Many years ago, my parents and brother were driving home late at night, full speed on a highway, when a large rock thrown off an overpass struck their car’s windshield. There was a time when an impact like that would have shattered the windshield glass, likely leading to a tragic accident and – for me – a painful childhood. But, thanks to the modern miracle of laminated safety glass, the windshield did not shatter; it only cracked. The rock rolled away, my dad maintained control of the car, and the three of them got home safe and sound.
One of the lessons of that night is that in many applications, the mechanical strength of glass is every bit as important as its transparency. However, there’s a reason we don’t often see literal glass ceilings. The problem is that glass breaks before it bends – even the tiniest fracture spreads rapidly in all directions until the entire pane shatters. In engineering terms, glass is strong (it can withstand a lot of stress before cracking) but not tough (it has little damage tolerance after the onset of cracking). This is in contrast to sheets of metal or plastic that can deform to accommodate small defects, making them generally tougher materials.
To a group of researchers at Lawrence Berkeley National Laboratory and Caltech, this begged the question: is it possible to engineer tougher glass by making it behave more like metal? The answer, it turns out, is yes. By tinkering with alloyed mixtures of metals and glass, the group stumbled upon a new type of glass that ranks as one of the world’s strongest and toughest known materials. The key ingredient in the mix is palladium, a metal with a high degree of plasticity that offsets the natural tendency of glass to shatter. The resulting material is both flexible like a metal and hard like conventional glass, a combination of toughness and strength that is extremely valuable but rare for a single material to possess.
So now the team can start counting up the billions of dollars they are set to make off their invention, right? Actually, not so fast. The samples tested in their study were only a few millimeters in diameter, and fundamental limitations in the fabrication process prevent significantly larger samples from being made. The best metallic glass sample contained five elements (palladium, silver, phosphorus, silicon, and germanium), and the complexity of this material complicates study beyond trial-and-error methodology. However, the research team is optimistic that the strategies they developed will lead to the discovery of even stronger (and hopefully larger) glass materials.
Press release from ScienceDaily
Demetriou MD, Launey ME, Garrett G, Schramm JP, Hofmann DC, Johnson WL, & Ritchie RO (2011). A damage-tolerant glass. Nature materials, 10 (2), 123-8 PMID: 21217693