Laser-acoustics. A method for testing coatings and material surfaces

The laser-acoustic technique based on surface acoustic waves has proved to be an interesting method for testing coatings and material surfaces. The central point of the laser-acoustic technique is to measure the dispersion curve of the surface acoustic wave that represents a so-called signature for micro-structural gradients perpendicular to the surface. The dispersion curve is the spectrum of the phase velocity of the surface acoustic wave depending on frequency. For a completely homogenous material, the phase velocity is constant, i. e. does not depend on frequency. For coated materials, the phase velocity varies with frequency. The dispersion spectrum may ascend or descend, depending on the combination of film and substrate material. Fitting a theory to the measured curve enables the Young's moduli of both film and substrate to be deduced. The physical principles of the method are described. The capability of the laser-acoustic method is demonstrated for three examples.