The acoustic microscope was developed as a tool for studying internal microstructure of non-transparent solids; it is now widely used in detection of cracks and subsurface defects (Briggs, 1992; Zinin nad Weise, 2003).
Imaging a subsurface defect in a coating/substrate system using an acoustic microscope. The output signal of the microscope reaches its maximum when the real focus in the coating is on the top of the spherical defect.
A theoretical analysis of acoustic microscopy of spherical cavities in solids has been developed by Lobkis et al. (1995) Reference (Zinin nad Weise, 2003) has addressed the application of SAM for imaging subsurface defects inside solids for three different types of materials used in modern industry: soft materials (epoxy, concrete, polymers, etc.), superhard materials (diamond, boron nitride, and superhard amorphous carbon etc.), and intermediate case (amorphous carbon films, TiN films etc.). The images of subsurface defects in soft materials can be easily interpreted using a simple ray optical approach (Zinin et al., 2002). For stiff materials, image interpretation for subsurface defects becomes difficult because the contrast of the image at GHz is mainly determined by Rayleigh surface waves (Atalar, 1985). For these materials, the Rayleigh wave penetrate deeper than the focal points of longitudinal and shear waves. The interpretation of acoustical images acquired in moderately hard materials such as DLC films is more complicated than in softer materials. For DLC films of several microns (1-3 mm) thick, the wavelength of the longitudinal and shear wave at 1 GHz in the film is greater than the film thickness. The 1 GHz images of the defects inside the coating are therefore near-field images (Fei, et al, 2004). The ray theory does not provide the location of the defects, and a rigorous solution should be used (Lobkis et al., 1995).
Acoustical images of a flat Cr-DLC coated specimen measured at 1.3 GHz with different defocuses of: (a) 0 mm, (b) -2 mm, (c) -4 mm, and (d) -6 mm. The field view of the images was 200 mm´130 mm. Defects inside circles are subsurface defects whereas those inside rectangles are surface features .
Conventionally, SAM images show variations of the amplitude of the acoustical signal. Reinholdtsen and Khuri-Yakub (1991) measured amplitude and phase of the SAM signal at low frequency (3 to 10 MHz) to improve subsurface images. By modifying the two-dimensional point spread function, the transverse resolution was improved by about 20%, and the obscuring effect of surface roughness from images of subsurface features was eliminated.References