Tribological Properties of Polydopamine-Modified Black Phosphorus Nano-Additives Based on Soybean Oils

Surface modification is an important way to improve the dispersion stability and lubrication performance of black phosphorus (BP) in lubricating base oils. In this paper, black phosphorus/polydopamine (BP-PDA) nanocomposites were prepared based on the self-polymerization of dopamine in an alkaline environment provided by Tris buffer and NaOH. The crystal structure, particle size distribution and physicochemical composition of the BP-PDA composites were systematically analyzed using atomic force microscopy (AFM), scanning electron microscopy (SEM), dynamic light scattering (DLS), X-ray diffraction (XRD), Raman spectroscopy (Raman) and X-ray photoelectron spectroscopy (XPS). In addition, the tribological and dispersive properties of BP-PDA in soybean oil were investigated using a UMT-TriboLabball-on-disk tribometer with steel/steel as the sliding contacting partner. The friction and wear mechanism of BP-PDA composites in soybean oil was emphasized. Firstly, the effect of additives on friction properties was investigated at different concentrations, and the results showed that the friction coefficient was found to increase and then decrease with the increase of concentration at a load of 10 N, a rotational speed of 200 r/min, and a temperature of 22~25 ℃. The optimum concentration was 0.16% by mass fraction. Next, it was explored that the addition of BP-PDA dispersed oil with a mass fraction of 0.16% reduced the friction coefficient and the width of the abrasion marks by about 43% and 52%, respectively, compared to soybean oil. Through the analysis of the morphology and composition of the friction products on the surface of the friction marks, it was found that during the sliding process, BP-PDA as an additive could catalyze and induce the generation of amorphous carbon films from soybean oil molecules under the action of high contact pressure and shear force as well as frictional heat. With the sliding process, the amorphous carbon film and other friction products form a composite tribo-film. The tribo-film is mainly composed of amorphous carbon, BP and PDA, and phosphorus oxides. The formation of the composite tribo-film avoided direct contact at the sliding interface, which resulted in reduced friction and wear. In addition, the dispersion of BP-PDA in soybean oil was investigated, and the dispersion stability of PDA modified BP was improved from 7 days to about 20 days compared with that of pure BP because of the presence of hydrogen-bonding groups in both PDA and soybean oil. This study provided ideas for the design of additives in biomass lubricants, and provided data support and theoretical guidance for revealing the friction-reducing and anti-wear mechanism of BP nanocomposites in biomass lubricants.