Development of new PVD coatings for magnesium alloys with improved corrosion properties

The use of magnesium alloys has enormous potential in terms of weight saving, resource availability and sustainability. This contrasts with the need to protect the surface from wear and corrosion, which is necessary to increase the depth of application of this promising material in industrial applications. The general requirements for the surface protection and surface functionalization can be fulfilled in particular by PVD coatings. However, so far there are no industrial PVD coatings available that meet the requirements for corrosion protection of the magnesium substrate. Common PVD coatings cannot be applied to magnesium due to the risk of severe galvanic corrosion between substrate and coating. In previous studies, TiMgN was identified as a prospective layer material for the corrosion protection of magnesium alloys as well as for the functionalization of the surface with regard to wear resistance and decoration. In the present work, the TiMgN coating concept has been applied to an industrial 4-cathode magnetron sputter unit of the type Cemecon CC800/9-HiPIMS. As substrate material the magnesium alloy AZ31hp was used. In addition to pure TiMgN, the rare earth elements Y and Gd were alloyed to the TiMgN coating in order to improve the corrosion properties. The influences of coating composition, target design and sputtering mode (Direct Current DC and High Power Impulse Magnetron Sputtering - HiPIMS) were examined with regard to the chemical, mechanical and structural properties as well as the corrosion properties. It will be shown, that the corrosion properties of the TiMgN coated magnesium alloys depend significantly on the deposition conditions and on the coating microstructure, while the defect density seems to be of minor influence. Alloying of Gd to the coating leads to a further increase in corrosion resistance to more than 300 h in the salt spray test, while alloying of Y shows no positive effect. (C) 2014 Elsevier B.V. All rights reserved.