NDE of a subsurface defect in a magnetic disk head and surface defects fabricated on metal plate by photoacoustic microscopy

In this paper, three independent contributions on NDE of subsurface and surface defects with the same photoacoustic microscope (PAM) are summarized. The PAM of this research uses optical scanners for beam scanning. The only difference is that a compact closed type PA cell is used in NDE of magnetic disk heads, whereas a quasi-open type large PA cell is used in the other two experiments. A crack in a ferrite region and a bubble in a glass filler of a magnetic head are observed with the PAM with a resolution of up to 2 micrometers. Photoacoustic (PA) imaging of closely located surface defects fabricated on a metal plate clearly reflects depth information of individual defects. PA signal intensities of simulated pitting corrosions fabricated on a metal plate vary with both depth and diameters of holes. The characteristics are interpreted with a simple model where PA signal is proportional to a surface area exposed to a laser beam.