Abstract
Sliding contact experiments and first-principles calculations were performed to elucidate the roles of structural defects and water dissociative adsorption process on the tribo-chemical mechanisms responsible for low friction of graphene. Sliding friction tests conducted in ambient air and under a dry N2 atmosphere showed that in both cases a high running-in coefficient of friction (COF) occurred initially but a low steady-state COF was reached only when the sliding was continued in air with moisture. Density functional theory (DFT) calculations indicated that the energy barrier (Eb) for dissociative adsorption of H2O was significantly lower in case of reconstructed graphene with a monovacancy compared to pristine graphene. Cross-sectional transmission electron microscopy of graphene transferred to the counterface revealed a partly amorphous structure incorporating damaged graphene layers with d-spacings larger than that of the original layers. DFT calculations on the reconstructed bilayer AB graphene systems revealed an increase of d-spacing due to the chemisorption of H, O, and OH at the vacancy sites and a reduction in the interlayer binding energy (EB) between the bilayer graphene interfaces compared to pristine graphene. Thus, sliding induced defects facilitated dissociative adsorption of water molecules and reduced COF of graphene for sliding tests under ambient and humid environments but not under an inert atmosphere.
Highlights
The reduction of friction by using carbon-based materials either in the bulk form, or as surface coatings, is an important technological alternative to the usage of liquid lubricants[1,2,3]
chemical vapour deposition (CVD) deposited multilayer graphene tested using a pin-on-disk tribometer at 1N showed a high coefficient of friction (COF) of 0.52 in a dry N2 atmosphere, but exhibited a low steady state COF value of 0.11 in an air atmosphere with 45% RH16.The computational studies based on density functional theory (DFT) showed that hydroxyl groups on pristine graphene could reduce the adhesion between graphene layers contributing to low friction[17], the relation between adhesion and friction should be interpreted with caution, as it has been suggested that the friction force would depend on other factors such as adhesion energy hysteresis[18]
Cross-sectional transmission electron microscopy (TEM) studies revealed that a tribolayer was formed on the counterface and consisted of an amorphous carbon matrix with intermittent stringers of graphene stacks; the d-spacing between the graphene layers was larger compared to that of the pristine graphene
Summary
The reduction of friction by using carbon-based materials either in the bulk form, or as surface coatings, is an important technological alternative to the usage of liquid lubricants[1,2,3]. Low COF values of polycrystalline diamond (PCD) and diamond-like carbon (DLC) coatings observed under the ambient conditions were often attributed to the saturation of the dangling carbon bonds at the contact surfaces by dissociatively adsorbed gas molecules[9,10,11]. Low-load (1 nN) sliding experiments conducted using friction force microscope showed that the multilayered graphene with more than four layers (produced by mechanical exfoliation) exhibited a COF of 0.2 when tested under an ambient atmosphere with a relative humidity (RH) of 25% compared to a higher COF of 0.4 for the monolayer graphene[14]. The low COF of nanocrystalline diamond in water vapour and hydrogen atmospheres were attributed to OH and H termination of surface C atoms. It was shown that vacancy formation and adsorption of the hydroxyl groups would reduce the strength and elastic modulus of graphene[28]
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