In the standard cops 'n robbers shoot-out scenes from Hollywood or video games, police frequently take a defensive position behind the open doors of police cruisers. The real fiction, however, is the protection afforded by a cruiser door. Police might as well take cover behind a cereal box, especially with the current proliferation of military-style semi-automatic firearms or assault weapons.

The spread of assault weapons in the streets of America poses a threat to law enforcement that no police car door can defend against, requiring prudent agencies to dress patrol cars in extra armor to protect officers against this high firepower danger.

To combat the threat from assault rifles, some police departments are beginning to outfit their patrol fleets in special armor, usually by attaching door skins made of protective material such as Kevlar or G-Lam to the outside of the door. Another means of protection includes installing lightweight ceramic boron carbide tiles inside door panels. These materials provide a radical increase in protection from small arms fire, projectiles, fragmentation and shrapnel when compared to the original equipment.

Boron carbide has been in use since the 1960s for body armor, helicopter seats and other protective applications. The principal appeal of ceramic tile is that it reduces the weight of passive armor systems while increasing the ability to defeat ballistic threats.

But if an Oak Ridge National Laboratory (ORNL) innovation hits the streets, police cruisers, military helicopters, combat vehicles and VIP limousines could all soon make an improved armor protection fashion statement. ORNL researchers, located in Oak Ridge, Tennessee, have fabricated ceramic boron carbide armor tiles that demonstrate superior ballistic performance compared to commercial material currently available.

Modern ceramic armor material is lighter than metal - including titanium - and two to three times as hard, making it the preferred armor protection.

Tests at the lab show tiles made of ORNL's boron carbide ceramic and facings made of polymer matrix composites provide superior ability to stop armor-piercing bullets than commercial ceramic armor. In tests performed at a ballistics range, the ceramic tiles sandwiched by four layers of a polymer matrix composite stopped .30-caliber armor-piercing bullets traveling up to 2,800 feet per second.

"The hardness of the ceramic fractures the bullet, making it easier to stop small fragments," says Steve Nunn of ORNL's Metals and Ceramics Division.

On the level
"With these ceramic technologies, we have demonstrated the equivalent of National Institute of Justice (NIJ) Level IV protection at less than 6 pounds per square foot areal density and Level III protection at less than 5 pounds per square foot," Nunn states.

A medium-size vest plate (small arms protective insert, SAPI) is about 0.8 square feet. This means that a vest providing NIJ Level III protection would weigh only about 8 pounds plus the weight of the vest fabric. "In direct comparison, for NIJ threat Level III protection, the armor weight can be reduced by more than 60 percent compared to rolled homogeneous armor (RHA) steel," Nunn says.

The weight reduction for NIJ threat Level IV may be even higher, which is good news for those charged with dressing law enforcement personnel in suitable armor. The standard Humvee weighs 6,000 pounds. The "up-armored" version being procured for deployment in Iraq adds 3,800 pounds in steel plate armor and a bullet-resistant windshield. "The ORNL boron carbide armor system could reduce the armor weight by more than a ton," Nunn says.

Besides making vehicles easier to transport by air and capable of operation on soft sand or mud terrains, lowering vehicle weight reduces power requirements, fuel consumption, wear and maintenance. "As an example, for an advanced Army helicopter, it was estimated that each pound that could be removed from the aircraft weight would translate to $4 million in savings over the life of the aircraft," Nunn says.

Show me the money
The problem is, ORNL's armor research is currently in development limbo and has not been fully optimized.

While the program that originally supported this research and development is no longer providing research funding, Nunn is presently submitting proposals and contacting agencies that may be interested in continuing this lightweight armor development project.

"It's important to note that the goal of our original project was to improve the performance of boron carbide ceramics for armor applications, not to design an optimized armor," Nunn explains. "We clearly accomplished that goal."

The next step is to apply what's been learned to designing an improved armor system that will provide maximum protection at the lowest possible weight.

Once the project receives further funding, Nunn and colleagues plan to conduct tests to shed more light on why ORNL's tile provided for up to 24 percent better performance than commercially available ceramic tiles, and why ORNL's composite facing improved the ballistic performance of a commercial armor tile by 40 percent.

High velocity impact
ORNL's boron carbide ceramic was tested in direct comparison with boron carbide ceramic that was obtained from two commercial suppliers of ceramic armor tiles, Nunn explains.

All of the tiles evaluated in the comparison tests were of the same size and thickness (4 inches by 4 inches by 0.245 inches) and had Spectra Shield Plus spall covers and backing plates.

Then, a universal receiver was used to fire .30-caliber armorpiercing bullets at the tiles.

"Variable powder charges were used to change the projectile velocity, which was measured at the target position using a chronograph," Nunn says. The results of the tests showed that the V-50 rating (defined as the velocity at which a given projectile will have a 50-percent probability of penetrating a given material) of the ORNL boron carbide achieved a 24 percent higher performance rating than one of the commercial suppliers and 11 percent higher than the second.

To be able to observe improvements in performance, thin ceramic tiles were used to insure that tiles would fail at less than 2,800 feet per second.

ORNL's tile provided for up to 24-percent better performance than commercial ceramic tiles.

Another method that was evaluated at ORNL for improving the performance of boron carbide ceramic armor was the use of a polymer matrix composite (PMC) facing material.

According to Nunn, this material was applied directly to the two faces of the tile prior to applying the Spectra Shield Plus spall cover and backing plate.

Variations of the PMC facing that were evaluated included the type of reinforcing fibers, the matrix material, the number of composite plies and the orientation of the fibers in the individual plies. Then, various PMC combinations were compared on the baseline boron carbide ceramic tiles from a commercial supplier.

"Versus the .30-caliber round, all of the variations improved the V-50 of the armor tile when compared to the same tile without a PMC facing," Nunn says. One of the PMC facings increased the V-50 by more than 40 percent, stopping the projectile at over 2,875 feet per second (876 meters per second) with an areal density of 5 3/4 pounds per square foot. In spite of such encouraging results, the PMC facing has not yet been optimized due to lack of funding. Nunn believes that ballistic performance can be improved even more with additional research and development efforts.

What's next
Nunn would like to combine the ORNL boron carbide with the best PMC facing. "This combination was not tested before the previous program ended," he says. The improvements that were observed individually may combine to provide even greater increases in ballistic impact performance and lower the areal density needed to defeat typical threats.

"If the effects are additive, the .30-caliber round at 2,800 feet per second could be defeated with an armor areal density of less than 4.3 pounds per square foot," Nunn says.

Nunn would also like to optimize the ceramic material. "Because there were clear differences in the ballistic performance of the three boron carbide materials that were tested, it may now be possible to identify in the laboratory the material characteristics that control ballistic impact response," he says.

The PMC facing material also needs to be optimized. "New, improved fibers are now available and may significantly improve the ballistic performance of armor tiles with a PMC facing," Nunn says.

Additionally, there are many more variations of fiber, matrix and layering configurations that can be studied to understand how the PMC functions and to optimize the effect on improving ballistic impact performance.

"The PMC facing needs to be evaluated on other, lower-cost ceramic materials to see if comparable effects are observed," he says.

Shattering facts
Researchers from Johns Hopkins University and the U.S. Army Research Laboratory say they have figured out why boron carbide shatters so easily when struck by powerful ammunition, as reported in the March 7, 2003 issue of the journal "Science."

By observing the atomic structure of boron carbide fragments retrieved from a military ballistic test facility, the team discovered higher-energy impacts cause tiny bands of boron carbide to change into a more fragile glassy form.

This transformation to a glassy material (called high-impact pressure amorphization) has previously been seen in minerals and semiconductors, but this is the first report of such behavior in a ceramic as hard as boron carbide.

According to the researcher, the extremely high velocities and pressures associated with impact of a high-powered projectile appear to cause microscopic portions of the material's crystalline lattice structure to collapse. "It's like having a sturdy table and suddenly kicking the legs out from underneath it," says Mingwei Chen, associate research scientist in the Department of Mechanical Engineering at Johns Hopkins and lead author of the "Science" article.

Having found why boron carbide - whose hardness approaches that of a diamond - abruptly loses its protective capabilities, the research- ers believe they may have opened a door toward development of a new form of the material that will do a better job of keeping soldiers and police officers safe. If it could stand up to higher-energy threats, military experts believe the material would find wide use as a lightweight armor material for military, police, diplomatic and other vehicles.

Since boron carbide is a man-made material, altering the way it is manufactured could produce a better barrier. The question now is: how should the boron carbide be changed?

"We intend to try modifying the material's grain structure, it's chemistry and the additives used in making it," says James McCauley, a senior research scientist at the Army Research Laboratory at Aberdeen Proving Ground in Maryland, and a co-author of the journal article.

The goal will be to have the amorphization occur at higher impact pressures so the armor would provide better protection against a wider range of threats.

The Johns Hopkins work has no immediate impact on Nunn's efforts at Oak Ridge.

"The Johns Hopkins paper relates to our work in that we're both talking about the same basic ceramic material, boron carbide," Nunn says.

While it may provide some insight to suggest ways to improve the ballistic impact properties of boron carbide in the future, the work described is preliminary in nature and not everyone agrees with the conclusions of the authors, he says.

Last December Nunn responded to an NIJ call for proposals and sent a proposal to continue on ORNL's armor work, but focused on body armor for high level threats instead of helicopter protection. The proposal was turned down. Among the reviewers' comments were statements that high level threats were a low priority concern for law enforcement officers.