Spatial Resolution of the Scanning Acoustic Microscope for Subsurface Imaging of Samples

Abstract

The increasing use of thermomechanical ceramic parts in industry calls for new non-destructive evaluation -methods, which need to be tailored to such materials. The brittleness of ceramic materials (with mechanical toughnesses in the range from 1 to 5 MPa ✓m), together with their generally high Young’s modulus, lead to the occurrence of severe mechanical stresses, even for moderate strains, particularly in the vicinity of the external surfaces. So, small sized defects become very critical in these locations, and the control means must exhibit high spatial resolutions. For this purpose, we use the scanning acoustic microscope. This technique has been first developed at a working frequency around 1 GHz. This leads, in some materials, to a wavelength comparable to that of light and therefore, to a spatial resolution approaching that of an optical microscope. At these high frequencies, the acoustic attenuation enables the propagation over some hundreds of wavelengths, that is, a fraction of a millimeter. Therefore, the method allows the observation of very thin slices (by transmission) or of the surface of samples (by reflection) and is not suitable for testing materials in bulk. For this reason, we chose to develop a scanning acoustic microscope operating at a 100 MHz frequency. Using a non-destructive testing method, based upon a focused beam, provides an adequate spatial resolution (say 0.1 mm) for ceramics at this frequency, together with a sufficient exploration depth (say 5 to 10 mm).