Questions on the strength of graphene
The claim that graphene is 200 times stronger than steel is widely repeated. University of Cambridge Professor Harry Bhadeshia set the record straight at a London Materials Society lecture held at the Royal School of Mines. James Perkins reports.
Steel expert Harry Bhadeshia is flattered that the material he has spent his professional life researching is held in such a high regard that it is the substance against which everything else is measured. But the claim that graphene is 100 (or 200, or 300) times stronger than steel is 'absolute rubbish', according to the University of Cambridge metallurgist. The claim, which stems from research conducted in 2008, is repeated ad nauseum in reportage of graphene, encouraged by many university media departments.
This comparison is based on shaky foundations and, as Bhadeshia says, design involves a combination of parameters depending on the application. 'It is nonsense to talk about the dimension of a new material on the basis of just one property. A design problem is not interesting at all unless there are many solutions for the same parameters. If you are talking just about strength, it is a very boring problem to work on.'
How strong is graphene?
In 2008, researchers from the engineering department at Columbia University, USA conducted the first strength test on graphene and published the results in Science in a paper called Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene. The team placed pristine graphene flakes over small circular holes etched in silicon and tested their strength using nanoindentation, which involved pushing on the centre of each flake with a diamond-tipped atomic force microscope with a radius of 20nm. The studies found that the intrinsic strength of the material was 130GPa. In a quote published with the associated press release, Professor James Hone said, 'Our research establishes graphene as the strongest material ever measured, some 200 times stronger than structural steel.' The same team, in a 2013 Science paper titled High-Strength Chemical-Vapor–Deposited Graphene and Grain Boundaries, again used nanoindentation to prove CVD-grown graphene had a tensile strength of 118GPa.
A paper published in Nanoscale in August 2015, Nanoindentation cannot accurately predict the tensile strength of graphene or other 2D materials, casts some doubt on whether the results from this method are accurate but, even using the measurements from 2008 and 2013, Bhadeshia shot down the claim that graphene is 200 times stronger than steel.
The steel comparison
By comparing the strength of graphene to A36 structural steel, for example, which has an ultimate tensile strength of 400MPa, graphene is 325 times stronger than steel, but Bhadeshia explained that comparisons such as this are not acurate.
A fairer test would be against the iron whisker Brenner grew in 1956, in which he found an ultimate tensile strength of 13GPa. In theory, Bhadeshia said, it should be possible to achieve an ultimate tensile strength of 21GPa in an ideal crystal of iron. 'Divide 130 by 21 and that is only 6.19 times the strength of steel, not 100 times the strength of steel. Of course, this is the intrinsic strength of graphene, which means if I make it bigger than one micrometre, it is going to collapse. Steel I can make very, very big.'
Like for like
To compare the strength of a nanomaterial with another that has proved itself as one of the most useful bulk materials created by the human race is unfair on a number of fronts.
As Bhadeshia explained, 'The thing to notice is the strength collapses when you scale the particle by micrometers, and even nanometers. As soon as you scale the particle up, the strength collapses, and that is because the probability of finding a defect in a sample increases as the particle size is larger. This was a very well established fact in 1956.'
Other stresses a material needs to stand up to include fatigue, compression, torsion in phase, axial loading and centripetal forces. It also needs to fail in a safe, and predictable way. 'Anyone who tells you that they have created a new material because it is very strong is really talking nonsense and we shouldn't be saying this kind of thing to students,' said Bhadeshia.
Graphene is still great
Just because the claim of graphene's strength is dubious, does not mean it is not an impressive material with fantastic properties. Bhadeshia said, 'I am 100% sure we should work on it and it should be heavily funded for research. To claim things that are not true is what I am against.'
The University of Manchester is a proponent of the 200-times stronger than steel claim and Materials World approached the university for comment.
'The strength of a material is determined by how firmly two atoms are bound together. Carbon atoms have a high level of electrical attraction which gives graphene its strength,' a spokesman said.
'The University of Manchester has described graphene as 200 times stronger than steel based on the intrinsic properties of mono-layer single-crystals rather than their performance in devices or composite materials.'What do you think about the comparison of graphene's strength compared to steel? Email firstname.lastname@example.org, or tweet us @materialsworld