Characterization of multilayer coatings of aluminium, tungsten and molybdenum on steel substrate using laser-induced breakdown spectroscopy

Abstract

Laser induced breakdown spectroscopy (LIBS) has been applied for the depth profiling of multilayer metallic coating structure which is simulated to the ITER-like wall materials in fusion devices such as EAST tokamak. The coatings were produced by magnetron sputtering and characterized by LIBS using a Nd:YAG laser at 1064 nm with pulse duration of 5 ns. The light from the resulting plasma was analyzed as a function of both number of laser pulse (N) and elemental spectral line intensity. The depth profile of the multilayer coating was determined by three different spectral signal processing methods; intensity background subtraction, intensity normalization and linear Pearson correlation. The intensity normalization method provides the best outcomes/solutions for thickness measurement and analysis of interfaces of multilayer coatings among other methods. The evolution of spectral intensity representing different materials was properly examined for 90 consecutive laser shots at the same spot. The ablation behaviour of the coating materials was analyzed in terms of ablation rate under the laser pulse of various fluences. In a much deep crater, some factors originating from laser ablation, such as material re-deposition and plasma shielding effect were analyzed. As a comparison, the depth, shape and diameter of craters were also measured using SEM and a profilometer. The influence of laser power density on the ablation rate and crater profile was investigated. The elemental composition, inside the crater, was verified by the EDX technique. The results indicate that LIBS has a potential power for in-situ depth profiling of multilayer coatings on the wall of the EAST.