Glass may be hard, but it's all too easy to break, as anyone who's seen a shattered window knows. But now scientists have discovered that they can make glass 200 times tougher than normal by making it 'weaker' — using a laser to etch wavy micro-cracks into an otherwise solid surface.
The discovery, described last month in Nature Communications, borrows secrets from mollusk shells, which use very brittle, breakable materials to create some of nature's toughest structures.
Seashells lined with iridescent mother-of-pearl are more than just pretty — they're a remarkable feat of microengineering, said study co-author François Barthelat, a mechanical engineer at McGill University in Quebec, Canada. Animals with such shells somehow use brittle, crumbly chalk (known formally as calcium carbonate) to build armor that can protect them along unforgiving reefs and rocky shorelines.
"Nature is very good at making materials with wonderful microstructures — almost perfect structures," Barthelat said. "As engineers, it's very hard to duplicate."
The secret is in the architecture of the nacre, the iridescent material lining the inner surface of certain mollusk shells. It's made of 95% chalk — hexagonal plates of calcium carbonate in a crystalline form called aragonite, and they interlock rather like Lego blocks. But the boundaries between these hard layers are much weaker, filled with soft, protein-rich material that can deform when energy passes into it. Thus, the brittle aragonite tablets don't shatter with every impact, because the supposed 'weak' layers dividing the hard tablets allow the shell to dissipate energy and stop cracks from propagating.
Barthelat and his colleagues wondered if they could learn the design secrets of mother-of-pearl and apply them to a famously brittle, breakable material: glass. Glass is hard but it isn't tough — its atoms are randomly organized, lacking any structure, and so it's very easy to shatter.
But Barthelat wasn't about to try to build a shell, brick by microscopic brick. Instead of trying to build with tiny pieces, the researchers pulled the more general design principles from what they saw in the seashell. They used a laser to engrave tooth-like squiggles into smooth glass, creating patterned cracks in its structure. Just as the the nacre's 'weak' protein-rich boundaries dissipated energy and kept the chalky structure from shattering, these curving cracks in the glass would divert and channel cracks in the brittle glass so it could not fracture any further.
The concept is similar to why stamp sheets are perforated: When you rip down a sheet – causing a 'crack' in the paper – the little trails of holes guide the tear straight down the page.
Treating the glass this way – making it 'weaker' – actually makes it 200 times tougher, the scientists found. The researchers also filled the cracks with polyurethane, but they say it's not even really a necessary ingredient.
The team used this bio-inspired method on the kinds of glass slides you put on samples under a microscope, but it should be able to scale up for, say, dinnerware, windowpanes and even car windshields – anywhere that shattering glass could present a dangerous prospect.
They're not the only ones looking to nature to build tougher or stronger materials: A team of researchers recently designed tiny structures that were based on the architecture inside bones.