Residual porosity in pressureless infiltration reduces mechanical integrity.
**Comparative Analysis of Methods**
| Property | Powder Metallurgy | Infiltration |
|------------------------|----------------------------|----------------------------|
| CNF Distribution | Uniform but may agglomerate | Excellent, continuous network |
| Porosity | Moderate (5-10%) | Low (<5%) with pressure |
| CNF Loading | Limited (<10 vol%) | High (up to 30 vol%) |
| Interfacial Bonding | Weak without coating | Strong due to metal flow |
| Scalability | High | Moderate (complex setup) |
**Optimizing Strength-to-Weight and Wear Resistance**
1. **CNF Alignment**:
Unidirectional alignment during extrusion or preform fabrication enhances load-bearing capacity along the alignment axis. Al-CNF composites with aligned CNFs show 10-15% higher tensile strength than randomly oriented ones.
2. **Interfacial Engineering**:
- **Coating CNFs**: Electroless nickel or copper plating improves wettability and bonding.
- **In-Situ Reactions**: For Mg matrices, forming MgO or Mg2Si at the interface strengthens bonding but must be controlled to avoid brittleness.
3. **Hybrid Methods**:
Combining powder metallurgy with partial infiltration (e.g., spark plasma sintering followed by gas pressure infiltration) reduces porosity while preserving CNF integrity.
**Performance Metrics**
- **Al-CNF Composites**:
- Tensile strength: 250-400 MPa (vs. 100-150 MPa for pure Al)
- Wear rate: 1.2-2.5 x 10^-5 mm³/Nm (vs. 6-8 x 10^-5 mm³/Nm for pure Al)
- **Cu-CNF Composites**:
- Hardness: 120-150 HV (vs. 60-80 HV for pure Cu)
- Thermal conductivity: Maintains 80-90% of pure Cu’s conductivity due to CNF pathways.
- **Mg-CNF Composites**:
- Specific strength: 150-200 kN·m/kg (vs. 100-120 kN·m/kg for pure Mg)
- Corrosion resistance: Improved by 30-40% due to reduced grain boundary activity.
**Conclusion**
Powder metallurgy and infiltration are effective for embedding CNFs in Al, Cu, or Mg matrices, each offering distinct advantages. Powder metallurgy suits moderate CNF loadings with scalable production, while infiltration achieves superior bonding and higher reinforcement fractions. Both methods significantly enhance strength-to-weight ratios and wear resistance, making CNF-metal composites viable for high-performance applications. Future work should focus on interfacial optimization and hybrid processing to further improve properties.