Tribological response of MoS₂ coated and oxy-nitrided samples with alternative extreme pressure and anti-wear additives

Oxy-nitriding and MoS₂ coatings are widely used surface modification techniques in industry to improve the friction and wear characteristics of components and interacting surfaces. To ensure optimum performance of components within tribological environments, it is crucial to ensure compatibility between surfaces and lubricants, as the break-down of the lubricating film could lead to seizure between interacting components. This study analyses the interaction of extreme pressure and anti-wear additives with two modified surfaces. The friction and tribofilm formation behaviour were investigated using a Mini Traction Machine fitted with a 3D Spacer Layer Imaging Method. The chemical composition of the species formed after testing was analysed using X-ray photoelectron spectroscopy. This study found that the properties of the modified surfaces and their interaction with the various lubricant additives impact their tribological performance. With the oxy-nitrided samples, no real effect on friction was observed when using the sulphurised olefin or tricresyl phosphate additives, mostly due to lack of interaction with the less reactive iron nitride layer and their roles as anti-wear additives. However, when using the sulphurised olefin additive with the MoS₂-coated sample, a significant reduction in friction was observed with time. This was mostly likely due to the combined effect with the formation of FeS and MoS₂ within the tribofilm.