Zinc coatings on steel produced by ion beam assisted deposition

World-wide, only about 30% of cold-rolled products are coated by conventional methods such as hot-dip galvanising and electrodeposition. The demand for coated steel strip is expected to increase in the next few years and it has been demonstrated that dry PVD technologies can be applied continuously or in a semi-continuous way to produce corrosion-resistant coatings with excellent properties and performance. PVD technologies are also attractive because of reduced environmental constraints compared with conventional methods. Ion beam assisted deposition (IBAD), a relatively new PVD technique, offers the possibility of controlling process parameters independently, and can produce coatings with tailored properties. The aim of this work has been to evaluate the use of IBAD as a prospective method to produce Zn coatings on steel strip. Zinc coatings were deposited by IBAD onto 1 mm IF-steel sheet, at a thickness of approximately 5 μm. Various angles (10 °–50 ° to the ion beam), evaporation rates (1–3 nm s −1) and ion beam energies (500–1000 eV) were selected, while the beam intensity was kept constant at 25 mA. This was to maintain the I/A arrival ratio at a sufficiently low value to obtain good deposition rates when taking into account sputtering effects. A bending test was conducted to test the formability of the coating. Any cracking or delamination that occurred was observed under an optical microscope and a chemical test was used to determine the amount of revealed substrate. This test showed the lack of interconnected pores in the Zn coatings. Furthermore, all IBAD films showed good adhesion and formability with no signs of delamination. Topographical observation of the coatings was undertaken using SEM, which was also used to study the coating thickness and uniformity. These observations showed the deposited coatings to be uneven and of widely varying thicknesses. These variations are discussed in terms of the effects of sputtering and evaporator-substrate distance. From SEM micrographs it could be seen clearly that ion bombardment during deposition influences the structure of the produced film, changing the structure from a highly columnar growth to a fine, even-grained structure. The corrosion resistance was tested using a standard ASTM B117 salt spray test, with a 3.5% NaCl salt solution. This test showed a dependence of the corrosion resistance on the film thickness, with little variations due to the different morphology or microstructure of the films