Improving the performance of a proportional 4/3 water-hydraulic valve by using a diamond-like-carbon coating

Today, there are several water-hydraulic, power-control systems already available on the market. Their components are usually made of stainless steel, which ensures satisfactory performance under mild, conventional operating conditions. However, for more demanding operating conditions and long-term, low-friction and low-wear performance, they do not provide the required performance. One of the possible ways to improve the performance of stainless-steel components in water-hydraulic systems is to coat them with diamond-like carbon (DLC), since this material is well known for its excellent low-friction and low-wear characteristics and also provides very good performance under water-lubrication conditions. In this study, real-scale lifetime tests with 2.3 million cycles were performed on a hydraulic test rig with a proportional 4/3 directional control water-hydraulic valve. Two types of contacts in the valve were tested: the steel-spool/steel-sleeve and the DLC-spool/steel-sleeve. The wear behaviour of the valve was evaluated with a scanning electron microscope (SEM) and internal leakage measurements. In the real-scale lifetime tests the wear and the damage on the DLC-coated spool were significantly lower than on the steel spool. Furthermore, in agreement with this, the internal leakage in the DLC-spool/steel-sleeve valve was almost constant throughout the tests, while in the steel-spool/steel-sleeve valve the leakage slowly, but steadily, increased. The steel/steel and DLC/steel contacts were also separately evaluated in pin-on-disc model tribological tests under water-lubricated conditions for a comparison and for a better understanding of the tribological mechanisms. In agreement with the real-scale tests, the DLC/steel contact showed improved friction and wear performance in comparison with the steel/steel contact. (C) 2012 Elsevier B.V. All rights reserved.