The remarkable toughness of hexagonal boron nitride unmasked

PICTURE: A scanning electron microscope image shows branching cracks in a 2D hexagonal boron nitride (h-BN) single crystal. The experiments and computer modeling of Rice University and Nanyang Technological University have shown … see After

Credit: Image courtesy of J. Lou / Rice University

HOUSTON – (June 2, 2021) – It’s official: hexagonal boron nitride (h-BN) is the iron man of 2D materials, so resistant to cracking that it defies a century-old theoretical description still used by engineers to measure toughness.

“What we have observed in this material is remarkable,” said Jun Lou of Rice University, co-author of a Nature article published this week. “No one expected to see this in 2D materials. That’s why it’s so exciting.”

Lou explains the importance of the discovery by comparing the fracture toughness of h-BN with that of its better-known cousin, graphene. Structurally, graphene and h-BN are almost identical. In each, the atoms are arranged in a flat network of interconnected hexagons. In graphene, all atoms are carbon, and in h-BN, each hexagon contains three nitrogen atoms and three boron atoms.

The carbon-to-carbon bonds in graphene are the strongest in nature, which should make graphene the strongest substance there is. But there is a catch. If even a few atoms are out of place, graphene’s performance can range from extraordinary to poor. And in the real world, no material is free from defects, Lou said, which is why fracture toughness – or resistance to crack growth – is so important in engineering: it describes exactly how a real-world material can resist before it fails.

“We measured the fracture toughness of graphene seven years ago, and it’s actually not very fracture resistant,” Lou said. “If you have a crack in the mesh, a small load will only break that material.”

In short, graphene is brittle. British engineer AA Griffith published a fundamental theoretical study on fracture mechanics in 1921 which described the fracture of brittle materials. Griffith’s work described the relationship between the size of a crack in a material and the amount of force required to grow the crack.

Lou’s study in 2014 showed that the tensile strength of graphene could be explained by the proven Griffith criterion. Considering the structural similarities of h-BN to graphene, it was also expected to be brittle.

This is not the case. The breaking strength of hexagonal boron nitride is about 10 times that of graphene, and the behavior of h-BN in breaking tests was so unexpected that it defied description with Griffith’s formula. Showing precisely how he behaved and why took more than 1,000 hours of experiments in Lou’s laboratory in Rice and equally painstaking theoretical work led by co-author Huajian Gao at Nanyang Technological University ( NTU) in Singapore.

“What makes this work so exciting is that it unveils an intrinsic hardening mechanism in a material that is supposed to be perfectly brittle,” Gao said. “Apparently, even Griffith couldn’t predict such drastically different fracture behaviors in two brittle materials with similar atomic structures.”

Lou, Gao and their colleagues traced the extremely different behaviors of the materials to slight asymmetries resulting from h-BN containing two elements instead of one.

“Boron and nitrogen aren’t the same, so even if you have this hexagon, it’s not exactly like the carbon hexagon (in graphene) because of this asymmetric arrangement,” Lou said.

He said the details of the theoretical description are complex, but the result is that the cracks in the h-BN tend to branch out and spin. In graphene, the tip of the crack goes straight through the material, opening links like a zipper. But the asymmetry of the network in h-BN creates a “bifurcation” where branches can form.

“If the crack is branched, it means it is spinning,” Lou said. “If you have this spinning crack, it basically costs extra energy to push the crack further. So you’ve effectively hardened your material making it much harder to propagate the crack.”

Gao said, “The inherent asymmetry of the network gives the h-BN a permanent tendency for a moving crack to get out of its way, like a skier who has lost the ability to maintain a balanced posture to go straight ahead. . “

Hexagonal boron nitride is already an extremely important material for 2D electronics and other applications due to its heat resistance, chemical stability and dielectric properties, which allow it to serve as both a base support and insulating layer between the electronic components. Lou said the surprising toughness of h-BN could also make it the perfect option for adding tear resistance to flexible electronic components made from 2D materials, which tend to be brittle.

“The 2D materials electronics niche is the flexible device,” Lou said.

In addition to applications like electronic textiles, 2D electronics are thin enough for more exotic applications like electronic tattoos and implants that could be attached directly to the brain, he said.

“For this type of setup, you have to make sure that the material itself is mechanically strong when you bend it,” Lou said. “The fact that h-BN is so fracture resistant is great news for the 2D electronics community, as it can use this material as a very effective protective layer.”

Gao said the findings could also point to a new path for making strong mechanical metamaterials through engineered structural asymmetry.

“Under extreme loads, failure may be inevitable, but its catastrophic effects can be mitigated through structural design,” Gao said.

Lou is Professor and Associate Department Head in Materials Science and Nanotechnology and Professor of Chemistry at Rice. Gao is a Distinguished University Professor in NTU Engineering and Science Schools.

Rice-affiliated co-authors are Yingchao Yang, now an assistant professor at the University of Maine, Chao Wang, now at the Harbin Institute of Technology in China, and Boyu Zhang. Other co-authors include Bo Ni of Brown University; Xiaoyan Li from Tsinghua University in China; Guangyuan Lu, Qinghua Zhang, Lin Gu, and Xiaoming Xie from the Chinese Academy of Sciences; and Zhigong Song of the Agency for Science, Technology and Research in Singapore and formerly of Tsinghua and Brown.


The research was supported by the Department of Energy (DE-SC0018193) and simulations were performed on resources provided by the National Science Foundation’s Extreme Science and Engineering Discovery Environment (MSS090046) at the Center for Computing and viewing from Brown University.

Links and Resources:

The DOI of Nature the paper is: 10.1038 / s41586-021-03488-1

A copy of the document is available at: https: //do /ten.1038 /s41586-021-03488-1

High resolution IMAGES are available for download at:

https: //news /04 /0503_TOUGH-223hBN12-lg.jpg

LEGEND: A scanning electron microscope image shows branching cracks in a 2D hexagonal boron nitride (h-BN) single crystal. Experiments and computer modeling from Rice University and Nanyang Technological University have shown that the asymmetry of the h-BN network allows cracks to follow branching paths, which effectively strengthens the 2D material by rendering more difficult the propagation of cracks. (Image courtesy of J. Lou / Rice University)

https: //news /04 /0503_TOUGH-fig-lg.jpg

LEGEND: Computer simulations at Nanyang Technological University in Singapore have helped explain the unexpected fracture toughness of 2D hexagonal boron nitride. The material’s intrinsic toughness comes from slight asymmetries in its atomic structure (left), which produce a permanent tendency for mobile cracks to follow branched paths (right). (Image courtesy of H. Gao / NTU)

https: //news /04 /0503_TOUGH-jlbz7005-lg.jpg

LEGEND: Experiments by Rice University materials scientists Jun Lou (left) and Boyu Zhang have shown that 2D hexagonal boron nitride is surprisingly resistant to cracking. (Photo by Jeff Fitlow / Rice University)

https: //news /04 /0503_TOUGH-jl7001-lg.jpg

LEGEND: Jun Lou (Photo by Jeff Fitlow / Rice University)

https: //news /04 /0503_TOUGH-bz6982-lg.jpg

LEGEND: Boyu Zhang (Photo by Jeff Fitlow / Rice University)

https: //news /04 /0503_TOUGH-hgzs-lg.jpg

LEGEND: Computer simulations by Nanyang Technological University materials scientists Huajian Gao (left) and Zhigong Song helped explain the unexpected fracture toughness of 2D hexagonal boron nitride. (Image courtesy of H. Gao / NTU)

https: //news /04 /0503_TOUGH-hex-lg.jpg

LEGEND: In graphene and hexagonal boron nitride (h-BN) atoms, the atoms are arranged in a flat network of interconnected hexagons. In graphene, all atoms are carbon. In h-BN, each hexagon contains three nitrogen atoms and three boron atoms. (Image courtesy Rice University)

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