Effect of CrSiN thin film coating on the improvement of the low-speed torque efficiency of a hydraulic piston pump

The hydraulic pump used for an electro-hydrostatic actuator operates only when it is directed to compensate for the control error between the input command and the output position. Therefore, its operation is unsteady and intermittent, which leads to poor lubrication of the internal parts. The steel-base cylinder barrel of an axial piston type pump rotates on a soft bronze valve plate with a slide contact. Therefore, the unsteady lubrication of these components can cause rapid wear of the valve plate. In this study, a CrSiN thin film was deposited on the cylinder barrel surface in order to improve its tribological contact with the valve plate under low-speed mixed lubrication conditions. The CrSiN-coated cylinder barrel showed a much lower friction coefficient and wear rate of the valve plate than the original uncoated one. In addition, its friction coefficient was independent of the normal load, while that of the original cylinder barrel increased as the normal load increased. If these positive phenomena are represented by the torque efficiency of the test pump, an improvement of more than 1.3% can be achieved at a discharge pressure of 300 bars and a rotational speed of 100 rpm. The wear rate of the valve plate mated with the CrSiN-coated cylinder barrel was almost negligible.