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02 Jul 2019 Belle Dumé / Physics World – materials research update
A new type of glass created by mimicking the nacre (or mother-of-pearl) structure in mollusc shells is ductile yet tough and highly resistant to impacts. The material could come in useful for countless applications, including windows, windshields, solar panels and touchscreens. The technique used to make it is also relatively easy and scalable, which means that it could readily be manufactured in industrial volumes and at a reasonable cost.
Glass has outstanding optical, thermal, chemical and electrical properties as well as being hard and durable, but it is inherently brittle and has poor impact resistance. Although lamination and tempering can improve its impact resistance so that it can be used in applications such as car windshields, for example, these techniques do not unfortunately reduce brittleness.
Bricks and mortar
Researchers led by François Barthelat of the Department of Mechanical Engineering at McGill University in Canada have now succeeded in duplicating the way mollusc shells make use of a brittle mineral (calcium carbonate) and turn it into the tough structure of their shells. These shells, which protect the soft bodies of molluscs from strong predator jaws, are made of nacre. This material has a complex hierarchical structure in which flat polygonal tablets or “bricks” of calcium carbonate are embedded in an organic biological polymer (protein) “mortar” and arranged in sheets.
Nacre is resistant to impacts thanks to the fact that the bricks in the material can slide past one another when stress is applied, so dissipating the energy of an impact and preventing shattering.
Laser engraves hexagonal patterns
Barthelat and colleagues engineered a laminated glass with similar properties using borosilicate glass sheets (220 μm thick) layered and bonded together using 125-μm-thick interlayers of the polymer ethylene-vinyl acetate, which is transparent. “This material is the same as traditional laminated glass except that we engrave a hexagonal pattern in each glass layer using a precision-focused ultraviolet laser beam,” explains Barthelat. “We then assemble the engraved glass layers with the polymer and precisely shift each layer with respect to the previous layer.”
This technique allows the researchers to obtain a staggered arrangement of the hexagonal tablets in 3D, much like a 3D brick wall. The mortar between the hexagons is the polymeric layer.
Two to three times more resistant
The bioinspired glass is two to three times more resistant to impacts than laminated and tempered glass while having the same static strength, says Barthelat. It also breaks in a “graceful” fashion, which means that instead of breaking catastrophically with many cracks and shards like regular glasses, it dents and deforms. “A window made of our material can thus be damaged or be punctured in a small area leaving the rest of the window intact. This damage tolerance is very different to that in regular glass, in which any local damage destroys the entire window.”
The researchers confirmed, using micro-computed tomography (CT) images, that it is indeed the sliding mechanism between the bricks in the material and the viscoelastic properties of the polymer matrix that give the glass its exceptional properties.
The glass is not perfect though and one of its main shortcomings is that it might be too ductile for some applications – like in large windows, for example. The researchers say that this problem could be overcome by adding layers of plain glass to the material, but this might make it as prone to breaking as normal glass even though it is more impact-resistant overall.
Powerful source of inspiration
“Our results prove again though that nature is a powerful source of inspiration for engineering materials design,” Barthelat tells Physics World. “And that controlling materials and architecture, even on the scale of millimetres (rather than the micro- or nanoscale), can produce outstanding properties and high-quality structures.”
The technique used to make the glass also proves that removing material and applying seemingly weakening processes can actually improve a structure, he adds. “Ask any glass scientist and they will tell you that shooting a powerful laser into a piece of glass is a bad idea because it creates defect and decreases strength, but we have shown that the opposite can be true.”READ MOREGetting to business with coatings
Towards bendable versions
“Glass has excellent optical properties and is a durable material, making it the material of choice in numerous applications, but glass components in a structure are always the “weakest links”. With our material, we address this drawback and have made glass tougher and more resistant to impact. This could extend its range of applications and operating conditions.
The McGill researchers, reporting their work in Science 10.1126/science.aaw8988, say they would now like to make nacre-glass materials in other shapes – such as curved and ultrathin structures for use in touch-screens, for example. “We would also like to develop bendable versions of our glass,” reveals Barthelat. “A thin plate of this glass would flex to large deformations and then recover without damage.” This type of structure might be used to make foldable screens.
Belle Dumé is a contributing editor to Physics World