Composite Materials

Composite Materials

Particle-reinforced composites

Fiber-reinforced composites

Laminar composites

Mechanical properties

High performance composites

Other composites

Resources

Composite materials, or shortened to composites, are microscopic or macroscopic combinations of two or more distinct engineered materials (those with different physical and/or chemical properties) with a recognizable interface between them in the finished product. 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. Wood composites are commonly seen examples of composite materials.

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 very high tensile strengths, but because of theirsmall 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 (about10%) 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).

Particle-reinforced composites

A particle has no long dimension. Particle composites consist of particles of one material dispersed in a matrix of a second material. Particles may have any shape or size, but are generally spherical, ellipsoidal, polyhedral, or irregular in shape. They may be added to a liquid matrix that later solidifies; grown in place by a reaction such as age-hardening; or they may be pressed together and then interdiffused via a powder process. Theparticles may be treated to be made compatible with the matrix, or they may be incorporated without such treatment. Particles are most often used to extend the strength or other properties of inexpensive materials by the addition of other materials.

Fiber-reinforced composites

A fiber has one long dimension. 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). Other fibers include metal, ceramics, and polymers. The fibers can be used as continuous lengths, in staple-fiber form, or as whiskers(short, fine, perfect, or nearly perfect single crystals). Fiber-reinforcement depends as much on fabrication procedure as on materials.

Laminar composites

Platelets or lamina have two long dimensions. Laminar composites include plywood, which is a laminated composite of thin layers of wood in which successive layers have different grain or fiber orientations. The result is a more-or-less isotropic composite sheet that is weaker in any direction than it would be if the fibers were all aligned in one direction. The stainless steel in a cooking vessel with a copper-clad bottom provides corrosion resistance while the copper provides better heat distribution over the base of the vessel.

Mechanical properties

The mechanical properties of composite materials usually depend on structure. Thus these properties typically depend on the shape of inhomogenities, the volume fraction occupied by inhomogenities, and the interfaces between the components. The strength of composites depends on such factors as the brittleness or ductility of the inclusions and matrix.

For example, failure mechanisms in fiber-filled composites include fracture of the fibers; shear failure of the matrix along the fibers; fracture of the matrix in tension normal to the fibers or failure of the fiber-matrix interface. The mechanism responsible for failure depends on the angle between the fibers and the specimen’s axis.

If a mechanical property depends on the composite material’s orientation, the property is said to be anisotropic. Anisotropic composites provide greater strength and stiffness than do isotropic materials. However, the material properties in one direction are gained at the expense of the properties in other directions. For example, silica fibersin a pure aluminum matrix produce a composite with a tensile strength of about 110, 000 psi (pounds per square inch) along the fiber direction, but a tensile strength of only about 14, 000 psi at right angles to the fiber axis. It therefore only makes sense to use anisotropic materials if the direction that they will be stressed is known in advance.

Isotropic material are materials properties independent of orientation. Stiff plateletinclusions are the most effective in creating a stiff composite, followed by fibers, and then by spherical particles.

High performance composites

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, within 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. However, 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, rodlike 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. However, 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 forstructural aerospace applications. Epoxies aregenerally 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 conditionsthan 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.

Other composites

In addition to the examples already given, examples of composites materials also include: (1) Reinforced and pre-stressed concrete, which is a composite of steel and concrete. Concrete is itself a composite of rocks (coarse aggregate), sand (fine aggregate), hydrated Portland cement, and usually, voids. (2) Cutters for machining made of fine particles of tungsten carbide, which is extremely hard, are mixed with about 6% cobalt powder and sintered at high temperatures. (3) Ordinary grinding wheels, which are composites of an abrasive with a binder that may be plastic or metallic. (4) Walls for housing, which have been made of thin aluminum sheets epoxied to polyurethane foam. The foam provides excellent thermal insulation. This composite has a higher structural rigidity than aluminum sheets or polyurethane foam alone. The polyurethane foam is itself a composite of air and polyurethane. (5) Underground electrical cables composed of sodium metal enclosed in polyethylene. (6) Superconducting ribbons made of Nb₃ Sn deposited on copper. (7) Synthetic hard superconductors made by forcing liquid lead under pressure into porous glass fibers. (8) Microelectronic circuits made from silicon, which are oxidized to form an insulating layer of SiO₂ . This insulating layer is etched away with hydrofluoric acid, and phosphorous is diffused into the silicon to make a junction. Aluminum or another metal can be introduced as a microconductor between points. The microelectronic circuit is thus a tailored composite.(9) Ceramic fiber composites including graphite or pyrolytic carbon reinforced with graphite fibers; and

KEY TERMS

Fiber— In terms of composite fillers, a fiber is a filler with one long dimension. More specifically, a fiber is a complex morphological unit with an extremely high ratio of length to diameter (typically several hundred to one) and a relatively high tenacity.

Lamina (platelet)— In terms of composite fillers, a lamina is afiller with two long dimensions.

Matrix— The part of the composite that binds the filler by virtue of its cohesive and adhesive characteristics.

Particle— In terms of composite fillers, a particle is a fillerwith no long dimension. Particles may have any shape or size, but are generally spherical, ellipsoidal, polyhedral or irregular in shape.

borosilicate glass lithium aluminum silicate glass ceramics reinforced with silicon carbide fibers. It was possible to drive a tungsten carbide spike through such a composition without secondary cracking in much the same way that a nail can be driven through wood.

Resources

Books

Backman, Bjorn F. Composite Structures, Design, Safety, and Innovation. Amsterdam, Netherlands, and Boston, MA: Elsevier, 2005.

Delhaes, Pierre. Fibers and Composites. London, UK, and New York: Taylor & Francis, 2003.

Nethercot, David A. Composite Construction. London, UK, and New York: Taylor & Francis, 2003.

Sperling, Leslie H. Introduction to Physical Polymer Science. Hoboken, NJ: Wiley-Interscience, 2006.

Randall Frost