Synergistic mechanism of graphene and ZIF-8 composite additive in grease

Abstract

Abstract Grease lubrication with compound additives can resolve the challenges of precision retention in mechanical transmissions operating under heavy loads and spatial constraints. The compound additives generate a synergistic effect, enhancing the friction-reducing performance of the grease. This study investigates the frictional properties of greases containing varying mass fractions of graphene (Gra) and zeolitic imidazolate framework-8 (ZIF-8) via experimental testing and molecular dynamics (MD) simulations. The influence of additive composition and concentration on the lubricating performance of grease is analyzed through tribological experiments. MD models of lubricating grease with different nano-additives are established to explore the boundary lubrication mechanism of composite additives. It is found that lamellar Gra and spherical ZIF-8 particles can improve the frictional behavior of lithium-based grease. The performance of the lubricating grease is optimal when the mass fraction of the additive is 2.0 wt%. The average coefficient of friction for the base grease is 0.130. The coefficient of friction decreased to 0.100, 0.100, and 0.088 with the addition of 2.0 wt% Gra, ZIF-8, and Gra/ZIF-8 composite additive, respectively. Correspondingly, the wear rates are reduced by 34.8%, 39.8%, and 46.3% compared to the original grease. The Gra/ZIF-8 composite additive system demonstrates the best overall performance, with experimental and simulation results showing good agreement. MD simulations revealed that the flexible spherical structure of ZIF-8 within the composite additive can undergo significant deformation under contact stress, adapting to surface irregularities, increasing the effective contact area, and performing functions such as rolling, load-bearing, and filling. Additionally, ZIF-8 mitigates the curling deformation of Gra under external forces, forming a stable protective tribofilm on the friction surfaces. The synergistic interaction between Gra and ZIF-8 significantly enhances the load-carrying capacity of the lubricating film, reduces the friction coefficient, and minimizes wear.