Susceptibility of Poly–Alpha Olefin and Diisooctyl Sebacate Base Oils With Different Polarities to Oleic Acid Surface–Capped Ultra-Small Cerium Oxide Nanoparticle

A key issue for nano-additive to effectively exert tribological function is to ensure its entrance to and adhesion on the frictional contact surfaces. But the adhesion of nano-additive on the rubbed surfaces under oil lubrication faces the challenge of the competitive adsorption of lubricant base oil thereon. In this study, oleic acid–modified ultra-small cerium oxide (OA-CeO₂) nanoparticle was synthesised by one-pot liquid-phase surface-modification method in the presence of OA as the surface modifier. The susceptibility of non-polar poly–alpha olefin 6 (PAO6) and polar diisooctyl sebacate (DIOS) base oils to the as-prepared OA-CeO₂ nano-additive was investigated, and the effect of the OA-CeO₂ nano-additive on the friction-reducing and anti-wear abilities of the two kinds of base oils towards a steel–steel sliding contact was investigated with four-ball friction and wear tester. Furthermore, the tribological mechanism of the adsorption and deposition of the OA-CeO₂ nano-additive on the surface of friction steel and the competitive adsorption of base oil were discussed. Characterisations by scanning electron microscopy, transmission electron microscopy and Fourier transform infrared spectroscopy demonstrate that the as-prepared OA-CeO₂ nanoparticle is of a spherical shape and has an ultra-small average size of 1.2 nm. As the lubricant additive in PAO6 and DIOS base oils, the OA-CeO₂ nano-additive exhibits different tribological properties, which is attributed to the difference in the base oils' polarity. Namely, the CeO₂ nanoparticle in the non-polar PAO6 base oil is more easily adsorbed on the rubbed surface of the steel–steel sliding contact, thereby forming the CeO₂ deposition film to improve the tribological properties of the base oil. However, the CeO₂ nanoparticle added in polar DIOS base oil is difficult to form the CeO₂ deposition film, because of the competitive and preferential adsorption of the polar base oil on the rubbed steel surface. Therefore, it is imperative to select the base oils with proper polarity to better exert the friction-reducing and anti-wear functions of the OA-CeO₂ nano-additive.