Balancing oxygen-containing groups and structural defects for optimizing macroscopic tribological properties of graphene oxide coating.

• A lamellar GOCr coating is constructed by an ultrafast self-assembly method. • Oxygen-containing groups and structural defects of coating are tuned by hydrazine reduction. • In air, the best lubrication is achieved by balancing functional groups and defects. • In vacuum, the lubrication performance is mainly influenced by defects. Graphene oxide (GO) coating is a potential candidate for improving friction-reduction and wear-resistance capabilities of moving mechanical systems. However, GO coating with diversity of oxygen-containing groups and defects on GO nanosheets dramatically affect its lubrication application. In this work, a facile, green, and ultrafast self-assembly method based on metal coordination interaction has been developed to construct lamellar GOCr coatings. Moreover, hydrazine vapor reduction was selected to tune the oxygen-containing functional groups and defects in a controllable manner while with minimal damage to the substrate. The macro-tribological behavior of the prepared coatings is investigated in both air and vacuum environments to explore the roles of oxygen-containing functional groups and defects on their tribological performances. The results show that, the oxygen-containing functional groups and unstable defect sites with high chemical activity can form hydrogen bonds with water molecules in air, leading to quick lubrication failure. Therefore, the prepared coating exhibits the best lubrication performance when the oxygen-containing functional groups and defects on GO nanosheets reach an optimal balance. In contrast, the lubrication performance of all coatings in vacuum is better than that in air environment, strongly demonstrating that the lubrication property is mainly influenced by defects due to the absence of water molecules in vacuum environment. [ABSTRACT FROM AUTHOR]