Carbon fibers and graphite fibers both exhibit excellent mechanical properties. The tensile strength of graphite fiber is approximately twice that of glass fiber, while its thermal resistance far surpasses that of glass fiber. Fabrics woven from graphite yarn retain high mechanical strength even when heated to 2,500°C. Consequently, when used as a reinforcing material in fiberglass composites, they enable the production of high-performance, heat‑resistant, ablation‑resistant reinforced plastics.

　　Carbon fiber was first produced as early as the nineteenth century. The filament that illuminated the world’s first electric light bulb was made of carbon fiber. Shortly thereafter, some researchers carbonized cotton to obtain carbon fiber, but the resulting material exhibited poor performance and failed to attract significant attention. It was not until 1958 that a method was proposed to produce carbon fiber by carbonizing rayon. Two years later, Japanese scientists led by Akio Fujino made substantial improvements to the process of manufacturing carbon fiber from polyacrylonitrile (PAN) fibers, and this approach eventually became the primary method for the industrial production of carbon fiber today.

　　Their proposed method is as follows: First, polyacrylonitrile fibers are pre‑oxidized by heating them in air at 200–300°C while under tension. Subsequently, under an inert gas atmosphere, the pre‑oxidized fibers are heated to 1000°C for carbonization; during this process, elements such as hydrogen, nitrogen, and oxygen are volatilized and removed from the fiber’s molecular chains, leaving behind graphitized carbon fibers whose carbon atoms are arranged in a structure similar to graphite. The pre‑oxidation step in this manufacturing process is referred to as graphitization. Because of this graphitization treatment, the arrangement of carbon atoms becomes more akin to that of graphite, and the breakage of carbon atom chains during subsequent carbonization is also reduced. As for how the molecular chains of the synthetic fibers transform into carbon fibers with a graphite‑like structure after pre‑oxidation and carbonization, this remains an issue that has not yet been fully elucidated; however, the following explanation does have some basis:

　　Originally, the polyacrylonitrile molecular chains were coiled; under tensile stress, they uncoil along the direction of stretching, forming a series of nearly parallel, zigzagging chains. At elevated temperatures, one or two adjacent chains link together to form rings, while atoms other than carbon are removed, yielding a fiber composed of hexagonal carbon rings. This process can be represented by chemical symbols on page 95.

　　Boron fiber is also a novel reinforcing material that emerged in the 1960s, exhibiting exceptionally outstanding performance. It is a nonmetallic substance, typically appearing as a brown powder or a gray, lustrous crystal. Moreover, it is an extremely hard material, with a hardness nearly comparable to that of diamond. In 1959, the American scientist Tarry first proposed using chemical vapor deposition to produce boron fibers, a suggestion that promptly received strong support from the U.S. Air Force Materials Laboratory. After several years of research and development, boron fiber rapidly evolved into one of the most remarkable high‑strength, high‑stiffness reinforcing materials.