The presented review summarizes nearly two decades of studies on femtosecond laser-induced breakdown spectrometry (fs-LIBS).

The space-time dynamics of thermal melting, subsurface cavitation, spallative ablation, and fragmentation ablation of the silicon surface excited by single IR femtosecond laser pulses is studied by...

Single vs. multimode laser beams have been compared for laser ablation on steel samples. Laser plasma properties and analytical capabilities (precision, limit of detection) were used as key parameters for comparison. Peak fluence at focal spot has been observed to be higher for Gaussian beam despite ∼14-fold lower pulse energy. A comparison of Gaussian and multimode beams with equal energy was carried out in order to estimate influence of beam profile only. Single mode lasing (Gaussian beam) results in better reproducibility of analytical signals compared to multimode lasing while laser energy reproducibility was the same for both cases. Precision improvements were attributed to more stable laser ablation due to better reproducibility of beam profile fluence at laser spot. Plasma temperature and electron density were higher for Gaussian laser beams. Calibration curves were obtained for four elements under study (Cr, Mn, Si, Cu). Two sampling (drilling and scanning procedures) and two optical detection schemes (side-view and optical fiber) were used to compare Gaussian and multimode beam profile influence on analytical capabilities of LIBS. We have found that multimode beam sampling was strongly influenced by surface effects (impurities, defects etc.). For all sampling and detection schemes, better precision was obtained if Gaussian beam was used for sampling. In case of single-spot sampling better limits of detection were achieved for multimode beam. If laser sources have the same wavelength and equal energy then quality of laser beam becomes a crucial parameter which determined laser ablation and analytical capabilities of LIBS.

The influence of sample temperature on the laser ablation process and analytical capabilities of laser induced breakdown spectroscopy (LIBS) was studied systematically. We were interested in estimating the analytical capabilities of a LIBS probe which was developed for in situ elemental analysis of molten metals during the production of metal parts by additive manufacturing. The sample surface was heated locally (4 mm diameter spot) in an argon atmosphere using a powerful continuous wave fiber laser while a nanosecond laser ablated only a small spot (0.3 mm diameter) in the high temperature area. This procedure can be utilized for high temperature LIBS studies of any samples including materials with high melting temperatures (tungsten, aluminum oxide etc.). Low alloy steel samples were heated up to their melting points and LIBS experiments were carried out in a wide temperature range (25–1600 °C). LIBS plasma imaging revealed that the size and brightness of the expanding plume did not depend on solid sample temperature but these parameters increased manifold for molten steel ablation. Comparison of plasma emissivity, temperature and electron density did not reveal any meaningful differences in the case of LIBS measurements for room temperature (25 °C), hot solid (1050 °C) and molten (1550 °C) targets, and thus enhanced plasma emissivity was attributed to greater ablation rate. The surface temperature impact on LIBS analytical capabilities was studied by comparing calibration curve linearity, limits of detection and the root mean square error of cross-validation for chromium, silicon and manganese. High temperature of the sample surface favored greater atomic/ionic line emission but molten sample LIBS analysis revealed poor limits of detection for all elements, which was explained by strong fluctuations of the laser ablation process at the liquid metal surface.

The feasibility of in situ quantitative multielemental analysis and production failures detection by laser induced breakdown spectroscopy (LIBS) has been demonstrated during direct energy deposition process in additive manufacturing. Compact LIBS probe was developed and equipped with the laser cladding head installed at industrial robot for real-time chemical quantitative analysis of key components (Ni, W) during the synthesis of high wear resistant coatings of nickel alloy reinforced with tungsten carbide particles. Owing to non-uniform distribution of tungsten carbide grains in the upper surface layer the only acceptable choice for LIBS sampling was made to the melt pool at growing clad. Laser ablation at powder particles above melt pool was insignificant for LIBS plasma properties due to low intensity and low probability of plasma breakdown at powder particles. No impact of LIBS sampling on cladding process and clad properties was observed according to optical and scanning electron microscopies. The feasibility of in situ LIBS quantitative elemental analysis of key components (tungsten and nickel) has been demonstrated during the cladding process. LIBS analysis results were in good agreement with offline measurements by electron energy dispersive X-ray spectroscopy and X-ray fluorescence spectroscopy. Finally, LIBS technique was demonstrated to be a good tool for real-time detection of cladding process failures (poor laser beam quality, undesirable variation of components concentrations).