Behavior of DLC coated low-alloy steel under tribological and corrosive load: Effect of top layer and interlayer variation

In many industrial applications components are subjected to mechanical load, while being exposed to corrosive 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 development 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 resistance. 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 transitions 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 environment and in contact with an Al2O3 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 improve the fatigue strength of the compound. Thus, even after 106 impacts the coating was proved to be still impenetrable for an electrolyte that could lead to corrosion of the substrate.