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.

False hope

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.