Low temperature deposition of Cr(N)/TiO 2 coatings using a duplex process of unbalanced magnetron sputtering and micro-arc oxidation

Abstract

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 TiO 2 coatings is in the range of only 600–900 HV 0.05 and their wear resistance is thus limited. Nevertheless, from the viewpoint of improving load support, a thick TiO 2 coating with a hardness of approximately 900 HV 0.05 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 TiO 2 layer was deposited on a Ti6Al4V alloy substrate (using MDO) for load support, and Cr(N) hard coatings were deposited on top of the TiO 2 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)/TiO 2 coatings were tested. The results indicate that duplex Cr(N)/TiO 2 coatings can provide a combination of good wear resistance and load-bearing capacity as well as corrosion resistance.