On Graphene Armor
A lot of ink has been spilled on the subject of graphene armor. For example:
Such articles show little understanding of the facts. A deeper reading should indicate that graphene will probably never make for an armor material.
To get right to the point, there are five reasons for this:
– Production difficulties.
– A low fracture toughness.
– A propensity for brittle failure much along the lines of ceramic materials.
– A relatively high density at 2.2 gm/cc.
– Zero (macro-) hardness, low (bulk-) stiffness.
The mechanical properties attributed to graphene, e.g. a superlative tensile strength and modulus, are typically totally theoretical, or were taken from the analysis of microscopic samples. Ceramic materials often have high theoretical tensile strengths — very typically in the 2000-3000 MPa range — and small single crystals can indeed be quite strong. But, in reality, with bulk polycrystlline ceramic test articles, tensile failure will set in at 200-300 MPa. This is because microstructural defects, which can’t be avoided in bulk ceramics, act as crack initiation sites. Once stress levels cross a certain threshold — which threshold is always far lower than the theoretical tensile strength of the material — cracks will rapidly radiate outwards from those defects.
The behavior of bulk graphene mirrors that of ceramic materials: Graphene might have a theoretical tensile strength of 150 GPa, but, when you actually measure a large-enough sample, it’ll inevitably be much weaker than that, as the smallest defect in the carbon lattice will initiate a brittle failure cascade.
In a very real sense, graphene is a chain, and it’s only as strong as its weakest link. It seems it is impossible to avoid weak links in graphene, just as it is effectively impossible to avoid imperfections, such as pores, in bulk ceramic materials. (A perfect ceramic would be very strong and, more than that, it would be damage tolerant.)
The details surrounding the brittle behavior of graphene are described at length in this paper. And a lot more can be inferred from it.
Graphene’s high density is also a problem. For it is sure to be much weaker than the 150 GPa advertised — but, to merely be competitive with materials such as Dyneema on a weight basis, it must out-perform them by more than double!
There’s more. Individual sheets of graphene have no hardness, and very low stiffness. Even though they have an extremely high (theoretical) tensile modulus — in fact, graphene is said to be the stiffest known material — stiffness rises as the cube of thickness. So a steel sheet that’s 1.8mm thick is ~20% thinner than a 2.2mm sheet, but it’s ~60% less stiff. A steel sheet that is, like graphene, one atom thick would be, for all practical intents and purposes, almost infinitely less stiff than that 1.8mm sheet.
When it comes to stiffness, thickness simply matters more than theoretical or intrinsic mechanical properties. And there is a fundamental qualitative difference between the “stiffness” of a two-dimensional material like graphene and the stiffness of a bulk material like steel.
What I mean can readily be seen in UHMWPE fiber-resin laminates, such as Dyneema and Spectra Shield. These polyethylene fiber materials are, at least theoretically, very stiff in that they have a high tensile modulus — but, in reality, a sheet of Dyneema is as flexible as a plain sheet of printer paper. This is wholly on account of its low thickness; the fact that it’s not a true bulk material, but a laminate comprised of very thin, and therefore flexible, fibers embedded in a flexible resin.
Graphene and UHMWPE are similar in that their properties are size-dependent. And graphene is like UHMWPE in that it’ll never be able to wholly replace ceramic or metallic materials in armor. A certain degree of bulk rigidity, hardness, and compressive strength are required to fracture or erode steel bullet cores.
Although it’s difficult to run ballistic tests with graphene, high-quality graphene sheets have been evaluated in small and limited ballistic experiments. Contrary to the breathless headlines and all of the hype, the results of those experiments were not impressive, nor do they show any real promise or future potential.
In the study that was described in the first article linked-to above, graphene was directly compared to gold and PMMA — which are, I must strongly emphasize, not suitable materials for ballistic armor. Graphene is apparently about twice as good as PMMA at typical ballistic impact velocities.
The data from these microscopic experiments were extrapolated and translated into bulk/macroscopic “data.” That this methodology is spurious should go without saying. Even so, the authors of the paper write: “The specific penetration energy for multilayer graphene is ~10 times more than literature values for macroscopic steel sheets at 600 meters per second” — which sounds very interesting indeed, until you realize that the steel they’re comparing graphene with is low-strength, low-toughness 304 stainless steel — a grade of steel that’s nothing at all like an armor steel. It wouldn’t surprise me to learn that Hardox 600, a common abrasion-resistant industrial grade of steel, performs 10x better than 304 stainless in the same sort of experiment! A boron carbide and Dyneema ceramic armor plate would surely perform >20x better than that same sheet of 304 stainless.
In a different paper, a raw steel plate was compared with a second type of steel plate that was coated with 15-layers of graphene embedded within a polyurea resin matrix. The coated plate exhibited enhanced ballistic performance… by 4%. An almost completely insignificant result.
So graphene is, as yet, unimpressive on its own, and unimpressive as a coating material.
Interestingly, people have added graphene to ceramic materials, in an attempt to reinforce and toughen them. In small amounts, up to around 1% by volume, graphene does indeed impart a toughening effect. This paper is representative — and it has great SEM images. But the thing to note is that even the best graphene-toughened alumina is inferior to grades toughened via other means, such as zirconia-toughened alumina. ZTA can, in fact, be more than twice as tough as the graphene-reinforced material. And there are process and sintering limitations associated with the use of graphene as reinforcement in ceramics and metals; it’s tough to disperse it without damaging it, and it’s chemically reactive.
As SiC whiskers are already pretty darn useful for toughening alumina and many other ceramics, and as many ceramics can be toughened via secondary-phase approaches, graphene doesn’t bring anything new to the CMC table.
Then there’s the big question of whether what’s sold as graphene is actually graphene.
As Derek Lowe describes in that post, the global graphene supply network is evidently a very long way away from being able to provide industry with graphene of reasonable quality. If the current situation is anything to go by, it may be decades before large and relatively clean sheets are available at prices that would be suitable for use in armor. And, even at a low price, its potential seems very limited.