Ultrasonic condition monitoring using thin-film piezoelectric sensors

Thin-film low-profile sensors have been investigated for ultrasonic condition monitoring. The sensors are made by growing a thin film of aluminium nitride onto the component to be monitored. The transducers can be engineered to operate in passive or active mode from 200 kHz to 400 MHz. New or existing applications based on ultrasonic pulse-echo techniques or acoustic emission testing can make use of the sensors, including monitoring of high-temperature plant or machinery. The sensors have been demonstrated on various component materials such as stainless steel, ferritic steel, aluminium, titanium and silicon carbide. The piezoelectric material used, aluminium nitride, has a very high Curie temperature so the devices can be used up to 600C. Examples are presented of devices operating in pulse-echo and passive detection modes, which could be used for permanent monitoring of parts which would normally require maintenance outage in order to be inspected. In addition, because the typical thin-film transducers are only 840 m thick, sensors can be placed in locations previously impossible to access. The operating frequency of the thin-film devices has been investigated by simulation using an equivalent circuit model.