In the world of materials, strength (the amount of force a substance can withstand) and toughness (its capacity to resist fracturing) are not merely different attributes; they're very difficult to achieve together. Now a collaboration of researchers from Caltech and the Department of Energy's Lawrence Berkeley National Laboratory has created a form of glass that has both qualities. It's stronger and tougher than steel or, indeed, any other known material. The material features palladium, a metal whose possible use in glasses was recognized 45 years ago.
"It's probably the best damage-tolerant material we've seen," says Robert Ritchie, a professor at the University of California, Berkeley, who tested the new material. He says no one has ever achieved such toughness from 100 percent glass and that the potential exists to mass-produce the glass.
Julia Greer, an assistant professor of Materials Science at Caltech, who was not involved with developing the material, says, "it has potential to overcome the limitations the metallic glasses always had."
The work is outlined in a study published this week in the journal Nature Materials. Marios Demetriou, a professor at Caltech and lead author of the paper, says the work involved finding a particularly strong version of the simplest form of glass, called marginal glass, and then turning it into the even stronger form known as bulk glass.
"What we did here is find a very, very tough marginal glass made of palladium with small fractions of metalloids like phosphorus, silicon, and germanium, which yielded one-millimeter-thick samples. And we just said, let's add very little of something that will make it bulk without making it brittle," says Demetriou. By adding 3.5 percent silver to this marginal glass, Demetriou was able to increase the thickness to six millimeters while maintaining its toughness.
"The Achilles' heel of these metallic glasses is that when you pull them in tension or try to deform them somehow, they fail catastrophically," says Greer. This occurs through the formation of what's termed "shear bands," small defects which coalesce into vein-like patterns that rapidly evolve as cracks, causing the glass to break under extremely small strains. However, according to the researchers, the palladium glass generates so many of these bands that they form a blocking pattern that prevents cracks from propagating without impairing the material's overall properties.
Ritchie says it may be possible to combine other elements to make even better glasses. John Lewandowski, a professor of metallurgy at Case Western Reserve University, says, "One of the results from this project will be to spur a lot of work in related areas, examining the details, modeling it, analyzing temperature effects or what happens when you test it."
The limitation is palladium's very high cost. Therefore, Ritchie says, although there are countless structural applications that could utilize this material's high strength and toughness—like automotive and aerospace components—many of them will prove impractical in the marketplace.
Demetriou is more optimistic. He believes there's already demand for metallic glass and says a product like a dental implant made from the stuff could be available within the next five years. He says this would offer a "superior alternative" to traditional implants made of noble metals, which are softer and stiffer and thus more likely to wear or cause bone atrophy.
The first step is convincing a manufacturer that the material possesses "unique and unusual attributes," he says. Then a series of tests of its performance, longevity, and biological compatibility will be needed before ultimately determining whether the pricing would be competitive.
As for making large-scale structures like bridges, Demetriou says cost would probably prevent that. But he has hopes of developing something cheaper. "If we develop an iron or copper alloy with these properties," he says, "I'll tell you this: we will put steel out of business forever."
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