Preparation and tribological properties of core-shell Fe3O4@C microspheres

To improve the dispersion property of the magnetic Fe3O4 nano-lubricant additives, the carbon layers were successfully decorated on the surface of Fe3O4 to fabricate the magnetic core-shell Fe3O4@C microspheres by a simple in-situ precipitation method combined with subsequent hydrothermal carbonization. A series of analysis technologies were utilized to characterize the morphology, structure and components of Fe3O4@C microspheres. Interestingly, the results showed that Fe3O4 core with diameter of about 0.5–0.8 μm is composed of numerous and small-sized Fe3O4 nanoparticles, and the carbon layer with the thickness of about 17 nm is decorated on the surface of Fe3O4. When considered as lubricant additives for sunflower oil, 0.25 wt% Fe3O4@C microspheres dispersed in sunflower oil exhibit significantly reduction in friction coefficient and wear scar width up to 15.4% and 31.7% compared to neat sunflower oil, respectively. Even at high load and sliding speed, a stable friction coefficient as low as 0.077 could be maintained. The improved tribological performance of Fe3O4@C microspheres can be ascribed to the core-shell structure and the synergistic effect between carbon layer and magnetic Fe3O4 nanoparticles.