Water hydraulics are environmentally compatible and energy efficient compared to conventional hydraulic systems using oil as a working fluid. Because of its low viscosity, however, water only slightly shows hydrodynamic film lubrication under a mixed lubrication regime. Therefore, metallic materials used in conventional oil hydraulics cannot be used, simply because of a lack of proper lubrication. A recent technical approach to this tribological problem is to use high-performance materials such as Si-containing ceramics or engineering plastics. These materials are, however, expensive, hard to machine, or have insufficient strength. Another approach is to apply a coating that shows lubricating properties under water lubrication on a metallic substrate. Hydrogenated diamond-like carbon (DLC) coatings were deposited using unbalanced magnetron sputtering (UBM) equipment with different hardnesses. Effects of coating hardness on tribological properties were investigated with tribotests under water lubrication. Results showed that the wear volume increased rapidly during the initial running-in process, but remained nearly constant after the running in process. The ball wear rate increased as the hardness of the DLC coating increased when metals (stainless steel and brass) were used as counter parts. In contrast, the UHMWPE ball wear rate was independent of the DLC hardness. TEM analysis and nano indentation measurements were conducted of the transfer layer on the counter bodies contact surfaces. The transfer layer consisted mainly of Fe, O and C. The low friction of DLC coating is attributed to this low hardness transfer layer, which acts as a boundary lubricating layer with low shear strength.
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