A fully spray processed embedded composite thermocouple for the use at high temperatures and harsh environments

We present a fully spray processed polymer based temperature sensor which can be embedded in the organic coating of metallic machine components. The main aim of the sensor is to measure the temperature in such coatings. This could be used, e.g., in friction bearings for condition monitoring. For the present demonstrator, the sensor element is embedded on a steel substrate. Here, the sensing element is a thermocouple, which is made out of two conductive paints with carbon black and silver as organic or metallic filler particles, respectively. The used carbon black paint is custom-made and uses polyamide-imide as polymer binder which serves also as the polymer backbone of the insulation and the encapsulation layer. The commercially available silver paint is based on polyimide which yields the desired bond strength with respect to the insulation and encapsulation layer. The investigated thermocouples are characterized on a temperature test rig with and without top coating up to a junction temperature of 200 °C. Subsequently, the influence of prolonged heat treatment (in total 16.5 days at 200 °C) on the thermocouple sensitivity is investigated. Finally, the thermocouple cross-sensitivity study of the pressure influence on the temperature sensitivity is performed. First a test was made with pure pressure load of 40 MPa without a temperature gradient between junction and terminal. The final cross-sensitivity measurement was performed in a climatic chamber including a pressure test rig which is able to apply a load of 200 MPa at a maximum temperature of 141.5 °C.