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The rapid qualitative and quantitative stratigraphic analysis of metal coatings is of substantial importance in modern industrial applications. Laser-Induced Breakdown Spectroscopy (LIBS) is presented as an attractive option to obtain fast and reliable stratigraphic results. There are still urgent optimization needs particularly in respect to metal coating systems. In this investigation, radiation of the harmonic wavelengths at 1064, 532, 355 and 266 nm of a nanosecond pulsed Nd:YAG laser were used to ablate a steel sheet galvanically coated with a 20 μm layer of a Ni-Co alloy. Atomic emissions from the resulting plasmas were recorded and converted into stratigrams by the linear correlation coefficient method. For all wavelengths, the average ablation rate as a function of laser fluence, as well as the effective absorption coefficient were determined and compared to predictions based on a simple wavelength-independent heat diffusion model. This study enables a deeper mechanistic understanding and an optimization of a high-precision stratigraphy of metal coating systems.

LIBS is a proven method for elemental analysis. It needs little to no sample preparation and relies solely on optical access to the sample. LIBS has therefore been handled as a promising analytical method for layered or structured samples with the potential to replace metallurgic cross-sections followed by SEM/EDX raster screenings. Due to the strong dependency of the ablation process on changing crater geometries, laser fluence, and plasma dynamics (e.g. laser absorption and scattering by the plasma, material dependent optical depth, reflecting coefficients, scattering processes, etc.), attributing an absolute depth to the number of laser pulses applied to the sample is not trivial. In this work, we show absolute depth recognition based on single-spot, non-scanning optical coherence tomography on LIBS craters produced with a standard LIBS setup. The results are verified with the use of ex-situ (electron-)microscopic techniques.