Structural Color Sensors with Thermal Memory: Measuring Functional Properties of Ti-Based Nitrides by Eye

To ensure safe and reliable operation of materials with high thermal loads, e.g., in turbines or thermal solar collectors, it is key to detect material degradation. Here, a sensor concept providing a direct optical feedback of thermally induced hardness and resistivity changes in transition-metal-nitride functional coatings is presented. The sensor concept relies on a lossy Gires-Tournois interferometer configuration using thermally induced detuning of a highly absorbing state in the optical spectrum as feedback. It is demonstrated for the specific case of TiAlN coatings that such detuning is due to a symmetry-breaking structural phase transition, which is accompanied by the formation of saturated structural colors.