Boron is Harder Than Diamond
by : Philip Ball
You don’t often break a diamond. So when in 2003 Dave Mao cracked a tooth of his diamond anvil, he knew something extraordinary must have happened. Together with his daughter Wendy and other colleagues at the Geophysical Laboratory of the Carnegie Institution for Science in Washington DC, he was using the device to test materials at pressures many millions of times higher than those at the Earth’s surface – higher even than in our planet’s core – by squeezing them between two tiny diamond jaws.
Behind the glitz, diamond is just a form of carbon. It is, however, by common consent the hardest material known. The substance in the Maos’ test cell had also begun as pure carbon. It was plain old graphite – the soft, slippery stuff that is used for pencil leads and lubricants. Clearly, something had happened in the anvil cell to make it awesomely hard.
It seemed the Maos might accidentally have succeeded where many before had failed. Had they made the first superhard material that matched or even surpassed diamond? Probably not, as it turned out. Six years and several twists later, though, that feat might at last have been achieved, though not with pure carbon. If the latest reports are right, the hardness crown has changed hands at last.
Why the fuss? Diamond’s hardness has served us well enough over the years. Diamond-studded saws and drills have been around since at least the time of the Napoleonic wars. They are not even particularly expensive any more, since researchers at the General Electric Company in Schenectady, New York, discovered in the 1950s that you can make synthetic diamonds by subjecting softer carbon-rich materials to immense temperatures and pressures.
Unfortunately, diamond doesn’t always cut it. In particular, it does not cut steel: the carbon just dissolves in the hot iron, reacting to form iron carbide. This susceptibility to heat and chemical attack is one reason why we are on the lookout for alternatives to diamond, says Artem Oganov, a materials physicist at Stony Brook University in New York. Diamond is also electrically insulating, which can be a limitation. “It would be good to have a range of superhard materials that have other properties,” says Oganov, such as metallic or semiconducting characteristics.
Finding rivals has been a frustrating business. In part that is because although all of us know a hard object when we bump into one, working out what makes things hard is – well, hard. “Intuitively, covalent bonds and high bond-strength are a requirement,” says Mao. Covalent bonds are one of the ways in which atoms link up to build molecules, large and small. They form when the smeared-out cloud, or “orbital”, occupied by an electron belonging to an atom overlaps with one belonging to another. In general, the better the overlap, the stronger the bond. Hardness typically seems to arise in materials that have strong, short bonds.
Most of us know a hard object when we bump into one – but not what makes it hard
So might there be some tweak to carbon’s atomic arrangements that would optimise the strength of its bonds and make a material harder than diamond? In the late 1980s, Marvin Cohen, a materials physicist at the University of California, Berkeley, thought so. He theorised that the strong bonds linking atoms in a hypothetical crystalline compound of carbon and nitrogen dubbed beta carbon nitride should make it particularly hard. But despite extensive efforts, the material has been difficult to synthesise, and its hoped-for hardness has never been demonstrated.
And what of Dave and Wendy Mao’s miraculously hardened graphite? Unfortunately, the usual technique for determining a material’s structure – bouncing X-rays off the sample and looking at the resulting diffraction pattern – proved extremely difficult for the tiny quantities in the diamond anvil cell. Opening the device to take a closer look didn’t help, either, because the material morphed back into graphite as soon as the pressure was released.
Whatever it is, a material like the Maos’ altered graphite could have its uses. Imagine, for example, an impact-resistant “smart skin” that, while normally soft and flexible, becomes the hardest thing going under a large force. Until we know what a material looks like at the atomic scale, however, reliable fabrication remains a problem.
Mao and theorist Yanming Ma and colleagues at Jilin University in Changchun, north-east China, recently proposed that the transformed graphite has a structure they call monoclinic carbon. This M-carbon forms when graphite sheets buckle and form extra chemical bonds between the layers (Physical Review Letters, vol 102, p 175506). The resulting structure, they calculated, should be almost as hard as diamond – although not quite. It is also strikingly similar to a form of carbon made by shining strong laser light onto graphite, reported in May this year by Katsumi Tanimura and colleagues at Osaka University, Japan (Physical Review Letters, vol 102, p 087402). The Japanese team did not attempt to assess the hardness of their material.
This piece of unfinished business aside, only one material has been claimed so far to crack the diamond ceiling – diamond itself. A nanocrystalline form of diamond, sometimes called aggregated diamond nanorods, was described in 2003 by Tetsuo Irifune and his colleagues at Ehime University in Japan. Since then, Natalia Dubrovinskaia and her colleagues at the University of Bayreuth in Germany have found that a tip made of these nanorods could scratch regular diamond, seemingly indicating a greater hardness.
So much for carbon. But who says we need it? In a quest for completely different superhard materials, Richard Kaner at the University of California, Los Angeles, and his team have been exploring the nether regions of the periodic table. Their first stop was the element osmium, each atom of which has eight “valence” electrons available for covalent bonding – the highest number known. More electrons, they reasoned, meant stronger bonds and perhaps superhardness. In 2005 the strategy seemed to bear fruit as the team discovered that osmium diboride, a repeating structure of one osmium atom bound to two boron atoms, is indeed very hard – although still only about a quarter as hard as diamond.
Two years later, they claimed that rhenium diboride was even harder, though still not a match for diamond. Rhenium is osmium’s neighbour in the periodic table, and although its valence electron density is smaller, crucially it could make shorter, and therefore stronger, bonds. Kaner’s claim has not gone undisputed.
Meanwhile, attention was switching back to the lighter end of the periodic table, home to many elements that can form short, strong bonds. One such is boron, which sits just one berth over from carbon. The idea that boron has superhardness potential goes back at least to 1965, when Robert Wentorf, one of the General Electric team that made synthetic diamond, claimed to have made superhard crystals of boron at a pressure of 100,000 atmospheres and a temperature of 1500 °C. He couldn’t work out what the material’s structure was, though, and the idea was shelved for 40 years.
“People were basically scared of boron,” says Oganov by way of explanation. Boron forms several complex structures that are hard to tell apart. What’s more, it reacts with nearly everything, and even a trace of impurities can drastically change the structure and properties of the boron crystal.
It was only in February this year that a team led by Oganov published a structure for the superhard boron crystal – a repeating pattern of 28 boron atoms they called B28 (Nature, vol 457, p 863). In May, Dubrovinskaia and her team announced that they had made large crystals of B28 that were about half as hard as diamond (Physical Review Letters, vol 102, p 185501). Close, but still no diamond necklace.
Boron and on
Pure boron is not the last word, though. Boron nitride – in which boron is combined with nitrogen – forms analogues of all the known carbon phases. There is a soft variant called h-BN, which is made of sheets of hexagonal rings just like graphite, and finds similar use as a lubricant. Then there’s cubic boron nitride, or c-BN, which has a structure similar to diamond, and for a long time has played second fiddle only to diamond in hardness.
There is a third version, too, known as wurtzite or w-BN, which is comparable to a diamond-like form of carbon known as lonsdaleite. It had been made since the 1970s by using high pressure or explosive shock waves to squeeze h-BN, but had only been fabricated in quantities too small for its hardness to be measurable by any conventional means. In 2007, however, Dubrovinskaia and her colleagues succeeded in making a mosaic of w-BN crystals which they claimed had a hardness comparable to that of diamond (Applied Physics Letters, vol 90, p 101912).
They thought that the material’s hardness came about because its crystals were tiny – just 10 or so nanometres across. Many crystalline materials get harder as the grains that make up their crystals get smaller, because grain boundaries prevent the movement of defects in the packing of atoms. But earlier this year, Changfeng Chen of the University of Nevada, Las Vegas, and his colleagues offered another explanation. They think that w-BN may be inherently hard, because it can transform into another, stronger structure when another material presses into it. The pressure causes chemical bonds to flip into a different arrangement which looks like that of c-BN, but has its network of atomic bonds ideally positioned to resist stress (Physical Review Letters, vol 102, p 055503).
“It’s a bit like someone changing their body posture in response to applied stress so that they can carry a higher load,” says Chen. As a result, the material becomes even harder than diamond, at least in theory. What’s more, Chen and colleagues figured that the same thing that happens to w-BN should happen for lonsdaleite, its carbon counterpart. That could actually be harder to scratch and indent than diamond itself.
Meanwhile Vladimir Solozhenko, now at the University of Paris, and his co-workers had the idea that it might be best to throw all the most promising elements that have popped up in most hard materials to date – carbon, boron and nitrogen – into the pot. In 2001 they reported that one particular combination, BC2N, has a hardness midway between c-BN and diamond (Diamond and Related Materials, vol 10, p 2228).
Until all such options have been explored, there is still plenty to play with, and no reason to think that diamond is as hard as it gets. In any case, new materials don’t have to surpass diamond in order to be useful: c-BN has been used as an abrasive and in cutting tools for many years. Though only half as hard as diamond, it is by far the best material for grinding through steel.
It is true, too, that the real challenge in industries that use superhard materials, including construction, mining and aerospace engineering, is to find materials that are not just hard, but cheap and easy to make as well. In the end, perhaps, the real crown will go not to the material that cuts diamond, but to the one that undercuts it.
Philip Ball is a freelance science writer based in London
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