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[March 2003]
Researchers Solve Ballistic Mystery in Ceramic Armour
Ballistics experts in recent years have puzzled over a troubling loss of impact resistance in boron carbide, sometimes used in protective armour. The material does an excellent job in 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 (Baltimore, MD, USA) and the US 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 amorphisation, or transformation to a glassy material, has previously been seen in minerals and semiconductors but the researchers say that they are the first to report such behaviour 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 the door towards 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 armour material for military, police, diplomatic and other vehicles.
The research was funded by the US Army Research Laboratory through the Center for Advanced Metallic and Ceramic Systems in Johns Hopkins' Whiting School of Engineering.
ENDS