Composite Materials
There are benefits to compositing two or more materials. Composite materials take advantage of the unique properties of each component and are designed to optimize the strength, density, electrical properties, and cost of the materials.
Many conventional materials such as plywood, reinforced rubber, and reinforced concrete are composites since they consist of combinations of materials. Concrete, with its cement matrix and rock aggregate, is a composite. To many, however, the term composite material refers to a specific category of advanced composites.
Advanced composites have emerged primarily as a result of the aircraft and spacecraft industries, in which there are extremes in strength and weight requirements. New metals have been created that not only meet this demand, but also are capable of resisting fatigue, corrosion, and vibration.
Fiber-reinforced metal-matrix composites (MMCs) consist of high-performance metallic reinforcements--glass, boron or graphite filaments--in matrices of aluminum, titanium, magnesium, copper, or another material. MMCs are created by diffusion bonding, casting, hot molding, or by plasma spraying the matrix around the reinforcement, which can be in the shape of particles, whiskers, or fibers.
MMCs were first used in 1974 in the United States space shuttle project. A boron-aluminum composite was used in tubing that went into the craft. This same technology was passed on commercially and has come to be used in the manufacture of many products, from pistons and connecting rods to tennis racquets and fishing rods.
Boron and silicon carbide are the most common of the very high modulus reinforcements. They were developed for the United States Air Force Materials Laboratory by Texaco and United Technology corporations.
Composites can be found in the textile industry. Felt is a type of composite consisting of interlocking fibers created by the use of heat, moisture, and pressure. Wool, or wool combined with other fibers, is mechanically or chemically meshed together while the fibrous mass is kept warm and moist. The three types of felts are pressed, needled, and woven. Felt is a popular fabric because of its similarity to fur.
Composites are becoming common in plastics. The same reinforcement materials--glass, boron, and graphite--are used to increase the strength and stiffness of plastics. Polyvinyl chloride is used as a laminate for metal. It protects metal from corrosion and allows colors to be added. Plastic composites can be molded into shapes, needed for kitchenware and electrical insulation, uses that require light weight, strength, and flexibility.
Super-strength composites are employed in the construction industry. Bridge and building designs that demand that larger spaces be spanned require synthetic materials to meet the new challenges. This requirement extends to the machinery and equipment used in building or roadway construction. New types of reinforced concretes containing fibers are being used for highway overpasses, especially in areas that are earthquake prone. High-strength metal girders are used to support bridge decks and in stress-skin highrise buildings.
A main concern in the development of composites is that an adequate bond be achieved between the matrix and the introduced material. There must also be an equitable distribution of the composited materials. Fibers can be created by spinning, drawing (pulling), extrusion, and deposition. They are then combined with the host matrix by controlled mixing.
Material compositing probably began when the Egyptians mixed straw in clay to strengthen their unbaked bricks. Anticipating modern construction practices, the Romans began to mix volcanic ash into cement to create a subaqueous building material in about 200 B.C. Today laboratories with multimillion dollar budgets are continuing the tradition as they attempt to bring composites into the age of high technology.
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