Acoustic microscope for surface characterization

The use of acoustic microscopy for nondestructive thickness measurements of surface layers is discussed. The approach is based upon the thickness dependence of the surface wave velocity in a layered medium. Using an acoustic material signature from a focused beam microscopy, one can determine the surface-wave velocity. This technique also has the potential of providing information on coating defects and surface adhesion. An experimental facility is being established for the characterization of the relatively thick coatings that are protecting surfaces from corrosion and erosion. Thickness in the range of 0.01 to 0.25-mm require an operating frequency between 5 and 200-MHz. An experimental system capable of operation at discrete frequencies between these limits was constructed and initially used to obtain the acoustic material signature of aluminum on beryllium from which the surface wave velocity is deduced. This measured velocity compares favorably with calculated results.