In recent years, Tata Steel Europe has increased their focus on Physical Vapor Deposition (PVD) for the zinc coating application of their steel substrates, as an alternative to Hot Dip Galvanizing (HDG). PVD offers some benefits over HDG like multilayer structures and lower heat impact on the steel.
To achieve sufficient coating adhesion strength of the zinc coating, before deposition the steel substrate is normally cleaned and activated by a plasma sputter unit. As this sputtering takes place in the vacuum chamber, like the deposition, it is prone to precipitation of sputtered material in the vacuum chamber. If the PVD process is scaled up to an industrial coating line, the volumes of sputtered material and precipitated material will become problematic for the service reliability. Therefore it has been investigated whether an acid etching surface pre-treatment step before the vacuum chamber could reduce the needed plasma intensity, and thereby decrease the sputtered volume in the vacuum chamber.
A range of acid etching times and plasma sputtering times were tested, to obtain the range in which the coating adhesion was sufficient. To test the coating adhesion, two (automotive) tests were used. It was found that by pickling, the plasma sputter intensity could not be reduced. So the coating adhesion strength seemed not directly related to the pickling time, for the particular steel used in this project.
After the limits of good adhesion were determined, the characterization started to identify what in the elemental composition or the surface morphology could determine whether there was good adhesion or not. It was found that (even very short) pickling completely removes the surface enrichment of the first 50 nm, while plasma sputtering only lowers the surface enrichments. It was found that plasma sputtering does not influence the morphology, while pickling smoothens the surface out, with increasing pickling. As the oxygen concentration profile did not change significantly as function of pickling time, but its enrichment thickness was about equal to the minimum plasma sputter depth, it is thought that the oxygen concentration is the major influence on good and bad adhesion.