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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. |
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