Research On Nanoscale Friction And Wear Mechanism Of Graphene Coatings On Single Crystal Iron

As an emerging solid lubricating coating,graphene(Gr)is expected to meet the special requirements of lubricating materials,such as ultra-thin,low-friction,high thermal conductivity,electrical conductivity and environmental friendliness,in the filed of precision micro-motion components that is represented by micro/nano electromechanical systems.At present,research on the lubricating properties of graphene has not fully explained the huge difference between graphene’s high strength and low wear resistance.At the same time,the focus of the current research is mostly on graphene itself.But exploring the mechanism of the enhanced wear resistance of substrate by graphene coatings at atomic scale is also crucial for in-depth understanding of the lubricating properties of graphene and assessing the state of substrate.The current research shows that the interface bonding strength between substrate and graphene is an important factor affecting the wear resistance of graphene,and graphene can bind strongly to native iron surfaces which can help to reduce the wear of graphene.Thus,in the view of the use of iron-based magnetic materials in micro/nano-electromechanical systems,this thesis mainly conducts fundamental research on the lubrication mechanism of graphene coating on iron substrate and lays a theoretical foundation for subsequent applications.The specific content includes the following aspects:In order to study the inhibition mechanism of graphene coating on the wear of substrate as well as the lubrication failure mechanism of graphene,molecular dynamics simulation models of nano-indentation,nano-scratch and graphene-lubricated rough surface were established.The wear debris generated during the sctatching of iron substrate was identified by the combined method of atomic critical displacement and velocity.And a recognition program,based on modified centrosymmetric parameters,atomic coordination number and graphene polycyclic search method,was compiled to identify the defects in worn graphene.These methods lay the foundation for the theoretical analysis in subsequent research.Through the nanoindentation simulation of the graphene/Fe system,the influence of graphene coating on the stress distribution in the iron substrate surface under a rough indenter is analyzed.that is,graphene can reduce the contact stress through "stress homogenization effect" and increase the real load area of the iron substrate,thereby suppressing the nucleation of dislocations during indentation.Through the analysis of the energy distribution and dislocation characteristics in plastic zone,it is found that the graphene coating can also suppress the normal uplift of the atoms around the indenter as well as the movement of dislocations along horizontal direction,thereby suppressing the pile-up effect,increase the load bearing capacity and surface hardness,reduce the depth of plastic zone.Through the above comprehensive effects,the contact damage of iron substrate can be reduced.The influence of graphene coating on the formation of wear debris during nanoscratch was discussed.Graphene can suppress the formation of wear debris by suppressing the off-plane extrusion effect of the substrate atoms that are located at the front of the scratch.Based on the analysis of the internal stress and displacement fields inside iron substrate,it is found that graphene coating can induce downstream dislocation lines at the front of the scratch which is beneficial for reducing dislocation pile-up,thereby inducing the annihilation of the dislocation.Therefore,the depth of the plastic zone and its vacancy defects can both decrease with the increase of graphene thickness,which is conducive to improve the fatigue wear resistance of the substrate.Graphene coating can also isolate the direct contact of asperities between iron surface and thus reduce the interfacial shear component of friction force,can reduce the plough component of friction force by reducing the scratch depth,and can inhibit the formation of debris and subsequently reduce the chip component of friction,and finally reduce the total friction force.The lubrication properties of graphene between randomly rough iron substrate were simulated.The adhesion strength between grapheen and iron decreases with the increas of surface roughness of substate.The shear of Fe-Fe interface cannot be eliminated thus it is the main source of friction force even the subtrate surface is isolated by graphene coatings.The mechanism of lubrication failure of graphene under low load conditions is analyzed,and it is found that the chain self-damage induced by dangling bonds at the edges of graphene and the carbon-carbon bonding in the folded or crumpled graphene are the main mechanisms of graphene wear.A ball-on-disk tribotest of graphene lubricated high purity iron was carried out.Variation of the Raman spectroscopy of the gradual transformation of graphene to amorphous carbon was observed.