Harder Than Diamond?

The only thing harder than diamond has been the task of coming up with an even tougher material. Despite physicists' predictions that some exotic materials should be harder, researchers have thus far failed to make enough of them to test their hardness. Now a team of theoretical physicists from Korea and the United States is suggesting a new, and possibly easier approach: Give diamond a tougher coating. In the current Physical Review Letters, they calculate that adding a thin layer of boron atoms will create a surface nearly 20% tougher than diamond alone. If the idea pans out, it could pave the way to making harder protective coatings for everything from machine tools to computer disk drives.

Diamond is a crystalline form of carbon in which each carbon atom is bonded to four others in a sturdy tetrahedron, a shape that somewhat resembles a child's jack, in this case with a single atom in the center with the others surrounding it. A diamond's surface looks like a bunch of jacks sitting on a tabletop, with each jack resting on a base of three of its atoms and the fourth sticking up in the air. Directly above the fourth atom is a final atom--the one that's actually on the surface. Usually this capping atom is hydrogen.

Seungwu Han and Jisoon Ihm of Seoul National University in Korea, and Steven Louie and Marvin Cohen of the University of California, Berkeley, knew that if they could find a bonding arrangement that allowed atoms to sit more closely together than they do in the tetrahedrons, that material would likely be harder. In their model they inserted boron atoms, which can form these tighter bonds with carbon atoms. Unlike diamond-bound carbon, boron prefers to bind only to three other atoms. So their idea was to remove the topmost hydrogen atoms and the central carbons to which the hydrogens are bonded, and replace these with borons. Their calculations then showed that a measure related to the hardness, known as the bulk modulus, for this surface was 18% larger than for a simple diamond surface.

"It looks interesting," says Charles Lieber, a materials researcher at Harvard University, who cautions that he has not had time to study the paper in detail. However, Lieber adds that "it certainly would be doable to try experimentally." If such experiments succeed, diamond's sparkling performance may be in for a little competition.

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