Preparation and tribological properties of dual-coated CuO nanoparticles as water based lubricant additives

Adding nanoparticles in water-based lubricant is an effective method to improve the tribological properties of water-based lubricant. However, the use of water-based lubricant is limited because of the poor dispersion stability of nanoparticles in water. In this paper, CuO nanoparticles were synthesized by homogeneous precipitation method and then dual-coated with sodium oleate and alkylphenol polyoxyethylene ether in sequence in order to be dispersed stably in water. The samples were characterized via X-ray diffraction, Fourier transform infrared spectra and Transmission electron microscopy. The dispersion stability was researched by the standing for 3 months, centrifuging experiments with high-speed and zeta potential of nano-CuO in water. The tribological properties of the dual-coated nano-CuO as water based lubricant additives were evaluated with an MSR-10D four-ball tribotester and scanning electron microscope was used to analyze the worn surface. The results show that the dispersion stability of CuO nanoparticles in deionized water can be enhanced obviously by surface dual-coating process, and the surface-modified CuO nanoparticles can remarkably improve the anti-friction and anti-wear abilities of deionized water. Compared with the deionized water, the friction coefficient and the wear scar diameter of the deionized water added dual-coated CuO nanoparticles are significantly reduced, and the dual-coated CuO nanoparticles in the deionized water has an optimum concentration in order to obtain the excellent tribological properties under a certain load. Meanwhile, the optimal additive concentration of dual-coated CuO nanoparticles enhances with the increase of the applied load which contributes to the separation of nanoparticles between rough surfaces.