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 ce ntral point of the laser-acoustic technique is to measure the dispersion cu rve of the surface acoustic wave that represents a so-called signature for microstructural gradients perpendicular to the surface. The dispersion curv e is the spectrum of the phase velocity of the surface acoustic wave depend ing on frequency. For a completely homogeneous 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. Fitt ing a theory to the measured curve enables the Young moduli of both film an d substrate to be deduced. The physical principles of the method are descri bed. The capability of the laser-acoustic: method is demonstrated for three examples.