While graphene is being hailed as the strongest material in the world - 200 times stronger than steel - and presents plenty of potential due to its lightweight nature and unique mechanical and electrical properties, a new study by researchers at the Lawrence Berkeley National Laboratory reveals that although it is strong, its toughness, or resistance to fracture, is very low.
"This material certainly has very high strength, but it has particularly low toughness - lower than diamond and a little higher than pure graphite," Robert Ritchie, co-author of the study, said in a press release. "Its extremely high strength is very impressive, but we can't necessarily utilize that strength unless it has resistance to fracture."
The results were determined through a statistical theory that the team developed in order to determine the toughness of polycrystalline graphene made from chemical vapor deposition.
"It's a mathematical model that takes into account the nanostructure of the material," Ritchie said. "We find that the strength varies with the grain size up to a certain extent, but most importantly this is a model that defines graphene's fracture resistance."
While toughness is a material's resistance to fracture, strength is a material's resistance to deformation, and these two properties often don't come together due to their mutually incompatible properties.
"A structural material has to have toughness," Ritchie said. "We simply don't use strong materials in critical structures - we try to use tough materials. When you look at such a structure, like a nuclear reactor pressure vessel, it's made of a relatively low-strength steel, not an ultrahigh-strength steel. The hardest steels are used to make tools like a hammer head, but you'd never use them to manufacture a critical structure because of the fear of catastrophic fracture."
Despite these results, many of the potential graphene applications will require strength more than toughness. Furthermore, Ritchie and his team are currently examining the effects of adding hydrogen to graphene.
"We don't know a lot about the fracture of graphene, so we're trying to see if it's sensitive to other atoms," he said. "We're finding the cracks grow more readily in the presence of hydrogen."
The findings were published in the Jan. 28 issue of Nature Communications.
