What carbon fiber is
Carbon fiber is a thin filament, typically several micrometers across, made almost entirely of carbon. The carbon atoms are arranged in graphite-like layers that are mostly aligned along the length of the filament. That alignment is what gives the fiber high stiffness and strength for its weight along its axis. Across the fiber it is much weaker, which is why carbon fiber is not used on its own but bundled into tows and embedded in a matrix.
Starting materials
Carbon fiber is not made by drawing out carbon itself, since carbon does not melt. Instead it is made by converting an organic precursor fiber into carbon. The common precursors are:
- Polyacrylonitrile (PAN). The most widely used precursor. It produces fibers with a favorable balance of strength, stiffness and cost.
- Pitch. Derived from petroleum or coal tar. Some pitch-based fibers are especially stiff and conduct heat well, but they are generally more brittle.
- Rayon. An older precursor, used historically and still in some specialty products.
The precursor choice shapes the fiber’s properties and price.
The production steps
The details differ between producers, but the sequence is broadly similar for PAN-based fiber.
- Spinning. The PAN polymer is dissolved and spun into continuous filaments, which are stretched so the molecules line up along the fiber.
- Stabilization (oxidation). The filaments are heated in air at moderate temperatures, often in the range of a couple hundred degrees Celsius. This changes the polymer into a heat-stable structure that will not melt in the next stage. It is slow and must be controlled carefully, because the reaction gives off heat.
- Carbonization. The stabilized fiber is heated in an oxygen-free atmosphere, typically nitrogen, at temperatures well above 1000 degrees Celsius. Most non-carbon atoms leave as gases, and the remaining carbon forms layered, graphite-like structures.
- Graphitization (optional). Heating to still higher temperatures improves the alignment of the layers, which tends to raise stiffness but can affect strength and cost.
- Surface treatment. The surface is lightly oxidized or otherwise treated so that resin will bond to it.
- Sizing. A thin coating protects the fiber during handling and helps it stick to the matrix.
Because each stage takes energy and time, and the gases released need handling, carbon fiber is considerably more expensive than glass fiber.
What makes it useful
- High stiffness relative to weight.
- Good strength along the fiber direction.
- Low thermal expansion in the fiber direction, which helps dimensional stability.
- Good fatigue behavior in many designs.
- Resistance to many chemicals.
On the other hand, carbon fiber is brittle, does not yield the way metal does, and can cause galvanic corrosion when it touches certain metals in the presence of moisture. A broader look at how it compares with glass and aramid is given in the discussion of fiber reinforcement.
From fiber to part
Fibers are supplied as continuous tows, woven fabrics, unidirectional tapes or chopped material. They are combined with a resin, most commonly epoxy, to make the polymer matrix composites that most people mean by “carbon fiber.” The orientation of the fibers is chosen to carry loads in specific directions, so engineers build parts from layers set at different angles. The methods are covered in the discussion of composite layup. The quality of the bond at the fiber-matrix interface strongly influences how the finished part behaves.
Carbon fiber can also be embedded in metals, ceramics or carbon, giving the family of materials that includes carbon-carbon composites.
Typical uses
- Aerospace. Wing and fuselage structures, control surfaces and interior parts, where weight savings translate into fuel savings. This is a major part of aerospace materials today.
- Sporting goods. Bicycle frames, rackets, fishing rods and golf shafts.
- Automotive. Used in racing and some premium vehicles, with growing interest in lightweight components more broadly.
- Wind energy. Some turbine blade components, especially in long blades where stiffness matters.
- Pressure vessels. Wrapped tanks for gas storage.
- Industrial equipment. Robot arms, rollers and other moving parts where low inertia helps.
- Medical devices. Radiolucent components such as imaging tables, since carbon is largely transparent to X-rays.
Challenges in service
Carbon fiber composites can fail in ways that metals do not. Layers may separate, impact damage can hide beneath an intact-looking surface, and drilling or cutting can fray the fibers. Machining carbon fiber without delamination calls for sharp, wear-resistant tools, which is one reason diamond-coated tools are common in this field. Understanding composite failure modes is part of safe design.
End-of-life handling is another concern. Cured thermoset composites cannot simply be remelted, so composite recycling methods such as grinding, pyrolysis and solvent processes are still developing.
Place in the carbon family
Carbon fiber sits between disordered and crystalline carbon: its layers resemble graphite, but they are small, turbostratic and imperfectly stacked. The overview of carbon allotropes explains how this fits with diamond, graphene and the other forms.