Water-lubricated behaviour of AISI 440C stainless steel and a DLC coating for an orbital hydraulic motor application

The low-speed, high-torque, orbital hydraulic motor is a component that often uses mineral oil to convert hydraulic energy into the rotational motion of a shaft. The main purpose of our research was to investigate the possibility of using water, which is a much less environmentally damaging working fluid than oil. The limitations of water lubrication were overcome with an appropriate surface-engineering approach. The surface coated with diamond-like carbon (DLC) significantly reduced both the friction and the wear. The lowest coefficients of friction in this study, taking into account all the tested parameters, were obtained when the stainless-steel/DLC (SS/DLC) contact was tested in water. The friction of the SS/DLC in water was even lower than the friction of the SS/DLC or SS/SS contact in oil. Furthermore, the wear coefficient of the SS/DLC in water was comparable to the wear coefficient of the SS/SS in oil. The surface hardness, surface roughness and applied force were also varied, but they did not play such an important role as employing the DLC coating. The SS/DLC contact can thus be considered as a very promising solution for an orbital hydraulic motor from the tribological and environmental points of view. •The coefficient of friction, the wear loss and the wear coefficient of SS/SS and SS/DLC were investigated.•Three different applied forces, two surface roughnesses and two surface hardnesses were considered.•The lowest friction was obtained for the SS/DLC contact in water, taking into consideration all the tested parameters.•The wear coefficient of the SS/DLC contact in water was comparable to the SS/SS contact in oil.