Performance of Cholesterol-Derived Ionic Liquid Composite Boron Nitride Nanosheets as Anti-Friction and Anti-Wear Additives

With the elevation of environmental awareness and the implementation of sustainable development strategies, more new requirements have been proposed for the development of the lubricants, and green, environmental compatible lubricants or lubricant additives present a promising requirement. In recent years, renewable lubricating products derived from biomass have huge application potential in replacement of petroleum-based products. A variety of lubricating products with natural products as raw materials have been established in part of the industrial applications. However, these bio-based lubricating products have some shortcomings, such as difficulty in preparation, poor compatibility, high cost, and particularly poor performance. Therefore, some two-dimensional nanomaterials are usually used to enhance and coordinate the performance of bio-lubricants. In order to harmonize both enhanced tribological properties and renewable abilities of lubricant, huge efforts have been paid to develop the new lubrication system and reveal the internal mechanism of action.　In this study, a cholesteryl-guanidine salt ionic liquid (IL) denoted as ColC₄TMG was designed and synthesized with natural renewable cholesterol as a precursor by proton transfer reaction, with cholesterol derivative as anion and tetramethylguanidine (TMG) as cation. As well as nano-sized hexagonal boron nitride nanosheets (h-BNNS) as friction modifier additives were prepared by mechano-chemical peeling method. The structural characteristics of synthesized IL were characterized by nuclear magnetic resonance, high resolution mass spectrometry, and Fourier transform infrared spectroscopy. The structures of the prepared BNNS were confirmed by transmission electron microscope and X-ray diffraction. They were used alone or in combination as the friction reduction and anti-wear additives in PEG 200 with different concentrations. The viscosity and thermal stability of lubricant samples were analyzed by stabinger viscometer and simultaneous thermogravimetry. Their lubrication performance for steel/steel friction pairs at 25 ℃ was evaluated and compared by multifunctional friction tester with a ball-on-disc reciprocating configuration. The morphology and element state of worn scars were analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy to investigate the evolution of wear and tribochemical reactions. By combining the above results, the lubrication mechanism of IL compound BNNS as an additive was explored. 　The results showed that it was feasible that with cholesterol as a precursor, short-chain carboxylic acid was first introduced to cholesterol skeleton through esterification reaction, and then functional IL could be prepared through proton transfer reactions with amines. And BNNS with a size of about 70~80 nm was successfully prepared to improve the dispersion of nano-additives. The lattice distance of BNNS between adjacent fringes was about 0.25 nm, corresponding to 100 crystal planes of BN crystal, which proved that the crystal structure of BN was not destroyed during the mechano-chemical peeling treatment. It was obvious that IL composited BNNS (IL@BNNS) as an additive showed better dispersion stability compared with a single BNNS additive in basic oil, which was attributed to the branching of the alkyl chain of the IL to the oxidation bond on the BNNS surface.