Investigation of thin-film ultrasonic transducers for high-temperature applications

Ultrasonic transducers based on aluminium nitride (AlN) piezoelectric thin films have been proposed for high-temperature (>400°C) defect monitoring of industrial plants and components. To form the transducers, c-axis oriented AlN films, 6 μm thick, were grown on carbon steel substrates by RF sputter deposition. If the films are deposited directly onto components, then the problems of high-temperature couplant layers can be avoided. Two approaches were taken to testing the devices. In the first method, the devices were heated on a hot plate up to 400-500°C and tested by pulse-echo measurements at high temperature and on returning to room temperature. It was shown that AlN thin-film transducers could maintain their performance in pulse-echo mode at temperatures approaching 500°C. A deterioration in performance below 500°C was attributed mainly to degradation of the top electrode. In the second method, film and substrate were heated in an oven to 600°C and subsequently characterised by X-ray diffraction and scanning electron microscopy to identify any structural change in the films and to investigate the transducer failure mode. These tests showed no significant change in the AlN film after heating. The high temperature durability of the transducers, however, was limited by heat-induced surface oxidation of the carbon steel substrate at the elevated temperatures, which led to decoupling of the film from the substrate.