New Ultrahard Cubic Boron Nitride Challenges Diamond as a Cutting Tool

by Rachel Nuwer

Materials Research Society | Published: 17 January 2013

ultrahard-cubic-boron-nitride-220 An HRTEM image of microstructure in a nanotwinned cubic boron-nitride (nt-cBN) sample synthesized at 15 GPa and 1800°C. Interlaced domains of lamellar {111} type nanotwins are clearly visible. A more detailed description of this figure can be found at the end of this article. Image credit: Wentao Hu, Bo Xu, and Dongli Yu from Yanshan University. Click image to enlarge.

A diamond may be forever, but its place as the hardest material around may be challenged by a new crystal on the block. Cubic boron nitride, researchers found, can be taken to new levels of ultrahigh hardness by reducing its characteristic microstructural scale beyond measures previously achieved. Superhard, tough and stable, the refined cubic boron nitride’s applications may exceed even those of diamonds.

“Now, we can make this material as hard as single crystal diamond,” says Yanbin Wang, a senior scientist at the University of Chicago’s Center for Advanced Radiation Sources. “In some ways, it’s a better tool than diamonds, especially given its high fracture toughness.”

The key to this achievement was forming ultrafine nanotwins to reduce the characteristic microstructural scale of the cubic boron nitride, as reported by Wang and his colleagues Yongjun Tian, Bo Xu, Dongli Yu, Yanming Ma, and others in a recent issue of Nature. According to the Hall-Petch effect, a material’s hardness increases with its reduction in grain size, since it is much more difficult for deformation defects such as dislocations to propagate across grains or domain boundaries than within a single crystal grain.

But previous experiments showed that, after a certain threshold, the Hall-Petch effect breaks down, causing smallness to become a weakness rather than an advantage. Finding that perfect size—not too big, not too small—tends to stymie research on ultrahard materials.

Wang and his colleagues noticed, however, that most people were trying to reduce grain size of cubic boron nitride from its graphite form, which the material assumes in ambient settings. To create the ultrahard phase change, scientists put the normally soft material under high pressure and temperatures until it shifts from its original graphite structure to a diamond-like substance. But in those experiments, the size reduction always plateaued at around 14 or 15 nanometers.

Instead of beginning with graphite, the researchers wondered, what would happen if onion-like boron nitride—a synthesized graphite-like compound containing zigzagging rather than flat layers of boron and nitrogen—was used instead? To find out, they applied heat and pressure to the onion-like boron nitride, which produced 50 to 100 nanometer grains. Upon further examination, they realized the new material had very fine twin domains—or tiny crystals with mirroring relations to one another —with a thickness of only about 3.8 nanometers. Because of lower boundary energies across the twin domains, they could achieve a finer microstructual size control through forming twinned structures within cubic boron nitride grains. “The atomic arrangement is still cubic but it has a hierarchical microstructure,” Wang says. “We learned a lot over the past five years, and finally everything seems to make sense.”

They went on to produce nanotwinned cubic boron nitride bulk samples and tested their product in the lab. Though still considered the second hardest material after diamonds, the smaller domain size significantly increased the material’s hardness, which can be attributed to the combined contributions from the Hall-Petch effect and quantum confinement effect. Unlike diamonds, however, the nanotwinned cubic boron nitride displays fracture toughness comparable to commercial cemented tungsten carbide. “It’s very hard but at the same time it’s very tough,” Wang says. “It does not want to crack.”

Diamonds also tend to react with transition metals, meaning they have limited applications as cutting tools for certain materials. In contrast, the new product lacks this limitation. The synthesis of nanotwinned cubic boron nitride does not require as much pressure and heat as in previous studies, and Wang hopes to lower these parameters even further so that industrial high pressure equipment can be used to make the product.

For now, Wang and his colleagues at Yanshan University have synthesized bulk samples of several millimeters and plan to create larger versions that can be tested as standard cutting tools. Eventually, the new material could find use in cutting alloys, reinforcing drill heads or assisting in other industrial applications.

Diamonds may also benefit from the discovery. If the same method could be applied to polycrystalline diamond, those materials, too, might be able to achieve previously unattainable hardness. “I’m confident we can improve the diamond properties,” Wang says. “We’re currently thinking about that.”

Read the abstract in Nature  here.

Figure caption: An HRTEM image of microstructure in a nanotwinned cubic boron-nitride (nt-cBN) sample synthesized at 15 GPa and 1800°C. Interlaced domains of lamellar {111} type nanotwins are clearly visible. The upper-right inset shows a photograph of three as-synthesized nt-cBN bulk samples, which are 2 mm in diameter (d) and transparent.  The lower-left inset shows indentation curves represented by Vickers hardness (HV) versus loading force F (in N) relations for a polycrystalline nt-cBN bulk sample (gold), a cBN single crystal (d= 0.3 mm; red) and a synthetic diamond single crystal (d = 3 mm; cyan). The inset inside the hardness plot is an optical micrograph showing radial cracks produced at a load of 19.6 N in nt-cBN, from which the inferred fracture toughness is 12.7 MPa-m1/2 , significantly higher than the typical value for cemented tungsten carbide (~10 MPa-m1/2). Image credit: Wentao Hu, Bo Xu, and Dongli Yu from Yanshan University. Click image to enlarge.

 

 


Comments (1)

Please log in to your MRS account to comment using the Log-in button at the top of this page. You can create an MRS account here.
  • Where can I purchase nanotwined C-BN? I see tons of publications about it but nothing about manufacturing on commercial bases. Please, advise. Edward - Sr RD engineer
  • Edward , 07 Jun 2013