Carbon Fiber

What is Carbon Fiber

Carbon fiber is a man-made reinforcing fiber made by thermally converting an organic precursor into a filament that is more than 90 percent carbon by mass. Within each filament, the carbon atoms form turbostratic graphitic layers that are highly oriented along the fiber axis, which is why the fiber is extremely strong and stiff in tension lengthwise but weak across its diameter. A single filament is only about 5 to 7 micrometers across, roughly one-tenth the diameter of a human hair, so thousands of filaments are gathered into a tow before any practical use.

On its own a tow is a soft, flexible bundle. The engineering material of interest is the composite that results when the fiber is embedded in a matrix, almost always a polymer such as epoxy, giving a carbon-fiber-reinforced polymer (CFRP), the carbon-reinforced member of the broader FRP composite family. The fiber carries the load while the matrix transfers stress between filaments, holds them in alignment, and protects them. This is why a carbon fiber datasheet quotes both a bare-fiber property and a composite property normalized to a fiber volume fraction, commonly 60 percent. For example, Toray lists T700S at about 4,900 MPa tensile strength as a fiber, but roughly 2,860 MPa as a 60 percent unidirectional composite.

The defining advantage of carbon fiber is specific performance, meaning strength or stiffness divided by density. Fiber density is about 1.75 to 1.95 grams per cubic centimeter, far below the 2.7 of aluminum alloy or 7.8 of carbon steel, so a CFRP part of equal stiffness can weigh a fraction of a metal one. This is the property that made carbon fiber indispensable to aerospace: the Boeing 787 airframe is roughly 50 percent composite by weight, and each aircraft contains on the order of 23 tonnes of carbon fiber, contributing to a fuel-efficiency improvement of about 20 percent over the metal aircraft it replaced.

The history of the material runs from Thomas Edison's carbonized bamboo lamp filaments in 1879, through the first modern high-strength fibers developed at the Royal Aircraft Establishment in the United Kingdom and at Union Carbide in the United States during the 1960s, to the polyacrylonitrile (PAN) process refined by Japanese producers from the 1970s. Toray commercialized the T300 grade that remains a reference baseline today. Since then the field has split into a high-performance aerospace branch driven by ever-higher strength and modulus, and a large-tow industrial branch driven by lower cost for wind energy, automotive, and pressure vessels.

In market terms, world carbon fiber demand was on the order of 150 to 160 thousand tonnes per year in the mid-2020s, with aerospace and defense historically consuming close to half of high-performance grades and wind energy now the largest single industrial driver. Three forces decide whether a project can justify the material: the price per kilogram, which remains many times that of structural metals, the manufacturing route, since CFRP parts are laid up and cured rather than cast or machined, and the qualification burden, because each grade and lot must be traceable for structural use.

Precursors and Manufacturing

Almost every carbon fiber on the market begins as one of two precursors: polyacrylonitrile (PAN) or pitch. The precursor sets the ceiling on what properties the finished fiber can reach, so it is the first branch in any classification. PAN-based fiber accounts for roughly 90 percent of global production and dominates because it yields the highest tensile strength. Pitch-based fiber is a specialty route prized for very high modulus and thermal conductivity. A third, minor route uses regenerated cellulose (rayon) and survives mainly in ablative and friction applications.

The PAN process runs in four stages. First, oxidative stabilization holds the white PAN precursor under tension in air at about 200 to 300 degrees Celsius, cross-linking the polymer so it will not melt. Second, carbonization in an inert nitrogen atmosphere at roughly 1,000 to 1,500 degrees Celsius drives off non-carbon atoms and leaves a fiber that is around 92 to 95 percent carbon, the strength-optimized condition. Third, optional graphitization at 2,000 to 3,000 degrees Celsius raises the carbon content above 99 percent and sharply increases modulus at some cost to strength, producing the high-modulus M-series grades. Fourth, surface treatment and sizing prepare the fiber to bond with resin. Tension is held throughout to keep the graphitic ribbons aligned along the axis.

The pitch process melt-spins mesophase pitch, a liquid-crystalline tar derived from petroleum or coal, through a spinneret. Because the mesophase is already partly ordered, the resulting graphitic structure is far more perfect than PAN can achieve, which is what lets pitch fiber reach moduli of 600 to 900 GPa and beyond, along with very high axial thermal conductivity. The trade-off is lower tensile strength and greater brittleness, because the same large, well-ordered crystals that raise stiffness also create cleavage planes. The table below contrasts the two routes.

For most structural buyers the practical conclusion is simple: specify PAN unless the part is governed by deflection, dimensional stability, or heat conduction rather than by strength. Pitch fiber is rarely the right answer for a load-bearing tube or panel, but it is the only answer for a satellite mirror bench that must not bow under thermal gradients. Producers such as Mitsubishi Chemical (Dialead) and Nippon Graphite supply the high-modulus pitch grades, with Dialead K13C2U reaching about 900 GPa modulus.

One further consequence of the manufacturing route is consistency. Because properties depend on tension, temperature profile, and precursor purity, structural users qualify a fiber against a specific datasheet revision and often a specific production line. A grade name like T800 has persisted across process generations, so the lot certificate, not just the grade label, is what a quality system tracks. Surface treatment and sizing chemistry, applied at the end of the line, are equally part of the specification because they govern how the fiber bonds in the laminate.

Modulus Grades and Classification

Within PAN fiber, the industry sorts grades by tensile modulus into standard modulus (SM), intermediate modulus (IM), high modulus (HM), and ultra-high modulus (UHM). Modulus measures stiffness, the resistance to elastic stretch, and it is the property that most cleanly separates the price tiers. A second axis is tensile strength, where high-strength variants exist within each modulus class. Toray encodes this with two families: the T-series for high strength and the M-series for high modulus, the latter using a J suffix to mark grades with improved strain to failure.

The table below lists verified fiber properties from public Toray and Hexcel datasheets. Values are bare-fiber, impregnated-tow results. The same grade may show slightly different numbers across datasheet revisions, so confirm against the current sheet before a purchase order.

Standard modulus, near 230 GPa, is the workhorse class. T300, T700, and AS4 sit here. These grades combine good strength, reasonable cost, and high strain to failure, which means they tolerate damage and impact better than stiffer grades. They are the default for pressure vessels, sporting goods, general industrial structures, and the skins of cost-sensitive parts. Within the class, the high-strength variants such as T700S buy more reserve strength at modest added cost.

Intermediate modulus, near 290 to 325 GPa, is the aerospace primary-structure default. T800, T1100, and IM7 deliver both very high strength, above 5,500 MPa, and noticeably higher stiffness than standard modulus, letting designers reduce laminate thickness and weight. The premium over standard modulus is justified where every kilogram saved has high value, as on aircraft wings and fuselage frames or on filament-wound aerospace pressure vessels.

High and ultra-high modulus, from roughly 350 GPa to 900 GPa, trade strain and strength for extreme stiffness. As the table shows, modulus climbs while strain to failure falls below 1 percent, so these fibers are brittle and intolerant of point loads or sharp radii. They are used where deflection or dimensional stability rules, such as satellite trusses, optical benches, antenna reflectors, and high-speed robotic arms. The pitch-based Dialead K13C2U, at about 900 GPa, also brings the high thermal conductivity that makes it useful for spacecraft thermal management.

Tow, Weave, and Product Forms

Carbon fiber reaches the buyer in several physical forms, and the form is as much a part of the specification as the grade. The base unit is the tow, a continuous untwisted bundle of filaments. From tow, producers make dry woven fabric, unidirectional tape, chopped and milled fiber, and resin-impregnated prepreg. Each form suits a different fabrication method and surface requirement, so the same T700 grade can arrive as a 12K tow on a spool, a 200 gram-per-square-meter twill fabric, or a frozen prepreg roll.

Tow size is named by filament count in thousands. A 3K tow holds 3,000 filaments, a 12K tow holds 12,000, and large or heavy tows reach 24K, 48K, and 50K. Filament diameter stays at 5 to 7 micrometers regardless of count, so a larger tow is simply a fatter bundle. Small tows weave into fine, smooth fabrics favored for cosmetic surfaces and aerospace laminates, but cost more per kilogram because spinning and handling thousands of fine bundles is expensive. Large tows lay down fiber far faster and cost less, which is why wind-turbine spar caps, automotive parts, and civil reinforcement standardized on 24K and above. The table below summarizes the trade-off.

Woven fabric is described by weave pattern and areal weight in grams per square meter (gsm). A plain weave interlaces every tow over and under, giving a stable but stiff and less drapable cloth. A 2x2 twill drapes better around curves and is the most common decorative and structural weave. Typical fabric weights run from about 90 gsm light cloth to 600 gsm heavy cloth, with 200 gsm 3K twill being the familiar visible-weave material. Unidirectional fabric and tape place all fibers in one direction for maximum strength along the load path, held together by light stitching or a thin binder.

Prepreg is fabric or unidirectional tape pre-impregnated with a precise, partially cured resin, usually epoxy. It gives the most consistent fiber-to-resin ratio and the best mechanical properties, which is why aerospace structures are built from it, but it must be stored frozen, has a limited out-life at room temperature, and is cured under heat and pressure in an autoclave or oven. Dry fabric, by contrast, is infused with resin at layup using wet layup, resin transfer molding (RTM), or vacuum infusion, trading some property consistency for lower storage cost and larger part sizes.

The bridge between fiber and resin is sizing, a thin polymer coating applied during fiber production at roughly 0.5 to 1.5 percent of fiber weight. Sizing reduces abrasion and fuzz, controls static, and above all is chemically matched to a resin family so the fiber bonds well in the laminate. Epoxy-compatible sizing is the default, but vinyl ester, phenolic, bismaleimide, and thermoplastic systems each require their own formulation. A fiber whose sizing does not match the chosen resin will show poor interlaminar shear strength even if the bulk fiber is perfect, so the sizing code is a mandatory line on the purchase specification.

Key Specification Parameters

Reading a carbon fiber datasheet is a core purchasing skill, because two sheets can quote different numbers for the same fiber depending on the test basis. Manufacturers report tensile properties from resin-impregnated tow tests under ASTM D4018 or ISO 10618, where a tow is collimated, impregnated with epoxy, cured, and pulled to failure, with results back-calculated to the fiber. Single-filament strength follows ISO 11566, and tow nomenclature and linear density follow ISO 13002. The same fiber can read higher in a single-filament test than on its impregnated-tow datasheet, so comparisons are only valid when all candidates quote the same standard. Eight parameters drive selection.

Tensile strength is the maximum stress the fiber sustains before fracture, in megapascals. It ranges from about 3,530 MPa for T300 to 7,000 MPa for T1100G among PAN grades. Note that fiber strength is far higher than the strength of the finished laminate, which is diluted by the resin and by the fiber volume fraction, typically 55 to 65 percent, so always distinguish the fiber number from the composite number on the same sheet.

Tensile modulus is axial stiffness in gigapascals, from about 230 GPa for standard modulus to 588 GPa for M60J and up to 900 GPa for pitch fiber. Modulus is the property that defines the grade class and most strongly drives price. Because carbon fiber is essentially linear-elastic to failure with no yield, modulus and strength together fix the strain to failure, the third key parameter.

Strain to failure (elongation), in percent, is strength divided by modulus. High-strength standard and intermediate grades reach 2.0 to 2.2 percent, giving useful damage tolerance, while ultra-high-modulus grades fall below 1 percent and are brittle. A low strain to failure is the warning sign that a fiber must be handled with generous radii and protected from point loads and impact.

Density is about 1.75 to 1.81 grams per cubic centimeter for standard and intermediate PAN grades, such as 1.80 for T700S, rising toward 1.9 or above for high-modulus and pitch fibers. Low density is the basis of the material's specific-strength advantage, so density is always paired with strength and modulus when comparing against metals on a weight basis.

Filament diameter is 5 to 7 micrometers, for example about 7 micrometers for T700S and 5.2 micrometers for IM7. Smaller diameter raises surface area and can improve resin bonding and apparent strength, but makes the fiber more delicate to handle. Filament count (the K number) sets the tow size and, with it, fabric fineness, deposition rate, and price.

Carbon content reflects the degree of carbonization, roughly 92 to 95 percent for strength-optimized grades and above 99 percent for graphitized high-modulus grades. Sizing type and content close the list: the sizing must match the intended resin, and its weight fraction, around 0.5 to 1.5 percent, affects both handling and the fiber-resin bond. The table below shows how three representative grades compare across these parameters.

Selection Decision Factors

To turn the preceding chapters into a specific fiber order, work through the decision sequence below. Most selection errors come not from a single wrong value but from deciding the grade before the part's governing requirement is clear. These eight steps can serve as a fixed RFQ template for a structural composite program.

1. Governing requirement: First decide whether the part is strength-driven, stiffness-driven, or thermal. Strength-driven parts point to high-strength PAN such as T700 or T800. Stiffness-driven parts point to intermediate or high modulus. Thermal or dimensional-stability parts point to pitch fiber. This single choice sets the precursor and modulus class before any number is compared.
2. Modulus class and grade: Map the requirement to standard modulus near 230 GPa, intermediate near 290 to 325 GPa, or high modulus from 350 to 900 GPa. Each class step roughly multiplies price and reduces strain to failure, so do not oversize modulus; a stiffer fiber than needed wastes money and reduces damage tolerance.
3. Tow size and product form: Choose tow count and form together. Cosmetic or fine aerospace laminates use 3K to 12K woven or prepreg; wind, automotive, and civil parts use 24K to 50K large tow for deposition rate and cost. Decide between prepreg (best consistency, frozen storage, autoclave cure) and dry fabric with infusion (lower cost, larger parts).
4. Resin system and sizing: Confirm the sizing code matches the matrix family, whether epoxy, vinyl ester, phenolic, bismaleimide, or thermoplastic. A mismatch silently cuts interlaminar shear strength, so the sizing line is mandatory on the specification.
5. Test basis and traceability: Require that all quoted properties use the same standard, ASTM D4018 or ISO 10618 for tow, ISO 11566 for single filament. For structural use, demand lot certificates traceable to a production line, not just a grade name, because grade labels persist across process changes.
6. Service environment: Account for operating temperature, ultraviolet and moisture exposure, and impact risk. Bare carbon fiber tolerates high temperature, but the epoxy matrix usually limits service to about 120 to 180 degrees Celsius; higher service needs bismaleimide or other high-temperature resins. Add a glass-fiber isolation ply wherever the laminate contacts aluminum or steel, because carbon fiber is strongly cathodic and accelerates galvanic corrosion of the metal, and prefer titanium alloy or stainless steel fasteners rather than aluminum at such joints.
7. Qualification and certification: For aerospace and pressure-vessel use, the fiber must already be qualified to the relevant material specification and the producer must hold the necessary approvals. Pressure vessels follow standards such as ISO 11119 and ISO 11515 for composite cylinders; aircraft parts follow the airframer's allowables database. Verify these before committing a grade.
8. Total cost and supply security: Add fiber price, layup and cure cost, scrap rate, and the risk of single-source supply. High-performance grades are sometimes export-controlled and capacity-constrained, so confirm lead time and a qualified second source. A fiber that saves a few dollars per kilogram but cannot be resupplied on schedule is the more expensive choice over a program's life.

One last commonly overlooked dimension is manufacturer support and serviceability: datasheet revision control, lot traceability, application-engineering help with layup and cure, and the willingness to share design allowables. The main structural suppliers are Toray, which now includes the former Zoltek large-tow business, Hexcel with the HexTow AS4 and IM7 lines, Teijin with the Tenax range, Mitsubishi Chemical, SGL Carbon, and Solvay. For high-modulus pitch fiber the principal sources are Mitsubishi Chemical Dialead and Nippon Graphite. Qualifying a fiber is a multi-month effort, so the supplier's stability and documentation discipline matter as much as the headline strength number.