Published online 19 April 2007 | Nature | doi:10.1038/news070416-13

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Scratching diamond just got easier

Ultra-hard material made in the lab without high pressures.

Rhenium diboride powder hot from a furnace (left), and a superhard pellet made from electric heating (right).Rhenium diboride powder hot from a furnace (left), and a superhard pellet made from electric heating (right).Richard B. Kaner and Jonathan B. Levine/UCLA

There's hard, and then there's superhard. Researchers have designed and made a material capable of scratching diamond — and done it without resorting to harsh, high-pressure methods.

Diamond is the hardest material known and is used for drilling and cutting other hard materials. But diamond has its drawbacks — it can't practically be used to cut anything containing iron because a chemical reaction during the high-speed drilling creates iron carbide, which ruins the diamond blade. Alternatives to diamond have been made, but creating them usually requires incredibly high pressures, which is expensive and inconvenient.

The second-hardest material known is boron nitride, with the boron and nitrogen atoms arranged in a cubic structure; this one requires high pressures to make.

"Our goal was to find new hard materials by design, rather than by trial and error," says Sarah Tolbert at the University of California, Los Angeles, who was part of the research team that made the new superhard, ultra-incompressible material: rhenium diboride.

Mix and match

Covalent bonds — strong bonds formed when atoms share their electrons — increase a material's hardness. Diamond, for example, only has these types of bonds. Metals, however, can be incompressible without being so hard because their bonds have more flexibility: they resist if squeezed from all directions at once, but can be deformed if pricked by a sharp point in one specific direction.

Tolbert and her colleagues figured that introducing an atom that likes to form short covalent bonds into a material that is known to be very incompressible should lead to a superhard and ultra-incompressible material.

They started by looking for a metal that had lots of electrons - electrons repel each other so having lots of them makes a material hard to compress. Their trail led them to rhenium, which has plenty of electrons to offer up, and is known to be very incompressible.

Boron atoms fitted nicely between the atoms of rhenium to form short stiff covalent bonds. "You can insert boron without changing the crystal structure very much," says Tolbert. The resulting material, rhenium diboride, had been made for other reasons before, but no one realized quite how hard it was, says John Badding at Pennsylvania State University.

Measured up

When Tolbert and her colleagues measured their rhenium diboride they found it to have a hardness of about 48 gigapascals (GPa); that sounds quite low compared with the mega 70-100 GPa strength of diamond, but it's comparable to the strength of the current second-place holder, boron nitride, which gets a strength rating of about 45-50 Gpa. And the new material is made quite easily in the lab, under ambient pressure, they report in Science1.

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Furthermore, there are hints that the new stuff could be even harder than their measurements suggest: the team found that in some circumstances it could actually scratch diamond. "To scratch diamond you have to have something that is at least in the neighbourhood of the hardness of diamond," says Badding.

That is probably because the material seems to show a different hardness when tested from different angles. Rhenium diboride has a hexagonal structure, and the crystals show different values for various hardness measurements when tested either perpendicular or parallel to the axis of the hexagon, the team reports.

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  • References

    1. Chung H -Y., et al. Science, 316. 436 - 439 (2007).