Scanning Acoustic Microscopy of Partly Embedded Cracks in Polycrystalline Alumina

Reflection scanning acoustic microscopy (SAM) at 1 GHz has been used to examine artificially induced cracks in alumina samples. The results are compared with the corresponding optical micrographs and scanning electron microscope images. For a sample with a 0.08 μ m centre-line average (CLA) surface finish, the SAM working with a 400 μ m frame size, showed that a crack that appeared to be about 150 μ m in length in the corresponding optical image was, in fact, at least 200 μ m in length. For a sample with a good engineering finish of 0.15 μ m CLA, the SAM was able to reveal crack detail to a degree comparable with that revealed by an optical microscope working under optimum conditions. On a sample with a highly polished surface it was possible to distinguish between cracks and grain boundaries by using the V(z) response of the SAM, in agreement with theory.