Definitive strength and fracture toughness of graphene revealed

A team of UCLouvain and WEL Research Institute, in collaboration with University of Trento, has recently made a breakthrough in the research on graphene towards its application in different technologies. Although graphene is always depicted as the strongest material ever, this claim results from fundamental studies dealing with perfect or almost perfect samples or with not fully significant statistical/experimental approaches due to inherent complexity associated to testing of a single atomic layer, nearly 150,000 thinner than a human hair. As for any material, graphene contains defects and these defects lead to a drop of the cracking resistance compared to the perfect case. Following the rational used for all engineering structures, a fracture mechanics approach was used to establish fail-safe design rules for graphene-based elements. Fracture mechanics relies on the testing of specimens containing a sharp crack and on the determination of the fracture toughness which quantifies the resistance of the material to crack propagation. The toughness expresses the energy per unit area spent in the fracture process zone, also named fracture energy or critical energy release rate.

The challenge was extreme: how to prepare single atom thick but “large” specimens with or without a crack, load it, determine under direct tension the extreme (especially in the case of defect free samples) critical loads and displacements associated to cracking, repeat this a sufficiently large amount of times to generate data with statistical significance and possibly make the link with the microstructure defects?

This has been made possible by the extension of the patented UCLouvain on-chip nanomechanical approach relying on microelectronic fabrication methods, successfully applied for the first time to graphene, and allowing the simultaneous testing of hundreds of specimens.

Both original fracture mechanics “crack-on-chip” and “tension-on-chip” conditions have been studied.

The major findings of this study are

1. the definitive toughness of monolayer graphene is equal to 20J/m2 (a very high value for very thin materials),
2. a maximum fracture strain higher than 11.5% has been extracted which is the highest fracture strain, experimentally measured, ever reported in the literature under uniaxial tension conditions for large graphene samples,
3. the highest strength was found to be 110 GPa (compare with the high strength of steel, of about 1-2 GPa), close to the theoretical value and corresponding to defect-free graphene as also confirmed by thermodynamics and quantized fracture mechanics (a generalization of classical continuum fracture mechanics for taking into account the discrete nature of matter),
4. this is associated with the world record stored specific elastic energy of 6 GJ/m3 (comparable to that of TNT explosives)
5. the fracture of graphene is inherently controlled by unavoidable 1.4 nm-sized defects that can be related to 5-7 pairs that will ultimately set the maximum operating loads of any device relying on graphene,
6. the design of graphene-based devices produced under best processing condition should be based on a strength equal to 50-60 GPa and fracture strain of 5-6% as a result of the newly statistically experimentally revealed critical defects, as confirmed by quantized fracture mechanics.

This study is published in the journal Nature Communication

Sahar Jaddi, M. Wasil Malik, Bin Wang, Nicola M. Pugno, Yun Zeng, Michael Coulombier, Jean-Pierre Raskin, Thomas Pardoen, Definitive engineering strength and fracture toughness of graphene through on-chip nanomechanics, Nature Communication (2024) 23-25673 (NCOMMS-23-25673)

DOI : 10.1038/s41467-024-49426-3