Making nearly diamond hard metals

The end product of a hard material is interesting but it is the understanding of how to tune materials to make them superhard that is more impressive and even more interesting.

Researchers have designed and created superhard materials that can be made without using high pressure. There are two ways to make super-hard materials that are “ultra-incompressible,” meaning they are resistant to shape deformation, which is a necessary condition for hardness: One is to imitate diamond by using carbon and combining it with boron or nitrogen to maintain short bonds; the other is to look for metals that are already incompressible and try to make them hard, said Kaner. He and his colleagues are developing the second approach.

“Our idea is to combine an incompressible metal, which happens to be soft, with short covalent bonds to make it hard,” said Kaner, who is a member of the California NanoSystems Institute (CNSI) at UCLA, which encourages cross-disciplinary collaboration to solve problems in nanoscience and nanotechnology.

They made Rhenium diboride. It is as incompressible as diamond in one direction, and in the other direction, just slightly more compressible.”

Rhenium is a fairly dense, soft metal, which is next to osmium on the periodic table of chemical elements. “We formed short covalent bonds, pushing the rheniums apart by just 5 percent from where they were in rhenium metal, making it both incompressible and very hard. The rhenium-rhenium distance expanded by only 5 percent from the metal — that’s the key to this Science paper.

At low applied forces, the hardness of rhenium diboride is equivalent to cubic boron nitride, the second-hardest material known, Kaner said. At higher applied forces, rhenium diboride is a little bit below that.

“Our material is hard enough to scratch diamond, and much harder than osmium diboride,” he said.

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