Composite Materials

A composite material is a microscopic or macroscopic combination of two or more distinct materials with a recognizable interface between them. For structural applications, the definition can be restricted to include those materials that consist of a reinforcing phase such as fibers or particles supported by a binder or matrix phase. Other features of composites include the following: (1) The distribution of materials in the composite is controlled by mechanical means; (2) The term composite is usually reserved for materials in which distinct phases are separated on a scale larger than atomic, and in which the composite's mechanical properties are significantly altered from those of the constituent components; (3) The composite can be regarded as a combination of two or more materials that are used in combination to rectify a weakness in one material by a strength in another. (4) A recently developed concept of composites is that the composite should not only be a combination of two materials, but the combination should have its own distinctive properties. In terms of strength, heat resistance, or some other desired characteristic, the composite must be better than either component alone.

Composites were developed because no single, homogeneous structural material could be found that had all of the desired characteristics for a given application. Fiber-reinforced composites were first developed to replace aluminum alloys, which provide high strength and fairly high stiffness at low weight but are subject to corrosion and fatigue.

An example of a composite material is a glass-reinforced plastic fishing rod in which glass fibers are placed in an epoxy matrix. Fine individual glass fibers are characterized by their high tensile stiffnesses and a very high tensile strengths, but because of their small diameters, have very small bending stiffnesses. If the rod were made only of epoxy plastic, it would have good bending stiffness, but poor tensile properties. When the fibers are placed in the epoxy plastic, however, the resultant structure has high tensile stiffness, high tensile strength, and high bending stiffness.

The discontinuous filler phase in a composite is usually stiffer or stronger than the binder phase. There must be a substantial volume fraction of the reinforcing phase (~10%) present to provide reinforcement. Examples do exist, however, of composites where the discontinuous phase is more compliant and ductile than the matrix.

Natural composites include wood and bone. Wood is a composite of cellulose and lignin. Cellulose fibers are strong in tension and are flexible. Lignin cements these fibers together to make them stiff. Bone is a composite of strong but soft collagen (a protein) and hard but brittle apatite (a mineral).

High performance composites are composites that have better performance than conventional structural materials such as steel and aluminum alloys. They are almost all continuous fiber-reinforced composites, with organic (resin) matrices.

Fibers for high performance composites

In a high-performance, continuous fiber-reinforced composite, fibers provide virtually all of the load-carrying characteristics of the composite, i.e., strength and stiffness. The fibers in such a composite form bundles, or filaments. Consequently, even if several fibers break, the load is redistributed to other fibers, which avoids a catastrophic failure.

Glass fibers are used for nonstructural, low-performance applications such as panels in aircraft and appliances to high-performance applications such as rocket-motor cases and pressure vessels. But the sensitivity of the glass fiber to attack by moisture poses problems for other applications. The most commonly used glass fiber is a calcium aluminoborosilicate glass (E-glass). High silica and quartz fibers are also used for specialized applications.

Carbon fibers are the best known and most widely used reinforcing fiber in advanced composites.The earliest carbon fibers were produced by thermal decomposition of rayon precursor materials. The starting material is now polyacrylonitrile.

Aramid fibers are aromatic polyamide fibers. The aramid fiber is technically a thermoplastic polymer like nylon, but it decomposes when heated before it reaches its projected melting point. When polymerized, it forms rigid, rod-like molecules that cannot be spun from a melt. Instead they have to be spun from a liquid crystalline solution. Early applications of aramid fibers included filament-wound motor cases, and gas pressure vessels. Aramid fibers have lower compressive strengths than do carbon fibers, but their high specific strengths, low densities, and toughness keep them in demand.

Boron fibers were the first high-performance reinforcement available for use in advance composites. They are, however, more expensive and less attractive for their mechanical properties than carbon fibers. Boron filaments are made by the decomposition of boron halides on a hot tungsten wire. Composites can also be made from whiskers dispersed in an appropriate matrix.

Continuous silicon carbide fibers are used for large-diameter monofilaments and fine multifilament yarns. Silicon carbide fibers are inherently more economical than boron fibers, and the properties of silicon carbide fibers are generally as good or better than those of boron.

Aluminum oxide (alumina) fibers are produced by dry spinning from various solutions. They are coated with silica to improve their contact properties with molten metal.

There is usually a size effect associated with strong filaments. Their strengths decrease as their diameter increases. It turns out that very high strength materials have diameters of about 1 micrometer. They are consequently not easy to handle.

Matrices for high performance composites

The matrix binds fibers together by virtue of its cohesive and adhesive characteristics. Its purpose is to transfer load to and between fibers, and to protect the fibers from hostile environments and handling. The matrix is the weak link in the composite, so when the composite experiences loading, the matrix may crack, debond from the fiber surface, or break down under far lower strains than are usually desired. But matrices keep the reinforcing fibers in their proper orientation and position so that they can carry loads, distribute loads evenly among fibers, and provide resistance to crack propagation and damage. Limitations in the matrix generally determine the overall service temperature limitations of the composite.

Polyester and vinyl esterresins are the most widely used matrix materials in high performance continuous-fiber composites. They are used for chemically resistant piping and reactors, truck cabs and bodies, appliances, bathtubs and showers, automobile hoods, decks, and doors. These matrices are usually reinforced with glass fibers, as it has been difficult to adhere the matrix suitably to carbon and aramid fibers. Epoxies and other resins, though more expensive, find applications as replacements for polyester and vinyl ester resins in high performance sporting goods, piping for chemical processing plants, and printed circuit boards.

Epoxy resins are used more than all other matrices in advanced composite materials for structural aerospace applications. Epoxies are generally superior to polyesters in their resistance to moisture and other environmental influences.

Bismaleimide resins, like epoxies, are fairly easy to handle, relatively easily processed, and have excellent composite properties. They are able to withstand greater fluctuations in hot/wet conditions than are epoxies, but they have worse failure characteristics.

Polyimide resins release volatiles during curing, which produces voids in the resulting composite. However, these resins do withstand even greater hot/wet temperature extremes than bismaleimide matrices, and work has been underway to minimize the void problem.

The thermoplastic resins used as composite matrices such as polyether etherketone, polyphenylene sulfide, and polyetherimide are very different from the commodity thermoplastics such as polyethylene and polyvinyl chloride. Although used in limited quantities, they are attractive for applications requiring improved hot/wet properties and impact resistance.