Carbon fiber is an exceptionally strong, stiff, and lightweight material constructed of thin, strong crystalline filaments of carbon. It is created by heating and stretching carbon strands into long, thin fibers that are subsequently woven or braided together to produce a fabric. Carbon fiber materials are well-known for their high strength and low weight, and they are frequently employed in a wide range of applications where these properties are crucial, like in the aerospace, marine, and automotive sectors, as well as in sporting goods and other consumer items.
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What is Carbon Fiber?
Carbon fibers, also known as CF or graphite fiber, are fibers with a diameter ranging from 5 to 10 micrometers (0.00020–0.00039 in) and consisting primarily of carbon atoms. Generally, carbon fiber is five times stronger than steel and twice as stiff. Carbon fibers provide many benefits, including high stiffness, high tensile strength, a high strength-to-weight ratio, strong chemical resistance, high-temperature tolerance, and low thermal expansion. Carbon fiber is particularly widespread in aerospace, civil engineering, military, racing, and other competitive sports due to these qualities. Nevertheless, they are more costly than other fibers involving glass, basalt, or plastic fibers.
In the process of creating a carbon fiber, carbon atoms are bonded together in crystals that are roughly parallel to the fiber’s long axis. This is because this crystal alignment offers the fiber a high strength-to-volume ratio. Thousands of carbon fibers are grouped to make a tow that may be used alone or woven into a fabric. Carbon fibers come in various weaves, such as plain, twill, etc.
Carbon fibers are typically mixed with other materials to create a composite. When impregnated with a plastic resin and baked, for example, it creates a carbon-fiber-reinforced polymer (commonly called carbon fiber). This produced material has a high strength-to-weight ratio and is exceptionally stiff but slightly brittle. Carbon fibers are frequently combined with other materials, such as graphite, to create reinforced carbon-carbon composites with extremely high heat tolerance.
Carbon fiber-reinforced composite materials are utilized in manufacturing aircraft and spacecraft parts, racing car bodywork, golf club shafts, bicycle frames, fishing rods, vehicle springs, sailboat masts, and other components requiring light weight and high strength.
How is Carbon Fiber Made?
The manufacturing process of carbon fiber is part chemical and part mechanical. Here is a detailed description of how carbon fiber is made:
Step 1: Preparing Raw Materials or Precursor
To manufacture carbon fiber, using an organic polymer precursor is a must. The precursor is the term used to refer to the raw material utilized in producing carbon fiber. Approximately 90% of carbon fibers manufactured are derived from polyacrylonitrile (PAN). The remaining 10% of the carbon fibers are made from rayon or petroleum pitch. These raw materials are classified as organic polymers, characterized by long strings of molecules bound together by carbon atoms.
Step 2: Stabilizing and Carbonizing Fibers
After preparing the precursor, it is pulled into long fibers. The carbon fiber manufacturing process begins with carbonization. Before carbonization, the fibers must be chemically altered to transform their linear atomic bonding to a more thermally stable ladder bonding. This is performed by heating the fibers in air to roughly 390-590° F (200-300° C) for 30-120 minutes. This causes oxygen molecules to be taken up by the fibers from the air, resulting in a rearrangement of their atomic bonding pattern.
After stabilizing the fibers, they are heated to around 1,830-5,500° F (1,000-3,000° C) for several minutes in a furnace filled with an oxygen-free gas mixture. In extremely high temperatures, the absence of oxygen protects the fibers from burning. When fibers are heated, they lose non-carbon atoms. Then, the remaining carbon atoms form tightly bonded carbon crystals aligned parallel to the fiber’s long axis.
Step 3: Treating the Fiber Surface
Following the carbonization process, the fibers exhibit a surface that fails to bond well with the epoxies and other substances used in composite materials. In this case, the surface of the fibers is slightly oxidized to improve bonding properties. Introducing oxygen atoms to the surface enhances chemical bonding properties and additionally etches and roughens the surface for improved mechanical bonding properties.
The fibers can be oxidized by immersing them in different gases such as air, carbon dioxide, or ozone, as well as liquids such as sodium hypochlorite or nitric acid. Alternately, the fibers may be electrolytically coated by making them the positive terminal in a bath having various electrically conductive substances.
Step 4: Sizing the Fibers
Following surface treatment, protective coatings are applied to the fibers to prevent damage while winding or weaving. This is referred to as sizing. The selection of coating materials is based on their compatibility with the adhesive employed to make composite materials. Commonly used coating materials encompass a range of substances such as epoxy, polyester, nylon, urethane, and several others.
Then, the coated fibers are carefully coiled onto cylinders known as bobbins. The bobbins are placed into a spinning machine, where the fibers undergo a twisting process to form yarns of different sizes.
Brief History and Background of Carbon Fiber
In 1860, Joseph Swan successfully created carbon fibers for the first time, which were subsequently utilized in the production of light bulbs. In 1879, Thomas Edison employed a process involving the high-temperature baking of cotton threads or bamboo slivers, resulting in the carbonization of these materials into an all-carbon filament. This filament was then utilized in one of the first incandescent light bulbs designed to be heated by electricity. In 1880, Lewis Latimer produced a dependable carbon wire filament for the incandescent light bulb, which was heated using electricity.
In 1958, Roger Bacon developed high-performance carbon fibers at the Union Carbide Parma Technical Center outside Cleveland, Ohio. The production of such fibers involves the application of heating strands of rayon till they get carbonized. However, carbon fiber’s brittleness restricted its application for a long time. Since then, the fabrication of carbon fiber has become considerably more effective. Carbon fiber has a tensile strength of 4,000 MPa and a modulus of 400 GPa. This has expanded its utility into a vast array of applications.
Properties of Carbon Fiber
Carbon fiber is lighter and stronger than metals for many uses. Its excellent properties suit many structural, automotive, aerospace, and specialty applications. Let’s take a closer look at these properties.
High Strength-to-Weight Ratio
One of the features of carbon fiber is its high strength-to-weight ratio. Carbon fiber is extremely strong, stiff, and light. Any material possesses a high strength-to-weight ratio if it is strong and lightweight. Materials with high strength-to-weight ratios include aluminum, titanium, magnesium, carbon and glass fiber, and high-strength steel alloys.
Carbon fiber filaments are electrically conductive, allowing the material to be used in applications like lightning strike protection. However, carbon fiber conductivity can potentially accelerate galvanic corrosion in fittings. This issue can be reduced by careful installation.
Corrosion-resistant and Chemically Stable
While carbon fiber is resistant to deterioration, it is important to note that epoxy, a commonly used material in carbon fiber composites, is susceptible to damage from sunlight and requires protection. There is a possibility that other matrices in which the carbon fiber is embedded might exhibit reactive.
Rigid or Stiff
The rigidity or stiffness of a material may be measured by its Young modulus, which characterizes the extent to which the material undergoes deformation when subjected to stress. Carbon fiber-reinforced plastic is over 4 times stiffer than glass-reinforced plastic, 20 times stiffer than pine, and 2.5 times stiffer than aluminum.
Carbon fiber exhibits better fatigue resistance and damage tolerance than composite materials like fiberglass. Tensile fatigue damage is shown as a decrease in stiffness with increasing stress cycles. Failure is impossible when cyclic stresses coincide with fiber orientation.
The tensile strength refers to the greatest stress a material can bear when stretched or pulled before necking or failing. Due to their internal flaws, carbon fiber does not always fail at the same stress level. But small strains would cause them to fail.
Low Thermal Expansion
The coefficient of thermal expansion of carbon fiber is very low compared to metals. This means parts made from carbon fiber experience minimal dimensional changes due to temperature variation. The low thermal expansion of carbon fiber makes it ideal for situations where small movements are required.
Non Poisonous and X-Ray Permeable
Carbon fiber composite materials are not poisonous and allow X-rays to pass through them. This makes carbon fiber suitable for medical applications, such as prosthesis use, implants, tendon repair, x-ray accessories, surgical instruments, etc.
Carbon fiber can be integrated into firefighting protective clothing. One example is nickel-coated fiber. Because carbon fiber is chemically inert, it may be employed in situations involving fire and corrosive agents.
Strong covalent bonds create the layers in the fiber. The sheet-like aggregations easily facilitate the propagation of cracks. When the fibers bend, they will fail at low strain.
Types of Carbon Fiber
Based on raw materials, mechanical properties, and final heat treatment temperature, carbon fibers can be classified into the following categories:
Types Based on Raw Materials
There are two types of carbon fiber based on raw materials.
PAN-based Carbon Fiber
PAN-based carbon fiber is created by PAN ( Polyacrylonitrile) precursor carbonization. This carbon fiber has high tensile strength and elastic modulus. It is widely used for structural material composites in aerospace, industrial fields, and sports / recreational products.
Pitch-based Carbon Fiber
Pitch-based carbon fiber is another type of fiber. This carbon fiber is formed by the carbonization of oil/coal pitch precursor and possesses many properties ranging from low elastic modulus to ultra high elastic modulus. Fibers with ultra high elastic modulus are widely used in high stiffness components and other applications that need high thermal or electric conductivity.
Types Based on Mechanical Properties
Ultra High Elastic Modulus Type (UHM)
The ultra high elastic modulus type is commonly called UHM. The tensile elastic modulus of this type equals or exceeds 600 GPa. Its tensile strength is 2,500 MPa or above. The ultra high modulus carbon fiber is exclusively used in the most advanced engineering applications where extreme stiffness per unit weight is necessary, like satellites, drones, and competition sports equipment.
High Elastic Modulus Type (HM)
The high elastic modulus type, or HM, has a tensile elastic modulus ranging from 350 to 600 GPa. And its tensile strength is at least 2,500 MPa. It is commonly utilized in aerospace, automotive, and advanced sporting goods. High elastic modulus carbon fiber enables the creation of lightweight components without compromising strength.
Intermediate Elastic Modulus Type (IM)
In the intermediate elastic modulus type (IM), the range of the tensile elastic modulus is between 280 and 350 GPa. The tensile strength of this material is equal to or exceeds 3,500 MPa. It is employed in industries that require enhanced performance, such as aerospace, automotive, and sports equipment manufacturing.
Standard Elastic Modulus Type (HT)
The standard elastic modulus type, commonly referred to as HT, is being discussed here. The range of the tensile elastic modulus is between 200 and 280 GPa. The material’s tensile strength is estimated at least 2,500 MPa or above. It suits diverse applications, from automotive components and sporting equipment to wind turbines and aircraft structures.
Low Elastic Modulus Type (LM)
The low elastic modulus type (LM) has a tensile elastic modulus equal to or less than 200 GPa. The tensile strength of the material is equal to or less than 3,500 MPa.
Types Based on the Final Heat Treatment Temperature
Regarding the final heat treatment temperature, carbon fibers are categorized as:
High-heat-treatment Carbon Fibers (HTT)
High-heat-treatment carbon fibers (HTT) are associated with high-modulus type fiber and require a final heat treatment temperature higher than 2000°C.
Intermediate-heat-treatment Carbon Fibers (IHT)
Intermediate-heat-treatment carbon fibers (IHT) are characterized by a final heat treatment temperature typically equal to or more than 1500°C. These fibers are often associated with high-strength type fiber.
Low-heat-treatment Carbon Fibers (LHT)
Low-heat-treatment carbon fibers (LHT), in which the final heat treatment temperature does not exceed 1000°C. These materials have a low modulus and strength.
Types of Carbon Fiber Weaves
Carbon fiber frequently comes in the form of a woven fabric. Carbon fiber fabric has a variety of weaves. Some commonly used types of weave are twill, satin, and plain.
A plain weave carbon fiber sheet exhibits a symmetrical appearance with a small checkerboard pattern. The tows are interlaced in an over/under pattern in this weave. The narrow distance between interlaces contributes significantly to the stability of the plain weave. Fabric stability refers to the capacity of a fabric to retain its weave angle and fiber orientation. Because of its notable stability, the plain weave is not optimally suitable for layups with intricate contours. In addition, it lacks the pliability exhibited by many other weaves. Plain weave fabrics are typically appropriate for two-dimensional curves, tubes, and flat sheets.
Twill weave is more pliable and capable of shaping to intricate contours. It is superior to harness satin weave in terms of fabric stability, yet not as good as plain weave. Following a tow strand in a twill weave goes over a set number of tows and then under the same number. Applying the over/under pattern forms a diagonal arrowhead, sometimes called a “twill line”.
Since ancient times, the satin weave has created silk fabrics with exceptional draping characteristics and a seamless, smooth appearance. The drapability of composites enables them to form and wrap around intricate contours readily. This fabric has low stability due to its high formability. The commonly used satin weaves in the textile industry are the 4 harness satin (4HS), 5 harness satin (5HS), and 8 harness satin (8HS). Formability will increase as the number of satin weave increases, while fabric stability will diminish.
Applications of Carbon Fiber
The unique benefits of carbon fiber make this material find numerous cutting-edge applications across multiple industries. Here is an overview of the applications and uses of carbon fiber:
Aerospace – Carbon fiber is widely used in aerospace applications due to its high strength-to-weight ratio, stiffness, and corrosion resistance. Major uses include aircraft fuselages and wings, helicopter blades, rocket casings, and payload fairings. Using carbon fiber in airplanes can reduce weight by up to 20% for some aircraft. Barely Visible Impact Damage (BVID) is a disadvantage of employing carbon fiber in aircraft. This unseen damage has the potential to impact a component’s safety. It requires a great deal of training and testing to detect BVID.
Automotive – As costs come down, carbon fiber is increasingly used in high-end and racing automobiles to reduce weight while maintaining strength and rigidity. Uses include body panels, hoods, roofs, spoilers, and structural components. Carbon fiber can help improve fuel efficiency and handling. Formula 1 race cars rely heavily on carbon fiber.
Sports Equipment – Many high-end bicycles now utilize carbon fiber frames and components to decrease weight and improve performance. Carbon fiber is also used for golf club shafts, tennis rackets, hockey sticks, and other specialty sports gear. In addition, carbon fiber hard hats, clothing, protective gear, and carbon fiber strain are examples of carbon fiber applications. Because racing sports often use carbon fiber helmets and shoes.
Civil Engineering – Carbon fiber reinforcing bars and cables can strengthen concrete and replace traditional steel reinforcement. This reduces the weight of concrete structures and increases their tensile strength. Carbon fiber wraps and laminates can strengthen structural elements like columns and bridges.
Consumer Electronics – Carbon fiber makes some high-end consumer electronics like laptops, phones, tablets, tripods, and headphones lighter and more durable. Carbon fiber device cases and structural components add strength and heat dissipation to these products. Also, the material is especially useful when electromagnetic transparency is needed.
Medical Devices – The biocompatibility and strength of carbon fiber have led to uses such as orthopedic implants, prosthetic limbs, and MRI machines. Carbon fiber-reinforced polymers allow precision parts like bone plates to be produced. Because carbon fiber appears transparent in X-ray images, it is used in various X-ray and imaging devices. Additionally, prosthetic limbs constructed from carbon fiber are strong, lightweight, and comfortable.
Military – Carbon fiber, first used for lighting in naval ships, is now employed in everything from missiles and drones to helmets and tent posts. Carbon fiber’s primary advantages for the military are its strength and lightweight nature, which allows for simpler transportation and increased energy saving.
Carbon fiber comprises thin strands of carbon tightly bound together to form a highly durable, very strong, and exceptionally light material.Its high strength-to-weight ratio, rigidity, and heat tolerance make it an ideal choice when light weight and high strength are critical in industries like aerospace, medical, construction, sports, marine, and military applications. Carbon fiber parts can be manufactured in complex shapes and precisely tailored designs to optimize strength. This unique material delivers the critical advantage of dramatic weight reduction without sacrificing durability.
Carbon fibers – From Wikipedia
Type of Carbon Fiber Products and their Special Features – From The Japan Carbon Fiber Manufacturers Association (JCMA)
Carbon Fibres: Production, Properties and Potential Use – From Material Science Research India
PAN-based carbon fibers have emerged as the most extensively utilized carbon fiber. Globally, over 90% of commercial carbon fibers are made from PAN precursor fibers.
When comparing their respective strength-to-weight ratios, carbon fiber is stronger than steel. Although steel and carbon fiber both have an elastic modulus of 200 GPa, steel is five times heavier. Carbon fiber may be preferable in many applications due to its high strength-to-weight ratio.
Ultra high modulus carbon fiber is exclusively used in highly advanced engineering applications like satellites, UAVs, and specialized competition sports equipment where extreme stiffness per unit weight is imperative.