Effect of thermal annealing on the physico-chemical and tribological performance of hydrophobic alkylated graphene sheets

Abstract

There has been increasing interest in modification of graphene for application in the field of lubrication to reduce friction and wear on moving mechanical assemblies. In this work, a facile and effective surface modification approach of graphene oxide (GO) sheets by octadecyl amine followed by annealing at 150 °C to form superhydrophobic functionalized GO (f-GO) has been demonstrated. We also validated the effect of temperature on the structural and wetting behaviour of alkylated sheets with increase in annealing temperature (450 °C). The restoration of graphitic conjugated nanosheets of f-GO and emergence of superhydrophobicity in modified graphene (f-GO₁₅₀) and structural alteration in f-GO₄₅₀ after thermal treatment were confirmed by UV-Vis, FT-IR, and Raman spectroscopy, powder X-ray diffraction (XRD), differential scanning calorimetry (DSC) analysis and water contact angle (WCA) measurements. The thermal annealing at 150 °C distorted the long hydrocarbon chains on f-GO that reduced the surface energy and increased the surface roughness, and f-GO exhibited a high WCA of ∼158.8°. A further increase in annealing temperature to 450 °C reduced the hydrophobicity (WCA ∼ 92.8°) as most of the attached carbon chains were removed from the graphitic sheets. The dynamic light scattering technique was used to measure the particle size in an oil suspension and the particle size distribution was found to be in correlation with the wetting behaviour of the particles and also suitable for the effective control of tribological behaviour in sliding lubrication. We investigated the load bearing ability of pristine and annealed f-GO under both ambient and lubricated conditions in varying pressure regimes with a pin/ball-on-disc tribometer. The superhydrophobic f-GO₁₅₀ was found to decrease friction and wear in dry/lubricated contacts by forming a thicker and continuous film on the sliding interface, which is supported by FESEM/EDX and corresponding Raman analysis.