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).

The potential of the method of fluorescence spectroscopy for the detection of damage and diseases of fruits and vegetables was studied. For this purpose, the spectra of fluorescence of healthy and rotten apples and potatoes have been investigated. Excitation of samples was carried out using a continuous semiconductor laser with a wavelength of 405 nm and a pulsed solid-state laser with a wavelength of 527 nm. Peaks in the region of 600–700 nm in rotten samples were shifted towards shorter wavelengths for most samples in both modes of spectroscopy. The differences in the fluorescence spectra of a healthy and rotten apple surface have been revealed to be in the spectral range of 550–650 nm for 405 nm continuous excitation. When exposed to a laser in a pulsed mode (527 nm), the contribution of the 630 nm peak in the spectrum increases in rotten samples. The observed differences make it possible to use this method for separating samples of healthy and rotten fruits and vegetables. The article paid attention to the influence of many factors such as sample thickness, time after excitation, contamination by soil and dust, cultivar, and location of the probing on fluorescence spectra.