Iron ions induced self-assembly of graphene oxide lubricating coating with self-adapting low friction characteristics

Graphene oxide (GO) is a promising solid lubricant for moving mechanical systems to implement reliable and effective lubrication due to its easy modification and excellent tribological properties. However, obtaining highly dense GO coatings simultaneously with high hardness and extensibility is still a challenge for available selfassembly techniques, and these characteristics are beneficial to tribological properties. Inspired by the advanced abrasion resistance system of mussel byssus threads, a surprisingly simple and versatile metal ions (namely Fe2+ and Fe3+ ions) directed self-assembly strategy has been developed to rapidly fabricate GO coatings. GO as a solid lubricant is highly sensitive to the interface structure and test environment. Consequently, the influence of the valence state of iron ions on the macroscopic tribological properties of GO-based coatings in air, nitrogen gas and vacuum environments are comparatively investigated. The results show that the GO-Fe3+ coating with a highly compact characteristic and unique granular microarchitecture resulting in higher hardness and strain capacity displays a lower friction coefficient and remarkable wear resistance than the GO-Fe2+ coating. Additionally, because water molecules generate strong hydrogen bonding gradually disrupting the ordered layered structure, both GO-Fe2+ and GO-Fe3+ coatings exhibit a higher friction coefficient in air than in nitrogen gas and vacuum.