Crack imaging in alumina: a scanning acoustic microscope study

A scanning acoustic microscope (SAM) operating at 750 MHz has been used to image artificially induced cracks in polycrystalline and single crystal specimens of alumina. Although the resolution of the instrument used in this work, at 1.6m̈m, was not as good as that of an optical microscope, significantly more information about the crack characteristics was revealed acoustically. Cracks of finite width act as scattering points for acoustic waves and thus sources of contrast in the SAM image, even when their width is less than the theoretical spatial resolution. In polycrystalline samples it was found that defocussing the SAM both above and below the focal plane, utilising different regions of the V(z) response, enabled contrast due to grain boundaries, cracks, and individual grains to be distinguished. It was also possible to identify different modes of cracking. For the single crystal samples, distinctive contrast was obtained from both horizontal and vertical sub‐surface cracks within one Rayleigh wavelength of the surface. In most materials, features such as these are not readily detectable by other techniques.