Behavior of DLC coated low-alloy steel under tribological and corrosive load: Effect of top layer and interlayer variation Kirsten Bobzin a , Nazlim Bagcivan a , Sebastian Theiß a , Raphael Weiß a, ⁎, Udo Depner b , Torsten Troßmann b , Jörg Ellermeier b , Matthias Oechsner b a Surface Engineering Institute, RWTH Aachen University, Kackertstr. 15, 52072 Aachen, Germany b Zentrum für Konstruktionswerkstoffe, State Materials Testing Institute Darmstadt, Institute for Materials Technology, TU Darmstadt, Grafenstr. 2, 64283 Darmstadt, Germany abstract article info Available online 6 November 2012 Keywords: PVD DLC Multilayer Wear Corrosion In many industrial applications components are subjected to mechanical load, while being exposed to corro- sive environments. In order to cope with the resulting tribo-corrosion, both corrosion and wear resistant steels are often resorted to. Since those materials are expensive and often difficult to machine, the develop- ment of protective coatings deposited on less expensive and easily machinable materials, is of high interest. Due to their chemical stability diamond-like carbon (DLC) coatings deposited via physical vapor deposition (PVD) seem to be appropriate to offer corrosion protection in addition to their well-established wear resis- tance. This paper deals with the development of DLC multilayer coatings consisting of alternating a-C and chromium based layers and an a-C:H top layer. The coatings were deposited on low-alloy steel (AISI 4140) using reactive magnetron sputter ion plating (MSIP) technology to investigate the possibility of improving the properties concerning tribological and corrosive load. The mechanical and tribological properties of the top layer were analyzed depending on the acetylene gas flow. Furthermore, the influence of different transi- tions from the a-C to the chromium based layers on the fatigue strength was investigated. The applicability of the DLC coatings in corrosive environments was proved using potentiodynamic polarization tests in artificial seawater. The tribological analyses regarding continuous sliding wear using a pin-on-disk tribometer show that the developed DLC coatings lead to very low wear rates as well as friction coefficients in aqueous envi- ronment and in contact with an Al 2 O 3 counterpart, nearly independent of the acetylene gas flow. Moreover, investigations in an impact tribometer with maximum initial Hertzian stress of about 10 GPa show that pure metallic chromium layers with a graded chromium and carbon containing transition to the a-C layers im- prove the fatigue strength of the compound. Thus, even after 10 6 impacts the coating was proved to be still impenetrable for an electrolyte that could lead to corrosion of the substrate. © 2012 Elsevier B.V. All rights reserved. 1. Introduction In many technical applications, components are subjected to superimposed corrosive and mechanical loads. The economic impact of subsequent degradation and failure triggered by corrosion and wear cannot be neglected. Studies show an overall annual economic loss caused by corrosion of about 3% of the world gross domestic product [1]. In accordance with [2], wear induced costs can be esti- mated to approximately 1–2% of the gross domestic product of the industrial nations. In order to cope with superimposed corrosion and wear, it is often required to resort to corrosion and at the same time wear resistant steels. Since those materials are not only expensive but also difficult to machine, the development of protective thin coatings, which can be deposited on less expensive and easily machinable materials, is of high interest. Due to their chemical stability against acids as well as bases at room temperature, diamond-like carbon (DLC) coatings deposited via physical vapor deposition (PVD) or plasma enhanced chemical vapor deposition (PECVD) are suitable candidates to offer corrosion protection [3–5]. However, DLC coatings often contain a certain amount of flaws and pores leading to the exposure of the substrate to the environment and thus, to corrosion of the substrate [6–9]. Multilayer architectures are known to result in a reduction of pores reaching from the surface of the coating to the substrate, so that corrosion is reduced as well [8,10–12]. Uematsu et al. proved that a multilayer architecture consisting of three DLC layers with a layer thickness of 4 μm consecutively deposited via PECVD leads to an improvement of corrosion fatigue strength of an Mg alloy substrate, because through-film thickness defects are removed [11]. Kok et al. used nanoscale multilayer C/Cr coatings deposited with different bias voltage [13] on stainless steel (316SS) to investigate their Surface & Coatings Technology 215 (2013) 110–118 ⁎ Corresponding author. Tel.: +49 241 80 95267; fax: +49 241 80 92941. E-mail address: weiss@iot.rwth-aachen.de (R. Weiß). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.08.075 Contents lists available at SciVerse ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat