The Effect of Nano-CeO₂ Additive on the Properties of Lithium Complex Grease

Nanoparticle additives have been extensively studied and shown to significantly enhance the tribological performance of lubricating greases. Rare earth nanoparticles exhibit superior performance compared to other types of nanoparticles. Compared with traditional additives, rare earth nanoparticles possess excellent extrusion, anti-wear, and environmentally friendly properties. In this research, to further improve the tribological performance of lithium complex grease, nano-CeO₂ with different mass fractions were added to the lithium complex grease. The nano-crystalline morphology and structure of nano-CeO₂ were characterized using X-ray diffractometer (XRD), Fourier transform infrared spectrometer (FT-IR), and scanning electron microscope (SEM). A four-ball friction and wear tester were used to test the tribological properties of the prepared nano-crystalline, and the friction coefficient was automatically evaluated by a computer connected to the test machine. The changes in the surface morphology and diameter of the worn spot on the steel ball after the friction experiment were observed using SEM, inverted metallographic microscope, and three-dimensional surface profilometer. Meanwhile, X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) were used to test the typical element changes on the lubricating grease and steel ball surfaces, and to analyze the mechanism of nano-CeO₂ on the tribological performance of the lubricating grease. The results showed that when the mass fraction of nano-CeO₂ added to the lubricating grease was 0.8%, the best anti-wear and anti-friction performance was exhibited. The friction coefficient and worn spot diameter decreased by 22.5% and 12.1%, respectively, compared with the original lubricating grease. Analysis of the friction coefficient curve showed that nano-ceria can significantly improve the friction stability of the lithium complex grease. SEM observations revealed that the addition of nano-CeO₂ significantly improved the morphology of worn spots on the steel ball surface and reduced the formation of pits and scratches. Three-dimensional surface scans found that the roughness of the worn spot on the steel ball surface was reduced by 38.1% compared with the original lubricating grease, and the average and maximum depths of the worn spot decreased by 56.9% and 51.3%, respectively. Nano-CeO₂ had a polishing and self-healing effect on the worn surface. At the same time, during the friction process, some nano-CeO₂ particles were transferred to the steel ball surface, dispersing certain pressure stress. Testing the thermal conductivity of the lubricating grease revealed that the thermal conductivity of the lithium complex grease increased with the addition of nano-CeO₂. The results of EDS and XPS showed that nano-CeO₂ promoted the formation of other metal oxides on the steel ball surface, which formed a stable chemical film on the worn surface, reducing the direct contact between the friction pairs and further reducing the severe wear of the steel ball.