Adamantite! Rearden Metal! Uru! Durasteel! Dalekanium! Unobtanium! Thousands of fictional characters have fought and died for these equally fictional super-materials. So what is the real-life strongest substance on our puny, sun-warmed planet?
Mankind's pursuit of the "strongest" material hasn't exctly been a concerted, organized effort, but it figures into history in incredibly profound ways. Hell, anyone who's played Age of Empires or Civilization—or read a book—knows that historians name entire eras after materials. The Iron (and steel) Age followed the Bronze Age, which followed the Copper Age. Materials got stronger, and humanity advanced. The two were hugely correlated.
It's been a while since we've had a good ol' material-based epoch. Too long! So let's find the next one. Goodbye Age of Computers, hello Age of ________.
To say that a material is "strong" describes so many different things that it can end up meaning nothing. A piece of chalk is stronger than a piece of string cheese in one way, but not another. Spiderweb may be stronger than steel in a particular test, but it's still a bendy, silky mess. So what do we mean when we talk about strength?
Mark Hersam, Professor of Materials Science (and Chemistry) at Northwestern University, has an answer. Well, a few answers: "Typically when people talk about strength, they're talking about the amount of stress that needs to be applied before fracture. It's possible they could also be referring to how quickly it deforms." It's also possible that they could be talking about compressive strength, which is a measure of how easy it is for a material to get crushed. You could even be referring to impact strength, which is a material's ability to sustain a sudden smack. Arg! Make up your minds, people!
Fortunately, the 20/20 hindsight of history can guide us forward. Remember our ages? Iron, steel, copper, bronze—these are materials that we make things out of, be they structures or instruments. Hard things. Tough things: swords or trucks or skyscrapers or bridges. We're looking for hardness, yield strength—that's resistance to deformation, like denting or stretching—and tensile strength, which basically refers to bending. A material of world champion strength would be an optimum hybrid of these two qualities.
Tungsten carbide is phenomenally hard and has great yield strength, but is worryingly brittle when smashed or bent. Osmium alloys follow that same trend: extremely hard, but shatterable and lacking in true tensile strength. Diamonds are harder than either, but nearly impossible to work with or use in practical products and very, very rare. (Ever heard of a molded diamond? A diamond fighter jet? Right.)
Titanium alloys can be flexible and boast high tensile strength, but aren't as hard as steel alloys. Amorphous alloys like Liquidmetal, Apple's new squeeze, are among the strongest materials all-around, with respectable hardness, huge tensile strength and resistance to fatigue—though it's not superlative in any way. Every metal has its weaknesses, is what I'm saying.
The real super-materials, well, they don't quite exist yet. But we're getting closer. You've probably heard a lot about graphene in the last few weeks, after a couple of scientists won Nobel Prizes for their work with the material. You've probably heard that it's hard. (200x harder than steel.) That it's flexible. That it conducts electricity, and happens to be transparent. What you've probably heard, in a nutshell, is that it's awesome. It has immediate and obvious applications in electronics, and Andre Geim, one of the recipients of the prize, went so far as to tell the AP, "[Graphene] has all the potential to change your life in the same way that plastics did."
The test that two years ago led to the declaration of graphene as the world's strongest material was a test of a one-atom-thick sheet of the stuff. Had the experiment taken place on a (theoretical) larger scale, the numbers would scale up favorably. An inch-thick block of material with graphene's properties would be virtually indestructible. Thing is, such a sheet can't exist.
"The property [of strength] is nominally independent of the geometry, but the problem is that the application definitely depends on the geometry," says Hersam. And what he means, in English, is that while graphene in amazingly strong, this strength can't easily be adapted to real, useful applications. Graphene is by definition a sheet of carbon atoms, bonded in a particular way, but always one atom thick. If you try to stack these sheets together, they can bond, but not well. Stacked graphene is called flake graphite—pencil lead, basically. And you can't build a bridge or a gun or a satellite out of pencil lead.
But wait! All is not lost for graphene. Not nearly. "Graphene has to be incorporated into some matrix—a polymer, or a metal," says Hersamin. Basically, in order for it to be used in the real world, the stuff needs to become a component of something else. The trick will be maintaining its outstanding mechanical properties while it's dispersed inside another material.
This, he says, is an open question. Everybody's excited about graphene, but nobody's building graphene smartphones—yet. "If you can stitch the graphene into a plastic or into a metal in such a way that the load of the plastic or metal would transfer to the graphene, so that the graphene in reenforcing the bulk of the material," then you'd might have yourself a super-material.
Carbon nanotubes have long been held to have similar potential, but progress hasn't been great. You'll see carbon-nanotube gear at the top echelons of some sports, but it's a luxury, and its benefits—particularly when it comes to strength—aren't exactly huge. In order to get the nanotubes to bond with other materials, the bonding within the nanotubes themselves has to be compromised somewhat. It's a bit of a Catch-22, and graphene is every bit as vulnerable to the same fate; because nanotubes, to put it crudely, are just rolled-up sheets of graphene.
Some worry that graphene will suffer this same fate: a lonely material unable to find a suitable matrix-mate for years, the promise of its youth spread thin over time as enthusiasm for its charms wanes... But for now, the mood is optimistic. Some researchers, Hersam says, already believe they've come up with a way to stitch the graphene into a matrix without drastically changing its mechanical properties, but their thesis is new, and so far untested. It does seem to be the case, though, that the two-dimensionally arranged carbon atoms in graphene will be more forgiving than nanotubes, as far as material scientists are concerned.
The take-away is that graphene gear will never be possible, but something that is approximately like graphene may one day be used to build all kinds of things. And even then, it'll be difficult to say that it's the strongest material in the world. Its carbon-cousin, diamonds, may still be harder; certain metals may have slightly higher tensile strength. It might not claim a single record, or make for a great title for an era (The Graphene Composite 1.0 Age?), but it'll be strong. Not the hardest, or the most resistant to deformation. Just.. really strong.