Composites

Reinforcements allow designers to tailor materials that will meet specific requirements.

The concept of composites is not really new. Wood is considered to be a composite of cellulose fibers bonded by a matrix of natural polymers, mainly lignin. Egyptians reinforced mud with straw to make bricks. Concrete can be classified as a ceramic composite in which stones are dispersed among cement. And in the 1940s, short glass fibers impregnated with thermosetting resins, known as fiberglass, became the first composite with a plastic matrix.

Synergism enables reinforcing materials to improve properties of the matrix in composite materials. Designers now have the ability to tailor materials and their reinforcements to meet specific requirements. Composites also offer the designer a chance to build in only as much strength as they need where they need it.

All of these developments mean a larger and more complicated materials-choice menu. This diversity has made plastics applicable to a broad range of consumer, industrial, automotive, and aerospace products. It has also made the job of selecting the best materials from such a huge array of candidates quite challenging.

Chapters on Composites:

Thermoplastic Composites Thermoset Composites Laminated Plastics Metal-matrix Composites Ceramic-matrix Composites Fibers

Glass, mineral fibers: Glass fibers used in reinforced compounds are high-strength, textile-type fibers, coated with a binder and coupling agent to improve compatibility with the resin and a lubricant to minimize abrasion between filaments. Glass-reinforced thermoplastics are usually supplied as ready-to-mold compounds. Molded products may contain as little as 5% and as much as 60% glass by weight. Pultruded shapes (usually using a polyester matrix) sometimes have higher glass contents. Most molding compounds, for best cost/performance ratios, contain 20 to 40% glass.

Practically all thermoplastic resins are available in glass-reinforced compounds. Those used in largest volumes are nylon, polypropylene, polystyrene, ABS, and SAN, probably because most experience with reinforced thermoplastics has been based on these resins. The higher performance resins -- PES, PEI, PPS, PEEK, and PEK, for example -- are also available in glass-fiber-reinforced composites, and some with carbon or aramid fibers as well.

Glass-fiber reinforcement improves most mechanical properties of plastics by a factor of two or more. Tensile strength of nylon, for example, can be increased from about 10,000 psi to over 30,000 psi, and deflection temperature to almost 500°F, from 170°F. A 40% glass-fortified acetal has a flexural modulus of 1.8 × 10 (to the 6th power) psi (up from about 0.4 × 10 (to the 6th power), a tensile strength of 21,500 psi (up from 8,800), and a deflection temperature of 335°F (up from 230 °F). Reinforced polyester has double the tensile and impact strength and four times the flexural modulus of the unreinforced resin.

Also improved in reinforced compounds are tensile modulus, dimensional stability, hydrolytic stability, and fatigue endurance. Deformation under load of these stiffer materials is reduced significantly; deformation tests must be conducted at 4,000 instead of 2,000 psi stress (used for unreinforced materials) to produce usable results.

Fiber reinforcement of a resin always changes its impact behavior and notch sensitivity. The change may be in either direction, depending on the specific resin involved. But even when the change is an improvement, these properties may not be high enough for certain demanding applications. This need has led to the development of impact-modified compounds -- specifically, nylon 6 and 6/6 alloys, a nylon 6/6 copolymer and a polypropylene copolymer -- with up to 50% improvement over reinforced unmodified compounds. While the impact properties of a glass-reinforced compound are not always superior to those of the unreinforced compound, the reinforced modified compounds are always superior to the reinforced unmodified grades.

Molded glass-reinforced and mineral-reinforced plastics are used in a broad range of structural and mechanical parts. For example, glass-reinforced nylon, because of its strength and stiffness, is used in gears and automotive under-the-hood components, while mineral-reinforced nylon is used in housings and body parts because it is tougher and has low warpage characteristics. Polypropylene applications include automotive air-cleaner housings and dishwasher tubs and inner doors. Polycarbonate is used in housings for water meters and power tools. Polyester applications include motor components -- brush holders and fans -- high-voltage enclosures, TV tuner gears, electrical connectors, and automobile exterior panels. Camper tops, pallets, and hand luggage are typical applications of reinforced HD polyethylene.

The newest glass-reinforced compounds are the long-fiber materials. These compounds, available principally in a nylon 6/6 base resin, are fabricated by pultrusion. The injection-moldable pellets thus contain fully wetted fibers equal in length to the pellet -- typically 0.400 in. This compares to 0.030 to 0.060-in. fiber lengths in conventional, short-fiber products. In fiber loads of 50% by weight, mechanical properties are improved dramatically over those of the short-fiber compounds. Long-fiber-reinforced compounds are available in the U.S. from ICI Advanced Materials, Polymer Composites Inc., and Dexter Composites.

Continuous-fiber glass-reinforced polypropylene is available in sheet form, for stamping or hot-flow forming of large, thin-wall parts such as automotive front-end retainer panels, oilpans, fender liners, upper grille panels, and station-wagon load floors, and for lawn-mower shrouds, luggage, and housings and guards for farm equipment and snowmobiles. The product is marketed by Azdel Inc. The company also markets a glass-reinforced PET polyester sheet product and plans to add sheet materials based on other resin matrices.

Another new composite form is a sheet material in various thermoplastic matrix resins developed by Du Pont. Reinforcement is unidirectional but discontinuous glass, carbon, or aramid fibers.

Carbon fibers: Carbon-fiber-reinforced compounds are available in a number of thermoplastics, including nylon 6/6, polysulfone, polyester, polyphenylene sulfide, polyetherimide, polyetheretherketone, and ETFE and PFA fluorocarbons.

The carbon-fiber-reinforced materials, at two to four times the cost of comparable glass-reinforced thermoplastics, offer the ultimate in tensile strength (to 35,000 psi), stiffness, and other mechanical properties. Compared to the glass-reinforced materials these compounds (10 to 40% carbon) have a lower coefficient of expansion and mold shrinkage, and improved resistance to creep and wear. Strength-to-weight ratios are also higher.

Carbon-fiber reinforcement also makes plastic compounds partially conductive. In compounds containing small amounts of carbon, this characteristic is useful for applications where static charges cannot be tolerated. Compounds containing higher percentages of carbon can be used for applications such as business-machine housings to shield the equipment from electromagnetic interference (EMI). Attenuation of electromagnetic radiation in carbon-fiber-reinforced nylon, for example, has been reported to be 36 to 40 dB in the frequency range from 50 kHz to 1 GHz.

Commercially available structural-carbon fibers are derived either from polyacrylonitrile (PAN) fibers or a special petroleum pitch. PAN-derived fibers have been available for several years and, for several of the lower modulus varieties, large databases have been developed through their use in aerospace programs. These fibers are generally selected for their high strength and efficient property translation into the composite.

The pitch-based fibers are newer and, while they are not as strong as the low-modulus PAN fibers, the ease with which they are processed into high-modulus components makes them attractive for stiffness-critical and thermally sensitive applications.

Pultrusion technology first provided long-glass fiber-reinforced composites with high-performance capabilities. Following close on the heels of this development are new long-carbon fiber-reinforced composites with even higher properties. Specifically, flexural and tensile moduli are the highest ever measured in discontinuous fiber-reinforced thermoplastic composites.

High stiffness-to-weight ratios and greater wear resistance allow these composites to compete against many metals. In addition, long-carbon composites are excellent candidates for applications requiring electrostatic dissipation and EMI shielding. Tests on some of the new composites have produced surprisingly high (60 dB and above) electromagnetic radiation attenuation values.

Aramid fibers: Aramid fibers, with greater specific strengths than steel or aluminum, should be an ideal reinforcement for thermoplastic resins. However, chopped aramid fibers do not adhere as well as the conventional glass or carbon-fiber reinforcements. Proprietary sizing systems aid in wetting of the fiber, but extensive fiber damage results in properties for the composite that are less than spectacular.

On the plus side, aramid-fiber-reinforced composites have low warpage, excellent wear and abrasion resistance, low coefficient of friction, and low thermal expansion. In addition, the mechanical properties of the composites are relatively uniform in all directions.

Aramid-fiber-reinforced composites have been applied in applications such as chain snubbers and guides, where low wear and mating-surface wear are important. They have also been evaluated successfully in gears, bearings, compressor vanes, and pump impellers.

In the past, two drawbacks have made aramid-reinforced thermoplastic composites impractical. First was the difficulty of getting the chopped fiber dispersed in the resin because the high surface energy of the lightweight fibers inhibited wetting at the fiber/resin interface. This and the unusually low bulk density of commercially available aramid fiber did not allow processing in conventional compounding equipment. A second problem was the fiber damage that occurred, both during mixing and injection molding. Damage is usually severe because of the low compressive strength of aramid fiber.

The first of these problems has now been minimized, and fibers can be incorporated uniformly into a resin matrix. Consequences of the fiber damage show up, of course, as reduced properties which are similar to, or lower than, those of their glass-reinforced counterparts.

There is a silver lining, however, to the dark cloud of disappointing mechanical properties. The aramid-reinforced composites have been found to have low warpage, good wear and abrasion resistance, low thermal expansion, and -- most important of all -- uniform mechanical and thermal properties in all directions. Conventional fiber-reinforced composites have lower shrinkage and higher strength and modulus in the flow direction. Not so with the aramid-reinforced materials. Properties, including mold shrinkage and thermal expansion, are nearly isotropic, regardless of flow pattern.

Internal lubrication: The first thermoplastics that were recognized for their inherent lubricity were nylon, acetal, and polytetrafluoroethylene (PTFE). These materials perform well, but for the more critical uses, their coefficients of friction may be too high, or wear may be too rapid.

The next generation of self-lubricated thermoplastics was formulated of various base resins that contained molybdenum disulfide, graphite, or PTFE particles to improve both lubrication and wear characteristics. Although wear resistance was indeed upgraded considerably, mechanical strength and dimensional stability of these compounds are often insufficient.

To minimize these deficiencies, reinforcing fibers of glass or carbon are added. The resulting composites are several times stronger than the unreinforced materials, and they are extremely stable in a wide range of service environments. But these materials too have a shortcoming: In service, a period of time is required for the internal lubricant to become exposed and to be burnished over the wear surfaces. During this run-in period, as a bearing or wear member is put into service, the unlubricated surfaces are in contact, and damage may occur.

Two approaches to eliminating these problems use silicone fluids to provide the lubrication function. Both of these, Migralube and Rimplast, are proprietary formulations. Thermoplastic composites can also be internally lubricated with a variety of systems to improve wear resistance. PTFE and silicone, separately or in combination, provide the best improvements in wear characteristics. Graphite powder and molybdenum disulfide are also used, primarily in nylons. The PTFE lubricants are specially modified to enhance their lubricious nature in the compound. The optimum level of lubricating filler varies depending on filler type and resin, but typical ranges are:
PTFE 15-20%
Silicone 1-5%
PTFE/silicone 15-20%
Graphite 10%
MoS2 2-5%

Addition of these lubricants further improves wear characteristics of good bearing materials such as nylon and acetal. The lubricants also allow the use of poor wearing, but close-tolerance materials, such as polycarbonate, in gear or bearing applications. Lubricants can be used by themselves or in conjunction with glass or carbon-fiber reinforcements.

Polytetrafluoroethylene and silicone fluids; glass, aramid, and carbon fibers; and graphite powder are the primary reinforcements and lubricants used in internally lubricated composites. The composites are based on engineering resins for injection-molded wear and structural parts.

Polytetrafluoroethylene (PTFE) lubricants dispersed into a thermoplastic base resin greatly improve surface-wear characteristics. Molecular weight and particle size of the PTFE lubricant are designed to provide optimum improvements in wear, friction, and PV values for selected resin systems. PTFE has the lowest coefficient of friction (0.02) of any known internal lubricant. Its static coefficient of friction is lower than its dynamic coefficient, which accounts for the slip/stick properties associated with PTFE/metal sliding action. During the initial break-in period, the PTFE particles embedded in the thermoplastic matrix shear to form a high-lubricity film over the mating surface. The PTFE cushions asperities from shock and minimizes fatigue failure.

Silicone fluids are chosen for their ability to perform as boundary lubricants and for partial compatibility with a particular base resin. The silicone is sufficiently compatible with the base resin to form an alloy, yet incompatible enough to cause migration to the surface of the compound. The silicone moves to the surface of a molded or extruded part by two mechanisms: diffusion by random molecular movement, and exclusion from the matrix (migration) because of its limited compatibility. The result of the migratory action is a continuous generation of a silicone film, which serves as a boundary, or mixed-film lubricant.

Glass fibers improve both short-term and long-term mechanical properties of a resin. The fibers also improve creep resistance, thermal conductivity, and heat-deflection temperature as well as the tribological properties of the base resin. The degree of improvement depends on the efficiency of the sizing system that bonds the resin to the fibers. Glass beads and unsized milled-glass fibers, on the other hand, increase the wear factor of the mating surface and the coefficient of friction.

Glass fibers are frequently used in combination with silicone and PTFE lubricants which offset the negative wear effects that the glass fibers have on surface characteristics. The use of silicone only, in conjunction with glass fibers, is not recommended, however. PTFE provides far more protection to the mating surface and should be used (with or without silicone) if the wear rate of the mating surface is important.

Carbon fibers added to thermoplastic resins provide the highest strength, modulus, heat-deflection temperature, creep, and fatigue-endurance values commercially available in composites. These property improvements, coupled with greatly increased thermal conductivity and low friction coefficients, make carbon fibers ideal for wear and frictional applications where the higher cost can be tolerated. In applications where the abrasive nature of glass fibers wears the mating surface, the softer carbon fibers can be substituted to reduce the wear rate. Carbon fibers can also be used in conjunction with internal lubricants to further improve surface characteristics of most thermoplastic resin system.

Another useful property of carbon-fiber-reinforced thermoplastics is their low volume and surface resistivities. Most resin systems reinforced with 15% or more carbon fibers can effectively dissipate static charge, which is a problem common to gears, slides, and bearings used in business-machine, textile, electrical, and conveying equipment.

Aramid fibers are stronger on a weight basis than steel or aluminum, but they are not as easy to work with as are glass and carbon. The high surface energy of the lightweight fibers makes wetting of the fibers and dispersion in resins difficult. Also, the fibers, having low compressive strength, are easily damaged in mixing and molding operations, which reduces composite properties. The first problem has been minimized somewhat with proprietary sizing/coupling systems, but fiber damage is still being worked on. Consequently, strength improvement in aramid-reinforced composites is not a reason to use these materials. But there are advantages offered by aramid/resin composites: Warpage and thermal expansion are low, abrasion resistance is high, and mechanical and thermal properties in molded parts are nearly uniform in all directions.

Graphite powders are low-friction, high-temperature solids traditionally used to lubricate moving metal parts where boundary lubrication is required. Their ability to lubricate depends on their structure, purity, and particle size. Properly selected graphite powders can be extrusion compounded with a variety of thermoplastics to provide coefficients of friction and wear factors between those of the base resin and the PTFE/silicone-lubricated versions. An important use for graphite-lubricated thermoplastics is in components that operate in aqueous environments.

Reinforced and/or internally lubricated compounds are used in a variety of applications. Polycarbonate-based composites are used for gear and bearing surfaces where accuracy is important and chemical resistance is not a severe problem, such as in cameras and office equipment. Acetal and PPS-based compounds have found wide acceptance in hostile environments such as gasoline metering and pumping devices. Nylon composites are often chosen for certain harsh environments because of their excellent chemical resistance.

Thermoset matrix systems dominate the composites industry because of their reactive nature and ease of impregnation. They begin in a monomeric or oligomeric state, characterized by very low viscosity. This allows ready impregnation of fibers, complex shapes, and a means of achieving cross-linked networks in the cured part. The early high-performance thermoset-matrix materials were called advanced composites, differentiating them from the glass/polyester composites that were emerging commercially in the 1950s. The "advanced" term has come to denote, to most engineers, a resin-matrix material reinforced with high-strength, high-modulus fibers of glass, carbon, aramid, or even boron, and usually laid up in layers to form an engineered component. More specifically, the term has come to apply principally to epoxy-resin-matrix materials reinforced with oriented, continuous fibers of carbon or of a combination of carbon and glass fibers, laid up in multilayer fashion to form extremely rigid, strong structures.

Resin systems: More than 95% of thermoset composite parts are based on polyester and epoxy resins; of the two, polyester systems predominate in volume by far. Other thermoset resins used in reinforced form are phenolics, silicones, and polyimides.

Polyesters can be molded by any process used for thermosetting resins. They can be cured at room temperature and atmospheric pressure, or at temperatures to 350°F and under higher pressure. These resins offer a balance of low cost and ease of handling, along with good mechanical, electrical, and chemical properties, and dimensional stability.

Polyesters can be compounded to be flexible and resilient, or hard and brittle, and to resist chemicals and weather. Halogenated (chlorinated or brominated) compounds are available for increased fire retardance. Low-profile (smoother surface) polyester compounds are made by adding thermoplastic resins to the compound.

Polyesters are also available in ready-to-mold resin/reinforcement forms -- bulk-molding compound (BMC), and sheet-molding compound (SMC). Bulk-molding compound is a premixed material containing resin, filler, glass fibers, and various additives. It is supplied in a doughlike, bulk form and as extruded rope.

Sheet-molding compound consists of resin, glass-fiber reinforcement, filler, and additives, processed in a continuous sheet form. Three types of SMC compounds are designated by Owens-Corning Fiberglas Corp., as random (SMC-R), directional (SMC-D), and continuous fiber (SMC-C). SMC-R, the oldest and most versatile form, incorporates short-glass fibers (usually about 1 in. long) in a random fashion. Complex parts with bosses and ribs are easily molded from SMC-R because it flows readily in a mold. SMC-C contains continuous glass fibers oriented in one direction, and SMC-D, long fibers (8 to 12 in. long), also oriented in one direction.

Moldings using SMC-C and SMC-D have significantly higher unidirectional strength but are limited to relatively simple shapes because the long-glass fiber cannot stretch to conform to a shape. These two types of SMC are usually, but not always, used in combination with SMC-R. Various combinations are available that contain a total of as much as 65% glass by weight. These materials are used for structural, load-bearing components.

High-glass-content sheet-molding compounds are also produced by PPG Industries, designated as XMC. These compounds contain up to 80% glass (or glass/carbon mixtures) as continuous fibers in an X pattern.

Epoxies are low-molecular-weight, syruplike liquids that are cured with hardeners to crosslinked thermoset structures that are hard and tough. Because the hardeners or curing agents become part of the finished structure, they are chosen to provide desired properties in the molded part. (This is in contrast to polyester formulations wherein the function of the catalyst is primarily to initiate cure.) Epoxies can also be formulated for room-temperature curing, but heat-curing produces higher properties.

Epoxies have outstanding adhesive properties and are widely used in laminated structures. The cured resins have better resistance than polyesters to solvents and alkalies, but less resistance to acids. Electrical properties, thermal stability (to 550°F in some formulations), and wear resistance are excellent.

Phenolics, the oldest of the thermoset plastics, have excellent insulating properties and resistance to moisture. Chemical resistance is good, except to strong acids and alkalies.

Reinforced phenolics are processed principally by high-pressure methods -- compression molding and continuous laminating -- because volatiles are condensed during the molding process. Recently developed injection-moldable grades, however, have made the processing of phenolics competitive with thermoplastic molding in some applications.

Silicones have outstanding thermal stability, even in the range of 500 to 700°F. Water absorption is low, and dielectric properties are excellent. Chemical resistance (except to strong alkalies) is very good.

Reinforcements: Glass is the most widely used reinforcing material in thermoset composites. Glass fiber, with a tensile strength of 500,000 psi (virgin fiber at 70°F), accounts for almost 90% of the reinforcement in thermosetting resins. Other reinforcements used are carbon, graphite, boron, and aramid (Kevlar) for high-performance requirements; glass spheres and flakes; and fibers of cotton, jute, and synthetic materials.

Glass fibers are available in several forms: roving (continuous strand), chopped strand, woven fabrics, continuous-strand mat, chopped-strand mat, and milled fibers (hammermilled through screens with openings ranging from 1/32 to ¼ in.). The longer fibers provide the greater strength; continuous fibers are the strongest.

All forms of glass fibers are produced in the standard E-glass reinforcement type. Some of the higher strength forms are also available in S glass, which has a tensile strength about one-third higher than that of E-glass. Cost of S-glass is considerably higher. S-2 Glass, a product of Owens-Corning, is a variant of S-glass, having the same batch composition but without the rigid, military quality-control specifications. Properties are similar to those of S-glass; cost is between that of E and S-glass.

Carbon fibers in composites can be long and continuous, or short and fragmented, and they can be directionally or randomly oriented. In general, short fibers cost the least, and fabrication costs are lowest; but, as with glass, properties of resulting composites are lower than those obtainable with longer or continuous fibers.

Milled fibers are the shortest carbon fibers used for reinforcement. They range in length from 30 to 3,000 microns, averaging approximately 300 microns. Mean L/D ratio is 30. Short chopped fibers (about ¼ in. long), with an L/D ratio of about 800, increase strength and modulus of a composite more than milled fibers do. Cost of a molding compound reinforced with ¼ -in. fibers is about twice that of one containing milled carbon fibers.

Long chopped fibers (to 2 in.) are often added to a thermosetting glass/polyester sheet-molding compound to increase the stiffness of compression-molded parts. Continuous carbon fibers provide the ultimate in performance and/or weight reduction. Continuous fibers are available in a number of forms including yarns or tows containing 400 to 160,000 individual filaments; unidirectional, impregnated tapes up to 60 in. wide; multiple layers of tape with individual layers, or plies, at selected fiber orientation; and fabrics of many weights and weaves.

The outstanding design properties of carbon fiber/resin matrix composites are their high strength-to-weight and stiffness-to-weight ratios. With proper selection and placement of fibers, the composites can be stronger and stiffer than equivalent thickness steel parts and weigh 40 to 70% less. Fatigue resistance of continuous-fiber composites is excellent, and chemical resistance is better than that of glass-reinforced systems, particularly in alkaline environments. Like most rigid materials, however, carbon-fiber composites are relatively brittle. The composites have no yield behavior, and resistance to impact is low.

Thermal characteristics of carbon fibers are different from those of almost all other materials. Linear expansion coefficients range from slightly negative for 30 million-psi modulus fibers to approximately -1.3 × 10 (to the 6th power) in./in.- °F for the ultrahigh-modulus fibers. This property makes possible the design of structures with zero or very low linear and planar thermal expansion -- a valuable characteristic for components in precision instruments such as telescopes and for the alignment requirements of aerospace antennas and similar critical parts. Transverse coefficients of expansion are quite different -- typically 15 × 10 (to the 6th power) in./in.- °F.

The thermal conductivity of ultrahigh-modulus pitch-based carbon fibers exceeds that of copper. When density differences are considered, the specific conductivities can be as much as eight times that of copper.

Applications: Glass-reinforced polyester composites are used for automobile body panels, seats and panels for transit cars, boat hulls, bathroom shower-tub structures, chairs, architectural panels, agricultural seed and fertilizer hoppers, tanks, and housings for a variety of consumer and industrial products. Glass/epoxy applications include filament-wound pipe and tanks, and circuit boards.

Phenolic-matrix composites are used in printed-circuit boards, gears, and in insulators and other components for electrical equipment and appliances. Melamine applications include circuit boards, thermal and electrical insulation, and components requiring good chemical resistance. Silicones are chosen for service as electrical and thermal insulation, such as in circuit boards requiring maximum heat resistance.

The aircraft industry was quick to see the advantages of carbon/epoxy composites, which offered light weight, high strength and modulus, and -- most important -- excellent fatigue performance. Most engineering and manufacturing experience with these composites has been gained in developing military-aircraft components. While the manufacture of composite components is still highly labor intensive and expensive, the technology has advanced in recent years to improve production quality and speed with the use of systems such as automated tape laying.

Nevertheless, the high cost of carbon fibers and of processing the composites has limited most uses of carbon/epoxy composites to aerospace components -- fuselage panels for military aircraft and cargo doors for the space shuttle, for example -- and high-priced sports equipment -- tennis-racquet frames, golf-club shafts, skis, and archery bows.

Laminated plastics are a special form of polymer-matrix composite consisting of layers of reinforcing materials that have been impregnated with thermosetting resins, bonded together, and cured under heat and pressure. The cured laminates, called high-pressure laminates, are produced in more than 70 standard grades, based on National Electrical Manufacturers Association (NEMA) specifications.

Laminated plastics are available in sheet, tube, and rod shapes that are cut and/or machined for various end uses. The same base materials are also used in molded-laminated and molded-macerated parts. The molded-laminated method is used to produce shapes that would be uneconomical to machine from flat laminates, where production quantities are sufficient to warrant mold costs.

Strength of a molded shape is higher than that of a machined shape because the reinforcing plies are not cut, as they are in a machined part. The molded-macerated method is used for similar parts that require uniform strength properties in all directions. Other common forms of laminated plastics are composite sheet laminates that incorporate a third material bonded to one or both surfaces of the laminate. Metals most often used in composites are copper, aluminum, nickel, and steel. Copper-clad sheets (one or both sides) for printed-circuit and multilayer boards comprise the largest volume of metal composite sheet laminates. Nonmetallics include elastomers, vulcanized fiber, and cork. Composite metal/plastic materials are also produced in rods and tubes.

Vulcanized fiber is another product often classified with the laminated plastics because end uses are similar. Vulcanized fiber is made from regenerated cotton cellulose and paper, processed to form a dense material (usually in sheet form) that retains the fibrous structure. The material is tough and has good resistance to abrasion, flame, and impact.

Resins: Phenolics are the most widely used resin in laminated plastics. These low-cost resins have good mechanical and electrical properties and resistance to heat, flame, moisture, mild acids, and alkalies. Most paper and cloth-reinforced laminates are made with phenolics.

Polyesters are used for both electrical and mechanical service requiring moderate heat resistance. The resins are usually mineral filled to improve dimensional stability and flame retardancy and to reduce cost.

Malemine resins are used primarily in electrical-grade laminates because of their excellent resistance to arcing and tracking, high mechanical strength, and good resistance to alkalies.

Epoxies are recommended for applications requiring resistance to chemicals and humid environments. They have low moisture absorption and good dimensional stability, mechanical strength, bond strength, and fungus resistance.

Silicones, used primarily with glass-cloth reinforcement, have very high heat resistance (to 550 °F). Laminates based on silicones have low moisture absorption, and they maintain their electrical properties over a wide range of service conditions.

Polyimide binders extend the use-temperature range of glass laminates upward. These resins can also withstand lengthy solder-bath exposure without blistering.

Reinforcements: Papers are the lowest-cost reinforcing materials used in making laminates. Types include kraft, alpha, cotton linter, and combinations of these. Papers provide excellent electrical properties, good dimensional stability, moderate strength, and uniform appearance.

Cotton cloth is used for applications requiring good mechanical strength. The lighter-weight fabrics are not as strong but have excellent machinability.

Asbestos, in the form of paper, mat, or woven fabric, provides excellent resistance to heat, flame, chemicals, and wear.

Glass-fiber reinforcements, in woven fabric or mat, form the strongest laminates. These laminates also have low moisture absorption and excellent heat resistance and electrical properties.

Nylon fabrics provide excellent electrical and mechanical properties and chemical resistance, but laminates reinforced with these materials lack dimensional stability at elevated temperatures.

Metal-matrix composites are either in use or prototyping for the Space Shuttle, commercial airliners, electronic substrates, bicycles, automobiles, golf clubs, and a variety of other applications. While the vast majority are aluminum matrix composites, a growing number of applications require the matrix properties of superalloys, titanium, copper, magnesium, or iron.

Like all composites, aluminum-matrix composites are not a single material but a family of materials whose stiffness, strength, density, and thermal and electrical properties can be tailored. The matrix alloy, the reinforcement material, the volume and shape of the reinforcement, the location of the reinforcement, and the fabrication method can all be varied to achieve required properties. Regardless of the variations, however, aluminum composites offer the advantage of low cost over most other MMCs. In addition, they offer excellent thermal conductivity, high shear strength, excellent abrasion resistance, high-temperature operation, nonflammability, minimal attack by fuels and solvents, and the ability to be formed and treated on conventional equipment.

Aluminum MMCs are produced by casting, powder metallurgy, in situ development of reinforcements, and foil-and-fiber pressing techniques. Consistently high-quality products are now available in large quantities, with major producers scaling up production and reducing prices. They are applied in brake rotors, pistons, and other automotive components, as well as golf clubs, bicycles, machinery components, electronic substrates, extruded angles and channels, and a wide variety of other structural and electronic applications.

Superalloy composites reinforced with tungsten alloy fibers are being developed for components in jet turbine engines that operate temperatures above 1,830 °F.

Graphite/copper composites have tailorable properties, are useful to high temperatures in air, and provide excellent mechanical characteristics, as well as high electrical and thermal conductivity. They offer easier processing as compared with titanium, and lower density compared with steel. Ductile superconductors have been fabricated with a matrix of copper and superconducting filaments of niobium-titanium. Copper reinforced with tungsten particles or aluminum oxide particles is used in heat sinks and electronic packaging.

Titanium reinforced with silicon carbide fibers is under development as skin material for the National Aerospace Plane. Stainless steels, tool steels, and Inconel are among the matrix materials reinforced with titanium carbide particles and fabricated into draw-rings and other high-temperature, corrosion-resistant components.

Compared to monolithic metals, MMCs have:

Higher strength-to-density ratios Higher stiffness-to-density ratios Better fatigue resistance Better elevated temperature properties -- Higher strength -- Lower creep rate Lower coefficients of thermal expansion Better wear resistance The advantages of MMCs over polymer matrix composites are: Higher temperature capability Fire resistance Higher transverse stiffness and strength No moisture absorption Higher electrical and thermal conductivities Better radiation resistance No outgassing Fabricability of whisker and particulate-reinforced MMCs with conventional metalworking equipment. Some of the disadvantages of MMCs compared to monolithic metals and polymer matrix composites are: Higher cost of some material systems Relatively immature technology Complex fabrication methods for fiber-reinforced systems (except for casting) Limited service experience

Reinforcements: MMC reinforcements can be divided into five major categories: continuous fibers, discontinuous fibers, whiskers, particulates, and wires. With the exception of wires, which are metals, reinforcements generally are ceramics.

Key continuous fibers include boron, graphite (carbon), alumina, and silicon carbide. Boron fibers are made by chemical vapor deposition (CVD) of this material on a tungsten core. Carbon cores have also been used. These relatively thick monofilaments are available in 4.0, 5.6, and 8.0-mil diameters. To retard reactions that can take place between boron and metals at high temperature, fiber coatings of materials such as silicon carbide or boron carbide are sometimes used.

Silicon carbide monofilaments are also made by a CVD process, using a tungsten or carbon core. A Japanese multifilament yarn, designated as silicon carbide by its manufacturer, is also commercially available. This material, however, made by pyrolysis of organometallic precursor fibers, is far from pure silicon carbide and its properties differ significantly from those of monofilament silicon carbide.

Continuous alumina fibers are available from several suppliers. Chemical compositions and properties of the various fibers are significantly different. Graphite fibers are made from two precursor materials, polyacrilonitrile (PAN) and petroleum pitch. Efforts to make graphite fibers from coal-based pitch are under way. Graphite fibers with a wide range of strengths and moduli are available.

The leading discontinuous fiber reinforcements at this time are alumina and alumina-silica. Both originally were developed as insulating materials. The major whisker material is silicon carbide. The leading U.S. commercial product is made by pyrolysis of rice hulls. Silicon carbide and boron carbide, the key particulate reinforcements, are obtained from the commercial abrasives industry. Silicon carbide particulates are also produced as a by-product of the process used to make whiskers of this material.

A number of metal wires including tungsten, beryllium, titanium, and molybdenum have been used to reinforce metal matrices. Currently, the most important wire reinforcements are tungsten wire in superalloys and superconducting materials incorporating niobium-titanium and niobium-tin in a copper matrix. The reinforcements cited above are the most important at this time. Many others have been tried over the last few decades, and still others undoubtedly will be developed in the future.

Matrix materials and key composites: Numerous metals have been used as matrices. The most important have been aluminum, titanium, magnesium, and copper alloys and superalloys. The most important MMC systems are:

Aluminum matrix Continuous fibers: boron, silicon carbide, alumina, graphite Discontinuous fibers: alumina, alumina-silica Whiskers: silicon carbide Particulates: silicon carbide, boron carbide Magnesium matrix Continuous fibers: graphite, alumina Whiskers: silicon carbide Particulates: silicon carbide, boron carbide Titanium matrix Continuous fibers: silicon carbide, coated boron Particulates: titanium carbide Copper matrix Continuous fibers: graphite, silicon carbide Wires: niobium-titanium, niobium-tin Particulates: silicon carbide, boron carbide, titanium carbide. Superalloy matrices Wires: tungsten

Characteristics and design considerations: The superior mechanical properties of MMCs drive their use. An important characteristic of MMCs, however, and one they share with other composites, is that by appropriate selection of matrix materials, reinforcements, and layer orientations, it is possible to tailor the properties of a component to meet the needs of a specific design.

For example, within broad limits, it is possible to specify strength and stiffness in one direction, coefficient of expansion in another, and so forth. This is rarely possible with monolithic materials.

Monolithic metals tend to be isotropic, that is, to have the same properties in all directions. Some processes such as rolling, however, can impart anisotropy, so that properties vary with direction. The stress-strain behavior of monolithic metals is typically elastic-plastic. Most structural metals have considerable ductility and fracture toughness.

The wide variety of MMCs have properties that differ dramatically. Factors influencing their characteristics include:

Reinforcement properties, form, and geometric arrangement Reinforcement volume fraction Matrix properties, including effects of porosity Reinforcement-matrix interface properties Residual stresses arising from the thermal and mechanical history of the composite Possible degradation of the reinforcement resulting from chemical reactions at high temperatures, and mechanical damage from processing, impact, etc.

Particulate-reinforced MMCs, like monolithic metals, tend to be isotropic. The presence of brittle reinforcements and perhaps of metal oxides, however, tends to reduce their ductility and fracture toughness. Continuing development may reduce some of these deficiencies.

The properties of materials reinforced with whiskers depend strongly on their orientation. Randomly oriented whiskers produce an isotropic material. Processes such as extrusion can orient whiskers, however, resulting in anisotropic properties. Whiskers also reduce ductility and fracture toughness.

MMCs reinforced with aligned fibers have anisotropic properties. They are stronger and stiffer in the direction of the fibers than perpendicular to them. The transverse strength and stiffness of unidirectional MMCs (materials having all fibers oriented parallel to one axis), however, are frequently great enough for use in components such as stiffeners and struts. This is one of the major advantages of MMCs over PMCs, which can rarely be used without transverse reinforcement.

Because the modulus and strength of metal matrices are significant with respect to those of most reinforcing fibers, their contribution to composite behavior is important. The stress-strain curves of MMCs often show significant nonlinearity resulting from yielding of the matrix.

Another factor that has a significant effect on the behavior of fiber-reinforced metals is the frequently large difference in coefficient of expansion between the two constituents. This can cause large residual stresses in composites when they are subjected to significant temperature changes. In fact, during cool down from processing temperatures, matrix thermal stresses are often severe enough to cause yielding. Large residual stresses can also be produced by mechanical loading.

Although fibrous MMCs may have stress-strain curves displaying some nonlinearity, they are essentially brittle materials, as are PMCs. In the absence of ductility to reduce stress concentrations, joint design becomes a critical design consideration. Numerous methods of joining MMCs have been developed, including metallurgical and polymeric bonding and mechanical fasteners.

Fabrication methods: Fabrication methods are an important part of the design process for all structural materials, including MMCs. Considerable work is under way in this critical area. Significant improvements in existing processes and development of new ones appear likely.

Current methods can be divided into two major categories, primary and secondary. Primary fabrication methods are used to create the MMC from its constituents. The resulting material may be in a form that is close to the desired final configuration, or it may require considerable additional processing, called secondary fabrication, such as forming, rolling, metallurgical bonding, and machining. The processes used depend on the type of reinforcement and matrix.

A critical consideration is reactions that can occur between reinforcements and matrices during primary and secondary processing at the high temperatures required to melt and form metals. These impose limitations on the kinds of constituents that can be combined by the various processes. Sometimes, barrier coatings can be successfully applied to reinforcements, allowing them to be combined with matrices that otherwise would be too reactive. For example, the application of a coating such as boron carbide permits the use of boron fibers to reinforce titanium. Potential reactions between matrices and reinforcements, even coated ones, is also an important criterion in evaluating the temperatures and corresponding lengths of time to which MMCs may be subjected in service.

Relatively large-diameter monofilament fibers, such as boron and silicon carbide, have been incorporated into metal matrices by hot pressing a layer of parallel fibers between foils to create a monolayer tape. In this operation, the metal flows around the fibers and diffusion bonding occurs. The same procedure can be used to produce diffusion-bonded laminates with layers of fibers oriented in specified directions to meet stiffness and strength requirements for a particular design. In some instances, laminates are produced by hot pressing monolayer tapes in what can be considered a secondary operation.

Monolayer tapes are also produced by spraying metal plasmas on collimated fibers, followed by hot pressing. Structural shapes can be fabricated by creep and superplastic forming of laminates in a die. An alternate process is to place fibers and unbonded foils in a die and hot press the assembly.

The boron/aluminum struts used on the space shuttle are fabricated from monolayer foils wrapped around a mandrel and hot isostatically pressed to diffusion bond the foil layers together and, at the same time, to diffusion bond the composite laminate to titanium end fittings.

Composites can be made by infiltrating liquid metal into a fabric or prearranged fibrous configuration called a preform. Frequently, ceramic or organic binder materials are used to hold the fibers in position. The latter is burned off before or during infiltration. Infiltration can be carried out under vacuum, pressure, or both. Pressure infiltration, which promotes wetting of the fibers by the matrix and reduces porosity, is often called squeeze casting.

Cast MMCs now consistently offer net or net-net shape, improved stiffness and strength, and compatibility with conventional manufacturing techniques. They are also consistently lower in cost than those produced by other methods, are available from a wide range of fabricators, and offer dimensional stability in both large and small parts.

For example, Duralcan has developed its "ice cream mixer" technology and process controls to the point where it produces up to 25 million pounds per year of aluminum composite billets. Investment casting has been modified at Cercast to cast Duralcan billets into complex, net-shape parts. Pressure casting produces net shapes with exceptional properties at Alcoa, while pressureless infiltration is used at Lanxide Corp. to fabricate net-shape components.

At the current time, the most common method used to make graphite/aluminum and graphite/magnesium composites is by infiltration. Graphite yarn is first passed through a furnace to burn off any sizing that may have been applied. Next it goes through a CVD process that applies a coating of titanium and boron which promotes wetting by the matrix. Then it immediately passes through a bath or fountain of molten metal, producing an infiltrated bundle of fibers known as a "wire." Plates and other structural shapes are produced in a secondary operation by placing the wires between foils and pressing them, as is done with monofilaments. Recent development of "air stable" coatings permits use of other infiltration processes, such as casting, eliminating the need for "wires" as an intermediate step. Other approaches are under development. A particularly important secondary fabrication method for titanium matrix composites is superplastic forming/diffusion bonding (SPF/DB). To reduce fabrication costs, continuous processes such as pultrusion and hot roll bonding are being developed.

Three basic methods are being used to make whisker and particulate-reinforced MMCs. Two use powdered metals; the other uses a liquid-metal approach, details of which are proprietary.

The two powder-metal processes differ primarily in the way the constituents are mixed. One uses a ball mill, the other employs a liquid to aid mixing, which is subsequently removed. Mixtures are then hot pressed into billets.

Secondary processes are similar to those for monolithic metals, including rolling, extrusion, spinning, forging, creep-forming, and machining. The latter poses some difficulties because the reinforcements are very hard.

The class of materials known as ceramic matrix composites, or CMCs, shows considerable promise for providing fracture-toughness values similar to those for metals such as cast iron. Two kinds of damage-tolerant ceramic-ceramic composites are being developed. One incorporates a continuous reinforcing phase, such as a fiber; the other, a discontinuous reinforcement, such as whiskers. The major difference between the two is in their failure behavior. Continuous-fiber-reinforced materials do not fail catastrophically. After matrix failure, the fiber can still support a load. A fibrous failure is similar to that which occurs in wood.

Incorporating whiskers into a ceramic matrix improves resistance to crack growth, making the composite less sensitive to flaws. These materials are commonly described as being flaw tolerant. However, once a crack begins to propagate, failure is catastrophic.

Of particular importance to the technology of toughened ceramics has been the development of high-temperature silicon carbide reinforcements. Although other reinforcement materials are available, such as glass and carbon fiber, metal whiskers, and alumina-based products, this discussion focuses on SiC-based products because they are more applicable to high-temperature use.

SiC fibers, which are capable of withstanding temperatures to about 1,200 °C, are manufactured from a polymer precursor. The polymer is spun into a fine thread, then pyrolized to form a 15- µ m ceramic fiber consisting of fine SiC crystallites and an amorphous phase. An advantage of the process is that it uses technology developed for commercial fiber products such as nylon and polyester. Two commercial SiC fiber products are Ube Industries' Tyranno fiber and Nippon Carbon's Nicalon fiber, both from Japan.

SiC filaments: are prepared by chemical vapor deposition. A thick layer of silicon carbide is deposited on a thin fiber substrate of tungsten or carbon. Diameter of the final product is about 140 µ m.

Although developed initially to reinforce aluminum and titanium matrices, SiC filaments have since been used as reinforcement in silicon nitride. The material is manufactured by Avco Specialty Materials/Textron in the U.S. and by Sigma Composite Materials in the Federal Republic of Germany.

SiC whiskers consist of a fine (0.5- µ m-diameter) single-crystal structure in lengths to 100 µ m. The material is strong (to 15.9 GPa) and is stable at temperatures to 1,800°C. Whiskers can be produced by heating SiO2 and carbon sources with a metal catalyst in the proper environments. These reinforcements are manufactured on a commercial scale in Japan by Tateho Chemical Industries and Tokai Carbon Co.

Although these materials are relatively new, at least one successful commercial product is already being marketed. An SiC-whisker-reinforced alumina cutting-tool material is being used to machine nickel-based superalloys. In addition, considerable interest has been generated in reinforcing other matrices such as mullite, silicon carbide, and silicon nitride for possible applications in automotive and aerospace industries.

Interface conditions: In addition to developments in reinforcement materials, advances in controlling the interfacial bond between matrix and reinforcement have led to further mechanical property improvements of ceramic-ceramic composites. The interfacial bond must be optimized to promote favorable toughening mechanisms such as crack deflection and crack bridging. Without proper interface control, a brittle polyphase material results, rather than a toughened composite.

Toughness improvements by interfacial modifications have been made in both fiber and whisker-reinforced systems. Interface control has resulted in the development of toughened fiber-reinforced glass-ceramic matrix materials at the United Technologies Research Center and of toughened fiber-reinforced zirconia-based materials at the Naval Research Laboratories. At present, interfacial control is more advanced for fiber-reinforced composites than for whisker-reinforced materials.

One current approach is to design the interface so it has a parting layer that promotes crack deflection parallel to the fiber length. The parting layer protects the fiber from damage by deflecting cracks, enabling the undamaged reinforcement to support load and bridge cracks during matrix failure. Thus, the composite does not fail catastrophically. Fracture morphology is comparable to that of the fibrous fracture of wood structures. Current materials being used for such interfaces are boron nitride and carbon, materials that have weak crystallographic orientations that preferentially delaminate.

Modifications to the interfacial zone of whisker-reinforced composites are in their developmental infancy because of the difficulty of applying thin coatings on fine whiskers. Studies at Oak Ridge National Laboratories have demonstrated that thermal treatments of whiskers prior to their incorporation into an alumina matrix can increase fracture toughness of the composite. In those materials, best toughness -- about 8.0 MPavm -- results with whisker surfaces modified to be carbon rich and oxygen poor.

Current mechanistic studies at the University of California/Santa Barbara are directed toward understanding the role of interfacial structure on toughness of ceramic-matrix composites. In addition, some investigators feel that the approaches used by the carbon-carbon community, such as applying various CVD coatings to seal off the fibers, may result in near-term solutions for improving toughness of fiber-reinforced ceramics.

In whisker-reinforced materials, the matrix usually seals off the interface region from the composite exterior. This protects the interface from oxidizing environments. However, once cracks are initiated, they allow access of atmospheric elements into the interior. As with fiber-reinforced materials, new interface compositions must be developed that are stable in oxidizing environments.

In addition, there is still a need to develop further understanding of the role of whisker interfaces on toughening mechanisms for ceramics. The requirements of fiber and whisker-reinforced systems appear to have many similarities.

Reinforcement needs: Although the current interest in ceramic-matrix composites has resulted from improved reinforcements, there is still a need for further developments. Specifically, reinforcements are needed for ceramic matrices for service at temperatures greater than 1,800°C.

Currently available polymer-derived fibers are limited because they deteriorate above 1,200°C. A program aimed at developing higher temperature fiber has been sponsored by the Dept. of Defense, combining the expertise of Dow Corning in silicon-based materials with that of Celanese in fiber technology. From this program has come a new fiber material that has higher thermal stability than commercially available fibers.

SiC filament material has limitations in oxidizing environments due to its carbon core and carbon surface coatings that oxidize above 600°C. These filaments are designed for use in aluminum and titanium matrices. A similar product, engineered for ceramic matrices, is needed.

SiC whiskers are a nearer-term reinforcement for commercial ceramic-matrix composites, having already demonstrated success in reinforcing alumina. As with the other reinforcing materials, the whiskers currently being produced are more appropriate for reinforcing metals. Current theory indicates that thicker whiskers (1 to 3 µ m) are more appropriate for ceramics. Such materials are now under development.

Dimox process: Ceramic matrix composites are steadily moving from the laboratory to initial commercial applications. For example, engineers are currently evaluating these materials for use in the hot gas zones of gas turbine engines, because ceramics are known for their strength and favorable creep behavior at high temperatures. Advanced ceramics, for example, can potentially be used at temperatures 400 to 900°F above the maximum operating temperature for superalloys.

Until recently, however, there has been more evaluation than implementation of advanced ceramics for various reasons. Monolithic or single-component ceramics, for example, lack the required damage tolerance and toughness. Engine designers are put off by ceramic material's potential for catastrophic, brittle failures. While many CMCs have greater toughness, they are also difficult to process by traditional methods, and may not have the needed long-term high-temperature resistance.

A relatively new method for producing CMCs developed by Lanxide Corp., Newark, Del., promises to overcome the limitations of other ceramic technologies. Called the Dimox directed metal oxidation process, it is based on the reaction of a molten metal with an oxidant, usually a gas, to form the ceramic matrix. Unlike the sintering process, in which ceramic powders and fillers are consolidated under heat, directed metal oxidation grows the ceramic matrix material around the reinforcements.

Examples of ceramic matrices that can be produced by the Dimox directed metal oxidation process include Al2O3 , Al2Ti)5, AlN, TiN, ZrN, TiC, and ZrC. Filler materials can be anything chemically compatible with the ceramic, parent metal, and growth atmosphere.

The first step in the process involves making a shaped preform of the filler material. Preforms consisting of particles are fabricated with traditional ceramic forming techniques, while fiber preforms are made by weaving, braiding, or laying up woven cloth. Next, the preform is put in contact with the parent metal alloy. A gas-permeable growth barrier is applied to the surfaces of this assembly to limit its shape and size.

The assembly, supported in a suitable refractory container, is then heated in a furnace. For aluminum systems, temperatures typically range from 1,650 to 2,100°F. The parent metal reacts with the surrounding gas atmosphere to grow the ceramic reaction product through and around the filler to form a CMC.

Capillary action within the growing ceramic matrix continues to supply molten alloy to the growth front. There, the reaction continues until the growing matrix reaches the barrier. At this point, growth stops, and the part is cooled to ambient temperature. To recover the part, the growth barrier and any residual parent metal are removed. However, some of the parent metal (5 to 15% by volume) remains within the final composite in micron-sized interconnected channels.

Traditional ceramic processes use sintering or hot pressing to make a solid CMC out of ceramic powders and filler. Part size and shapes are limited by equipment size and the shrinkage that occurs during densification of the powders can make sintering unfeasible. Larger parts pose the biggest shrinkage problem. Advantages of the directed metal oxidation process include no shrinkage since matrix formation occurs by a growth process. As a result, tolerance control and large part fabrication can be easier with directed metal oxidation.

In addition, the growth process forms a matrix whose grain boundaries are free of impurities or sintering aids. Traditional methods often incorporate these additives, which reduce high-temperature properties. And cost comparisons show the newer process is a promising replacement for traditional methods.

Fiber reinforcements dominate composites. The fiber industry is divided between natural fibers -- those from plant, animal, or mineral sources -- and synthetic fibers. Many synthetic fibers have been developed specifically to replace natural fibers because synthetics usually behave more predictably and are more uniform in size. Often, synthetic fibers are less costly than their natural counterparts. In the garment industry, for example, the acrylic and rayon fibers were developed to replace more costly natural wool and silk.

For engineering purposes, metal, ceramic, glass, and organically derived synthetic fibers are more significant. Nylon, for example, is used for belting, nets, hose, rope, parachutes, webbing, ballistic cloths, and as reinforcements in tires.

Metal and ceramic fibers are used in high-strength, high-temperature, lightweight composite materials for aerospace applications. Fibers improve the strength-to-weight ratio of base materials such as titanium and aluminum. Anisotropic properties can be designed into a part made from a fiber composite by selectively aligning the fiber/base layup.

Fibers of stainless steel or aluminum provide conductivity in plastic components for static-electricity dissipation or, with higher loadings, shielding from electromagnetic interference (EMI). Shielding is particularly important for housings of computers, copiers, and other business machines. Because only small levels of stainless-steel fibers are needed, base-resin properties remain relatively unchanged, and the composite has good colorability.

Among the strongest materials are metal fibers formed by controlled solidification and cold drawing. Some nonmetallic fibers such as aluminum oxide and silicon carbide are nearly as strong as metal fibers but have a higher modulus of elasticity. Fibers in a metal matrix combine the strength of the fiber with ductility or other characteristics of the matrix. Many combinations of properties are possible -- for example, tungsten fibers in a copper matrix add the strength of the fiber to the conductivity of the matrix. Aluminum oxide and silicon carbide are among several fibers added to aluminum to produce high strength-to-weight ratio composites.

One application using an alumina/silica ceramic-fiber reinforcement in an aluminum matrix is that of diesel pistons. The reinforcement, placed either in the combustion-bowl or ring-groove area, is a preform of Fiberfrax (from Carborundum Co.) ceramic fiber. The pistons are produced by the squeeze-cast process.

Glass fibers, the most widely used reinforcement for plastic and rubber products, are also the finest (smallest diameter) of all fibers, typically 1 to 4 microns in diameter. Because glass fibers have a large surface area in proportion to volume (a ¾ -in.-diameter bead of glass is stretched into over 97 miles of fiber) surface conditions of the fiber have a strong influence on its strength and behavior.

Most glass-reinforced products are made with E-glass (electrical glass), which has good electrical and mechanical properties and high heat resistance. E-glass is available as chopped fiber, milled fiber, continuous roving, woven roving, woven fabric, and reinforcing mat. Tensile strength is 500,000 psi modulus is 10.5 million, and elongation can be as high as 4.8%.

Applications are in many industries, ranging from tub/shower units and boat hulls to tanks, ducts, and automotive exterior panels. Fabrication of components, using both thermoplastic and thermoset matrix resins is done by all conventional molding processes.

For higher performance than provided by E-glass, S-glass offers 30% higher tensile strength and 18% higher modulus. S-glass is used in such applications as aircraft flooring, helicopter blades, and filament-wound pressure containers.

Carbon fibers offer the widest range of stiffness of any material -- from about 5 million to as high as 100 million psi. Most commonly used of these materials, however, are those fibers in the midrange, having moduli in the 30 to 40 million-psi range because they have the most useful balance of properties.

Used alone or as a part of a hybrid reinforcement with glass or aramid, carbon adds considerable strength and stiffness to engineering resins. In chopped form, molding procedures of carbon-reinforced composites is essentially the same as those used for glass-reinforced compounds. In tape form, often with an epoxy resin, the fibers are usually laid up as laminates, with the continuous fibers at various angles to one another.

In between chopped and continuous fibers are the recently introduced long-fiber-reinforced composites, which are available with either carbon or glass-fiber reinforcement. In these compounds (ICI Advanced Materials and Polymer Composites Inc.) the carbon fibers averaging about 0.5 in. long (same length as the pellets) provide strength values between those of the chopped and continuous-fiber-reinforced composites.

Because of their light weight and high strength and stiffness, carbon-reinforced composites are used in aircraft components. Their high-temperature properties qualify them for applications such as pump packings, bearings, and brake components. Sports equipment of "graphite" materials include skis, racquets, golf club shafts, and lightweight bicycle parts.

Aramid fibers (aromatic polyamides) are characterized by excellent environmental and thermal stability, static and dynamic fatigue resistance, and impact resistance. These fibers have the highest specific tensile strength (strength/density ratio) of any commercially available continuous-filament yarn. Aramid-reinforced thermoplastic composites have excellent wear resistance and near-isotropic properties -- characteristics not available with glass or carbon-reinforced composites.

Aramid fiber, tradenamed Kevlar (Du Pont), is available in several grades and property levels for specific applications. The grade designated simply as Kevlar is made specifically to reinforce tires, hoses and belting, such as V-belts and conveyor belts.

Kevlar 29 is similar to the basic Kevlar in properties but is designated specifically for use in ropes and cables, protective apparel, and as the substrate for coated fabrics. In short fiber or pulp form, Kevlar 29 can substitute for asbestos in friction products or gaskets. Fabrics of Kevlar 29 can be made into bullet-resistant vests. Clothing made from Kevlar 29 can be as heat resistant as that made from asbestos and also be extremely cut resistant.

Kevlar 49 has half the elongation (2.5%) and twice the modulus (18 × 10 (to the 6th power) psi) of Kevlar 29. Applications are principally in reinforcing plastic compounds used in lightweight aircraft boat hulls and sports equipment. Composites containing Kevlar are also used as interior panels and secondary structural parts, such as fairings and doors on commercial aircraft.

Kevlar 149 is a highly crystalline aramid that has a modulus of elasticity 40% greater than that of Kevlar 49 and a specific modulus nearly equal to that of high-tenacity graphite fibers. It is used to reinforce composites for aircraft components.

Nomex aramid fiber (also a Du Pont product) is characterized by excellent high-temperature durability with low shrinkage. It will self-extinguish and does not melt, retaining a high percentage of its initial strength at elevated temperatures. It is available as continuous filament yarn, staple, and tow. Nomex is used in military and civilian protective apparel, dry gas filtration, rubber reinforcement, and industrial fabrics. Nomex aramid fibers are also available as a paper for use in high-temperature electrical insulation and in resilient, corrosionproof honeycomb core for aerospace and other transportation applications.

Thermoplastic fibers are also used to reinforce composite materials. Two such families are Compet and Spectra fibers, both products of Allied-Signal Corp. Thermoplastic fibers are particularly effective where high-shear processing would degrade conventional glass-fiber reinforcement, thereby reducing performance of the composite.

Compet fibers of nylon and polyester provide excellent impact resistance, surface appearance, and abrasion and corrosion resistance. They were developed to provide a degree of toughness and impact strength in brittle thermoset resins. Two polyester grades provide regular and reduced shrinkage characteristics, and a nylon grade is particularly resistant to alkalis. Tensile strength of the grades ranges from 120,000 to 150,000 psi. Compet fibers are often used in hybrid reinforcement systems, along with a stronger, higher modulus fiber.

Spectra, a lightweight, high-strength, extended-chain polyethylene fiber, is claimed to be 10 times stronger than steel and 75% stronger than any other organic fiber available. Two grades of Spectra are available. One is a 1,200-denier fiber designed for high strength under intermittent loading conditions -- sports equipment, ballistic fabrics, and medical products. The other is a 650-denier fiber for high strength under continuous load -- sailcloth, high-tension ropes, and cables that must withstand flex-fatigue conditions. Tensile strength of Spectra fibers ranges from 370,000 to 430,000 psi.