At the centre of our planet, the weight of billions of tonnes of rock pushing down from above creates crushing forces that are more than three million times the atmospheric pressure here on the surface. Yet on an unassuming laboratory worktop in northern Bavaria, physicist Natalia Dubrovinskaia is able to exceed even these formidable pressures several times over, with a device she can hold in the palm of her hands.
The recent successes could have wide ranging impacts from breakthroughs in medicine to changing our understanding of distant worlds
With a few precise turns of a couple of screws at the top of a small metal cylinder she can generate pressures more than three times greater than those found in the Earth’s core. Amazingly, she and her colleagues at the University of Bayreuth have discovered a super-material capable of withstanding these phenomenal forces. It is so hard that it is capable of leaving a dent in diamond crystals – long regarded as the hardest material in the world.
Her new substance is the culmination of decades of a sort of modern day alchemy that has seen scientists tweak and tinker with the chemical structures of substances to alter their properties. It is a journey that has involved many false starts and blind alleys, but the recent successes could have wide ranging impacts from breakthroughs in medicine to changing our understanding of distant worlds.
Humankind’s love-affair with hard materials goes back to the earliest days of our species, when our ancestors used hard stones to shape other softer rocks into blades. These were later replaced by progressively harder metals until just over 2,000 years ago when the first steel was produced, which remained as the hardest-known material until scientists discovered they could coat tools in diamond at the end of the 18th Century.
Despite its obvious allure for jewellery, most processed diamonds are used to create ultra-hard coatings for tools and drills that resist wear. For the mining and oil industry, such diamond-tipped tools are an essential part of their operations – without them they could not burrow through the hundreds of metres of rock to reach the valuable resources beneath the Earth. “Hard coatings are needed for a variety of applications ranging from high-speed machine cutting tools, deep-sea drilling, gas and oil explorations to biomedical applications,” says Jagdish Narayan, chair of material science at North Carolina State University.
To understand what makes a material hard, it is necessary to look at the atomic structure of its crystals.
Diamonds are formed from the same carbon atoms that also make up the soft graphite found in the centre of pencils. The difference between these two forms of carbon is the arrangement of the atoms. Graphite is formed from sheets of carbon atoms arranged in flat hexagons, held together by weak attractive forces between each layer.
In diamonds, however, the carbon atoms are bound together in a tetrahedral formation, a shape that is extremely rigid. Combined with the strong carbon to carbon bonds, it makes diamond extremely hard.
The word “diamond” itself is derived from the ancient Greek adámas, or unbreakable. Yet diamond does break and crumble at high enough pressures. Tiny flaws in a crystal can also weaken it, making the diamond vulnerable to disintegration.
For scientists, this creates a problem – how do you study the behaviour of materials at pressures above the point when even the hardest naturally occurring substance on the planet begins to break apart? You need to find something stronger.
Perhaps unsurprisingly, the search for superhard materials begins by attempting to replicate the structure of diamond, but there are only a few elements able to bind together in this way.
One such material is boron nitride. Like carbon, this synthetic material comes in several different forms, but it is possible to replicate the structure of diamond by replacing the carbon atoms with nitrogen and boron atoms. First created in 1957 and known as cubic boron nitride, it was initially reported to be hard enough to scratch diamond – hopes that quickly dulled as later tests showed that it is less than half as hard as its carbon-based counterpart.
The next few decades saw a string of similar disappointments as scientists looked for other ways to bond those three elements – nitrogen, boron and carbon – in various structures. Thin films of one such material were produced in 1972, however, creating a form that mimicked the structure of diamond; the downside was that it involved complex chemistry and extremely high temperatures to produce. It was not until 2001 that a diamond-like boron carbon nitride was reported to have been produced by researchers at the National Academy of Sciences of Ukraine in Kiev with colleagues in France and Germany. But they found while the new material was harder than crystals of cubic boron nitride it was still fell short of diamond.
Then, seven years ago, Changfeng Chen, a physicist at the University of Nevada, and colleagues at Shanghai Jiao Tong University in China, thought they had hit on something that might topple diamond from its pedestal. They calculated that a bizarre hexagonal form of boron nitride, known as wurtzite boron nitride would be able to resist 18% more stress than diamond. This rare material has a similar tetrahedral structure to diamond and cubic boron nitride except the bonds form at different angles. Computer simulations of how this material might behave when put under pressure suggested some of these bonds are flexible and re-orientate themselves by about 90 degrees when under stress to relieve tension.
While the bonds in diamond respond in a similar way to stress, wurtzite boron nitride becomes nearly 80% stronger under higher pressures. The snag is that wurtzite boron nitride is rather dangerous to create – it only occurs naturally in the extreme heat and pressure of volcanic eruptions and has to be created synthetically in explosions that mimic these conditions, meaning it is notoriously difficult to obtain in sufficient quantities and it has yet to be tested. Similar problems have limited the potential study of a related substance, known as lonsdaleite, that should be able to withstand up to 58% more stress than standard diamond crystals.
It is only within the past couple of years that we are finally seeing some breakthroughs. In 2015, Jagdish Narayan, and his colleagues at North Carolina State University revealed they had melted a non-crystalline form of carbon known as glassy-carbon with a rapid laser pulse, heating it to 3,700C (6690F) before rapidly cooling it. This cooling, or quenching, step led to the name Q-carbon. What they had produced was a strange, but exceptionally strong amorphous form of carbon. Unlike other forms of carbon it is magnetic and glows when exposed to light.
The structure of the material itself was mainly constructed from diamond-type bonding but also had about 10 to 15% graphite-type bonding. Tests by the research team suggest Q-carbon could be at least 60% harder than diamond, but this has yet to be definitively confirmed. True hardness tests require samples to be indented with a point that is harder than the material under examination. When crushing a sample of Q-carbon between two sharpened diamond points, this creates a problem. “The diamond tips deformed during hardness measurements of Q-carbon,” said Narayan.
It is here that Dubrovinskaia’s superhard anvils enter the picture. Her new material is a unique form of carbon known as nanocrystalline diamond balls, and rather than being made from a single crystal lattice of carbon atoms like the gemstones we prize for engagement rings, it is made up of lots of tiny individual crystals – each 11,000 times smaller than the width of a human hair – that are bound to each other by a layer of graphene, the Nobel Prize-winning wonder-material made of a carbon layer just one atom thick.
It is the same as balancing 3,000 adult African elephants on a single stiletto heel
Whereas a diamond crystal will begin to give way at pressures of up to 120 Giga Pascals (GPa), the new material, however, can withstand at least 460 GPa. It can even survive when pressed together to generate pressures of up to 1,000 GPa. That makes these tiny spheres harder than any other known substance on the planet. To put that in perspective, that is the same as balancing 3,000 adult African elephants on a single stiletto heel. “This is the hardest of all the known superhard materials as it indents all of them,” said Dubrovinskaia.
These nanocrystalline diamond balls are also transparent, allowing them to act like tiny lenses through which researchers can peer at the material being crushed using X-rays. “It allows us to pressurise material under investigation and observe what happens,” says Dubrovinskaia. “Achieving ultra-high pressures opens new horizons for a deeper understanding of matter.”
Already Dubrovinskaia and her colleagues have applied this to study osmium, a metal that is among the most resistant to being compressed in the world. They found it could resist compression pressures of over 750 GPa. At this point, the inner electrons, which are normally tightly bound to the nucleus of the metal atom and are highly stable, began to interact with each other. The researchers believe this strange behaviour could lead the metal to change from being a solid into a previously unknown state of matter. They hope to investigate what properties this gives osmium in the future.
These superhard nanodiamonds go beyond simply giving new hardened edges to cut through rock and metal. Such nanodiamonds in powdered form are finding uses in the cosmetics industry as they are highly absorbent, holding on to active substances. They are also readily absorbed by the skin, carrying those substances with them. The medical industry is also beginning to explore ways of using them to carry drugs such as for chemotherapy into difficult to reach areas of the body. Research has also shown nanodiamonds can promote the growth of bone and cartilage.
Most profoundly, this recent work could also help unlock some of the mysteries of our solar system. They have organised an international conference of experts next month where some of these new possibilities will be discussed. While at the centre of the Earth, pressure is thought to reach up to 360 GPa, the largest planet orbiting our sun, the gas giant Jupiter, is thought to have pressures up to 4,500 GPa at its core.
At these pressures, elements start to behave in strange ways. Hydrogen – normally a gas on Earth – starts to behave like a metal, for example, and becomes capable of conducting electricity. Dubrovinskaia and Dubrovinsky hope their superhard diamonds could help us create those kinds of cosmic conditions. “We could start to model the interior of giant planets or extraterrestrial super-Earths outside our solar system,” said Dubrovinskaia. “I think it is even more fascinating we can do this in something we can hold in our hands.”
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