PHOTOACOUSTIC FREQUENCY-DOMAIN DEPTH PROFILOMETRY OF SURFACE-LAYER INHOMOGENEITIES - APPLICATION TO LASER PROCESSED STEELS

An observed change in the photoacoustic signal frequency response of laser processed stainless-steel and carbon steel samples with respect to unprocessed reference samples is reported. A recently developed thermal wave theory for depth profiling of bulk inhomogeneities (where the surface thermal diffusivity is known and is the same as the homogeneous reference material) in condensed phases with arbitrary, continuously varying thermal diffusivity profiles [A. Mandelis, S. B. Peralta, and J. Thoen, J. Appl. Phys. 70, 1761 (1991)] has been modified to obtain quantitative thermal diffusivity profiles extending from the surface into the bulk. Profiles obtained using this method, which is, in principle, of nondestructive nature, are consistent with the profiles obtained from destructive methods such as cross-sectional optical metallographic examination and microhardness testing.