Graphene Armor
Why Graphene Armor Doesn’t Exist and What it Might Look Like
Mountains of papers and articles have been written on the subject of graphene armor. From serious studies in scientific journals, to speculative and sensational click-bait, there’s far too much to review here. But all of it rests on the shakiest and weakest of foundations. Here’s why:
- The “strength” of graphene is an idealization. It’s not real.
The theoretical strength of a material is essentially the strength of that material’s atomic bonds. Thus the theoretical strength of a pane of common soda-lime silicate glass is approximately 40,000MPa. This over-states the actual measurable strength of that type of glass by roughly two orders of magnitude; defects in a real-world pane of glass, which act as crack nucleation points, reduce its strength to roughly 100MPa. Even the strongest fiberglass whiskers, which are as defect-free as modern technology can make them, almost never exceed a 6,000MPa tensile strength – ~15% of the material’s theoretical strength.
All other materials are similar in this regard. The theoretical strength of polymers and ceramics is typically >10x their actual measured strength. (And perfect ceramics that attain their theoretical strengths would be almost completely fracture-proof.) The same is largely true of metals, though this is slightly more complicated as a metal alloy is not a homogeneous substance. In any case, martensite’s strength ceiling is well over 3600MPa, and yet most martensitic steel alloys clock in at roughly half that value.
When people talk about the strength of materials, people talk about actual strength. Nobody believes that glass is an ultra-strong material because in principle it might be able to attain a 40GPa tensile strength. At least, this is usually the case – but it’s not the case with graphene. People, including the Nobel Prize committee, seem to believe that graphene actually exhibits its full theoretical strength. They call this its “intrinsic strength.” This is folly.
Graphene’s theoretical strength is roughly 110-130GPa – roughly 3x the theoretical strength of common window glass. It has even been measured at 130GPa in nanometer-sized samples with the aid of an atomic force probe. But, as with glass, the strength of bulk graphene is dominated by its defect load, and all forms of graphene are absolutely lousy with defects.
The problem, as Derek Lowe notes, is that bulk graphene is usually produced by exfoliating sheets of graphite with strong acids and oxidizing agents such as potassium permanganate. Yields are low, so the graphite-acid solution has to be centrifuged to isolate a graphene-like substance – but the violent process leaves it riddled with defects, and it has so many oxygen functional groups (mostly hydroxy -OH and epoxy -O-) attached to its margins and to its basal plane that it’s not really graphene at all, but rather graphene oxide.
Put it like this: Oxidizing the hell out of graphite and centrifuging it is never going to get you a material with a 130GPa tensile strength, a 40GPa tensile strength, or even a 6000MPa tensile strength. The theoretical strength of graphene oxide is ~31GPa, and samples tend to measure at roughly 230MPa. In the real world, it’s an awful lot weaker than fiberglass, and not much stronger than window glass.
You can get rid of the oxygen functional groups by treating your graphene oxide with another acid or hydrazine bath – or via thermal annealing, gamma ray irradiation, or electron beam bombardment – and this results in a substance called “reduced graphene oxide”. But though these sorts of treatment reduce the oxygen content of the material, they do nothing to reduce its high defect load. To the contrary, they increase that load further. Strength, correspondingly, declines.
There are other ways to make graphene, but they’ve never successfully been applied to bulk macroscopic material.
Graphene monolayers can be applied to transition metals such as copper and nickel via carbon vapor deposition, but the results are polycrystalline and often also highly loaded with defects. Metal foils are rarely smooth on an atomic level, and deposited graphene will likewise not be flat – it’ll follow the contours of the metal substrate, in a process that introduces flaws at grain boundaries. This kind of deposited graphene is also very difficult to transfer off the nickel or copper foil without suffering damage.
In all cases, the end result is yet another material that, just like glass, is nowhere close to its theoretical strength.
Wikipedia, AlexanderAlUS – Own work
- Bulk, macroscopic graphene materials have inevitably poor properties
There’s a curious thing about graphene: When you have one layer, it’s a very interesting material. When you have 5 layers, it’s much less interesting. When you have >10 layers, you’ve got graphite, not graphene.
Single-layer graphene is, of course, totally useless in armor applications. It’ll break if you throw a ping-pong ball at it. (It can barely support the weight of a ping-pong ball at rest, and that’s if you’re lucky and have good-quality graphene.) How do you get from that to a promising armor material?
There have been attempts to consolidate graphene layers into a bulk material with a thickness dimension. One of the earliest utilized spark plasma sintering, which is typically a way of manufacturing ceramic parts from ceramic powders. This experiment resulted in a bulk material – but one with extremely poor properties, hardly distinguishable from graphite.
This account largely encapsulates the story of bulk graphene materials. By Q4 2024, none had ever attained a tensile strength higher than 700MPa – and most attempts have resulted in a material with a lower tensile strength than pyrolytic graphite’s 120MPa.
In 2024, researchers noted that they made high-performance bulk graphene by mixing it with a large volume of silicon nanoparticles. The result was a bulk Si-C material with an “ultra-high” peak compressive strength of 1100MPa and a peak flexural strength of 440MPa. I’d argue that there’s an easier way to make a high-performance Si-C material: Silicon carbide has an average compressive strength of 3900MPa and an average flexural strength of roughly 390MPa. (Though it can be much higher if that property is optimized for.) It’s also a few orders of magnitude cheaper, and it’s no more brittle than graphene – in fact, it can exhibit a higher fracture toughness. Sometimes the mundane is simply superior to the exotic.
So bulk graphene made by hot-pressing or chemically reacting graphene sheets has come up short. What if we look to carbon fiber, aramid, UHMWPE, and fiberglass as examples to follow? What if we make a graphene-resin composite?
The problem is that it’s not even clear how to get there, even in principle. The above composites are derived from fibers that are bound and held in place with small amounts of resin, so that the end product is roughly 80-90% fiber and 10-20% resin. Graphene, however, is ideally one atom thick. (Note that the average carbon fiber, 500µm in diameter, has somewhere around 35,000 atomic layers.) If you bind graphene between two atomic monolayers of epoxy, you’ve already got at least as much resin as graphene. And I’ve never so much as heard of an atomic monolayer of epoxy; it’s probably not practically feasible, and it may not even be theoretically possible.
There’s simply no way to make a graphene-epoxy laminate that, like today’s typical armor and engineering composites, has <20% resin content.
Accordingly, as of Q4 2024 there are no examples of graphene composites that are anything like common engineering fiber-resin composites. There are zero high-strength materials that are >80% graphene by weight.
- It’s very expensive and impractical.
For something to work as an armor or engineering material, it has to be producible in fairly large commercial quantities, of let’s say at least a couple dozen tons per year. There’s a pretty long list of promising armor materials which can’t feasibly be manufactured in such quantities, and which therefore are all but unknown. Cubic scandium boride. Boron fibers. Various high-pressure silicon compounds. And I could go on all day. (Adept is actively trying to make and utilize some of these.)
There’s a reason armor engineers still get excited over ALON and MgAl2O4 transparent armor windows, rather than single-crystal diamond windows. Diamond windows are theoretically possible – in fact, as with some forms of graphene, also via CVD – but they’re not practical right now for cost reasons, and because the rate of production would be low.
Though graphene powders and flakes are produced in quantities of roughly 100 tons per year, that randomly oriented and defect-laden material has no strength and no utility as a primary armor material. Bulk high-strength graphene exists to the same extent that diamond windows exist – which is to say, barely if at all.
Even those powders and flakes are expensive, at a floor price of roughly $200/kg as of September 2024 – considerably more costly than “high end” armor materials like boron carbide and UHMWPE.
Which brings me to:
- Resins doped with graphene particles are not graphene, ceramics doped with graphene particles are no good
It’s possible to increase the strength of a resin or polymer by adding graphene particles to it, just as it’s possible to do the same with SiC whiskers, fiberglass strands, layers of montmorillonite nano-clay, etc. To call something “graphene armor” because it has 1-2% randomly-oriented graphene particles in one of its resin layers is laughable. I’d argue that it’s inherently deceitful. Besides, the strength gains are typically modest, if they even exist in the first place.
There have been some attempts to add graphene particles to ceramic materials, but there are significant problems with this on a conceptual level, and all known results have been poor, especially in light of the difficulties involved in embedding graphene into ceramic matrixes, and insofar as the important mechanical properties of those ceramics are concerned.
- Nevertheless, there is a potential future for graphene in armor
The meme that great graphene monolayers, or multi-layered bulk graphene materials, can stop bullets ought to be laid to rest. That said, there may nevertheless be viable and potentially useful ways to employ them.
Graphene particle-reinforced adhesives and polymers remain weak materials. They are and will remain of little interest as primary materials in armor systems. But graphene has been shown to reinforce copper in much the same way that it reinforces those plastics: At ~2% by weight, it’s associated with increased strength and hardness. Ductility is always reduced, but not always by very much. Copper-graphene is a form of metal-matrix composite (“MMC”) – where a small volume of a particle phase is suspended in a metallic matrix – and it’s a relatively successful one.
Copper is not the first thing that comes to mind when you think of metallic armor materials. Titanium, however, is right up there. As carbides tend to form at the metal-graphene interface, and as titanium carbide (TiC) is a remarkably strong material, it stands to reason that graphene-titanium composites may hold promise as high-performance armor materials. And, along just those lines, a research group recently produced a graphene-titanium composite with an impressively high tensile strength over 1500MPa.
Any titanium MMC with a tensile strength over 1500MPa is of great interest as an armor material. The best currently-available armor-grade titanium alloys cluster around a tensile strength of 1100MPa, so the reported value is a substantial improvement. It might even be possible to improve this concept further, after the manner of dual-hardness steel, by making thick “functionally graded” titanium MMC plates with graphene reinforcement on one side but not the other – a hard front to help disrupt and destroy the incoming threat, and a softer more ductile rear to mitigate fracture and absorb kinetic energy via plastic deformation, both combined in the same plate of material.
Titanium is by no means the only option. Every strong transition metal that forms stable carbides with good mechanical properties – like cobalt, tungsten, molybdenum, and even perhaps vanadium – may have potential as a graphene-reinforced metal matrix composite.
Unfortunately, graphene reinforcement is less likely to be beneficial in aluminum, magnesium, and iron alloys, as the carbides of those metals have poor mechanical properties. Aluminum carbide and magnesium carbide are remarkably poor – to such an extent that they’re considered highly deleterious impurities.
High-strength graphene MMCs would not be competitive with ceramic armor materials or systems at all, nor would they compete with UHMWPE or other strong fiber composites, but they might fill a different niche. One that immediately comes to mind is vehicular armor, particularly in structural load-bearing roles that ceramics are not suitable for.
Of course, this is highly speculative. For it to become reality, the price of graphene has to drop much further, and graphene MMCs have to be further validated, then made on an industrial scale. But stranger things have happened. Graphene’s day may be yet to come – not in large sheets, but as a way of reinforcing metals such as titanium.
