Characteristics of PVD-coatings on AZ31hp magnesium alloys

Magnesium-based light-metal alloys belong to a class of structural materials with increasing industrial attention. Magnesium alloys show the lowest density among the engineering metallic materials, low cost and large availability. However, the limitations according to mechanical strength and the low corrosion resistance restrict their practical application. To improve the surface hardness and the corrosion resistance, PVD-coating techniques offer possibilities to overcome these drawbacks. The paper presented reveals relevant mechanical and chemical properties of various PVD-coatings on high purity (hp) AZ31 magnesium alloy specimens. The industrially very important layer systems TiN, CrN, TiAlN, NbN-(TiAl)N, CrN-TiCN and the multi-layer composite AlN/TiN are discussed in detail. Furthermore, the protective effect of CrN/NbN superlattices has been studied. All of these coatings were deposited by d.c. magnetron sputtering. The sputtering processes have been performed by two different coating devices-PLS 500 (laboratory scale) and HTC 1000/4 ABS® (industrial scale). Both the mechanical behaviour and the corrosion resistance of the specimens have been studied after coating. The chemical composition of the thin films was analysed by GDOES. A special emphasis of the investigation was laid on searching for appropriate stripping procedures due to the fact that the substrate material is very reactive.