Systematic line selection for online coating thickness measurements of galvanised sheet steel using LIBS

LIBS can be used as an online method of characterizing galvanized coatings on sheet steel moving through a production line. The traversing sheet steel is irradiated with a series of single laser bursts, each at a different position on the sheet steel. An ablation depth in the same range as the coating thickness (about 10 mu m) is achieved by using a Nd:YAG laser at 1064 nm in collinear double-pulse mode. The coating thickness is determined from the ratio of the intensities of an iron line and a zinc line measured at a burst energy high enough to penetrate the coating with a single burst. Experiments at different burst energies were carried out to optimize the thickness resolution, and a method of systematically selecting iron and zinc lines was deduced, which is based on multivariate data analysis (MVDA) of the intensity ratios calculated for a set of 6 zinc lines and 21 iron lines. A temperature correction was applied, because the parameters of the plasma change with burst energy, and the influence of this on the thickness resolution is discussed. The ambient atmosphere present (air, Ar, N-2) as well as self-absorption of spectral lines both have an influence on the thickness resolution. At optimum conditions, a thickness measurement accuracy of better than 150 nm was obtained for a set of electrolytic galvanized sheet steels with coating thicknesses in the range 4.1-11.2 mu m.