Online coating thickness measurement and depth profiling of zinc coated sheet steel by laser-induced breakdown spectroscopy

In this study a new method for online analysis of the zinc coating of galvanized sheet steel based on laser-induced breakdown spectroscopy (LIBS) is presented. The coating is characterized with a series of single laser bursts irradiated on the traversing sheet steel, each on a different sheet steel position. To achieve an ablation depth in the range of the coating thickness of about 10 μm a Nd:YAG laser at 1064 nm in collinear double pulse mode was used. The depth information is obtained by control of the ablation depth by adjusting the burst energy using an external electro-optical attenuator. Concepts for the determination of the coating thickness and the chemical composition are presented. The achieved thickness resolution is estimated to about 400 nm for coating thicknesses of electrolytic galvanized sheet steel in the range of 3.2 to 11.2 μm. In the case of hot-dip galvanized sheet steel information about the depth profile of aluminium can be gained by the new method.