Our most sophisticated synthetic materials may boast unprecedented strength and durability but for something that puts them to shame, just visit a pond or tidal pool. The mineralized structure of nacre, a biological composite that makes up the shells of simple organisms such as mollusks, is stiffer and harder than anything we can make.
This observation has inspired researchers to examine such examples from an engineering perspective. Now a team at McGill University has created its own nacre-like composite that could set a new standard for the performance of glass we use in everyday applications, from cell phone screens to automobile windows.
Credit: Francois Barthelat
Their findings, which were published at the end of June in Nature Communications, highlight the distinctive internal make-up of nacre, which takes the form of an intricate array of interlocking tiles as opposed to the homogeneous, liquid-like consistency of conventional glass. The difference is crucial, since it gives nacre the ability to respond to impact by flexing, which is made possible as layers of these tiles slide under one another to absorb the energy from a blow and minimize the subsequent physical displacement. In contrast, laminated glass has nowhere to disperse this energy, which results in irreparable damage such as cracks or shattering.
The McGill group, led by Department of Mechanical Engineering Professor Francois Barthelat, built their own form of nacre by starting with .22 mm thick borosilicate glass plates. The profile of hexagonal tiles were carved out of the glass with a laser engraver, which produced trenches about 10 µm wide and 40 µm deep. After the etched surface was taped over, the tiles were separated much as one would break up a segmented candy bar, by rolling it over a curved cylinder that released them upon bending. A low-strength, highly deformable polymer — ethylene-vinyl acetate — was spread between the tiles, as well as serving as the interface between further layers of tiles would be placed on top of one another.
“The ‘mortar’ between the hexagons is the polymeric layer,” explains Zhen Yin, a PhD student who co-authored the published results. “This way we obtain a staggered arrangement of hexagonal tablets in three dimensions, much like a 3D ‘brick wall’.”
Impact tests that measured the force and displacement for conventional glasses as well as different types of nacre-like glass revealed just how significantly the latter was able to withstand this abrupt loading. One nacre-like variety experienced a maximum force that was less than half of what was undergone by a plain laminated glass, and afterward showed no visible signs of damage.
According to Yin, this capability bodes well for the prospect that this kind of glass could find its way into a variety of practical applications. There are some trade-offs between the strength of the glass and its ability to transmit light as well as a less robust form of glass, but he points to other areas in which transmissibility is not a major consideration.
“It could be used as in applications where high transparency is not necessarily required, such as medical implants or to make stronger and tougher robots,” he says. “The only question is to tailor the material architecture to best suit each specific application.”