Measurement of the coating thickness on the back side of double-sided coated structures by means of acoustic resonant spectroscopy

This paper describes a technique for measuring the thickness of the back side coating of a double-sided coated structure using acoustic resonance. The technique is used to observe the resonant frequency of high frequency ultrasound for a steel/coating/air structure. The resonant frequency of the transmitted ultrasound occurs when a quarter wavelength corresponds to the thickness of the back side coating, and the reflection coefficient has a minimum value. The top surfaces of the samples are covered with a coating about 30 μm thick and the thickness of the back side coating is in the range of 10.3–17.5 μm. The resonant frequencies for the examined samples are observed in the frequency range of 46.8–75.6 MHz, and the thickness of the back side coating can be accurately determined from the measured resonant frequency. Note that the coating on the top surface of the structure does not affect the thickness measurement accuracy of the back side coating.