Swash Plate Axial-Piston Hydraulic Motors: A Study of Surface Protective Treatments for the Piston/Cylinder Interface

Presently, there is a global push to improve the productivity of heavy duty machinery. With increasing demands to apply to stricter fuel and emission standards alongside increasing energy costs, it has become vital to maximise the energy efficiency of systems. Swash plate axial piston motors have inefficiencies up to 15% caused by fluid leakage and internal friction. Mechanical systems are being modified to reduce friction amongst components whilst allowing them to run at higher operating conditions and temperatures. This is where surface improvement technology plays a crucial role. By achieving compatibility between the modified surfaces and lubricant additives optimum efficient systems can be achieved. MoS2 coatings and nitriding heat treatments are applied to sample surfaces to improve their tribological properties. There are however only a few studies that focus on the tribochemical interactions of nitrided samples with lubricant additives. This study aims to investigate the impact of the properties of the modified surfaces on tribological and tribochemical interactions. The primary focus will be validating the application of the nitriding treatment to improve the durability of components and investigating the interactions with various lubricant additives in comparison to alternative treatments. To achieve this, tribological performance of the various samples and oils has been evaluated using the Cameron Plint TE77 tribometer in the boundary lubrication regime. This was followed by using the MTM-SLIM testing rig to validate the trends observed with the TE77 and add another level of complexity to the testing conditions. The MTM SLIM would allow visualisation of the formation and development of a tribofilm on the sample variants whilst using testing conditions similar to that used within a hydraulic motor. A number of surface analysis techniques were employed in this study such as Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX), X-ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy. They helped to characterise the tribofilms formed and understand the effect of the properties of treated surfaces. The results of this study indicate that the mechanical properties of the compound layer formed during the nitriding process had a significant impact on the sample tribological properties, allowing it to perform better than the untreated and MoS2 coated samples. However the inertness of this layer prevents the formation of a thick tribofilm which could further impact friction and wear. However the presence of additional layers upon the nitrided surface are shown to impact not only tribological behaviour but also tribochemical formation, due to the chemical compounds present. Critically the results showed that not only did the type of treatment applied to the samples impact the tribological behaviour but also the chemical interactions with the compounds within the lubricant played a significant role. This was shown with the MoS2 coated samples and sulphurised olefin additive, where the synergy between coating and FeS formed lead to the lowest friction system observed within this study. This study has shown that the oxy-nitriding process is an effective treatment to improve the tribological performance of the samples, with lower friction and wear being achieved compared to alternative samples. The presence of an oxide layer led to the formation of FeS2 within the tribofilm. However, due to the relative thinness of the tribofilm the impact on friction was minimal. The thickness of the tribofilm with the nitrided was influenced by the presence of nascent iron on the sample surface. The tribological influence of the oxide layer was minimal however with alternative layers such as FeS the friction and wear response the influence was significant. The project demonstrated that the various additive types could positively and negatively impact the friction and wear of the modified surfaces depending on its tribochemical interaction.