C&EN: COVER STORY - SCIENCE TRANSFORMS THE BATTLEFIELD

SCIENCE TRANSFORMS THE BATTLEFIELD

Advances in materials technology provide troops with ever-improving combat capabilities and levels of protection

Comic-book authors a turning green with envy. For years, superheroes and their futuristic special powers were exclusively the stuff of fantasy. But now, science and technology are beginning to enable ordinary humans to acquire some of the abilities of fictional superheroes. Once make-believe, these special powers are being conferred by advanced materials specially designed for military applications.

Imagine combat uniforms that switch from soft and pliable to firm and armored at the touch of a button. Or a battle suit that can function as an antenna, generate a steady supply of electricity, and relay vital information about the soldier wearing it. How about a military vehicle that can repeatedly deflect armor-piercing bullets at close range?

Equipment capable of performing these and other amazing feats is being developed by scientists and engineers working on a range of research programs. The projects aim to provide members of the armed forces with the highest level of protection and an overwhelming combat advantage. And the researchers are meeting those goals, in part, thanks to advances in ceramics, polymers, nanostructured substances, and other high-performance materials.

Ceramics have played a role in military applications for decades because of their strength, hardness, and other properties.

Ceramics have played a role in military applications for decades because of their strength, hardness, and other properties. In recent years, new types of materials and processes have led to advanced ceramics that perform better than conventional ceramics. The advances have raised the demand for these materials for use in vehicle and body armor and a growing number of other defense applications.

Compared with clays and other common ceramics, advanced ceramics have incredible properties. Ceramics used in coffee cups and fine china, for instance, exhibit flexural strengths--ability to withstand pressure without rupturing--of 5,000 to 10,000 psi. In contrast, sintered reaction-bonded silicon nitride can handle up to 140,000 psi.

When it comes to armor, ceramics have a distinct weight advantage over steel. Both materials can be used to stop bullets, but compared with typical steel densities of 7 to 8 g per cm³, ceramics, which generally weigh in at just 4 g per cm³ or less, are featherweights. Yet the lightweight materials save lives by shattering bullets and other metal projectiles into small fragments that are deflected.

REPLACING METAL armor with ceramics can lower the overall weight of an armored vehicle by as much as 60 to 70%, according to Joel P. Moskowitz, chief executive officer and president of Ceradyne, a manufacturer of high-performance ceramics based in Costa Mesa, Calif. Reduced weight translates to lower fuel consumption and increased maneuverability. For some commercial applications, less weight is a nice feature, but it's not essential, Moskowitz remarks. "For the military, lightweight materials can mean the difference between an armored helicopter being able to carry a payload or not being able to take off," he asserts.

S. Robert Skaggs, a retired program manager at Los Alamos National Laboratory, notes that, near the end of the Vietnam War era, researchers at Los Alamos and Lawrence Livermore National Laboratories, working with engineers at Coors Ceramics, Golden, Colo., developed body armor made from alumina (Al₂O₃), which has a density of roughly 4 g per cm³. Relative to steel, the ceramic breast and back plates, which were held in place by a special vest, offered lightweight protection. But at nearly 30 lb, the body armor was cumbersome and still rather heavy. "You couldn't maneuver in it," Skaggs remarks. "It was really only useful for sentry duty."


HIGH FLYING High-temperature stability and transparency to microwave radiation are among the properties that lead manufacturers to use silicon nitride-based ceramic radomes (front row) on Patriot missiles and fused-silica nose cones (back row) on Aegis missiles.
COURTESY OF CERADYNE

Since that time, other ceramics--such as boron carbide (B₄C), silicon carbide (SiC), silicon nitride (Si₃N₄), and aluminum nitride (AlN)--have been developed for various defense applications. For example, today's personnel body armor, which is lighter and contoured to fit better than the older alumina-based armor, is commonly made from boron carbide (2.5 g per cm³) and silicon carbide (about 3.2 g per cm³). Modern attack helicopters, such as the Apache, Blackhawk, Cobra, and other models, are fitted with boron carbide panels that fortify seats, walls, floors, and other parts of the cockpit to protect crews from gunfire. Similar protection is found on C-141 and C-17 cargo planes and other military aircraft.

Other combat vehicles are also protected by ceramic materials. Skaggs describes a project he worked on to fortify armored personnel carriers used by the U.S. Marine Corps. The vehicle bodies were made of hardened steel, but greater ballistic protection was required. So the already tough skin was further strengthened by covering the vehicles with ceramic tiles made from a high-temperature-processed alumina and a silicon carbide material that had been infiltrated with aluminum. Other types of ceramic armor tiles for various combat vehicles are available commercially. For example, Ceradyne manufactures tiles made of B₄C, SiC, TiB₂, and Si₃N₄. These types of materials are used today in the Army's Stryker and other light-armor vehicles.

A COMMON METHOD for preparing advanced ceramics is hot pressing. The procedure involves heating ceramic powders to some 2,000 °C while squeezing the materials in a several-hundred-ton press under an inert atmosphere. Moskowitz explains that the hot-pressing procedure avoids the need for the glassy, gluelike component typically used to hold teacups together.

In the sinter-reaction-bonding technique, silicon powder and small quantities of additives are transformed into the shape of a machine part, armor plate, or other product using conventional forming processes. Then the silicon parts are converted to silicon nitride by treating them with nitrogen in a high-temperature furnace for several days. The products are treated further at higher temperature and under high pressure to form strong, hard, durable sintered reaction-bonded products.

Strong and hard as they may be, ceramics don't act alone in protecting troops from high-velocity projectiles. "You need a hard surface that's backed by something that can absorb all the energy," Skaggs explains. Ceramic armor is backed by composites made of impact-resistant fibers, such as DuPont's Kevlar and other materials. The armor panels are designed to shatter bullets at the ceramic surface and trap the shrapnel in the backing fibers.

Kevlar's bullet-stopping capability stems from the structure of poly(paraphenylene terephthalamide) (PPTA), the polymer from which Kevlar is made, and fibers formed from the polymer. The molecule has a rigid structure due to conjugation along the chain, which leads to well-ordered liquid-crystalline solutions. The material, which is predisposed to lining up in solution phase, is ordered even further during the fiber-spinning process, resulting in tough fiber bundles.

Fundamental investigations of the structure of ceramics recently solved one of the mysteries of armor performance. For years, scientists were puzzled as to why boron carbide does such a good job of defending against low-energy projectiles but doesn't stand up to high-powered shots as well as would be expected on the basis of some of its materials properties.

Using high-resolution electron microscopy to examine boron carbide fragments from ballistics tests, Mingwei Chen and Kevin J. Hemker, mechanical engineers at Johns Hopkins University, and James W. McCauley, a senior research engineer at the Army Research Laboratory's (ARL) Aberdeen Proving Ground, in Maryland, discovered that upon high-energy impact, boron carbide undergoes an amorphization process [Science, 299, 1563 (2003)] previously unobserved for a ceramic as hard as B₄C.

The researchers discovered that areas within the specimen that appeared to be cracks turned out to be nanosized bands consisting of a new, glassy, amorphous form of boron carbide. They found that the hard substance fractures along the bands because the bands are weaker than the surrounding crystalline material.

In addition to their great strength, ceramics have other desirable properties that set them above other materials--sometimes literally. Missile nose cones (radomes) are made of ceramics such as fused silica and silicon nitride because the materials are transparent to microwave radiation and can withstand the tremendous heat and stress associated with traveling at supersonic speeds. The transparency enables missile guidance systems to emit and receive radiation through ceramic radomes and use the information to seek and destroy enemy missiles. According to Moskowitz, the latest version of the Patriot missile--Lockheed-Martin's PAC-3 system used in Iraq--is tipped with a Ceradyne radome.

TRAVELING AT FAR slower speeds and with their feet planted on the ground, foot soldiers--and their more pedestrian needs--are the focus of major research programs. One such effort is centered at the Institute for Soldier Nanotechnologies (ISN) at Massachusetts Institute of Technology. ISN, which was launched last year with a $50 million contract from the U.S. Army, sets as its ultimate goal creating the technology for a future battle suit that will combine high-tech capabilities with comfort and low weight (C&EN, March 25, 2002, page 12). Research at ISN is carried out by MIT scientists and engineers from several departments in conjunction with the Army Natick Soldier Center, Natick, Mass.; other Army research centers; and industry partners such as Raytheon and DuPont.

Traveling at far slower speeds and with their feet planted on the ground, foot soldiers--and their more pedestrian needs--are the focus of major research programs.


GET A LOAD OF THIS For a first-hand feel of the heavy loads carried by today's soldiers, MIT's Thomas suits up with the 82nd Airborne Division at Fort Bragg in North Carolina. Thomas and others are developing new materials that may reduce the weight of standard jump equipment, which includes primary (on back) and backup (in front) parachutes and other packs. PHOTO BY SLADE BEAUDOIN/U.S. ARMY

"We're focusing on protecting individual soldiers from ballistic, chemical, and biological hazards," says MIT materials science and engineering professor Edwin L. (Ned) Thomas, who directs the institute. By pooling a broad range of scientific expertise, ISN aims to exploit the unusual and versatile effects of nanoscale structures to develop highly functional lightweight materials. Thomas explains that the idea is to integrate the new materials into a "smart" uniform that will provide unprecedented protection and combat advantage to soldiers. He expects that much of the technology developed along the way will be transferable to police and fire services and to emergency workers.

Dynamic armor is one of several technologies that ultimately might be developed on the basis of today's investigations at ISN. Using plastic fibers with parallel hollow channels--a design developed at DuPont--it may be possible to make fabrics that change properties on demand. One approach would be to fill the channels with ferrofluids, colloidal suspensions of nanosized magnetic particles. Subjecting the fluids to a magnetic field would align the particles in the channels--in effect, solidifying the solutions reversibly--which in turn would cause the fibers, and hence the fabrics made from them, to switch from flexible to rigid.

A material of that type could remain flexible and comfortable to wear most of the time but switch to the rigid armored form in case of a ballistic threat. Thomas notes that the same concept could be used to aid wounded soldiers. For example, a portion of a soldier's garb could be transformed rapidly into a tourniquet to stop massive blood loss or into a splint to support a broken leg.

Smart fabrics seem like a great idea--but not if powerful magnetic field generators need to be lugged around to switch the materials on and off. Soldiers already carry more than 100 lb of gear in many cases. So MIT's Gerbrand Ceder, a professor of materials science and engineering, has begun investigating ways of using changes in pH or other chemical means for switching the magnetic states of the oxides of manganese, cobalt, and other transition metals from antiferromagnetic to ferromagnetic.


	IN THE LINE OF FIRE Amphibious assault vehicles are protected by lightweight ceramic armor that deflects bullets without adding the excessive weight of metal armor. GENERAL DYNAMICS PHOTO

"You want to use a material that sits right on the brink of transition," Ceder says, so that a small stimulus will cause a rapid transition. Ceder adds that working with nanoparticles is helpful in that regard because the small diffusion-length scales will facilitate fast reactions.Paula T. Hammond, an associate professor of chemical engineering, describes another line of research begun recently at MIT. Hammond and coworkers are developing thin-film assemblies of dendrimers and semiconductor nanoparticles to be used as sensitive and specific detectors for chemical and biological agents. The sensors are designed to respond to the presence of target analytes with a change in resistivity, which serves as the basis of detection. One of the goals of the project is to integrate the functional films into battle suits so that soldiers will be able to assess environmental hazards continuously. In related work, Hammond and Angela M. Belcher, an associate professor of materials science and engineering, are developing viral arrays for detection and remediation of harmful agents.

Elsewhere at MIT's newly formed institute, researchers have come up with a versatile waterproofing technique that can be used to keep soldiers dry and safe. Chemical engineering professor Karen K. Gleason developed a chemical vapor deposition procedure for building up films of polytetrafluoroethylene (PTFE) on many types of materials--including some that aren't amenable to common waterproofing methods. Recently, Gleason teamed up with MIT chemistry professor Alexander M. Klibanov to develop procedures for combining her waterproofing method with a microbe-killing fabric treatment pioneered by Klibanov's group.

In the Future Warrior concept envisioned by the Army's Natick Labs, protection from chemical warfare agents will be built directly into the high-tech battle suit. Scientists at MIT and elsewhere are working on various aspects of the project, but an all-in-one battle suit isn't expected anytime soon. So in the meantime, researchers continue working on less futuristic concepts for personal protection.

At ARL's Aberdeen Proving Ground, for example, scientists are looking for ways to make chemical protective clothing more breathable. Butyl rubber, a common material used for chemical protection, stands up to many harmful substances but also acts as a barrier to water vapor, making it hot and stressful for people who need to wear it. A candidate material under investigation as a substitute for butyl rubber is an ionic block copolymer made of a highly sulfonated poly(styrene-isobutylene-styrene). The material combines a flexible elastic chemical barrier and a hydrophilic water-permeable component. Recent studies show that the material can be processed as thin films and applied as a laminated coating on cotton-nylon fabrics.

ACTIVATED CARBON has been used in fabrics for years to provide soldiers with chemical and biological protection. Fabrics impregnated with carbon are permeable to water vapor and effective for adsorbing chemical warfare agents, but the protection comes at the cost of added weight and bulk. An alternative solution being studied at Natick Labs involves coating textiles with commercially available membranes that serve as a barrier to many chemical substances but are permeable to water vapor. The technology has led to lightweight one-piece suits that can replace the bulky coveralls still used for protection in harmful environments. In related work, Natick Labs scientists and their coworkers are using ion beams to modify the surface layers of selectively permeable membranes to render them more effective.

Ordinary-looking uniforms that are impermeable to harmful substances--or able to neutralize them--may become commonplace rather soon. But how about a uniform that generates its own electricity? The concept is not as futuristic as it may seem. In fact, prototypes may be ready for field-testing in the next year or so, according to Russell Gaudiana, vice president of research and development at Konarka Technologies, Lowell, Mass.

Konarka is developing polymer photovoltaic products for converting sunlight and indoor light into direct-current electrical power. The two-year-old company was founded to commercialize technology developed by the late Sukant K. Tripathy, a chemistry professor at the University of Massachusetts, Lowell. Tripathy came up with a low-temperature procedure for sintering TiO₂, which is central to fabricating dye-sensitized solar cells.

"Low-temperature sintering allows us to make solar cells on lightweight, flexible plastics," Gaudiana says. And the devices can be manufactured through simple high-speed coating procedures, he adds. In contrast to other types of photovoltaic cells in which the light-harvesting and electricity-generating layers are deposited on the rigid surfaces of glass or silicon, Konarka's cells can assume a large number of shapes and sizes and be fabricated in several forms. Gaudiana notes that the company recently filed a patent describing procedures for making photovoltaic fibers that can be woven into fabrics. With those capabilities in hand, buildings, tents, vehicles, and even uniforms can be used as surfaces on which to generate electricity.




GEARING UP FOR TOMORROW The U.S. Army Soldier System Center's Future Warrior sports futuristic-looking battle gear to inspire scientists to continue searching for new ways to equip soldiers for the battlefield.
COURTESY OF ARMY NATICK SOLDIER CENTER

Dye-sensitized photovoltaic cells were invented principally by Michael Grätzel of the Swiss Federal Institute of Technology, Lausanne. Gaudiana explains that the devices are made with 20- to 50-nm porous TiO₂ particles that are sintered to bridge them electrically, which enables TiO₂ to serve as an electron carrier. After the sintering process, ruthenium-based dye molecules are used to extend the sensitivity of TiO₂ into the visible-light region.

The Army, which is funding research at Konarka, is hungry for lightweight, renewable sources of power. The reason is that the growing array of sophisticated electronics carried into the field by today's soldiers requires an enormous amount of battery power. Computers, displays, night-vision goggles, communication devices, and other equipment all require reliable and stable sources of power. The new photovoltaic cells can provide electricity to run electronic equipment directly or to recharge batteries.

Much of the technology developed along the way will be transferable to police and fire services and to emergency workers.

Other fundamental investigations have led to films and coatings that protect military personnel and equipment. At Cornell University, for example, the research group of Christopher K. Ober, a professor of materials science and engineering, recently developed two types of materials--one hydrophilic, the other hydrophobic--to prevent fouling of ship hulls. Fouling from seaweed, barnacles, and slime-generating bacteria leads to excessive turbulence during ocean travel, which can reduce fuel efficiency by as much as 30%. The films consist of a block copolymer rubber and a liquid-crystalline component and can be applied by spray-painting.


	Gaudiana
	COURTESY OF KONARKA TECHNOLOGIES

On land, high-performance coatings are needed to protect fighting and ground-support vehicles and other types of equipment used by the armed forces. John Escarsega, Steven H. McKnight, Bruce K. Fink, John H. Beatty, and their coworkers at ARL's Aberdeen Proving Ground are developing durable and scratch-resistant camouflage paints that not only help hide military equipment but also resist chemical warfare agents. Additionally, the newest formulations can be thinned with water and emit only low levels of volatile organic compounds.

The ARL coatings scientists explain that the newer military paints are made of cross-linked polyurethane and polyurea binders that contain primary pigments to provide green, tan, brown, and other camouflage colors and extender pigments to make the paint flat (nonreflective)--a critically important property. They note that the chemistry of the binders and its interaction with the pigments are controlled through custom formulations not found in other paints. Replacing traditional silica-based extenders with polymeric materials has significantly improved the paints' scratch resistance and durability.

Today's high-tech battlefield equipment bears little resemblance to the equipment of a century ago. Continual advances in materials technology are providing members of the armed forces with ever-improving levels of protection and combat superiority. Ultimately, troops may acquire capabilities that seem to spring from the pages of comic books. Although it may be a long time before foot soldiers can move faster than a speeding bullet or leap tall buildings in a single bound, at the rate battlefield technology is progressing, the next century's warrior will surely give Superman a run for his money.