FIB and TEM observations of defects in hot-dip zinc coatings

The characteristic ability of FIB (focused ion beam) fabrication to remove materials from a very small and/or precisely located slab using an accelerated Ga ion beam were employed to prepare cross-sectional thin films of zinc-coated steel sheets composed of Fe-Zn intermetallic compounds. A few defects observed frequently on the galvannealed (GA) coating surface were analyzed. Streaky marks indicated the rich concentration of oxides at the interface between the galvannealed coating and the steel substrate. The annealing experiment indicated the existence of Mn and Si oxides on the steel substrate. The grain size of ferrite in the extreme surface of the substrate was smaller as compared with conventional IF (Interstitial Free) steels. Furthermore, some superlattice spots were observed at the fine ferrite grain. Defects that are termed 'wavy pattern' and 'dross' were also analyzed by the combination of the FIB and TEM (Transmission Electron Microscope) techniques.