Ultimate strength and toughness of graphene revealed
The University of Trento has participated in an international project where, for the first time, researchers were able to conduct hundreds of experiments to measure the ultimate mechanical properties of this material. They found that even a microscopic defect reduces graphene strength by fifty percent. The study was published in Nature Communications
According to scientists, today graphene is one of the most promising materials. Since it was discovered, it has been the focus of many studies and never ceases to surprise. Recently a group of researchers from UCLouvain made a breakthrough in research, revealing the definitive strength and fracture toughness of this material. The Italian part of the project was conducted by the University of Trento and, specifically, by Nicola Pugno, Professor of Solids and Structural Mechanics at the Department of Civil, Environmental and Mechanical Engineering, who has been studying this material for years and is part of the "Graphene Flagship", an ambitious European research initiative.
About graphene. Graphene is a two-dimensional material made up by a lattice of carbon atoms. Despite being as thin as an atom, it is ideally a hundred times stronger than steel. It is flexible, lightweight, waterproof, an excellent conductor of heat and electricity. These characteristics are known today thanks to numerical simulations and, in part, to the few experiments that have been conducted so far. The data however are incomplete and not much statistically significant because repeating the tests is very difficult.
To understand this complexity, imagine to carry out a tensile experiment on something very tiny, 150,000 times thinner than a human hair.
To know the resistance of the material, experts explain, you need two pieces of information: definitive strength and fracture toughness. These aspects provide the key properties of graphene.
The goal of the scientists involved in the project was to obtain this information from larger and more significant samples than the nanoscopic ones used so far, and to repeat the experiment multiple times to obtain statistically significant and definitive results. They succeeded in this.
About the research work. For the first time, the group of researchers conducted experiments on single sheets of a few hundred micrometers. This size is almost half a millimetre, which makes it visible to the naked eye.
Materials scientists know that perfection is hard to achieve. And, just like any material, graphene may have defects. These reduce strength in the fracture phase. Strength decreases in proportion to the size of the most critical defect, which in turn is proportional to the size of the sample that is being examined. Thus, smaller scale materials tend to be stronger because the likelihood of finding defects is lower. On the other hand, on a higher scale, even the strongest materials appear more fragile because the probability of imperfections increases.
The authors of the study used a quantized fracture mechanics approach which takes into account the initial defects of a material (microstructure cracks) and discrete nature of the material, invented by Nicola Pugno in the past.
Using microscopic chips patented by UCLouvain, the researchers simultaneously carried out hundreds of experiments on samples as thick as an atom, with and without cracks. Their goal was to measure the strength and toughness of the sample.
Interpreting the results. The role of the University of Trento in the project was to interpret the results of the experiments carried out using the aforementioned modelling approach. Nicola Pugno, an expert in bio-inspired nanomechanics, nanotubes and graphene and, more generally, in solids, structural and fracture mechanics, was involved by virtue of his proven experience and his numerous publications on the subject, as he is also the first who theoretically calculated the fracture toughness of graphene, which was ultimately confirmed today, 20 years later.
"The tests we carried out – he explains – show that, statistically, graphene sheets of this size often have a single defect, which measures just one and a half nanometers but which reduces the material's strength by fifty percent, as our previous calculations predicted."
"What the results of the study are saying – he adds – is that we must bear in mind that the strength of realistic graphene, that is to say, with its defects, is about half of that of the ideal graphene. It would be risky – he concludes – to consider defect-free graphene, because it is hardly ever so. We should always take into account at least one single defect, which may well be very small but still significantly affect the actual strength of the material."
Today graphene is used in various sectors, from aerospace, to biomedicine, to the construction sector. This study also opens the way to other applications that require very high resistance materials: from mechanics, to engineering, to sensors.
The study was published in Nature Communications and is available at https://www.nature.com/articles/s41467-024-49426-3