Here is a comprehensive, technical breakdown of carbon fiber materials-covering their atomic structure, manufacturing process, mechanical properties, and primary applications.
Carbon Fiber Structural Architecture. From：VectorMine / Getty Images
1. What is Carbon Fiber?
Carbon Fiber (or Graphite Fiber) consists of thin strands of carbon atoms measuring 5 to 10 micrometers in diameter. High-performance carbon fibers are at least 90% carbon by weight, bonded together in microscopic crystals aligned parallel to the fiber's long axis.
Key Structural Fact: The directional alignment of graphite layers gives carbon fiber its exceptionally high strength-to-weight ratio along its longitudinal axis, outperforming traditional metals like steel and aluminum.
2. Molecular Structure & Classification
Carbon fiber's strength comes from $sp^2$ hybridized carbon atoms forming two-dimensional hexagonal planar sheets (similar to graphene). These sheets fold together to form a microcrystalline structure.
Precursor Material Types
Carbon fibers are classified primarily by the precursor material used to create them:
PAN-based (Polyacrylonitrile): Represents over 90% of commercial production. Offers high tensile strength and excellent structural reliability.
Pitch-based: Produced from petroleum or coal tar pitch. Yields extremely high stiffness (tensile modulus) and thermal conductivity, though generally lower tensile strength.
Rayon-based: Historically significant, but rarely used today due to low carbon yield (~20–25%).
3. Manufacturing Process
Transforming liquid precursor polymer into structural carbon fiber involves five main stages:
1.Spinning：Precursor Formation.
Polyacrylonitrile (PAN) powder is mixed with solvents and spun into continuous filaments (tow), then stretched to align the internal polymer chains.
2.Stabilization：Thermal Cross-linking (200°C – 300°C).
The precursor fibers are heated in air. Oxygen bonds convert the linear molecular chain into a ladder-like ring structure, preventing melting during subsequent steps.
3.Carbonization：Non-oxidizing Heating (1,000°C – 1,500°C).
Fibers are heated in an inert atmosphere (nitrogen). Non-carbon atoms (hydrogen, nitrogen, oxygen) are expelled, leaving tightly bonded carbon rings.
4.Surface Treatment：Adhesion Enhancement.
The fiber surface is oxidized via electrolytic bathing to create chemical anchoring sites, ensuring strong mechanical bonding with resin matrices.
5.Sizing：Protective Coating.
A protective polymer coating (sizing) is applied to prevent strand damage during handling before weaving or impregnating with epoxy.
4. Mechanical Properties Comparison
To evaluate structural performance, carbon fiber is typically embedded in an epoxy matrix to form Carbon Fiber Reinforced Polymer (CFRP).
| Property | Standard Carbon Fiber (PAN) | Structural Steel (A36) | Aluminum Alloy (7075-T6) |
| Density (g/cm3) | 1.75 - 1.80 | 7.85 | 2.81 |
| Tensile Strength (GPa) | 3.5 - 4.9 | 0.40 | 0.57 |
| Tensile Modulus (GPa) | 230 - 300 | 200 | 71 |
| Specific Strength (kN.m/kg) | ~2,400 | ~50 | ~200 |
5. Key Advantages & Limitations
Advantages
Ultra-lightweight: Roughly 1/5th the density of steel and 2/3rds that of aluminum.
High Corrosion Resistance: Chemically inert to most acids, bases, and organic solvents.
Low Thermal Expansion: Near-zero Coefficient of Thermal Expansion (CTE), making it ideal for precision optical and space applications.
High Fatigue Life: Maintains structural integrity under millions of cyclic loads.
Limitations
Anisotropy: Mechanical strength exists primarily along the length of the fiber; off-axis strength requires multidirectional weaving or laminating.
Brittle Failure: Exhibits near-linear elastic response until catastrophic failure without prior plastic deformation.
High Cost & Recycling Complexity: Energy-intensive manufacturing and difficult post-consumer thermoset matrix recycling.