Intrinsic toughening and stable crack propagation in hexagonal boron nitride

Yingchao Yang, Zhigong Song, Guangyuan Lu, Qinghua Zhang, Boyu Zhang, Bo Ni, Chao Wang, Xiaoyan Li, Lin Gu, Xiaoming Xie, Huajian Gao () and Jun Lou ()
Additional contact information
Yingchao Yang: Rice University
Zhigong Song: Institute of High Performance Computing, A*STAR
Guangyuan Lu: Chinese Academy of Sciences
Qinghua Zhang: Chinese Academy of Sciences
Boyu Zhang: Rice University
Bo Ni: Brown University
Chao Wang: Rice University
Xiaoyan Li: Tsinghua University
Lin Gu: Chinese Academy of Sciences
Xiaoming Xie: Chinese Academy of Sciences
Huajian Gao: Institute of High Performance Computing, A*STAR
Jun Lou: Rice University

Nature, 2021, vol. 594, issue 7861, 57-61

Abstract: Abstract If a bulk material can withstand a high load without any irreversible damage (such as plastic deformation), it is usually brittle and can fail catastrophically1,2. This trade-off between strength and fracture toughness also extends into two-dimensional materials space3–5. For example, graphene has ultrahigh intrinsic strength (about 130 gigapascals) and elastic modulus (approximately 1.0 terapascal) but is brittle, with low fracture toughness (about 4 megapascals per square-root metre)3,6. Hexagonal boron nitride (h-BN) is a dielectric two-dimensional material7 with high strength (about 100 gigapascals) and elastic modulus (approximately 0.8 terapascals), which are similar to those of graphene8. Its fracture behaviour has long been assumed to be similarly brittle, subject to Griffith’s law9–14. Contrary to expectation, here we report high fracture toughness of single-crystal monolayer h-BN, with an effective energy release rate up to one order of magnitude higher than both its Griffith energy release rate and that reported for graphene. We observe stable crack propagation in monolayer h-BN, and obtain the corresponding crack resistance curve. Crack deflection and branching occur repeatedly owing to asymmetric edge elastic properties at the crack tip and edge swapping during crack propagation, which intrinsically toughens the material and enables stable crack propagation. Our in situ experimental observations, supported by theoretical analysis, suggest added practical benefits and potential new technological opportunities for monolayer h-BN, such as adding mechanical protection to two-dimensional devices.

Date: 2021
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DOI: 10.1038/s41586-021-03488-1

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