High-performance fibers that have been exposed to high temperatures usually lose their mechanical properties undetected and, in the worst case, can break at the very moment when lives depend on them. For example, fire department safety ropes or suspension ropes for heavy loads on construction sites. Empa researchers have now developed a coating system created using a sputtering process that changes color when exposed to high temperatures caused by friction or fire.
Three layers are required for the fiber to actually change color when exposed to heat. On the fiber itself, in the case of the research work PET (i.e. polyester) and VectranTM, a high-tech fiber, the researchers apply silver, which serves as a reflector. This is followed by an intermediate layer of titanium nitric oxide, which ensures that the silver remains stable. This is followed by the amorphous layer that causes the color change: germanium antimony tellurium (GST), which is only 20 nanometers thick. If this layer is exposed to elevated temperatures, it crystallizes; this changes the color impression, for example from blue to whitish. The color change is based on interference. Two different waves (e.g. light) meet and amplify or attenuate 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.
