Effects Of Environmental Moisture And Functional Groups On The Sliding Adhesion Behavior And Mechanical Properties Of Graphene Steps

Graphene as a solid lubricating material has been extensively studied,but the experimentally prepared graphene has a large number of atomic steps on the surface,and environmental molecules are easy to adsorb on the edges of these steps with dangling bonds.Both may have an impact on the tribological properties of graphene.Significant influence.We carried out Reax FF-MD simulation to study the influence and mechanism of environmental water molecules and step edges passivated by functional groups on the frictional behavior,wear resistance and load-bearing capacity of graphene steps.The main research contents are as follows:(1)The sliding friction process of the diamond probe sliding on the graphene step/HOPG surface and climbing the step was simulated,and the influence of environmental conditions(vacuum/humid environment),step edges passivated by functional groups(hydrogen/hydroxyl passivation)on the frictional behavior of the system was studied.The changes in the interface contact area and the number of water molecules at the interface were calculated.The different adhesion behaviors of the two functionalized step edges in a humid environment were explained.It was found that there were two different mechanisms of action between the step edges of functional groups and the formation of water molecules.The hydroxylated edges destroyed the interfacial water layers at the bottom of the tip by promoting the decomposition and adsorption of water molecules and forming a "physical filter",resulting in a difference in the adhesion behavior of the step models passivated by hydrogen atoms.Comparing different environmental condition models,it was found that the different forms of liquid water(water film,meniscus)had different effects on the interface adhesion force during the process of ascending the steps.(2)By performing a series of load calculations on the aforementioned diamond probe-graphene step/HOPG models,the initial critical load for the wear of the step edge functional group passivation model under different environments was obtained.By observing the evolution of atomic configuration,it was found that there are two main forms of wear failure.The hydroxylation step edge was prone to hydroxyl detachment,forming an interfacial bond with the probe surface,resulting in wear.And the hydrogenation step edge was abraded due to the furrowing of the step folds caused by the sliding of the probe.Comparing the critical loads of different environmental models,it was found that environmental water moleculars significantly enhanced wear resistance of the graphene steps through two mechanisms of action of water molecules.One was that the water molecules decompose and passivate the dangling bond atoms in the contact area.The second was that the interfacial water layer increases the height of the probe’s center of mass and reduces the hindrance of steps.(3)The indentation process of the diamond probe approaching the graphene step/HOPG surface along the normal direction was simulated.The effects of environmental conditions and functional groups(hydrogen/hydroxyl passivation)on the load-bearing capacity of graphene sheets were studied.The load-indentation depth curve was calculated,combined with the bond sequence change of the interfacial C-C bond,and the ultimate stress of each graphene sheet failure was obtained.By observing the nanoindentation process at the graphene steps,it was found that the environmental moisture can not only disperse the stress in the form of a water layer and improve the bearing capacity of the step layer,but also accelerate the crack propagation through the hydrolysis reaction and reduce the bearing capacity of each layer of HOPG.By observing the evolution of the atomic configuration,it was found that the hydroxyl groups at the edge of the step were easy to fall off and adsorb on the carbon atoms of the broken graphene dangling bonds,which weakened the C-C bond strength at the bonding site.As a result,the hydroxylation models had a smaller ultimate stress than the hydrogen passivation models.