Effect of Ni and Zn Elements on the Microstructure and Antibacterial Properties of Cu Coatings

In this work, 316L stainless steel samples were coated with copper (Cu) and German silver (Cu 17%Ni 10%Zn) to investigate the relation between their mechanical and antibacterial behaviors. The mechanical and material characteristics of the samples were studied by looking into the microstructure of the surface and the cross-section of the coatings, the surface roughness, and the adhesion strength between the coating layer and the substrate. The antibacterial behavior is then studied against gram-negative Escherichia coli and gram-positive Staphylococcus aureus. Two experiments were conducted to examine the antibacterial behavior. In the first experiment, the coated samples were covered with distilled water, whereas in the second experiment, the samples were tested without being covered with distilled water. The results show that German silver (Cu 17%Ni 10%Zn) had a higher antibacterial rate than copper (Cu) by around 10% for both gram-negative E. coli and gram-positive S. aureus. The reason is because a smoother surface is expected to limit the bacterial adhesion in most cases, and the German silver samples have a lower surface roughness (Ra) due to the higher thermal expansion value of zinc (Zn) compared with copper (Cu). A more in-depth look into the effect of various thickness of the coating with alloying elements (in this case nickel and zinc) on the antibacterial rate would be of great interest.