Composites
Composite materials consist of two or more distinct materials with a recognizable interface between them. The term composite is usually reserved for materials in which the distinct phases are separated on a scale larger than atomic, and in which the composite's mechanical properties have been significantly altered from those of the constituent materials.
Nature has been creating composites for millions of years. 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. The toughness and strength of bone arises from the dispersal of hard plate-like crystals of hydroxyapatite (mineral) in a soft matrix of collagen fibers (protein). As a composite, the resulting microstructure yields physical properties that even synthetic materials have been unable to match.
For the past several decades, scientists, taking their cue from nature, have been designing new materials with composite structures whenever there is a need to rectify the weakness in one material by the strength in another. Composites tend to be developed when no single, homogeneous material can be found that had all of the desired characteristics for a given application. Specific applications for composites now include structural materials such as concrete, high performance coatings, catalysts, electronics, photonics, magnetic materials, and biomedical materials, grinding wheels, underground electrical cables, superconducting ribbons, and ceramic fiber composites.
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 present to provide reinforcement. Examples do exist, however, of composites where the discontinuous phase is more compliant and ductile than the matrix.
Particle-reinforced composites consist of particles of one material dispersed in a matrix of a second material. The particles may have any shape or size, but they are generally spherical, ellipsoidal, polyhedral, or irregular in shape.
Fiber-reinforced materials are typified by fiberglass in which there are three components: glass filaments (for mechanical strength), a polymer matrix (to encapsulate the filaments), and a bonding agent (to bind the glass to the polymer).
The mechanical properties of composite materials usually depend on the composite's structure. Thus these properties typically depend on the shape of inhomogeneities, the volume fraction occupied by the inhomogeneities, and the interfaces between the components. The strength of composites depends on such factors as the brittleness or ductility of the inclusions and matrix.
High-performance composites often exhibit better performance than conventional structural materials such as steel and aluminum alloys. Such composites are almost all continuous fiber-reinforced composites, with organic (resin) matrices. Fibers used in high performance composites include glass fibers, carbon fibers, aromatic polyamide fibers, boron fibers, silicon-carbide fibers, and aluminum oxide fibers. Matrices include epoxy resins, bismaleimide resins, polyimide resins, and thermoplastic resins.
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