Interface fracture behavior of zinc coatings on steel: Experiments and finite element calculations

Hot-dipped galvanized steels are widely used in the automotive industry. The formability and damage resistance of zinc coatings depend strongly on their microstructure and adhesion to the steel substrate. In order to improve the mechanical performance of zinc coatings, the influence of their thickness, grain orientation and grain size on the zinc coating/steel substrate interface cracking behavior was studied. To this end, scanning electron microscopic observations during in situ tensile testing of zinc coated IF steel sheets were performed. After partial delamination of the zinc coating, cross sections of zinc coated steel were prepared to determine the location and extent of the interface cracking and the crystallographic orientation of the delaminated zinc grains. A two-grain model using a finite element method is proposed to analyze the zinc coating/steel substrate interface cracking behavior. In addition, the coating adhesion strength can be estimated based on this model. Both calculations and experimental observations show that: (i) a preferential zinc grain orientation with the [0001] direction parallel to the interface and (ii) a small grain size mitigate zinc coating/steel substrate interface cracking.