Preparation of DDP-Functionalized UiO-66 Nanoparticles and Tribological Properties as Lubricating Oil Additives

Nanoparticle additives have been widely researched and demonstrated to extensively improve the tribological performance of lubricating oil. Generally, the traditional nanoparticle as lubricating oil additives are mainly metals, metal oxides, sulfides, carbons, nanocomposites, rare earth compounds and so on. Compared with those traditional additives, metal-organic frameworks, as a category of novel organic-inorganic hybrid materials self-assembled from metal ions and organic ligands based on coordination bonds, have good mechanical properties, designable compositions and structures, as well as adjustable physicochemical properties, and thus have potential applications in lubricating oil. However, the feeble dispersity and compatibility between metal-organic frameworks and lubricating oils restricted their further development. In this research, we selected UiO-66 nanoparticles, a zirconium-based metal-organic framework with good chemical, mechanical and thermal stability, as a novel lubricating oil additives by combining them with dialkyldithiophosphoric acid (DDP) molecules with different molecular chain lengths and structures. We had characterized the nano-crystalline morphology, phase composition, chemical structure, hydrodynamic particle size and tribological properties of UiO-66@DDP-1/-2/-3 nanoparticles using transmission electron microscope (TEM), X-ray diffractometer (XRD), Fourier transform infrared spectrometer (FT-IR), dynamic light scattering (DLS) and high-frequency friction and wear tester. Three kinds of DDP molecules were assembled in coordination on the surface of UiO-66 nanoparticles without destroying the morphology, size, and nano-crystallinity of the pristine metal-organic framework. After DDP functionalization, the dispersion stability of UiO-66 nanoparticles in organic solvent and based lubricating oil was greatly improved. Using as a novel lubrication oil additive, UiO-66@DDP nanoparticles made an excellent contribution to improving the friction reduction and anti-wear performance of the lubricating system at specific concentrations, as well as increasing the load-carrying capability and comprehensive tribological performance of the base lubricating oil. Specifically, the optimal concentration of UiO-66@DDP-1, UiO-66@DDP-2, and UiO-66@DDP-3 were 0.6%, 0.4%, and 0.6% (mass fraction), respectively. Compared to the based lubricating oil, the 0.4% UiO-66@DDP-2 additive demonstrated the best overall tribological performance with a friction coefficient of 0.108 and a wear volume of 27 996 μm³. Meanwhile, the tribological performance of the 0.4% UiO-66@DDP-2 additive in the variable load, temperature and frequency measurements was also outstanding, with the upper limits of 300 N, 70 ℃, and 45 Hz, respectively. The mechanism of UiO-66@DDP nanoparticles for friction and wear reduction was investigated by X-ray photoelectron spectroscopy (XPS), which showed that all three kinds of UiO-66@DDP nanoparticles formed a tribofilm and contributed to the “ball bearing” effect. Our work provided insights into the future functional lubricating oil additives to facilitate the rational design of high-performance metal-organic frameworks-based materials as lubricant additives.