Ballistics experts in recent years have puzzled over a troubling loss of impact resistance in an extremely hard and lightweight ceramic material called boron carbide, sometimes used in protective armor. The material does an excellent job of blocking low-energy projectiles such as handgun bullets but shatters too easily when struck by more powerful ammunition.
Writing in the March 7 issue of the journal Science, researchers from Johns Hopkins and the U.S. Army Research Laboratory say they have figured out why this occurs. By observing the atomic structure of boron carbide fragments retrieved from a military ballistic test facility, the team discovered that higher-energy impacts cause tiny bands of boron carbide to change into a more fragile glassy form.
This high-impact pressure amorphization, or transformation to a glassy material, has been seen previously in minerals and semiconductors, but the researchers say they are the first to report such behavior in a ceramic as hard as boron carbide. The extremely high velocities and pressures associated with the 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 abruptly loses its protective capabilities, the researchers hope they 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 that boron carbide, whose hardness approaches that of diamond, would find greater use as a lightweight armor material for military, police, diplomatic and other vehicles.
Boron carbide is a man-made material, and altering the way it is manufactured could produce a better barrier. "The question now is, how should we try to change the boron carbide?" says James W. McCauley, a senior research scientist at the Army Research Laboratory at Aberdeen Proving Ground in Maryland and a co-author of the journal article. "We intend to try modifying the material's grain structure, its chemistry and the additives used in making it. The goal will be to have the amorphization occur at higher-impact pressures. Then the armor would provide better protection against a wider range of threats."
Boron carbide has been used since the 1960s for body armor, helicopter seats and other applications. In recent years, however, ballistics experts have become concerned about the material's tendency to significantly change its fracture characteristics when struck by high-powered firearms and more destructive types of ammunition. To figure out why this occurs, McCauley obtained fragments of a plate of boron carbide recovered from a recent series of ballistic tests at the Army Research Laboratory. He gave the fragments to Johns Hopkins researchers who collaborate with Army scientists and engineers through the Army Research Laboratory Materials Center of Excellence at Johns Hopkins.
Chen came up with a way to position ultra-thin edges of the fragments so that their atomic structure could be viewed through a high-resolution transmission electron microscope at Johns Hopkins. Chen analyzed his images with Kevin J. Hemker, a professor in the Department of Mechanical Engineering. Localized areas that initially appeared to be cracks in the material were found to consist of the new glassy form of boron carbide. Under normal conditions, atoms in boron carbide form a geometric pattern called a crystal lattice. In the 2-nanometer glassy bands, however, the atoms were in a jumbled or disordered arrangement.
"This discovery was very enlightening because it tells us that under extremely high pressures the crystal structure collapses and forms these nano-scale amorphous bands," says Hemker, co-director of the electron microscope lab and senior author of the Science article. "Then the material fractures along these bands because the glassy material appears to be weaker than the crystalline boron carbide."
Although the findings have immediate implications in the production of improved armor materials, the researchers pointed out that their observations also provide experimental evidence that extreme conditions in pressure, temperature and/or loading and quenching rates can lead to the creation of entirely new materials or structures with substantially altered physical and mechanical properties.
The research was funded by the Army Research Laboratory through the Center for Advanced Metallic and Ceramic Systems in Johns Hopkins' Whiting School of Engineering. The Electron Microscope Laboratory used in the study was funded by the W.M. Keck Foundation and the National Science Foundation.