Published online 5 May 2000 | Nature | doi:10.1038/news000511-1


Crunchy filling

Researchers may have spotted an elusive 'superhard' material between the layers of an atomic-scale sandwich, Philip Ball reports.

An international team of scientists may have tracked down one of the most eagerly hunted substances of recent years. Called 'beta-carbon nitride', this material was predicted in 1989 to be as hard as, or possibly even harder than, diamond, the hardest known substance. This would make beta-carbon nitride an extremely valuable engineering material, for example in cutting and grinding tools. But so far, all attempts to make the new material have failed.

Yip-Wah Chung at Northwestern University in Illinois and colleagues from California, Brazil and Italy report in Applied Physics Letters1, that they have spotted something closely matching its description in a composite structure made up of alternating layers of two materials, each just a few atoms thick.

Physicist Marvin Cohen of the University of California first proposed beta-carbon nitride in 1985. Cohen knew that hard materials like diamond contain atoms linked by short, strong bonds. He realised that bonds of this sort form between carbon and nitrogen atoms, and showed how atoms of these two elements could in theory join up into a stable three-dimensional lattice: a crystalline solid, in which the ratio of nitrogen to carbon would be about 1.3:1. In 1989, Cohen and co-worker A.-Y. Liu calculated that this solid should be super-hard.

Cohen's predictions have sparked off much controversy amongst materials scientists. Because of the potential technological importance of beta-carbon nitride, a great deal of research money and effort has been expended in trying to make it. Some researchers consider this to be a wasteful chase after a will o' the wisp; others even question whether the original predictions still bear up. But there is no lack of pursuers of the putative harder-than-diamond substance.

Several sightings have been claimed, but none has withstood close scrutiny, or produced a material that can be reliably made in large quantities. Still no one is sure if beta-carbon nitride can truly exist.

Chung and colleagues' results are the latest addition to the story. They have made a so-called superlattice, an alternating layered sequence of two materials, which the researchers say has features expected of Cohen's hypothetical material. One of these materials is zirconium nitride (ZrN); the other is a mixture of carbon and nitrogen. They denote the combination tentatively as CNx, indicating that the precise composition and structure is not fully clear.

Chung's group discovered several years ago that a layer of crystalline ZrN seems to provide a kind of template that guides carbon and nitrogen atoms on top into a hard, crystalline form. Thus they hoped that a very thin layer of carbon and nitrogen sandwiched between sheets of ZrN might be induced to adopt the structure of Cohen's crystalline solid.

So they grew a superlattice of dozens of alternating ZrN and CNx thin films, in which the latter were less than two thousandths of a millimetre thick -- enough to accommodate fewer than ten atomic layers. By bouncing electron beams off the layers, they show that the CNx films contain at least some orderly, crystalline regions (some previous attempts to make beta-carbon nitride have produced just disorderly atomic arrangements, which can't correspond to Cohen's material). They also calculate that the density of this material is about the same as that predicted for beta-carbon nitride.

But the strongest lines of evidence come from measuring the X-rays kicked out of the CNx layers by the energetic electron beam, and from the way that the material absorbs infrared radiation. These measurements suggest something about the way that the carbon and nitrogen atoms are joined together -- namely, that the bonds between them have the character of those predicted to occur in Cohen's material. Finally, the ratio of nitrogen to carbon in the films matches the prediction of about 1.3:1.

All of this looks promising, but stops tantalizingly short of providing definitive evidence that beta-carbon nitride has been found. So far, the sandwich is spread too thinly for physicists to clearly taste what it contains. 

  • References

    1. Wu,M. L. et al. Formation of carbon nitride with sp3-bonded carbon in CNx/ZrN superlattice coatings. Applied Physics Letters 76 2692 - 2694 2000. | Article | ISI | ChemPort |