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Nacre-inspired composites display optical transparency, fracture toughness

By Kendra Redmond September 25, 2019
Nacre-based composite (59% silica content by volume) that is (a) in contact with and (b) 5 mm above a backlit pattern. Credit: Magrini et al., Nature Communications.

Inspired by the structure of mother of pearl (nacre), researchers from ETH Zürich have demonstrated an approach to fabricating optically transparent composite materials that are tough, strong, and wear-resistant—normally an elusive combination. As they reported in Nature Communications, the composites have some of the highest strengths among glasses and a fracture toughness up to three times greater than that of common glasses.

Optically transparent materials such as silica- and soda-lime glasses are strong enough to withstand heavy loads, but they are prone to shattering—cracks spread quickly due to their brittle nature. Chemical and thermal treatments can increase the strength of a material but not its resistance to crack propagation, a property associated with fracture toughness. Recently, research has demonstrated that engraving microstructures on a brittle glass surface can increase its fracture toughness, but the microstructures act as defects that reduce the material’s strength.

Led by André R. Studart and Florian Bouville (now at Imperial College London), the ETH team took a nature-inspired approach to designing a material that is tough yet strong. The inside of mollusk shells and the outside of pearls are composed of nacre, a strong, tough, iridescent material. This combination of properties is achieved via a two-pronged approach: a chemical composition that offers optical functionality and a complementary microscale architecture that adds toughness to an otherwise brittle material. Studart calls this approach “a pathway to combine, in a single composite material, contradicting properties that would not be achievable using conventional approaches.”

Nacre is an inorganic–organic composite with a layered brick-and-mortar structure. Calcium carbonate “bricks” are linked together by thin mineral bridges and surrounded by a “mortar” of biopolymers. Research shows that this design adds toughness to a material, promoting crack bridging, deformation, and other behaviors that slow crack propagation.

To create a similar structure in an optically transparent material, Tommaso Magrini, a graduate student working with Studart, fabricated composites from silica glass flakes and a polymeric mixture of poly(methyl methacrylate) (PMMA) and phenanthrene (PHN) with the same index of refraction as silica. The glass flakes were dispersed in water and slowly settled into horizontally aligned layers. Then the material was dried, uniaxially compacted, and sintered to create bridges at the points where neighboring flakes were in contact. The result was a glass scaffolding of interconnected silica “bricks,” similar to the calcium carbonate structure seen in nacre. The polymeric mixture then infiltrated the scaffolding and was polymerized in situ.

By exposing the composites to different compressive strengths before sintering, the researchers were able to tune the density of the silica scaffolding. Their characterizations focused on composite samples with 35%, 45%, and 59% silica content by volume.

Mechanical tests showed that composite strength increased with silica content. At 59% silica content, composites displayed the strength of common glasses and transparent polymers. Their surface hardness was an order of magnitude lower than that of silica glass, but at least 10 times higher than that of transparent polymeric materials. Furthermore, the team found that cracks caused less structural damage to the new composites than to harder glasses. Bending tests and high-resolution imaging revealed that the fracture resistance of the composites increased with crack growth, yielding fracture toughness 2.5–3 times greater than that of common glass.

In the visible range, the researchers measured a total diffuse transmittance of 45–55% for 1-mm-thick samples. This is significantly less than typical optical glasses, but the researchers expected that optimizing the infiltration process could partially reduce the difference. Optical characterizations also revealed a significant hazing effect, likely due to air pockets in the material. This makes it best suited for applications close to a light source, such as protective display covers, say the researchers. They hope this research will spur efforts to quantify the structure–property relationships of the material, which could lead to enhanced mechanical and optical properties and a wider range of applications.

The ETH approach offers a simple and scalable route to bulk materials with a remarkable combination of transparency, strength, and toughness, according to Flavia Libonati, an expert in the toughening mechanisms of biological structural materials at the Politecnico di Milano. “This work demonstrates the possibilities of implementing microstructural nacre-like toughening mechanisms into glass-based materials, overcoming the intrinsic limitations of such materials, for example, brittle fracture, and offering a valid alternative to silica-based glasses for numerous applications,” she says.

Originally published in the September 2019 issue of MRS Bulletin.