Its impressive combination of hardness, toughness, and chemical stability opens up a wide range of industrial applications, such as drilling and exploring deeper and deeper into Earth’s interior.
A team led by Yongjun Tian from Yanshan University in China started with cubic boron nitride -- a superhard material that’s already used to cut hard stuff in situations where diamond completely fails. Unlike diamond and many other extremely hard materials, boron nitride is based on a latticework of boron and nitrogen atoms – not carbon.
To make the new material -- which they say is harder than some forms of diamond – the team modified structure of cubic boron nitride by decreasing the size of the grains.
Specifically, they reduced the scale of the structures within the material by generating features called ‘ultrafine nanotwins.’ Scientific American explains:
A nanotwin is a crystalline segment that mirrors the orientation of atoms on the other side of an interface (a so-called twin boundary) within a material. As such, a polycrystal made of nanotwin domains is a bit like a slab of plywood where the wood grain reverses direction in each successive layer.
- First, they used nested layers of nitrogen and boron atoms to create an onion-like material.
- Then, when compressed and subjected to intense heat and pressure (1,800 degrees Celsius and pressures of up to 15 gigapascals), the nanoparticles coalesced into tiny grains comprising numerous nanotwin domains.
- The resulting transparent lumps of cubic boron nitride (pictured) were riddled with nanotwin segments that are just 3.8 nanometers thick on average. (Previous attempts have managed to reduce grain sizes to 14 nanometers.)
Those samples had a measured hardness of up to 108 gigapascals. That’s slightly harder than synthetic diamond but less hard than polycrystalline diamonds that are made of nanoscale grains.
The work was published in Nature today.
[Via Scientific American]
Image from Tian et al., 2013