Natural diamond was first reported in India 2700 years ago and is the hardest material known to man. In the 1950s, scientists began synthesizing artificial diamond at high pressures and temperatures in the lab. Since then, they have been striving to create materials harder than natural diamond, but many thought that such a feat might just be impossible.

Now, a team led by Yongjun Tian of Yanshan University in Qinhuangdao is saying that diamond samples containing a high density of nanoscale twin domains are harder than natural or other synthetic types of diamond. “The new nanotwinned material is also more resistant to fracture, and so breaks the old adage for superhard materials: the higher the hardness of a material, the lower its fracture toughness,” says Tian.

Natural diamond’s hardness means that it has long been used in cutting and shaping tools, but the fact that it begins to oxidize at around 800 °C means that it quickly wears out at these high temperatures. This is one of diamond’s biggest drawbacks. Researchers have tried increasing the hardness of synthetic diamond by decreasing its grain size (down to around 10–30 nm), but these materials suffer from even poorer thermal stability. The new nanotwinned material, which has a nanoscale structure consisting not of tiny grains but of crystal twin domains that measure just 5 nm across, overcomes this problem too since it oxidizes at temperatures of more than 1000 °C.

Researchers normally synthesize nanostructured diamond using graphite as a precursor, explains Tian. The c-axis in the graphite crystal structure always lies perpendicular to the sp2 hybridized atomic layers of carbon – and at different positions across the atomic layers. When graphite transforms into diamond at high pressures and temperatures, only relatively large grains of nanodiamond can be obtained because neighbouring diamond grains with similar orientations can easily merge and become bigger. This is why the smallest grain size ever reported in synthetic diamond is around 10 nm.

Onion-like carbon nanoparticles

Instead of ordinary graphite, Tian and colleagues used carbon nanoparticles consisting of concentric graphite-like shells as precursors. These structures are called onion-like carbon nanoparticles. Like the c-axes in ordinary graphite, the c-axes in onion carbon lie perpendicular to the sp2-hybrized carbon layer – but not always. This time, they lie along different directions at different positions on the onion ring.

The researchers subjected these structures to high pressures of between 18 and 25 GPa and temperatures of up to 2000 °C and obtained a transparent material containing nanotwinned, nanocrystalline diamond with an average twin domain size of just 5 nm.

Puckered layers and stacking faults

“We require two conditions to form nanotwinned structures inside diamond nanograins,” explains Tian. “First, the diamond phase forming under high pressure and temperature must nucleate out quickly but grow slowly. The onion carbon precursors we used contain lots of puckered layers and stacking faults that provide ideal sites for nucleating diamond. And that is not all: the spherical atomic arrangement of the onion carbon also produces extra inner pressure that slows down the growth of diamond nuclei.”

Second, and also as important, the diamond phase generated “inherits” certain imprints from the onion carbon precursor, he adds. The distorted hexagonal phase of this carbon transforms into diamond via a martensitic-type mechanism by shifting carbon atoms. The different local areas in the sample have different orientations and the specific relationships between how the onion carbon and diamond are oriented at the outset are preserved as the transformation occurs – something that produces laminated nanotwins in the final diamond nanograins.

Higher hardness

The researchers found that the hardness of their new material is as high as 200 GPa. To compare, single-crystal diamond’s hardness lies between 60 and 130 GPa. Values for nanocrystalline diamonds without nanotwins are between 130 and 145 GPa.

“Our new nanodiamond might be used as an advanced tool material in a wide variety of machining, drilling and die applications in industry,” Tian told nanotechweb.org. “It would also be ideal in anvil cells because we expect that it could withstand pressures as high as 500 GPa or even 1 TPa, which is much more than that possible with the traditional diamond anvil cells routinely used in scientific instruments today.”

Plans to upscale

The team says that it is now busy working on further lowering the synthesis pressure for making nanotwinned diamond by using finer carbon onion precursors. “Our goal is to upscale our method so that we can produce industrial levels of the material.” Until now, the researchers have succeeded in making millimetre-sized pieces in the lab.

Another interesting line of research is to synthesize nanotwinned diamond-boron nitride composites,” he adds. “Boron nitride (BN) is a diamond-like material and these composites should have an oxidation temperature and hardness that lie between that of nanotwinned diamond and nanotwinned BN but they should be much more fracture resistant.”

The current research is detailed in Nature doi:10.1038/nature13381.