Double-strength glass may be within reach
25 September 2012
Processes that are common in the microelectronics and nanotechnology industry could produce glass that’s about twice as strong as the best available today.
Rice University chemist Peter Wolynes and Rice graduate student Apiwat Wisitsorasak determined in a new study that chemical vapour deposition (used industrially to make thin films) could yield a glass that withstands tremendous stress without breaking.
Their calculations are based on a modified version of a groundbreaking mathematical model that Wolynes first created to answer a decades-old conundrum about how glass forms. With the modifications, Wolynes’ theory can now predict the ultimate strength of any glass, including the common varieties made from silica and more exotic types made of polymers and metals.
Glass is unique because of its molecular structure. The molecules in glass are suspended randomly, just as they were as a liquid, with no particular pattern. The strong bonds that form between these randomly-arrayed individual molecules are what hold the glass together and ultimately determine its strength.
All glasses share the ability to handle a great deal of strain before giving way, sometimes explosively. Exactly how much strain a glass can handle is determined by how much energy it can absorb before its intrinsic elastic qualities reach their limitations. And that seems to be as much a property of the way the glass is manufactured as the material it’s made of.
Materials scientists have long debated the physics of what occurs when glass hardens and cools. In fact, the transition is one of their last great puzzles of the field. Cooling temperatures for particular kinds of glass are well defined by centuries of experience, but Wolynes argues it may be possible to use this information to improve upon glass’s ultimate strength.
The elastic properties of the finished product and the configurational energy (the positive and negative forces between the molecules) held in stasis by the “freezing” process determine how close a glass gets to the theoretical ideal — the most stable glass possible, he said.
“The usual impression of glass is that, relative to other materials in your life, it seems easy to break,” said Wolynes. “The reality is that when it’s freshly made and not scratched, glass is very strong.”
A chance encounter with a metallurgist last year made Wolynes think again about just how strong glass could be. “We had never worked on that kind of property, and the problem struck me as intriguing – and relatively simple in the framework of the theory we already had. We just hadn’t thought to calculate it,” he said.
Traditional glass is so ubiquitous that people rarely think about it (until it breaks). “Even though we now have Gorilla Glass and other tempering developments, they’ve been developed in a somewhat Edisonian fashion,” he said, noting that such hardened glasses commonly used in cell phones have a self-healing surface treatment that protects the glass itself from scratching. “Our paper is about what determines the limits on the strength of the glass, if there is no surface problem.
“In the early days, when people first measured the properties of glasses, they found they were easily breakable. Silica glass is very high-melting, so you’d expect it to be strong. Then they did finally figure out this was because cracks at the surface were propagating in. If they could eliminate the cracks, they would get much higher strengths.”
Current metallic glasses like the Liquidmetal famously licensed by Apple for consumer electronics “come to be about a quarter of this theoretical Frenkel strength,” Wolynes said. “So what is it that limits their strength? We ask whether the collective motions that go on in liquids as they’re becoming glasses are the same motions that are being catalyzed when we stress the material.
“Basically, we applied our theory for what determines how the liquid rearranges as it’s becoming glass. Add to that the extra driving force when you apply stress, and see what that predicts for the limit of how much it can be pushed before the atoms roll over each other” and the glass breaks, he said.
He noted the theoretical results closely match experimental ones for most materials. “The good news is, according to this theory, if you could make a material that is much closer to ideal glass – the glass you would get if you could make it infinitely slowly – then you would be able to increase its strength.” That may not be possible through traditional cooling of silica, metal and polymer glasses, which Wolynes’ and Wisitsorasak’s calculations indicate are approaching their limits.
But it might be possible through vapour deposition of atoms. “It would require tuning the deposition rate to the liquid/glass transition properties,” he said.
“Our theory says the best you can do with this is get about halfway to ideal glass,” which he said some experimentalists have demonstrated. “It’s possible there’s some loophole we don’t yet see that will let us get even closer to the ideal,” Wolynes said. “But at least, at this point, we can get halfway there. That means it would be possible, in principle, to get glass with at least twice the intrinsic strength of current glasses.”
Wolynes’ theory comes with a caveat, though. Glass hardened even to the point of near indestructibility can still be destroyed, and with dramatic effect. “If you could have something infinitely strong, then you’d never need to worry about it,” he said. “But there’s a little bit of a problem if you make something that’s very strong but can eventually break. It contains a huge amount of energy, so when it breaks, it fails catastrophically.”