Role of humidity in reducing sliding friction of multilayered graphene

The sliding friction of multilayered graphene was examined by conducting pin-on-disk tests in ambient air with different relative humidity (RH) levels (10–45% RH) and in dry N2 (0% RH) where a high coefficient of friction (COF) of 0.52 was measured. Tests carried out at 10% RH showed a low steady state COF of 0.17. Progressively lower friction values were observed when the RH of the atmosphere was increased, with the lowest COF of 0.11 reached at 45% RH. Micro-Raman analyses of sliding surfaces of graphene transfer layers formed on the Ti–6Al–4V counterfaces showed an increase in the intensity of D-band with an increase in the RH suggesting defect formations in the C-network as a result of sp2 to sp3 transformation and amorphization during sliding. The H and OH passivation of graphene, as indicated by X-ray photoelectron spectra (XPS) of the contact surfaces, was a possible reason for low friction of graphene in humid atmospheres. Transfer layers incorporated stringers of graphene stacks with d-spacings of 0.34–0.38 nm, which were larger than that of pristine graphite suggesting that repulsion between the adsorbed molecules caused an increase in the lattice spacing.