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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.

Incubation effects in the nanosecond laser ablation of metals exhibit a strong dependence on the thermal and mechanical properties of both the target material and the background gas or liquid. The incubation in air is controlled mainly by thermal properties such as the heat of vaporization. In liquid, the correlation of the incubation and the ultimate tensile stress of the metals suggests that incubation may be related to the mechanical impact on the solid material by the cavitation bubble collapse, causing accumulation of voids and cracks in the subsurface region of the ablation craters. At high ultimate tensile stress, however, the low sensitivity to the environment suggests that the mechanical impact is likely to play a negligible role in the incubation. Finally, the correlation between the incubation and the carbon content of alcoholic liquids may be explained by an absorptivity increase of the cavity surfaces due to carbonaceous deposits generated by laser-induced pyrolysis, or by the mechanical impact of long-living bubbles at higher dynamic viscosity of liquids.

Laser-induced depth profiling was applied to the investigation of galvanised steel sheets as a typical modern multi-layer coating system for environmental corrosion protection. The samples were ablated stepwise by the use of two different wavelengths of a frequency-converted Nd:YAG-laser, 266 nm and 532 nm, with a pulse duration of τ = 4 ns at fluences ranging from F = 50 to 250 J cm −2 . The emission light of the resulting plasma was analysed as a function of both penetration depth and elemental spectrum in terms of linear correlation analysis. Elemental depth profiles were calculated and compared to EDX-cross sections of the cut sample. A proven mathematical algorithm designed for the reconstruction of layer structures from distorted emission traces caused by the Gaussian ablation profile can even resolve thin intermediate layers in terms of depth and thickness. The obtained results were compared to a purely thermally controlled ablation model. Thereby light-plasma coupling is suggested to be a possible cause of deviations in the ablation behaviour of Al. The average ablation rate h as a function of fluence F for Ni ranges from 1 to 3.5 μm/pulse for λ = 266 nm as well as for λ = 532 nm. In contrast, the range of h for Al differs from 2 to 4 μm/pulse for λ = 532 nm and 4 to 8 μm/pulse for λ = 266 nm in the exact same fluence range on the exact same sample.

We show the temporal change in repassivation mechanism as a time-dependent linear combination of a high-field model of oxide growth (HFM) and the point defect model (PDM). The observed switch in transient repassivation current-decrease under potentiostatic control occurs independently of the active electrode size and effective repassivation time for all applied overpotentials. For that, in situ depassivation of plasma electrolytically oxidized (PEO) coatings on aluminium was performed with nanosecond laser pulses at 266 nm and the repassivation current transients were recorded as a function of pulse number. A mathematical model combines the well established theories of oxide-film formation and growth kinetics, giving insight in the non linear transient behaviour of micro-defect passivation. According to our findings, the repassivation process can be described as a charge consumption via two concurrent channels. While the major current-decay at the very beginning of the fast healing oxide follows a point-defect type exponential damping, the HFM mechanism supersedes gradually, the longer the repassivation evolves. Furthermore, the material seems to reminisce former laser treatments via defects built-in during depassivation, leading to a higher charge contribution of the PDM mechanism at higher pulse numbers.

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.

Homogeneous polycrystalline FexOy nanoparticles were generated by ablation of iron targets in water by nanosecond laser pulses at 532 nm. In ethanol, crystalline core‐shell Fe/FexOy structures with size medians around 20 nm were produced. The ablation of FeWxOy targets in water resulted in crystalline hollow shells and homogeneous FeWxOy nanoparticles. In contrast, amorphous core‐shell FeWxOy nanoparticles with a median size of 17 nm were produced in ethanol. The size distribution of both the FexOy and the FeWxOy particles showed a slight dependence on fluence and pulse number. This may be related to primary and secondary ablation and modification mechanisms., Partial financial support by the H2020 Action MSCA‐IF 656908‐NIMBLIS‐ESR is acknowledged. Further partial funding came from the National Science Foundation through Grant CMMI‐1301298, and the MAT2015‐67354‐R project of the Spanish Ministry of Economy and Competitiveness (MINECO).

In this study, galvanic coatings of Cu and Ni, typically applied in industrial standard routines, were investigated. Ablation experiments were carried out using the first two harmonic wavelengths of a pulsed Nd:YAG laser and the resulting plasma spectra were analysed using a linear Pearson correlation method. For both wavelengths the absorption/ablation behaviour as well as laser-induced breakdown spectroscopy (LIBS) depth profiles were studied varying laser fluences between 4.3–17.2 J/cm$$^2$$ at 532 nm and 2.9–11.7 J/cm$$^2$$ at 1064 nm. The LIBS-stratigrams were compared with energy-dispersive X-ray spectroscopy of cross-sections. The ablation rates were calculated and compared to theoretical values originating from a thermal ablation model. Generally, higher ablation rates were obtained with 532 nm light for both materials. The light–plasma interaction is suggested as possible cause of the lower ablation rates in the infrared regime. Neither clear evidence of the pure thermal ablation, nor correlation with optical properties of investigated materials was obtained.

Homogeneous polycrystalline Fe

Laser-induced plasma spectroscopy (LIPS) with a frequency-quadrupled Nd:YAG laser (266 nm, pulse duration: 4 ns) was applied to a metallic layer system consisting of an electrodeposited copper layer (30 μm) on an aluminium substrate. A stratigraphic model describing the emission signal in dependence of the pulse number was developed, which can explain several effects originating from laser ablation of various thin top layers by means of the Gaussian beam cross section character. This model was applied to trace elements through layers with thicknesses that are in the range of the resolvable depth, given by the single-pulse ablation rate, by means of empirical fitting functions. Additionally, the contribution of redeposited bulk material to the characteristic shape of emission-traces when averaging spot arrays with varying spacing could be quantified. This can be used to estimate cross-contamination in analytical applications where ablations need to be performed at close spacing.

In situ laser depassivation of plasma electrolytically oxididized (PEO) coatings on aluminium was investigated with nanosecond pulses. Ultraviolet radiation of 266 nm was chosen in order to achieve a high absorption in the dielectric coating. The additive accumulation of laser-induced material defects (incubation) affected the depassivation processes. Incubation occurred only at the edges of the ablation craters irradiated by the outer region of the Gaussian beam profile, where the local fluence is below the ablation threshold. The ablation rate in the spot center did not exhibit an incubation effect. Repassivation was interpreted by a linear combination of a high-field and a point defect growth model. At low overpotentials, field gradients affect the process driving the oxide growth at the buried interface. At high fields, corrosion reactions dominate at the oxide/solution interface.

The nanosecond-pulse laser-assisted generation of Ni/NiO

The nanosecond-pulse laser-assisted generation of Ni/NiOx core/shell nanoparticles (NPs) in water and alcoholic fluids can yield colloidal solutions without surfactants. The size distribution can be controlled by the nature of the alcohol, the number of laser pulses and the laser fluence. The incubation of the nickel target ablation in liquid contact shows a dependence on the carbon number of the respective alcohol. The laser-generated NPs consist of crystalline nickel cores with face-centred cubic patterns and stacking fault defects surrounded by nickel oxide shells. The solvent butanol, in contrast to ethanol and isopropanol, yields a narrow, nearly unimodal, size distribution. The majority of NPs have low size distributions, with medians in the range of 10-20 nm. These can be related to a metal ablation plume interacting with a supercritical liquid that decelerates the ejected material in a low-density metal-water mixing region. NPs in the range above 30 nm result in a minority distribution tail that strongly depends on the fluid nature, the pulse number and the fluence. This coarse NP set may be correlated with the rupture of a superheated molten-metal layer into larger entities.