A composite material (or just composite) is a mixture of two or more materials with properties superior to the materials of which it is made. Many common examples of composite materials can be found in the world around us. Wood and bone are examples of natural composites. Wood consists of cellulose fibers embedded in a compound called lignin. The cellulose fibers give wood its ability to bend without breaking, while the lignin makes wood stiff. Bone is a combination of a soft form of protein known as collagen and a strong but brittle mineral called apatite.
Humans have been using composite materials for centuries, long before they fully understood the structures of such composites. The important building material concrete, for example, is a mixture of rocks, sand, and Portland cement. Concrete is a valuable building material because it is much stronger than any one of the individual components of which it is made. Interestingly enough, two of those components are themselves natural composites. Rock is a mixture of stony materials of various sizes, and sand is a composite of small-grained materials.
Reinforced concrete is a composite developed to further improve the strength of concrete. Steel rods embedded in concrete add both strength and flexibility to the concrete.
Cutting wheels designed for use with very hard materials are also composites. They are made by combining fine particles of tungsten carbide with cobalt powder. Tungsten carbide is one of the hardest materials known, so the composite formed by this method can be used to cut through almost any natural or synthetic material.
Fiber: In terms of composite fillers, a fiber is a filler with one long dimension.
Matrix: The part of the composite that binds the filler.
Particle: In terms of composite fillers, a particle is a filler with no long dimension.
Some forms of aluminum siding used in homes are also composite materials. Thin sheets of aluminum metal are attached to polyurethane foam. The polyurethane foam is itself a composite consisting of air mixed with polyurethane. Joining the polyurethane foam to the aluminum makes the aluminum more rigid and provides excellent insulation, an important property for the walls of a house.
In general, composites are developed because no single structural material can be found that has all of the desired characteristics for a given application. Fiber-reinforced composites, for example, were first developed to replace aluminum alloys (mixtures), which provide high strength and fairly high stiffness at low weight but corrode rather easily and can break under stress.
Composites consist of two parts: the reinforcing phase and the binder, or matrix. In reinforced concrete, for example, the steel rods are the reinforcing phase; the concrete in which the rods are embedded are the binder or matrix.
In general, the reinforcing phase can exist in one of three forms: particles, fibers, or flat sheets. In the cutting wheels described above, for example, the reinforcing phase consists of tiny particles of cobalt metal in a binder of tungsten carbide. A plastic fishing rod is an example of a composite in which the reinforcing phase is a fiber. In this case, the fiber is made of threadlike strips of glass placed in an epoxy matrix. (Epoxy is a strong kind of plastic.) An example of a flat sheet reinforcing phase is plywood. Plywood is made by gluing together thin layers of wood so that the wood grain runs in different directions.
The binder or matrix in each of these cases is the material that supports and holds in place the reinforcing material. It is the tungsten carbide in the cutting wheel, the epoxy plastic in the fishing rod, or the glue used to hold the sheets of wood together.
High-performance composites are composites that perform better than conventional structural materials such as steel and aluminum alloys. They are almost all fiber-reinforced composites with polymer (plasticlike) matrices.
The fibers used in high-performance composites are made of a wide variety of materials, including glass, carbon, boron, silicon carbide, aluminum oxide, and certain types of polymers. These fibers are generally interwoven to form larger filaments or bundles. Thus, if one fiber or a few individual fibers break, the structural unit as a whole—the filament or bundle—remains intact. Fibers usually provide composites with the special properties, such as strength and stiffness, for which they are designed.
In contrast, the purpose of the matrix in a high-performance composite is to hold the fibers together and protect them from damage from the outside environment (such as heat or moisture) and from rough handling. The matrix also transfers the load placed on a composite from one fiber bundle to the next.
Most matrices consist of polymers such as polyesters, epoxy vinyl, and bismaleimide and polyimide resins. The physical properties of any given matrix determine the ultimate uses of the composite itself. For example, if the matrix melts or cracks at a low temperature, the composite can be used for applications only at temperatures less than that melting or cracking point.