Tribological Properties of Typical Zeolitic Imidazolate Frameworks as Grease-Based Lubricant Additives

In this study, three different zeolitic imidazolate framework (ZIF) materials of ZIF-8, ZIF-71, and MAF-6 were synthesized, and their tribological properties as base grease were evaluated. We demonstrate that the tribological properties of ZIFs are correlated with the framework density/pore volume and the substituted groups. Under 200 N, MAF-6 has the lowest wear volume value and ZIF-71 exhibits the lowest friction coefficient. As the load increased, MAF-6 maintained the best and most stable tribological performances among the three ZIFs additives; however, the properties of ZIF-71 worsened. Thus, MAF-6 exhibits the lowest density and largest solvent-accessible volume (SAV) and has the lowest elastic stiffness; it easily slides and adheres onto the wear surface, forming a tribofilm that reduces friction and prevents severe wear. ZIF-71 with chlorinated groups may enhance the tribological performance; as the load rises, its dense and stiff structure has an adverse effect. These results are expected to be helpful for designing future ZIFs with excellent lubrication properties.