TAGS: Smart Coatings
High-performance fibers that have been exposed to high-temperatures usually lose their mechanical properties undetected and, in the worst case, can tear precisely when lives depend on them. For example, safety ropes used by fire brigades or suspension ropes for heavy loads on construction sites. A team of researchers from Empa and ETH Zurich has developed a coating which changes color due to the physical reaction with heat, thus clearly indicating whether a rope will continue to provide the safety it promises in the future.
Changes in Coating Color at High-temperatures
Researchers from ETH Zurich and Empa developed a coating system in 2018 as part of a Master's thesis, which the Empa team was now able to apply to fibers. "
It was a process involving several steps," says Dirk Hegemann from Empa's Advances Fibers lab.
The first coatings only worked on smooth surfaces, so the method first had to be adapted so that it would also work on curved surfaces. Empa has extensive know-how in the coating of fibers - Hegemann and his team have already developed electrically conductive fibers in the past. The so-called sputtering process has now also been successfully applied to the latest coating.
Wafer-thin Layers with Great Effect
Three layers are required to ensure that the fiber actually changes color when heated. The researchers apply silver to the fiber itself, in this case PET (i.e. polyester) and Vectran™, a high-tech fiber. This serves as a reflector - in other words, as a metallic base layer. This is followed by an intermediate layer of titanium nitrogen oxide, which ensures that the silver remains stable.
And only then follows the amorphous layer that causes the color change: Germanium-antimony tellurium (GST), which is just 20 nanometers thick. When this layer is exposed to elevated temperatures, it crystallizes, changing the color from blue to white. The color change is based on a physical phenomenon known as interference.
Two different waves (e.g. light) meet and amplify or weaken each other. Depending on the chemical composition of the temperature-sensitive layer, this color change can be adjusted to a temperature range between 100 and 400 degrees and thus adapted to the mechanical properties of the fiber type.
Source: Empa