Composite Materials

Want to know the secret of Batman's success as a crime fighter? It's Robin. The fictional Caped Crusader didn't fight the dregs of Gotham City single-handedly--at least not in the TV series--because he could do his job more effectively with the Boy Wonder at his side. The Dynamic Duo, just as the name implies, worked as a team--each part complementing the other.

The idea that the whole can be greater than the sum of the parts isn't limited to superhero teams. It applies just as well to some real-world materials. By blending distinct components into composites, scientists and engineers make advanced materials with improved properties that outperform the constituents of the composites.

Plastics impregnated with glass or carbon fibers, for example, can be made tougher, stronger, and stiffer than the pristine plastic and fiber from which the composite is made. And by tailoring the composition and processing conditions, researchers can prepare custom materials endowed with combinations of properties that aren't found in other materials. The unique blend of properties has led manufacturers to replace steel and other conventional materials with advanced composites in many industries including aerospace, automobile, defense, and sports and leisure.

Blending two or more components to form composite materials isn't a new concept. Nature has been doing it for millennia. Bone, for example, a tough and rugged material, is a combination of brittle calcium phosphate and jellylike collagen. Other natural composites include shell, dentin, and tendon.

Synthetic composites have also been around for ages. But unlike natural composites, some of the oldest common synthetic examples, such as the interlocked wood and clay material used to build ancient dwellings, are based on mixing at a very coarse scale.

Finer composites--and the advanced properties they provide--were developed primarily after World War II in response to the aerospace industry's search for lightweight yet stiff replacements for common metals. Ultimately, the demand led to a large number of polymer-based materials, so-called metal-matrix composites, and other types of composites.

Although such materials have been used in commercial applications for decades, researchers continue to look for ways to improve materials' properties and performance and reduce costs. Some of the strategies include using new or modified components and developing new manufacturing processes. One approach that has drawn a lot of attention in recent years is incorporating nanometer-sized additives or nanoscale structure to make new types of engineering materials.

"ADVANCED COMPOSITES" typically refers to materials in which a polymeric resin (often called a matrix) serves as a kind of glue that holds a reinforcement material in place. Common matrix materials include epoxy, bismaleimide, polyimide, and phenolic (phenol-formaldehyde) resins. Reinforcements made from glass, carbon, boron, and other fibers impart stiffness and strength to the polymers, yet enable the products to remain lightweight.

Some 50 years ago, aircraft designers began taking advantage of the high strength-to-weight ratio associated with composites by replacing aluminum parts with others made from the newer materials. The design change helped reduce aircraft weight--thereby increasing fuel efficiency. In addition to lower weight, composites were also attractive to engineers because their resistance to corrosion and fatigue compares with metals.

The type of composite and extent to which the materials have been used in aircraft have changed over the years. For example, a few percent of fiberglass, which is a resin containing filaments made by drawing molten glass, was used in the 1950s in Boeing 707 passenger jets. By the 1960s, high-stiffness boron and graphite fibers embedded in epoxy resins became available, and the U.S. military focused on using these materials in rudders, ailerons, and other movable parts that control the motion of aircraft. Not long thereafter, boron fibers became widely used in the horizontal stabilizers of F-14 Tomcat fighter jets. And in today's F-22 fighters, carbon fiber composites and related materials compose nearly one-third of the jet's structure. Even greater reliance on composite materials is predicted for future military aircraft.

"We're moving from a force made up primarily of metal aircraft with riveted structures toward one that's making greater use of advanced compo sites," said Roland Cochran, polymers and composites branch head at the Naval Air Systems Command, Patuxent River, Md. Cochran's remarks were delivered last month in Washington, D.C., at the "Lightweight Materials for Defense" conference organized by the Institute for Defense & Government Advancement.

Cochran noted that some of the sophisticated capabilities of modern military aircraft wouldn't be possible without today's advanced composites. The V-22 (Osprey) tilt-rotor craft, for example, is able to take off, land, and hover like a helicopter, as well as reorient its rotors in midair and fly like a turboprop airplane. That kind of aeronautical split personality is due in part to the graphite-fiberglass rotors and other lightweight composite-based structures in the rotor system that are strong enough to tolerate high centrifugal forces yet remain slightly flexible.

Similarly, the extreme aerial maneuverability of F-18 fighter jets is partly due to composites used in the aircraft's wings, flaps, vertical and horizontal stabilizers, and other crucial parts.

Flying higher than military jets, satellites also benefit from composite materials. Cyanate-based resins are used by some designers, in preference to epoxies, because of the materials' inherent toughness, resistance to forming microcracks, and ability to withstand radiation damage.

BACK ON EARTH, polymer composites have played a role in transportation applications and in some sporting and leisure equipment for decades. But unlike the materials' rapid growth in popularity in the aerospace industry, composites have moved into terrestrial applications rather slowly. The primary obstacles have been the high cost of the materials and the labor-intensive operations and expensive fabrication equipment needed to process them.

Nevertheless, choosy customers have been enjoying some high-end products made from composites for years. Chevrolet's Corvette, for example, has been sporting a fiberglass body since the automobile became available in the early 1950s. And since the 1970s and 1980s, sports enthusiasts have benefited from the graphite-epoxy composites used to make lightweight yet strong racquetball and tennis racquets, golf clubs, and fishing rods.

Composites strengthened by carbon fibers form another group of materials that is finding its way into numerous applications outside of the aerospace industry. Just last month, for example, Lance Armstrong won a record-setting sixth victory in the Tour de France racing on a bicycle that features a frame and other components made from carbon composites.

According to fiber and composites manufacturer Hexcel, Stamford, Conn., Armstrong's bike, which includes Hexcel materials and was manufactured by Trek, Waterloo, Wis., weighed in at a mere 14.99 lb, the minimum allowable under international rules. The manufacturers note that Armstrong's bicycle was custom designed to be extremely lightweight yet capable of providing the requisite stiffness and strength demanded by the 21 zigzagging switchbacks of the famous Alpe d'Huez stage of the race.

Also taking advantage of carbon composites, but with two extra wheels and a whole lot more muscle than Armstrong's bicycle, the 500-horsepower Dodge Viper can really zip along--even by the high-speed standards of a Tour de France winner. New models of the high-performance and high-priced automobile are built with several carbon-composite-based parts made from materials manufactured by Quantum Composites, Bay City, Mich.

According to Quantum managers Michael Kiesel and Matthew Douglas, one of the keys to maximizing vehicle performance while keeping the weight as low as possible was selecting materials with just the right properties for the car components. For various elements of the Viper's fender support, door structures, and windshield frame, the carmaker chose parts made from carbon-based "sheet-molding" composite or blends of composites.

Kiesel and Douglas explained that sheet-molding composites are made by feeding carbon or glass fibers and a suitable resin into a roller system. The rollers compact the mixture and distribute the fibers into a thin uniform layer sandwiched between two plastic films.

The Viper's fender support, for example, is made from a composite that consists of a toughened vinyl ester resin and 1-inch-long carbon fiber tows (bundles of fibers) that are derived from poly(acrylonitrile). The fender support (or any sheet-molding part) is made via a compression-molding technique in which the sheet-molding compound is compressed in a mold and held at elevated temperature and pressure to cross-link the polymer.

Similar materials and methods are used to make the door structures and windshield frame. For the inner panels of the Viper's doors, which feature unconventional contours, the composites maker customized the stiffness by blending the vinyl ester carbon composite and a glass-based composite. And to prevent the convertible's windshield from deflecting when a driver grabs hold of the frame to climb out of the car, the company combined a fiberglass material with a tailor-made form of the carbon composite. The specialized material is made with carbon fibers that extend along the entire length of the windshield frame components and are aligned along a common axis to maximize stiffness.

In addition to the high strength-to-weight ratio that characterizes polymer composites, the materials offer other advantages relative to metals. For example, the ability to mold composites into complex three-dimensional shapes means that manufacturers can form a single composite-based part that takes the place of more than one metal part and several fasteners, pins, and bolts. Douglas estimated that as many as 20 metal parts would be needed to do the job of the Viper's six-part composite fender support.

FURTHERMORE, unlike many metals, polymer composites resist corrosion and thereby reduce the need for maintenance. Resistance to corrosive seawater was one of the key factors that led a team of boat designers to replace aluminum stern drives in saltwater fishing boats with parts made from one of Quantum's fiber-reinforced vinyl ester composites. Douglas noted that the material's high strength, stiffness, and impact resistance make it ideal for replacing metal components.

So what is it that makes carbon fibers so attractive to composite makers? As Kiesel pointed out, the modulus (stiffness) of commercial-grade carbon fibers is roughly 230 gigapascals, which is more than three times greater than the stiffness of common glass fibers. In addition, carbon fibers are only about 70% as dense as glass fibers. "Carbon fiber composites offer the possibility of even greater weight savings than glass-fiber composites," Kiesel summarized, "but the cost of the material is significantly higher."

At Oak Ridge National Laboratory and other institutions, researchers are working to lower the price. The primary objective is to develop methods for preparing carbon fiber-based composites that can be used to replace steel and aluminum in ordinary automobiles to make cars that are very lightweight and fuel efficient. One approach is to make the fibers using inexpensive starting materials. Another approach calls for developing new processing methods.

Manufacturers tend to be tight-lipped about the specifics of their fiber-processing techniques. But most fiber makers begin the process by forming filaments out of common materials such as rayon, poly(acrylonitrile), and pitch. The filaments are then pyrolyzed at high temperatures and subjected to controlled oxidation to make carbon fibers with micrometer-scale diameters. Several factors govern the fibers' properties, including carbon content (typically more than 90% carbon by weight), degree of crystallinity, and orientation of carbon planes.

Rather than using the common starting materials, scientists at Oak Ridge and their colleagues are investigating ways to use low-cost recycled materials such as lignin, a waste product of the pulp and paper industry. In addition, they are developing new processing techniques that rely on microwave heating of precursor materials in a plasma instead of using the common but less energy efficient thermal method. In one set of experiments using poly(acrylonitrile) as the starting material, Oak Ridge researchers found that the microwave technique could produce carbon fibers roughly four times faster while using 20% less energy than conventional methods.

Hearing that carbon fiber composites have turned into a low-cost commodity would be music to the ears of many types of manufacturers. The price remains high, but some researchers in the materials community hear the music anyway. In an application that's a little off the beaten path, scientists at the Air Force Research Laboratory (AFRL) at Wright Patterson Air Force Base, Dayton, Ohio, have teamed up with the Canadian Brass Quintet to study the performance of trumpet mutes and other musical instrument accessories made from carbon fiber composites. The investigation into vibrational frequencies and damping characteristics of the composites may lead to improvements in the design of musical instruments and aircraft structures.

It's hardly conventional to model military jet performance using musical instruments. Nonetheless, the study could be revealing. Tia Benson Tolle, chief of AFRL's structural materials branch, explained that "aircraft components are often excited acoustically, and vibrational modes, resonances, and damping can be critical to system performance." Acoustic properties are especially important, she noted, "in an aircraft environment, where excess vibration can cause material fatigue and unwanted noise levels."

Although the term "advanced composites" typically suggests polymer-based materials, metals also play an important role in composites. Pradeep K. Rohatgi, director of the Center for Composite Materials at the University of Wisconsin, Milwaukee, reported on composites in which various types of particles or fibers are added to a metal matrix to reinforce the material and improve its strength and other properties.

Aluminum, for example, can be reinforced with boron, carbon, silicon carbide (SiC), alumina (Al₂O₃), or graphite in such a way that the resulting composite is some 30 to 40% stronger and stiffer than pristine aluminum. In contrast, alloying methods, in which copper, zinc, and magnesium are dissolved in aluminum, increase aluminum's strength by just a few percent, Rohatgi pointed out.

Several processes can be used to prepare metal composites. In a vapor-phase method, metal is evaporated onto fibers. Alternatively, using solid (powdered) forms of the matrix and reinforcement materials, composites can be made by mixing, pressing, and sintering the starting materials. But according to Rohatgi, these methods tend to be expensive. So his research group developed low-cost, liquid-based methods in which fiber or particulate reinforcements are added to the metal in a molten state and the hot slurry is poured into a mold where it cools and solidifies. Rohatgi noted that the solidification method is well suited to forming large and complex parts.

Due to their low weight, high strength, abrasion resistance, and other desirable properties, metal-matrix composites have been selected by manufacturers for use in a variety of applications. Chrysler and other carmakers have used Al-SiC composites to make brake rotors for some high-end models. The material is also used in drive shafts and some aerospace applications. In addition, because of its high thermal conductivity and low coefficient of expansion, Al-SiC composites are used to make heat sinks for computers.

Low coefficient of friction is another property that can be engineered into metals. By embedding graphite particles in aluminum, the Milwaukee composites group prepared a lightweight and self-lubricating material that can be used to avoid a potentially debilitating mechanical problem. Rohatgi explained that in aluminum engines, aluminum pistons and cylinder liners can stick together during cold start-up and at other times when engine oil is scarce. But if the engine components are manufactured using the aluminum-graphite composite, the engine has built-in antiseizing protection.

In an ongoing effort to reduce the price of metal-matrix composites, Rohatgi and coworkers developed synthesis methods that make use of waste materials. Specifically, the group uses particles of fly ash--an aluminosilicate by-product that's produced in the U.S. in the 90 million-ton range annually from burning coal. By blending micrometer-sized, hollow fly ash particles with aluminum or lead, the group makes inexpensive, lightweight composites known as syntactic foams that are characterized by low density and high impact resistance.

A RECENT TREND in materials research has been to try to take advantage of nanoscale structuring to endow composite materials with unique properties. James S. Murday--superintendent of the chemistry division at the Naval Research Laboratory, Washington, D.C., and a representative of the National Nanotechnology Initiative--noted that the research effort is broad and extends to polymers, ceramics, metals, and other types of materials.

"We've seen several examples in which designing materials with nanoscale structure adds value and improves performance," Murday commented. For example, embedding low concentrations of carbon nanotubes in polymers has been shown to boost electrical conductivity by several orders of magnitude, he noted.

"But in order for the significance of these 'nanotech' findings to be elevated from 'important advance' to 'revolutionary contribution,' the chemistry community will have to play a critical role," Murday asserted. What's needed is a more detailed understanding of the relationship between materials properties and nanostructure and greater quality control in nano scaled materials, he explained. "Eventually, we need to be able to reach to our shelves and pull down catalogs of affordable, high-quality nanostructured products," he added.

At AFRL, research group leader Richard A. Vaia and coworkers are studying nanocomposites that one day may end up in those catalogs. According to Vaia, AFRL researchers are focusing on ways of modifying polymer composites with nanoscale materials to produce composites that exhibit unconventional combinations of properties.

For example, AFRL scientists are studying methods for using nanostructured, layered silicates to prevent gas diffusion through polymers used in fiber-reinforced composites. A key application of the work is future cryogenic gas storage tanks for aerospace use that are extremely lightweight and strong (like conventional carbon fiber composites) but do not require the metal gas-barrier liner that is used currently to make the vessels leakproof. Other research groups have used layered silicates to form flame-resistant polymer nanocomposites.

The AFRL team is also examining ways of using the silicates to reduce the coefficient of thermal expansion in resins, which in turn may reduce microcracking in composites. The team is also studying methods of using boron nitride and boron carbide nanotubes to modify thermal conductivity as well as carbon nanotubes to tailor electrical conductivity.

In a recent study, Vaia and coworkers studied the response to various stimuli of samples of polyurethane-carbon nanotube composites that had been deformed (stretched). The researchers found that, unlike the pristine polymer, which is an electrical insulator, composites containing low concentrations of nanotubes (1–3 wt %) shrink when electrical current is passed through them or upon exposure to infrared light [Nat. Mater., 3, 115 (2004)]. The nanocomposites are being studied for use in applications involving remote actuation, such as space deployment and unmanned aerial vehicles.

From satellites and jet fighters to golf clubs and fishing rods, custom-designed composite materials are being used in an enormous number of applications. As demand for composites increases, scientists will continue to coax polymers, ceramics, metals, and other substances into the materials science version of teamwork.