Posted on 10/18/2005 5:14:00 PM PDT by strategofr
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 The Johns Hopkins University 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 previously been seen 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 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," said 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?" said 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," said 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. The researchers said their findings showed the benefits that can emerge when university scientists and researchers in government laboratories work together at the interface of science and engineering.
"This has been a highly successful collaboration," McCauley said. "It's provided both scientific advances and practical assistance to the Army."
Have you let MIT know of your musings?
Very interesting. I must say though, does this require a a study? It doesn't take a scientist to figure out why a barrier will stop a dump truck traveling at 60MPH while that same barrier wont prevent a Dodge Viper traveling at 120MPH from passing through.
Instead of trying to modify the armor, why not put another layer of a separate material outside of the armor, which will slow the incoming projectile enough that the "hypervelocity impact induced phase change" does not occur?
um, they were wondering HOW is breaks under greater impact right? not WHY.
just about everything will break eventually as you increase the pressure/impact.
they should have used gooder english for writing this here article. its bad enough to change the meaning.
Jim McCauley... the inventor of transparent aluminum. Good man.
I disagree. While it may be true that "it doesn't take a scientist to figure that a barrier...", it is a much more difficult question to figure out "why a barrier..."
Maybe some of this stuff:
Air Force testing new transparent armor
Point taken. ;^)
I suggest hyper-dimensional phase-shifting armor.
Armors generally work by either breaking and catching the projectile (like boron carbide body armor) or by eroding the projectile away (like M1 tank armor). Armor of the latter variety doesn't generally slow the projectile down appreciably until it is nearly fully eroded, and so an eroding cover on B4C wouldn't help. On the other hand, a cover that could slow down the projectile would be a suitable replacement for B4C, and so just dispense with it altogether.
In fact, there are specialized cases where what you suggest may make sense and is done, but in general, adding a cover to the base armor, without diminishing the base armor is a recipe for overweight, overthick protection.
> I suggest hyper-dimensional phase-shifting armor.
Beam me up, Scotty.
> Perhaps if they dig deeper, they will discover the Rumsfeld/Cheny effect.
Is that the one where, if you are a real hard-ass, you don't need no steenken armor. Yeah, I think the chemical composition is M4C (Moron-Clymer).
Because that's not the question. The question is why the armor was performing less well than predicted for a given velocity. The study figured out what the problem was, and because we now know the problem, they'll be able to design more effective armors in the future.
>>"The question now is, how should we try to change the boron carbide?"
> Positive thinking: A 12 Step Program. Really this is the kind of question you should ask over at DU?
Material scientists talk like this all the time. It's quite unnerving to us Mech.E's
Easily defeated by a transphasic projectile.
Then it is no longer lightweight. It also becomes bulky.....
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