Magnetostrictive Cold Spray Sensor for Harsh Environment and Long-Term Condition Monitoring

Ultrasound sensors and inspection systems are frequently used to generate acoustic waves in metal structures capable of detecting and characterizing cracks, pits, erosion, inclusions, weld anomalies, and other material and structural features. One significant problem with piezoelectric transducers is the difficulty to achieve good coupling between the transducer and the surface being examined. This is particularly true in harsh conditions with high temperatures, cyclical hot and cold temperatures, highly radioactive fields found near nuclear reactors or spent nuclear fuel, caustic or corrosive fluids, and other extreme environmental conditions, as well as in long-term monitoring applications where repair or replacement of the sensor is difficult or expensive. Typically, coupling between the surface and the transducer is achieved with water, gel, grease, viscous shear coupling material, or pressure, which might not be possible or appropriate for long term applications in which the impedance-matching materials wear away, evaporate, or simply stop functioning due to changes in surface conditions. Fluid couplings can evaporate or drain away from the transducer-substrate interface; glue based couplings may foul or fail and are notoriously unreliable at high temperatures and in radioactive environments. This work explores the behavior of a magnetostrictive cold-spray patch that is metallurgically bonded to a stainless steel inspection target surface, and compares it to the performance of a standard adhesively-bonded ferrous-cobalt magnetostrictive strip solution. Cold-spray is a coating process where 10-100 micron diameter powdered metal is accelerated to Mach 2 to Mach 3 (2-3 x speed of sound) and impacted on the surface to be coated. Each powder particle forms a kinetic bond with the substrate or other coating particles to produce a metallurgically bonded layer. If the powder is nickel or cobalt with high magnetostrictive coefficients, this surface can serve as the base of a magnetostrictive sensor suitable for crack or pitting-damage inspection and monitoring that is not subject to temporal or environmental degradation. A commercially pure nickel (CPNi) cold-spray patch-based sensor on a 2 x 4 ft., 1/4 and 1/2 in. thick plate was contrasted with a more conventional FeCo adhesive strip to determine feasibility and relative efficacy of the two magnetostrictive substrates. The magnetostrictive coefficient of CPNi is reported as 25-60 ppm while cobalt has a magnetostrictive coefficient of 40-120. Moreover, the FeCo strip is prepared with a magnetic bias treatment. As might be anticipated, the FeCo strip showed similar to 7-19 dB stronger edge-wall reflection than the CPNi patch; however, both signals were readily detectable. This degree of different responses is manageable within the range of gain settings of commercial EMAT instruments. Based on the edge reflections, it is inferred that sensitivity to pit or crack flaws would be similarly detectable and, therefore, the cold-spray patch would be a viable alternative to a FeCo adhesive strip sensor that is not subject to adhesive degradation. Influences of magnetic bias and cold-spray patch thickness are also explored in this study.