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ECNDT 2006 - Th.3.4.1 Characterisation of Organic Thin Film Coatings on Automobile Steel Sheets by Photothermal Methods Thomas ORTH, Salzgitter Mannesmann Forschung GmbH, Duisburg, Germany Wilko FLÜGGE, Salzgitter Mannesmann Forschung GmbH, Salzgitter, Germany Jürgen GIBKES, AG FestKörperSpektroskopie, Ruhr-Universität, Bochum, Germany Abstract. In the nineties, the first generation of organic thin film coatings for corrosion protection of zinc-coated thin sheet steel have been introduced. The coating typically consists of a suspension of small zinc particles, embedded in a polymer matrix. In the scope of quality control, the characterisation of the resulting layer structure is of great interest, comprising not only a constant layer thickness and a local homogeneity of the coating, but also the depth distribution of the particles within the layer. Especially the latter parameter does have a direct influence on the spot weldability of the steel sheets. The present work shows, how photothermal methods like modulated infrared radiometry and photoacoustics can be used for a successful depth profiling of the thin film coatings. The sample surface is periodically heated using an intensity- modulated laser beam, and a thermal wave is induced in the layer system. By variation of the modulation frequency of the laser beam, the thermal diffusion length and, as a consequence, the penetration depth of the thermal wave can be adjusted. By a suitable evaluation of the amplitude and phase lag signals as a function of