Tribological properties of metal-organic framework nanosheets functionalized by oleic acid

Abstract

<p indent="0mm">Terephthalic acid and copper acetate were used as raw materials, and copper-based metal-organic framework (Cu-MOF) nanosheets were synthesized by ultrasonic-assisted self-assembly. Afterward, oleic acid (OA) molecules were successfully modified on the surface of Cu-MOF by ultrasonic treatment to acquire Cu-MOF@OA nanosheets. The morphology of Cu-MOF@OA maintained its sheet structure; however, the size of the nanosheets was reduced, and the agglomeration problem was considerably alleviated compared with Cu-MOF nanosheets. The Cu-MOF@OA nanosheets exhibited good dispersion stability in <sc>500 N</sc> base oil. The tribological tests indicated that Cu-MOF@OA showed the best tribological properties at the addition concentration of 1.0 wt.%. Moreover, the friction coefficient and wear volume were reduced to 3/5 and 1/6 of the base oil, respectively. Finally, scanning electron microscopy and X-ray photoelectron spectroscopy were applied to characterize the morphology and characteristic elements of wear scar lubricated by Cu-MOF@OA nanosheets and explore the lubrication mechanism. The results indicated that complex tribochemical reactions occur among Cu-MOF@OA, base oil, and iron substrate under the action of load and shear stress during the friction experiment, forming a tribofilm composed of nitrogenous substances, carbonaceous substances, and iron oxides, which reduce the friction and wear of metal surfaces.