Abstract
Oxy-nitriding is a widely used industrial process aiming to improve the tribological properties and performance of components. Previous studies have shown the effectiveness of the treatment with friction and wear performance, but very few have focussed on optimising this behaviour. The lubrication properties of several EP and AW additives were examined to investigate their effectiveness in improving the tribological properties of the layers formed after treatment. Previous studies showed the presence of an oxide layer on the sample could improve the effectiveness of the sulphurised olefin (SO) and tricresyl phosphate (TCP) additives. The friction and wear behaviour of oxy-nitrided samples were analysed using a tribometer and surface profiler. Scanning electron microscope, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy were employed to identify the morphologies and chemical compositions of the treated surface before and after testing. No real effect on friction was observed when using the SO or TCP additives, mostly due to lack of interaction with the less reactive iron nitride layer and their roles as anti-wear additives. However, when the zinc dialkyldithiophosphate-containing lubricant was used, a higher friction coefficient was observed. Greater improvements in anti-wear properties with the presence of additives in comparison with only using base oil were reported, with the TCP additive producing the lowest wear rates. The study effectively demonstrated that the additive package type used could impact the tribological and tribochemical properties of oxy-nitrided surfaces.
Highlights
Hydraulic motors/pumps are key components within hydraulic systems, but they are hindered by their inefficiency which in some cases can be up to 15% [1]
The layer can be composed of iron nitrides either as a single layer of γ′-Fe4N or ε-Fe2−3 N or as a mixed-phase compound layer composed of both types of nitrides. γ-Phases are seen to dominate the compound zone after nitriding, whereas
X-ray diffraction (XRD) scans (Fig. 5) across the QPQ pin sample showed that the oxide layer was primarily composed of Fe3O4, whereas the compound layer was predominantly composed of ε-Fe2−3N . with traces of γ′-Fe4N
Summary
Hydraulic motors/pumps are key components within hydraulic systems, but they are hindered by their inefficiency which in some cases can be up to 15% [1]. High friction between interacting components can cause excessive wear and may initiate seizure and complete failure of the motor/pump [2]. Tribology has a great influence in energy and material loss within the system, and through the reduction in friction and wear, the lifetime of components can be significantly increased [3]. Nitriding is recognised as an effective surface treatment technique for improving tribological and anti-corrosion properties alongside increasing the hardness of the material. This makes it ideal in machine parts, where the combination of a hard nitride layer and a lubricating film can effectively improve friction and wear behaviour [4]. The layer can be composed of iron nitrides either as a single layer of γ′-Fe4N or ε-Fe2−3 N or as a mixed-phase compound layer composed of both types of nitrides. γ-Phases are seen to dominate the compound zone after nitriding, whereas
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