Composite material. In technology, a composite is a combination of materials selected and manufactured to provide a range of properties not provided by any of the materials alone. Such properties include strength, stiffness, and the ability to endure conditions of prolonged or extreme stress. Composites are used in the manufacture of automobiles and airplanes as well as sports equipment, and are essential in space craft where extreme conditions must be endured.
Combined materials such as alloys have long been known, but the term composite is reserved for materials in which particles or short fibers ("whiskers") are dispersed in, or long fibers are woven in, some form of matrix. One familiar composite is FIBERGLASS. Fibers are commonly made of the elements carbon or silicon and their compounds, and if the composite does not have to experience extreme temperatures the matrix is usually some form of plastic.
When higher temperatures are involved, a metal matrix is chosen. It offers the advantage of greater strength but the disadvantages of heaviness and greater difficulty of manufacture. For extremely high temperatures, some form of ceramic matrix must be used. Since ceramics, in effect, cannot be melted, ceramic composites are generally produced by a process known as sintering (see CERAMICS).
Kevlar{Kev'lahr} Kevlar is the trade name of a strong synthetic fiber first developed by the DuPont Company in the early 1970s. Kevlar is lighter than nylon, has a tensile strength about five times that of steel, and is more resistant to abrasion than other high-strength fibers. It was designed for use with epoxy resins, as a reinforcement for laminates, and as a tread reinforcement in belted tires. It can be substituted for glass fabrics and is widely employed in the production of bulletproof garments, but because it is not a ceramic it cannot be used to reinforce metals. In the plastics industry Kevlar, Kevlar 29, and Kevlar 49 are categorized as "aramids," a special class of aromatic polyamides. Because of its toughness, the fiber requires special cutting and machine techniques to be worked. It is available in such forms as yarns, unwoven mats, and chopped fibers. The plastic has no melting point but decomposes at temperatures above 400 deg C (752 deg F); its specific gravity is 1.44 g/cu cm.