Synergistic effect of CuO/MoS ₂ nanoparticles as lubricant additives under boundary lubrication conditions

Lubricants containing nanoparticles have garnered significant attention due to their potential to improve the tribological performance of systems operating in demanding environments. In the current study, the enhanced stability of copper oxide (CuO) and molybdenum disulphide (MoS 2 ) nanoparticles (NPs) and their tribological performance, when used as lubricant additives, were experimentally tested to evaluate their synergistic effect on friction and wear performance. The oleic acid (OA) was used as a surface modifier agent to enhance dispersion stability. The influence of oleic acid on nanoparticle dispersion was evaluated using Dynamic Light Scattering (DLS) and Zeta Potential (ZP) techniques. The surface of nanoparticles was characterised by Fourier Transform Infrared Spectroscopy (FTIR) and Thermogravimetric Analysis (TGA). The tribological performance of the more stable nanolubricant, including friction reduction and wear resistance, was evaluated using a pin-on-reciprocating plate test rig. Surface characterisation of the wear tracks was performed using White Light Interferometry (WLI), Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM/EDX), X-ray Photoelectron Spectroscopy (XPS) and Transmission Electron Microscopy (TEM) to understand how the improved stability affected the worn surfaces and tribofilm properties. NPs stability study confirmed that the best stability could be achieved with concentrations of 1% CuO/MoS 2 NPs and 1.5 wt. % OA. Tribology results indicated that CuO and MoS 2 NPs individually could reduce friction and wear, whereas their combination exhibited enhanced friction and wear reduction. This synergy in tribological performance has been attributed to the combined effects of nanoparticle size and enhanced dispersion stability, which enable effective nanoparticle entrainment between the contacting surfaces, leading to the formation of a tribofilm that provides anti-wear protection and contributes to friction reduction.