Functionalized graphene-oxide nanosheets with amino groups facilitate macroscale superlubricity

Graphene-oxide (GO) has been recognized as an excellent lubrication material owing to its two-dimensional structure and weak interlayer interactions. However, the functional groups of GO that can contribute to anti-friction, anti-wear, and superlubricity are yet to be elucidated. Hence, further improvement in GO-family materials in tribology and superlubricity fields is impeded. In this study, macroscale superlubricity with a coefficient of friction of less than 0.01 is achieved by exploiting the high adhesive force between amino groups within aminated GO (GO-NH2) nanosheets and SiO2. It was observed that GO-NH2 nanosheets form a robust adsorption layer on the worn surfaces owing to the high adsorption of amino groups. This robust GO-NH2 adsorption layer not only protects the contact surfaces and contributes to low wear, but also causes the shearing plane to transform constantly from solid asperities (high friction) into GO-NH2 interlayers (weak interlayer interactions), resulting in superlubricity. A SiO2-containing boundary layer formed by tribochemical reactions and a liquid film are conducive to low friction. Such macroscale liquid superlubricity provides further insights into the effect of functional groups within functionalized GO materials and a basis for designing functionalized GO materials with excellent tribological performances.