Reconstruction of depth-dependent parameter profiles based on photothermal measurements

摘要

Microstructural changes in a near-surface zone, as they appear at the hardening of steel, machining of ceramics, laser treatments or wear, occur together with modifications of their thermal properties. Therefore, the influences of such processeson the surface layer become detectable non-destructively by photothermal means. Today, the thickness determination of the affected zone via calibration curves is regarded as state of the art, whereas an increasing attention is focused on the numericalevaluation of the depth-dependent parameter profile, i.e. the material changes as a function of depth, mostly obtained by destructive tests, as yet. This paper presents the theoretical basics of a parameter profile reconstruction, based on photothermallymeasured data. It compares analytical solutions, derived directly from the thermal diffusion equation (TDE), with numerical approaches using the Finite Difference Method (FDM), both aiming at a quick and non-destructive control of near-edge affectingprocesses. Both models are applied to photothermal measurements, leading to calculated depth profiles of hardness in steel, which are compared with data, destructively obtained from the same samples using Vickers indentation techniques.