Carbon Fibers: Manufacture, Properties, Structure & Uses

From the fuselage of a Boeing 787 Dreamliner to the shaft of a golf club, carbon fiber has become one of the most sought-after materials in modern engineering. Its combination of exceptional strength, low weight, and chemical stability has made it indispensable across industries that once relied entirely on metals. Understanding carbon fiber means understanding both its internal structure and the manufacturing processes that create it, because the two are inseparable.

What Are Carbon Fibers?

Carbon fibers are composed almost entirely of carbon atoms. They are black in color and have diameters in the range of several micrometers. Carbon fibers possess extremely high tensile modulus and strength and low density. Within the industry, the terms carbon fiber and graphite fiber are sometimes used interchangeably, but graphite fibers are actually a special type of carbon fiber that has a crystal, or graphitic, structure unique to true graphite fibers.

Carbon is a fiber that is at least 96% pure carbon. The fiber has exceptional heat resistance and does not ignite or melt. It maintains its full strength of 1.5 g/d after prolonged exposure to temperatures of more than 200°C. Carbon has a specific gravity of 1.4, a moisture regain of 10%, and an elongation of 10%. Carbon fibers have very low coefficients of thermal expansion, are chemically inert, and biocompatible. They also dissipate static quickly and are stiffer than many other fibers. It is much stronger and much lighter weight than steel and aluminum. It has superior fatigue resistance compared to all known metallic structures and is one of the most corrosion-resistant materials available.

Carbon fibers are heavier than other manufactured fibers but lighter weight than glass fibers or steel, materials with which they compete for many end uses.

Structure of Carbon Fibers

Carbon atoms can form molecules with various configurations such as carbyne, fullerene, nanotube, graphene, graphite, diamond, and amorphous carbon.

Carbon layers developed in some types of carbon fibers resemble a graphene layer, but unlike graphene contain many defects. These imperfect carbon layers tend to stick to each other, but the stacking of the layers is largely disordered. This type of carbon layer stack is called turbostratic carbon. The interlayer spacing of turbostratic carbon is larger than that of graphite and can be in the range of 0.35 to 0.38 nm. Amorphous carbon in carbon fibers completely lacks any long-range order of structure.

The properties of graphite and turbostratic carbon are extremely anisotropic. The high modulus and high strength properties of carbon fibers rely on the orientation of the carbon layer stacks, which are nearly parallel to the fiber axis. In addition to the orientation relative to the fiber axis, carbon layers show various alignments in the cross-section perpendicular to the fiber axis such as circumferential, radial, and random alignment.

The properties of carbon fibers depend on such factors as fractions, sizes, and orientation of carbon layer stacks, amorphous regions, and microvoids. In turn, these factors depend on the type of precursor, heat-treatment temperature, and the stretching of fibers during heat treatment.

Precursor Materials

Carbon-based materials possess high thermal and chemical resistances. Graphite, for example, does not change its state until heated to about 3,650°C at atmospheric pressure. This makes it difficult to produce carbon fibers directly from bulk carbon materials by melt or wet spinning.

Not all organic compounds are suitable precursors for carbon fiber. An appropriate precursor needs to meet several requirements: high yield of conversion to carbon fiber, the ability to keep fiber shape without causing volatilization or fusion during carbonization, and the ability to develop highly oriented carbon layers if high-performance carbon fiber is to be produced.

Organic compounds used as carbon fiber precursors include polyacrylonitrile (PAN), pitch, cellulose, and phenolic resin. Carbon fiber was originally used as the filament of incandescent lamps through a process of thermal decomposition of bamboo and cotton by Thomas Edison and Joseph Swan. Rayon-based carbon fibers were later developed for the aerospace industry. Today, PAN-based carbon fibers are seeing the largest demand.

Pitch is derived from coal tar or petroleum tar. Because the raw material is less expensive, substantial quantities of industrial- and general-grade materials are manufactured from pitch. Pitches are a mixture of organic compounds composed of condensed benzene rings and alkyl chains. The composition of a pitch varies depending on its source, for example, petroleum asphalt or coal tar. Extremely high-modulus carbon fibers can be derived from pitch, whereas extremely high-strength carbon fibers can be derived from PAN.

Manufacturing Process of Carbon Fibers

PAN-Based Carbon Fibers

PAN-based carbon fibers are produced through the following steps: wet spinning, stretching at an elevated temperature, stabilization at 200 to 300°C, carbonization at 1,000 to 1,500°C, and optional graphitization at 2,000 to 3,000°C.

Wet spinning is used, as the decomposition of PAN on heating renders melt spinning difficult. Stretching of PAN fibers before and during stabilization is important to develop the preferred orientation of carbon layers in the final carbon fiber structure.

A heat treatment called stabilization may be applied to the fiber before carbonization to develop chemical structures that do not suffer combustion, extensive volatilization, or melting during carbonization. Stabilization is usually carried out in air at relatively low temperature and is usually a lengthy process.

To convert the PAN fibers to carbon fibers, they are heated to temperatures greater than 1,000°C, driving off most of the hydrogen, nitrogen, and other atoms to leave only carbon. On further heating to 2,500°C, a crystalline graphite structure develops, which produces extremely high-modulus graphite fibers.

In PAN fiber, the graphite planes arrange themselves along the axis of the fiber instead of perpendicular, as is the case with pitch-based carbon fibers. Low extensibility along with high strength and modulus makes PAN-based fibers well suited for use with epoxy resin as composites.

The tensile modulus of PAN-based carbon fibers tends to increase with increasing heat-treatment temperature. The tensile strength of PAN-based carbon fibers also increases with increasing heat-treatment temperature up to about 1,300 to 1,500°C, and then decreases at higher temperatures.

Pitch-Based Carbon Fibers

Pitch-based high-performance carbon fibers are produced in the following steps: polymerization of pitch to mesophase pitch, melt spinning, stabilization at 200 to 350°C, carbonization at 1,000 to 1,500°C, and graphitization at 2,000 to 3,000°C.

Upon heating to a temperature above 300°C, pitch molecules polymerize, forming a liquid crystal called mesophase pitch. Pitch can be melt spun into fibers with a range of crystallinities. The more crystalline and oriented the precursor fibers, the stronger the final carbon fibers will be. Similar high-heat treatment is given to melt-spun pitch fibers to eliminate the hydrogen atoms.

Vapor-Grown Carbon Fibers

Chemical vapor deposition (CVD) is one method to produce carbon fibers from hydrocarbons such as ethylene, acetylene, benzene, and toluene with the aid of catalyst particles such as Fe, Co, and Ni. In a reactor heated at 500 to 2,200°C, hydrocarbons are decomposed at the surface of the catalyst particle and the resulting carbon atoms dissolve into the particles and then deposit in the form of a fiber. The carbon fibers produced with this method, which are called vapor-grown carbon fibers (VGCF), have diameters in the range from 15 to 150 nm.

Properties of Carbon Fibers

Carbon fibers have a tenacity in the range of 13 to 28 g/d and a very high modulus, both attributable to their crystalline structure. They are heavier than most other manufactured fibers but lighter than glass fibers or steel. The specific gravity is approximately 1.4, moisture regain is around 10%, and elongation is about 10%.

The fiber maintains its full strength of 1.5 g/d even after prolonged exposure to temperatures above 200°C. It does not ignite or melt, making it genuinely heat resistant in a way that most polymeric fibers are not.

The tensile modulus of PAN-based carbon fibers generally increases with rising heat-treatment temperature. Tensile strength also increases with temperature up to about 1,300 to 1,500°C, after which it begins to decrease. The single-fiber tensile strength of carbon fibers shows a broader statistical distribution than polymeric fibers, because strength depends on the distribution of flaws within the fiber. Shorter fibers statistically have fewer flaws and therefore tend to show higher average tensile strength than longer ones.

Carbon Fiber Reinforced Composites

Carbon fibers were originally developed as a reinforcement phase in composite materials and are typically used in carbon fiber-reinforced plastics (CFRP), produced by combining carbon fibers and a polymer matrix phase.

Interfacial bonding between the reinforcement fiber phase and the matrix is critical to the transference of an external load from the matrix to the reinforcement phase. Without sufficient interfacial bonding, the excellent fiber properties such as high modulus and high strength cannot be fully utilized. As-received carbon fibers show weak bonding to polymers, and this is improved by surface treatment. The interfacial bonding strength needs to be optimized, as bonding that is too strong leads to easy crack propagation and reduces tensile strength and toughness of the composite material.

When a carbon fiber in a unidirectional composite material is loaded in tension it breaks at the flaw. The tensile stress in the broken fiber, however, increases over some distance from the break and reaches the stress borne by the unbroken fibers due to stress transfer at the fiber/matrix interface. As a result, carbon fibers show a higher tensile strength in a composite material than in a bundle without a matrix.

In hybrid fiber composite materials, carbon fibers are used in combination with other fibers and a wider range of properties can be obtained. For example, by adding a small amount of glass or Kevlar fibers in CFRP, the impact toughness can be enhanced.

The flexibility of fibers and adjustability of fiber direction in the composite structure provides a way to process a stiff and strong material into the desired shape and helps to control the anisotropy of the properties. Specially designed fiber orientation angles allow a layered composite material to deform against stress in a very different way from isotropic materials. The fiber/matrix interface can be controlled to promote toughening mechanisms such as crack arrest and deflection, interfacial debonding, and fiber pull-out.

Processing Methods for CFRP

The processes to fabricate carbon fibers into CFRP are diverse depending on whether the matrix is a thermoplastic or thermosetting resin, whether the carbon fiber is continuous or staple, and how the fiber is arranged.

A prepreg is a moulding material composed of a thin sheet of unidirectional or woven fibers impregnated with a thermoplastic or thermosetting resin. Prepregs using thermosetting resins are usually stored under refrigeration conditions in order to suppress curing until ready to use. To produce a finished product, prepregs are cut and laid up onto a mould surface by hand or machine, and the resin is cured by heating under pressure in an autoclave.

Sheet moulding compound (SMC) is a thin sheet of short fibers impregnated with a thermosetting resin and used as the moulding material for compression moulding. In compression moulding, the required amount of SMC is placed in a preheated mould cavity, which is then closed and the resin is cured under pressure. Thermoplastic pellets containing short fibers are used as the moulding material with injection moulding. With injection moulding, the pellets are melted in a heated barrel, injected into the mould, and cooled for solidification.

Production methods without using moulding materials include pultrusion, filament winding, and resin-transfer moulding. In pultrusion, fibers are pulled through a thermosetting resin bath and a heated die with the desired cross-sectional shape and the resin is cured. Composite materials in the form of round and square tubing, channel, I-beam, and rods are produced with this method. With filament winding, fibers are pulled through a thermosetting resin bath and wound onto a mandrel. The mandrel is removed after the resin is cured or it becomes a part of the structure. Cylindrical structures and pressure vessels are produced with this method. In resin-transfer moulding, dry fibers are formed into a three-dimensional structure called a preform, which is placed into a mould cavity, filled with a low-viscosity liquid resin, and then cured.

Application of Carbon Fibers

Aerospace and Structural Uses

Carbon fiber-reinforced composites are used in lightweight structures for aircraft and spacecraft and as brake discs for jet airplanes. The Boeing 787 Dreamliner made by Boeing contains a large proportion of carbon fiber-based structure. The lighter-weight construction, versus all-metal body materials, saves fuel during flight. Bridges and buildings are also reinforced with carbon fibers, and graphite materials have been used for implantation to replace bone. Composite carbon fiber and resin is used in bridge repair and highway support columns.

Sporting Goods and Consumer Products

Carbon fibers are used in such diverse areas as sporting goods, the construction industry, the automotive industry, and medicine. Golf club handles, tennis racquets, fishing poles, skis, canoes, and kayaks all benefit from carbon fiber reinforcement. Carbon fiber is also used to reinforce lightweight metal components in athletic equipment like boat hulls, masts, oars, hockey sticks, snowboards, and bicycle bodies.

Industrial and Technical Applications

Carbon fiber is used in wind energy turbine blades and as a substitute for asbestos in technical products like brakes. It is used in radar-transparent military aircraft, communication satellites, and rocket-motor nozzles. Carbon is used as a coating of nylon for antistatic carpeting, upholstery, apparel, and technical brushes and belts. The ability to control the mechanical properties of composites in any particular direction renders them attractive in areas where anisotropy is useful, for example with pressure vessels designed to contain gases at high pressure.

Carbon Nanofibers and Nanotubes

A significant end use for carbon is in nanofibers and nanotubes of extremely tiny size. Carbon nanofibers and nanotubes have unusual properties: remarkable strength, high elasticity, low density, heat resistance, and large thermal and electrical conductivity. These nanomaterials have been described as having the strength of steel and the flexibility of a rubber band. Applications and potential applications include smart textile end uses such as mechanical relays and switches, thermal sensors, acoustic and pressure sensors, chemical sensors, medical sensors, radiation detectors, and acceleration sensors.

Carbon black nanoparticles from coconut shells, bamboo, and other organic materials are used much like activated charcoal. When incorporated at the 5% to 20% level in nylon and polyester fibers, the carbon black nanoparticles provide cooling action, ultraviolet protection, odor absorption or resistance, and static resistance for textile products. These composite fibers also have improved abrasion resistance and tensile strength.

Cost and Market Position

Carbon fibers add to the cost of many items because they are expensive, about five times the cost of PET polyester. The cost is, however, less than that of the aramids and some of the other high-tenacity, high-modulus fibers, which makes carbon fiber competitive within the high-performance fiber segment. Because the raw material for pitch-based fibers is less expensive, substantial quantities of industrial-grade carbon fiber are manufactured from pitch. The advancement in technology and manufacturing methods has reduced costs, and as a result the application of carbon fiber continues to increase.

Conclusion

Carbon fiber stands out among high-performance materials because of its unique combination of high strength, low weight, chemical inertness, and thermal stability. The choice of precursor, whether PAN or pitch, directly determines the balance of properties in the final fiber, and the heat-treatment conditions at each stage of manufacturing control the internal structure that makes those properties possible. As manufacturing technology continues to advance and production costs fall, carbon fiber is well positioned to move into a wider range of applications, from mass-market automotive components to broader structural uses in construction. A material that once belonged almost exclusively to aerospace is steadily becoming part of everyday engineering.