2.4 Composite Materials

Among the major developments in materials in recent years are composite materials. In fact, composites are now one of the most important classes of engineered materials, because they offer several outstanding properties as compared with conventional materials. A composite material is a combination of two or more chemically distinct and insoluble phases; its properties and structural performance are superior to those of the constituents acting independently.

For example, it is known that plastics possess mechanical properties that are generally inferior to those of metals and alloys—in particular, low strength, stiffness, and creep resistance. These properties can be improved by embedding reinforcements of various types (such as glass or graphite fibers) to produce reinforced plastics. Metals and ceramics, as well, can be embedded with particles or fibers, to improve their properties; these combinations are known as metal-matrix and ceramic-matrix composites. [6]

Applications of Reinforced Plastics. The first application of reinforced plastics (in 1907) was for an acid-resistant tank, made of a phenolic resin with asbestos fibers. Epoxies were first used as a matrix material in the 1930s. Beginning in the 1940s, boats were made with fiberglass. Major developments in composites began in the 1970s; those materials are now called advanced composites. They are typically used in military and commercial aircraft and in rocket components, helicopter blades, automobile bodies, leaf springs, drive shafts, pipes, ladders, pressure vessels, sporting goods, sports and military helmets, boat hulls, and various other structures and components. Glass- or carbon-fiber reinforced hybrid plastics are now being developed for high-temperature applications, with continuous use at about 300 °C (550 °F). The Boeing 777 is made of about 9% composites by total weight; that proportion is triple the composite content of previous Boeing transport aircraft. The floor beams and panels and most of the vertical and horizontal tail are made of composite materials. By virtue of the resulting weight savings, reinforced plastics have reduced fuel consumption by about 2%.

Metal-matrix Composites (MMC). The advantages of a metal matrix over a polymer matrix are its higher elastic modulus, its resistance to elevated temperatures, and its higher toughness and ductility. The limitations are higher density and greater difficulty in processing parts. Matrix materials in these composites are usually aluminum, aluminum–lithium, magnesium, copper, titanium, and superalloys. Fiber materials can be graphite, aluminum oxide, silicon carbide, boron, molybdenum, and tungsten. The elastic modulus of nonmetallic fibers ranges between 200 GPa and 400 GPa, with tensile strengths being in the range from 2000 MPa to 3000 MPa. Because of their high specific stiffness, light weight, and high thermal conductivity, boron fibers in an aluminum matrix have been used for structural tubular supports in the Space Shuttle orbiter.

Ceramic-matrix Composites (CMC). Composites with a ceramic matrix are another important development in engineered materials because of their resistance to high temperatures and corrosive environments. Ceramics are strong and stiff, and they resist high temperatures, but they generally lack toughness. Matrix materials that retain their strength up to 1700 °C (3000 °F) are silicon carbide, silicon nitride and aluminum oxide. Various techniques for improving the mechanical properties of ceramic-matrix composites, particularly their toughness, are being investigated. Applications are in jet and automotive engines, deep-sea mining equipment, pressure vessels, structural components, cutting tools, and dies for the extrusion and drawing of metals.