Low temperature deposition of Cr(N)/TiO2 coatings using a duplex process of unbalanced magnetron sputtering and micro-arc oxidation

The surface modification of light-weight metals (such as Mg-, Al- and Ti-alloys) using plasma technology is being increasingly investigated. We have recently shown that thick oxide coatings on Ti-alloys can be successfully deposited using a micro-arc discharge oxidation (MDO) technique. The hardness of the TiO2 coatings is in the range of only 600–900 HV0.05 and their wear resistance is thus limited. Nevertheless, from the viewpoint of improving load support, a thick TiO2 coating with a hardness of approximately 900 HV0.05 should provide a more adequate underlying load support layer for a hard top coat. Therefore, a duplex process of micro-arc oxidation and unbalanced magnetron sputtering has been investigated, in which a TiO2 layer was deposited on a Ti6Al4V alloy substrate (using MDO) for load support, and Cr(N) hard coatings were deposited on top of the TiO2 layer for wear resistance (using magnetron sputtering). The morphology and phase composition of the Cr(N) coatings were observed and analysed by scanning electron microscopy and X-ray diffractometry (XRD). The coating residual stress was also assessed using XRD. Mechanical and corrosion properties of the bilayered Cr(N)/TiO2 coatings were tested. The results indicate that duplex Cr(N)/TiO2 coatings can provide a combination of good wear resistance and load-bearing capacity as well as corrosion resistance.