Zarfl, C. (2019). Robust thin film strain gauges for high temperature sensing applications [Dissertation, Technische Universität Wien]. reposiTUm. https://doi.org/10.34726/hss.2019.41084
Piezoelectric sensors are excellent candidates to measure any dynamic pressure characteristics in air breathing engines. Due to their robustness and low temperature cross-sensitivity they can be operated in harsh environments and are able to reliably monitor high frequency pressure changes. Nevertheless, the lack of the possibility to determine any static pressure fraction is a main disadvantage of piezoelectric sensors. As a possible solution, the integration of high temperature stable strain gauges based on thin film technology is proposed in this work. After an overview of the state of the art in pressure sensing, the fabrication and characterization methods for metal thin films will be explained. Titanium aluminum oxynitride (TixAl1-xNyO1-y), deposited by magnetron sputtering will be presented as potential material system for high temperature stable thin lm strain gauges and compared to those made of platinum. The electromechanical properties of titanium aluminum oxynitride will be investigated in greater detail. By variation of the layer thickness, the metal to gas ratio and the reactive gas admixture in the sputter process, the electromechanical parameters are evaluated. The highest gauge factor of 3.2 at room temperature is found for the reactive gas admixture with no oxygen added (TiAlN), decreasing to 2.5 at 500C. In addition, no hysteresis-like behaviour of the gauge factor for tensile and compressive strain is observed, as present for platinum thin films. For TiAlN also the lowest linear temperature coeffcient of resistance with -3.82 +- 0.14 * 10-4K (-1) in the range from room temperature to 500C is measured. The main disadvantage of TiAlN is the high oxidation rate. In air, the relative change in resistance at 500C is 2.10 * 10 (-3) compared to 4.50 * 10 (-6) for platinum at 600C. Two concepts for the integration of thin film strain gauges into a piezoelectric sensor are presented. While the direct application on the piezoelectric single crystals turns out to be technically most challenging, the integration of a membrane for dual force transmission is the solution of choice. Strain gauges made of TiAlNO and platinum thin films are applied on this membrane and tested. Finally, a complete sensor package is realized with platinum meanders. This sensor is characterized in the pressure range from 1 to 50 bar and in the dynamic frequency range from 12 to 90 Hz. Furthermore, pressure measurements from room temperature up to 250C are performed. A linearity of the strain gauges of 4:2%FSO can be reached in this temperature range. In the sensor package a long term stability of the TiAlNO thin films for 1600 hours is proven at a temperature of 750C.