The effect of annealing treatment on the adhesion performance of a PVD coated ZnMg-Zn steel

ZnMg coatings can be considered as a potential candidate to protect steel car bodies from corrosion and degradation. However, single layer ZnMg coated steels suffer from the poor adhesion due to the presence of brittle Zn-Mg intermetallic phases in the coating. The insertion of a more ductile Zn layer as an intermediate layer between the steel substrate and the ZnMg layer turns up to improve the adhesion performance, considerably. In the present study, the effect of annealing treatments on the microstructure and adhesion strength of a ZnMg-Zn bi-layered coating have been investigated. The results indicate that the thickness of the Zn interlayer reduces with increasing the annealing time at a certain temperature. This can be related to the diffusion of Zn atoms from the interlayer towards the top ZnMg layer. The diffusion of elements across the interface promotes phase transformations in the ZnMg top layer. Such transformations can be controlled to enhance the adhesion performance. However, prolonged annealing can lead to the complete dissolution of the Zn interlayer and the formation of undesirable Fe-Zn intermetallic compounds, resulting in the adhesion drop. The diffusion phenomenon in ZnMg-Zn system has also been simulated to follow the mechanisms of phase transformations