Sliding properties of anodized aluminium alloy tested in water and hydraulic oil

Long operation and efficiency of hydraulic valves are often limited by the construction and design of the chosen valve. Faster switching of control valves can also be achieved by reducing the mass of the moving spool through the use of different materials. In addition to material and design optimization, current research and development in advanced hydraulic systems increasingly emphasise the use of environmentally acceptable lubricants. This study investigates alternative material pairs for a water hydraulic spool valve. Specifically, natural aluminium alloy (EN AW-6082) and anodized aluminium alloy were tested in water. The results were compared with reference measurements in hydraulic oil ISO VG46 and measurements of the conventional valve spool material, nitrided C45 steel. The tribological tests were performed in ball-on-disk reciprocating motion for 90 min, using load of 50 N and a stroke length 2.4 mm. The tests were performed at two sliding frequencies of 10 Hz and 40 Hz, resulting in linear speed of 0.05 m/s and 0.2 m/s. The results of the study show that anodized aluminium alloy has lower coefficient of friction (0.36) and reduced specific wear (1.49 × 10− 6 mm3/Nm) compared to natural aluminium alloy in water at lower sliding speed, with both measured values becoming comparable to the values measured for nitrided C45 steel. However, at higher sliding speeds the anodized aluminium alloy has even lower coefficient of friction (0.34) and gives better results in measured specific wear compared to natural aluminium alloy, but not at the same order of magnitude as measured for nitrided C45 steel. Under oil lubricated conditions, the tested sliding speed had no influence on the improved tribological properties of anodized aluminium alloy which were comparable to the measured values for nitrided C45 steel.