Carbon fiber is one of those materials that has always been related to cutting-edge applications, however, we don’t stop hearing about it lately. But what is carbon fiber?
As its name indicates, carbon fiber is a material composed of at least 92% carbon, whose structure can be crystalline, amorphous or partially crystalline . The crystalline form is identical to the structure of graphite, carbon atoms arranged in sheets with a regular hexagonal pattern.
It is well known that graphite is a very soft material. This is because the bond between the atomic layers of carbon with this structure is weak and they slide easily between them. On the contrary, they have a high modulus of elasticity in a direction parallel to the bond plane, showing great anisotropy of the material.
Taking advantage of this characteristic, the high modulus of carbon fiber lies in the fact that the carbon layers tend to be parallel to the axis of the fiber or, put another way, there is a preferred crystallographic orientation known as the fiber texture.
The greater the texture, the greater the density, carbon content, elastic modulus of the fiber, electrical and thermal conductivity parallel to the axis of the fiber and the lower the coefficient of thermal expansion and internal shear resistance.
The proportion of graphite in a carbon fiber can vary from 0 to 100%. So in the same carbon fiber we can find both crystalline and amorphous structures. The bonds of the latter are stronger, so the resistance will be improved, providing the excellent mechanical properties that we know from carbon fiber.
Obtaining carbon fiber
About 90% of the carbon fiber used is produced from a polyacrylonitrile (PAN) precursor, the rest from rayon or petroleum pitch. These precursors undergo chemical-mechanical transformations until the final product is obtained as carbon fiber.
An important phase is carbonization, in which the fibers are heated in an inert atmosphere at a temperature of approximately 1500 – 2500 ºC for several minutes in order to remove non-carbon atoms.
After this phase, a surface treatment is applied in which the surface is slightly oxidized in order to improve the bonding properties. In addition, an intermediate graphitization process can be added in which the product is heated above 2000 ºC in order to enlarge the grain and significantly increase the elastic modulus, making the fibers more fragile. Therefore, we can differentiate between high modulus fibers (HM) that provide greater rigidity or high resistance fibers (HT) capable of absorbing a greater amount of energy.
Carbon fiber properties
By varying the precursors, the percentages in composition and other production parameters, carbon fibers with certain characteristics will be obtained.
However, the properties of carbon fiber can be generally summarized as:
· High specific resistance
· High specific rigidity
· Light
· High impact resistance
· Good wear resistance
· Dimensional stability, thermal expansion close to zero
· Electrical conductivity, can produce galvanic corrosion
· Wide variety of carbon fiber formats
· Density: 1800 Kg/m3
The main advantage observed over conventional materials, such as steel or aluminum, is the high specific properties. That is, the relationship between resistance and rigidity and the weight of the component, and carbon fibers can be found up to 4 times higher in specific resistance than conventional steels.
Other benefits of carbon fiber are the flexibility it gives us in the design of components, as well as the optimization of the material due to the aforementioned anisotropy or different behavior depending on the direction.
Carbon fiber applications
Due to the processes necessary to obtain final carbon fiber products, the cost is high. However, the standardization of these processes is allowing a reduction in prices, enabling more diverse and everyday uses. Since carbon fiber began to be produced in 1963 for the construction of United Kingdom Ministry of Defense aircraft, applications in aeronautics and high-performance automotive have been constant and growing.
Nowadays, carbon fiber is a moderately accessible material, used in various applications as reinforcement of polymeric matrices (FRP). Some sectors are aerospace, naval, railway, biomedical, defense, automotive, construction, sports and electronics .
It is common to find bicycles, protective helmets, or even carbon fiber paddle tennis rackets.
