Tribological activity of ionic liquid stabilized calcium-doped ceria nanoparticles

Abstract

10% Calcium-doped ceria (CCO) nanoparticles have been synthesized by sol–gel method. Their surface has been modified by surfactants, sodium dodecyl sulfate and 1-decyl-3-methyl imidazolium bis(trifluoromethyl sulfonyl) imide to yield SCCO and IL-CCO respectively. Powder X-ray diffraction patterns of nanoparticles and surface modified nanoparticles are indicative of cubic phase of ceria. Fourier transform infrared spectra confirm the surface modification of nanoparticles, particularly with ionic liquid. Morphology of the as-prepared nanoparticles investigated by field emission scanning electron microscopy, transmission electron microscopy/high-resolution transmission electron microscopy reveals that there is decrease in size of nanoparticles from CCO followed by SCCO and then IL-CCO. Wrapping of nanoparticles by ionic liquid is apparent in the scanning electron microscopy (SEM) and transmission electron microscopic (TEM) images. The tribological activity of the well-characterized nanoparticles has been evaluated at the optimized concentration, 0.2% w/v in paraffin oil under ASTM D4172 and ASTM D5183 test conditions using a four-ball tester. Based on tribological parameters, mean wear scar diameter, average friction coefficient, load-carrying capacity, and loss of frictional power, their relative performance followed the order – IL-CCO > SCCO > CCO. Worn surface analysis by scanning electron microscopy/energy-dispersive X-ray spectroscopy, atomic force microscopy corroborated the tribological performance. The order of the activity could be correlated with the size of the nanoparticles. Moreover, lubricating properties of ionic liquid have been instrumental for the exalted activity of IL-CCO. The presence of heteroatoms of ionic liquid, nitrogen, oxygen, fluorine, sulfur along with calcium and cerium of nanoparticles in energy-dispersive X-ray (EDX) spectroscopy analysis of the wear scar surface lubricated with IL-CCO confirms the vital role of ionic liquid towards the tribological activity.