Super-tough material mimics metal and crystal

Technology 19 April 2007
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Ingots of rhenium diboride are formed by heating and mixing rhenium and boron

(Image: Richard B. Kaner/Jonathan B. Levine/UCLA)

A super-hard material that is tough enough to scratch diamond could be made cheaply and easily, a new study suggests. The material is made from the metal rhenium and the element boron and resembles both a metal and a crystal in structure.

Diamond is the hardest naturally occurring material known, although researchers have synthesised other substances to rival its hardness. These are generally crystals made from combinations of light elements, including carbon, nitrogen and boron, and are structurally similar to diamond.

These materials are better than diamond at some tasks, such as cutting steel, since the carbon in diamond reacts with steel to form iron carbide, dulling the cutting surface.


“But all the known super-hard materials are very expensive because they [have to be] made at high pressure,” says Sarah Tolbert of the University of California at Los Angeles, US. So, together with Richard Kaner and other colleagues, she took a new approach.

No pressure

The team created their material – rhenium diboride – without resorting to high pressures. “We wanted to change the ease with which hard materials are made,” Tolbert says.

First of all, they considered what makes substances hard. Metals are difficult to compress because their atoms carry dense coats of electrons that repel neighbouring atoms, although this means they can still bend. On the other hand, the atoms in stiff crystals such as diamond have bonds in rigidly prescribed directions, which create a strong, regular scaffold that resists bending, skewing or breaking.

Tolbert and colleagues devised a way to incorporate both these properties. They combined rhenium, a hard-to-compress metal, with boron, which forms strong bonds with rhenium to create a highly incompressible material that also has a rigid scaffold of bonds on the inside.

Melted material

They heated and mixed rhenium and boron powder in a furnace and ran a large electric current through the mixture. This quickly melted the materials, allowing ingots a few centimetres across to form.

The ingots appear shiny like a metal but are also extremely hard, as measured by how much they resist being indented by a diamond tip. The material is nearly as hard as cubic boron nitride and boron suboxide, two of the hardest materials known, and like them can scratch diamond. It should also be able to cut steel without reacting chemically with the iron.

“The novelty of this is that they’ve come up with this hybrid approach [to making extremely hard materials],” says Hector Lorenzana of Lawrence Livermore National Laboratories in California, US. “I think this is just the tip of the iceberg for this family of materials.”

Journal reference: Science (vol 316, p 436)

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