Laser-induced breakdown spectroscopy of copper, nickel and nickel-phosphorus coatings: influence of laser wavelength

Abstract

Laser-induced breakdown spectroscopy (LIBS) was demonstrated for depth profiling of model coating systems on steel such as copper, nickel and nickel-phosphorus. A Nd:YAG laser emitting at 1064 nm (pulse duration 7 ns), and at 532 nm (pulse duration 5 ns) was employed. The measured spectra were analysed with the Pearson linear correlation. The incubation, the increase of ablation rate and the decrease of the ablation threshold fluence with the pulse number, is comparatively low for all coating systems. That indicates, that the defects density responsible for this phenomenon is intrinsically high in these galvanic and electroless samples so that repeated pulsing does not lead to a drastic further defect density increase. The ablation rate depends strongly on the wavelength. This cannot be explained by the optical absorption of the substrates. The thermal diffusion length together with plasma shielding can explain the observed behavior semi-quantitatively. Plasma shielding affects the ablation process with the near-infrared wavelength in contrast to the visible case where it is negligible. These results show that the beam-plasma interaction not only plays an important role in the quantification of the ablation rate, the depth resolution and the analysis depth, but also in the plasma heating which itself affects the plasma emission efficiency. Thus, plasma shielding supports a higher depth resolution paralleled by a sensitivity increase due to plasma heating. Normally, a high stratigraphic resolution (low ablation rate) and a strong emission signal due to a high plasma volume and temperature exclude each other. However, the absorption of the plasma both reduces the fluence on the sample and the ablation rate, and heats the plasma, resulting in a higher emission efficiency.