Engineering and Design of Wear and Corrosion Resistant PVD Coatings Regarding the Exceptional Properties of Magnesium Substrates

Although magnesium alloys were popular in the first half of the 20th century, the bad corrosion properties prevented their breakthrough in industrial mass production. Since the technology for the production of high purity alloys was introduced in the 1970s, magnesium alloys became more and more in the focus of industrial attention. Today magnesium alloys are state-of-the-art in construction parts in automotive industry. Despite its outstanding properties like good castability, low density and nearly unlimited availability the negative aspects like weak corrosion and wear behaviour still limit the application of magnesiums in industry, due to the need of sufficient surface protection (E. Aghion, B. Bronfin, Mater. Sci. Forum2000, 350–351, 19). Today, plasma electrolytic anodisations are state-of-the-art (H. Haferkamp, “Magnesiumkorrosion—Prozesse, Schutz von Anode und Kathode”, in: Moderne Beschichtungsverfahren, F.-W. Bach, T. Duda, Eds., Wiley-VCH, Weinheim 2000, ISBN 3-527-30117-8, 242; M. Thoma, Metalloberflache1984, 38, 393; T. W. Jelinek, Galvanotechnik2003, 94, 46; A. Kuhn, Galvanotechnik2003, 94, 1114). They provide acceptable corrosion resistance and protect the magnesium from mechanical damage due to their high hardness. On the other hand, their high porosity limits their use in combination with electrochemically noble materials, leading to galvanic corrosion (J. Senf, “Untersuchung und Beschreibung von Magnesiumdruckgusslegierungen unter tribologischer, korrosiver und mechanisch-korrosiver Beanspruchung, Berichte aus der Werkstofftechnik”, Shaker Verlag, Germany 2001, ISBN 3-8265-8428-7). In addition, the high surface roughness of the plasma electrolytic anodisations restricts their use in tribological applications, particularly under sliding conditions (H. Hoche, “Grundlegende Untersuchungen zur Entwicklung von PVD-Beschichtungen auf Magnesiumlegierungen im Hinblick auf die Erhohung der Verschleisbestandigkeit und unter Berucksichtigung des Korrosionsverhaltens”, Dissertation, TU-Darmstadt D17, Shaker Verlag, Germany 2004). In order to achieve smooth surfaces with high quality, the PVD technology moves into the centre of interest. Since the 1980s PVD coatings are well established and widely used for different industrial applications, mainly for steel and tool coatings. The authors were the first who carried out serious studies on the development of PVD coatings for magnesium alloys in 1999 (J. Senf, “Untersuchung und Beschreibung von Magnesiumdruckgusslegierungen unter tribologischer, korrosiver und mechanisch-korrosiver Beanspruchung, Berichte aus der Werkstofftechnik”, Shaker Verlag, Germany 2001, ISBN 3-8265-8428-7; H. Hoche, “Grundlegende Untersuchungen zur Entwicklung von PVD-Beschichtungen auf Magnesiumlegierungen im Hinblick auf die Erhohung der Verschleisbestandigkeit und unter Berucksichtigung des Korrosionsverhaltens”, Dissertation, TU-Darmstadt D17, Shaker Verlag, Germany 2004). The extensive research activities lead to the recent development of a coating system, which provides both, good wear properties as well as good corrosion behaviour.