Effect of Mg concentration & layer thickness on the adhesion of ZnMg-Zn bi-layered PVD coatings

For many years zinc coatings have been regarded as one of the most effective corrosion protective coatings for steel. Recently, it has been shown that the addition of even small amounts of magnesium to the zinc coating can noticeably increase its corrosion protection performance. However, poor adhesion of ZnMg coatings to advanced high-strength steels is observed. The addition of a more ductile Zn interlayer between the steel substrate and ZnMg coating is a solution to improve the adhesion. In the present study, a series of ZnMg-Zn bi-layered coatings with different Mg concentrations (up to 14.1 wt.% Mg) and also different thicknesses of the Zn and ZnMg layers were prepared by a thermal evaporation process to investigate the adhesion performance/interfacial adhesion strength. The adhesion performance of the coatings was qualified by the BMW crash adhesion test (BMW AA-M223), while the interfacial adhesion strength at the ZnMg/Zn interface was quantified by scratch test. It is found that the interfacial adhesion strength decreases gradually with increasing the Mg content of the top layer. The novel finding is that the interfacial adhesion strength at the ZnMg/Zn interface is independent of the thickness of the Zn interlayer. However, the adhesion performance of a ZnMg-Zn bi-layered coating during bending test is a complex function of different parameters such as the thickness of Zn and ZnMg layers, interfacial adhesion strength and interfacial defects density.