Ultrasound interferometry for the evaluation of thickness and adhesion of thin layers

This work deals with the evaluation of thin layers by a technique called ultrasound interferometry, which is based on the resonance and antiresonance of ultrasound waves verified in the presence of some interfaces having different acoustical impedances. A Graphical User Interface (GUI), developed in matlab was used to simulate the calculation of several coating thicknesses over different substrates, like iron and aluminium. The results are based on the representation of reflection or transmission coefficients against frequency, thickness and velocity in the coatings. Introduction Ultrasound interferometry is a well known method, which is, frequently used to measure phase velocities. It is easily proved that changing the frequency of a probe transmitting into a medium with parallel faces, resonance and antiresonance of ultrasound waves can be visualised, in a continuous mode. Frequency values corresponding to two consecutive resonances and two consecutive antiresonances are directly correlated with the layer thickness and with the propagation wave velocity. The proposed goal required first the development of equations relating to the multiple internal reflections and transmissions of waves against frequency [1, 2]. In order to simulate this technique for an effective calculation of thicknesses of very thin layers, some tests were accomplished considering three propagation media: coupling, layer and substrate. The results illustrate essentially painting layers on different substrates like; iron, aluminium. A Graphical User Interface (GUI) was created to help simulating the behaviour of the frequency or abstract variable dependent reflection and transmission coefficients [3]. Any number of layers can be tested in the simulation tool. This technique has also great potential to test laminate adhesion, besides coating thickness evaluation. Theory Wave Propagation in three layers systems Figure 1 shows the schematic representation of the three layer system. In the absence of dispersion, the reflection (R) and transmission (T) amplitude coefficients from the medium 1 to medium 2 with acoustic impedance Z1 and Z2 respectively, are given by