Researchers in Germany have developed a graphene-based coating that changes colour when deformed or damaged. Its developers hope that it could help early detection of hidden damage to buildings, structures and vehicles.
The coating works in a similar way to the phenomenon that provides colour to the scales of iridescent fish. Rather than containing pigment, these derive their colour from their structure; periodically-arranged microscopic structures on their surface interfere with light as it strikes and is reflected from them, amplifying some visible wavelengths and suppressing others. The phenomenon is also seen in butterfly wings, peacock feathers, and the coloured cells in the skins of squid, cuttlefish and octopus.
Shang-Lin Gao, a specialist in composite materials at the Leibniz Institute of Polymer Research, led the project to develop a coating displaying this kind of structural colour based on regular nanometre scale flakes of graphene in semitransparent, parallel layers, which they applied to glass fibres for testing. When deformed, the layers compress and flatten, which changes the way light reflects from them and affects the colour. When undamaged, the coating is red; when deformed, it turns yellow, and when cracked on the microscale it turns green.
“Structural failure usually starts with tiny cracks and deformations,” Gao said. “Generally, these microscale cracks are hard to detect.”
The visible colour change, however, would provide an early warning and prevent sudden, catastrophic failure. Gao’s team explained its research in the Royal Society of Chemistry journal Material Horizons.