Study On The Tribological Properties Of (Oxidized) Graphene Films In Dry/Wet Environments

Graphene has ultra-high mechanical strength,easy-to-shear interlayer interface,and ultra-thin to a few atomic layer thickness,which can be developed and applied as a solid lubricating material.Environmental water molecules may be adsorbed to the friction interfaces and have an important impact on the tribological behavior of the system,but the mechanism of action has not yet been sufficiently studied.In this paper,first-principles molecular dynamics simulation methods are used to study the effects of interface water molecules and graphene surface functional groups on the tribological behavior of graphene/graphene systems and silica/graphene/silica systems.The main research work is as follows:The sliding friction process between graphene/graphene sheets was simulated,and the influence of interface water molecules(coverage rates of 0,100%)and graphene surface functional groups(hydroxyl groups,coverage rates of 0,25%)on the tribological behavior of the system was studied.Calculated and compared the friction change of each system in the sliding process;For the graphene/graphene system,the energy barrier between graphene sheets during the relative sliding is estimated by calculating the atomic overlap area(Registry Index),which explains the periodic change characteristics of the friction curve of the system.For the graphene oxide/graphene oxide system,the interfacial proton transfer between the upper/lower graphene oxide surfaces under a larger load was observed,and the number of interface protons transferred during the sliding process was further calculated.The results show that there is a certain correlation between the number of interface protons transferred and the friction force;the distribution and movement behavior of the interface water molecules are studied,and it is found that in the graphene oxide/graphene oxide system,the flow of water molecules is affected by the formation of hydronium ions on the surface.Under the influence of larger load,the flow velocity presents a trapezoidal distribution,which is similar to the macroscopic Newtonian fluid model.The sliding friction process of the silica/graphene/silica system under the action of extrusion-shearing was simulated,and the effects of interface water molecules(coverage ratios of 0,50% and 100%),graphene surface functional groups(hydroxyl,Coverage rate of 0,12.5%,25% and 50%),etc.on the tribological behavior of the system were studied.The friction change during the extrusion-shearing process was calculated,and the friction coefficients of the various models were compared.By comparing the relative sliding displacements of each part of the system,the position of shearing in the extrusion-shearing process is defined.It shows that the silica/graphene/silica system can undergo more complicated shear plane transitions.The fundamental reason is that the(oxidized)graphene sheet layer covering the silica substrate may slip relative to the substrate.During the extrusion-shearing process,the(oxidized)graphene sheet still has a rapid reciprocating motion similar to vibration.The speed fluctuation amplitude of the(oxidized)graphene sheet along the sliding direction is calculated,and the analysis shows that there is a positive correlation between the velocity fluctuation mean square error of the sheet layer and the coefficients.The distribution and orientation distribution of dipole moments of interface water molecules are studied.By calculating the difference in the number of hydrogen bonds formed between water molecules/upper substrate and water molecules/graphene oxide sheet,the effect of surface functional groups on the orientation distribution of water molecules is explained.