Zn–Cr alloy coatings produced by ion beam assisted deposition

Abstract

In the present work, Zn–Cr alloy coatings have been deposited on a Ti+Nb interstitial free steel by an IBAD process. Zn was deposited by a resistance-heated evaporation source, while Cr was simultaneously evaporated with an electron beam gun. Low-energy Ar ion beam bombardment was used during the process to sputter-clean the surface prior to deposition and during deposition to obtain a dense coating. Different Zn:Cr evaporation ratios ranging from 30:1 to 30:30 were applied to produce several Zn–Cr alloy coatings until a final coating thickness of about 7 μm was obtained. The process was carried out at room temperature and SEM observations showed a columnar growth of these coatings. The structure of these Zn alloy coatings was evaluated with XRD, which showed a change from the hexagonal Zn structure to a b.c.c. structure as the amount of Cr in the coating increased. Salt spray corrosion tests revealed that increasing the Cr content in the coatings improved significantly the corrosion resistance up to a maximum limit of about 20% wt., beyond which no substantial benefit could be observed. Mandrel bend tests showed a good adhesion of the coating to the substrate, and no delamination or cracks were observed. A dynamic microindentation test also showed that the presence of chromium significantly improved the hardness of the Zn coating up to a factor of 5, with a similar effect on the elastic recovery of the indentation.