Influence of Cr-Content on the thermoelectric and mechanical properties of NiCr thin film thermocouples synthesized on thermally sprayed Al2O3

Ni/Ni-Cr thin film thermocouples were deposited by means of DC magnetron sputtering for the application in plastic extrusion dies. In order to ensure the application of the thermocouples on complex surfaces, a thermally sprayed Al2O3 layer was used between the substrate and the thin films for thermal insulation and electrical isolation. The study includes the deposition, characterization, calibration, and validation of thin film thermocouples. The influence of the composition (10 at.-%, 20 at.-% and 30 at.-% Cr) was analyzed concerning mechanical and electrical properties. It was found out that the electrical resistance increases with the temperature and the Cr-content due to a decreasing crystallite size. By means of a scratch-tester and nanoindentation, a higher scratch load and hardness were observed for larger Cr-contents on the thermally sprayed Al2O3 coating. Despite the composition, the influence of the size as well as the thickness of the thermocouples were analyzed regarding the Seebeck coefficient. Independent from the size and Cr-content, all thermocouples feature a comparable Seebeck coefficient of about 40 μV/K. This high coefficient is similar to that of wire thermocouples, showing a low number of defects in the thin films. The ceramic coating features a thermal insulation of about 20 °C at 200 °C, allowing a more accurate measurement of the ambient medium.