Enhancing the Lift-Off Performance of EMATs by Applying an Fe₃O₄ Coating to a Test Specimen

Electromagnetic acoustic transducers (EMATs) are noncontact ultrasonic transducers. The transduction efficiency of a particular EMAT on a given specimen is dependent on the lift-off distance, which is the distance between the EMAT coil and the specimen surface. The transduction efficiency drops dramatically with increased lift-off distance, requiring EMATs to be in close proximity to the specimen, usually within a few millimeters. This article proposes a new EMAT method of applying an Fe3O4 coating to the test specimen and quantitatively studying the enhancement effect of Fe3O4 coating on lift-off distance. To eliminate the interference of the electrical and magnetic properties of the tested specimen, a nonmagnetic and nonconductive glass specimen is selected. The experimental results on a glass substrate coated with Fe3O4 demonstrate the feasibility of EMATs generating and receiving ultrasonic waves through the coating, by a magnetoelastic mechanism. The transduction efficiency of EMATs on an Fe3O4 coating does not increase linearly with the bias static magnetic field, and the maximum measured signal amplitude value occurs at a relatively low flux density of $\sim 0.12$ T. More specifically, it has been shown that the Fe3O4 coating can significantly enhance the lift-off distance of EMATs operating at 4 MHz to 8 mm on coated stainless steel. The performance of the Fe3O4 coating can be optimized, showing considerable potential to expand the application range of EMATs.