Thin-Film Piezoelectric Acoustic Emission Sensor With High Sensitivity Up to 650 °C

This article reports a high-temperature acoustic emission (AE) sensor enabling couplant-free and waveguide-free attachments on structures operating up to $650~^{\circ }$ C. The microfabricated sensor is constructed using silicon carbide as the substrate and aluminum nitride (AlN) as the piezoelectric film. The piezoelectric coefficient ( ${d}_{{33}}$ ) of AlN is measured using piezoresponse force microscopy (PFM) as 3.62 pm/V. The sensor exhibits an impedance response of ~1 k $\Omega $ in the 100-300-kHz frequency range, which is below the input impedance of conventional AE systems, causing a slight reduction in amplitude. Following the sensitivity and impedance characterizations, the sensor is tested inside a furnace at temperatures ranging from room temperature (RT) up to $650~^{\circ }$ C. Pencil lead break (PLB) and ball drop tests are used to simulate AE sources. The sensor is dry-coupled to the test surface using high-temperature wires and a stainless-steel fixture. The sensor sensitivity decreases slightly with increasing temperature, with a maximum reduction of 6 dB at $650~^{\circ }$ C. The sensor is evaluated for detecting creep damage in 316L stainless steel and demonstrated performance comparable to conventional sensors attached with waveguides. Compared with conventional bulk AE sensors, the key characteristics of this AlN-based thin-film AE sensor are its high-temperature functionality and couplant-free attachment, enabling direct placement near critical systems under elevated temperatures. This positioning mitigates the influences of long wave paths introduced by waveguides, enhancing the sensor's effectiveness in detecting the initiation and progression of damage. The developed sensor leverages the advantages of microfabrication, offering benefits such as mass production, low cost, and a compact footprint.