Tribological properties of ionic liquid modified MWCNTs, MoS₂, and their composite nanofluids

The machining process is generally accompanied by intense friction and heat generation. Excessive heat flux subsequently leads to thermal damage and shape defects on the workpiece, which will greatly reduce the service life of the tool. As a novel coolant, nanofluids can effectively improve the lubrication and cooling conditions in precision machining. This paper uses the ionic liquid 1-ethyl-3-methylimidazole tetrafluoroborate (EMImBF₄) to disperse multi-walled carbon nanotubes (MWCNTs) and molybdenum disulfide (MoS₂). The nanofluid with excellent tribological properties was prepared. The crystal structure of nanoparticles was analyzed by an X-ray diffractometer (XRD). The wettability and particle dispersibility of nanofluids were characterized by a Raman spectrometer, nanoparticle size potential analyzer and contact angle measuring instrument. Thermophysical properties were tested by a thermal conductivity measuring instrument and rheometer. Finally, a friction and wear tester and an ultra-depth-of-field microscope were used to analyze the friction properties of the prepared nanofluids. The following results are obtained. (1) After the MWCNTs or MoS₂ nanoparticles are modified by the adsorption of EMIm⁺ cations, the Zeta potential of the nanofluids is greatly increased, and the laminated structure formed by the adsorption of two nanoparticles increases the particle size distribution range. By this time, an electrostatic equilibrium area is formed around the nanoparticles, whereby the particles are effectively dispersed due to the steric hindrance effect. (2) MWCNTs, MoS₂, and their composite nanofluids are determined as pseudoplastic fluids, which are easy to spread and form films on metal (superalloy GH4169) surfaces with a minimum contact angle of 59.33°. After testing, the addition of nanoparticles and dispersants in the nanofluids did not cause a sharp increase in the viscosity, and the average viscosity was found to be as low as 1.49 mPa·s (25 °C), thus maintaining the flow advantages of water-based coolants while obtaining a higher thermal conductivity up to 1.02 W·(m·K)⁻¹ (25 °C). This is suitable for machining fields that require efficient flow heat transfer. (3) MWCNTs, MoS₂, and their composite nanofluids greatly enhance the anti-friction and anti-wear properties of the base fluid (deionized water), especially composite nanofluids containing two nanoparticles, which form a “bearing-like” effect by stacking the layered and tubular combined structures. Thus, the lubrication performance is optimal. Compared with the traditional water-based coolant, the average friction coefficient of the composite nanofluid is small (0.083). At the same time, the adhesive wear or abrasive wear on the surface of the workpiece is further reduced, the wear scar is narrow and shallow, and the volume wear rate is reduced by 72.33%.