Molecular Dynamics Simulation Study On Preparation Of Graphene-metal Composite Films

Due to the superior mechanical properties,graphene can be used to strengthen metal materials.Composite electroplating is an efficient and economic method for preparation of metal matrix composite film.However,the dynamic and successive growth processes can’t be detailed by experimentation to study the effect of graphene on electrodeposition,especially in atomic scale.Furthermore,poor dispersibility of graphene in solvents is a tough issue during experiments,and it is also difficult to precisely control the graphene size and number of graphene layers.In this paper,molecular dynamics(MD)simulations are conducted to model the composite electroplating process,making up for the deficiency of experimental methods.The growth process of composite coatings can be carefully presented here,as well as surface topography and internal structure.Friction coefficient and hardness of the coatings can be also measured.Because of the rough surface and cavities in plating coatings,a compact film is achieved by modeling magnetron sputtering process,and transport behavior of deposited atoms is investigated.In the end,the enhancement mechanism of graphene reinforcing metal materials is analyzed thoroughly by building the models,which are based on the structure of the films.(1)During the simulations of composite electroplating,the formation,growth and merging processes of aluminum atoms islands are dynamically studied.With addition of single layer grpahene(SLG),enrichment of graphene flakes gives rise to asperites in the coating,increasing the surface roughness.Al atoms depositing on graphene will form the close-packed plane on account of the honeycomb lattice of graphene,promoting the preferential growth of Al(111)plane,and confining the growth of original grains.As the number of graphene flakes grows,the amount of Al atoms in face-centred cubic(f.c.c.)order reduces from 5843 to 3225,Standard deviation of surface atomic height increases from 5.03 to 8.87 ?,the average friction coefficient first rises from 0.61 to 1.17 and then falls to 0.32,but the hardness first drops from 3.87 to 2.91 GPa and then jumps to 4.42 GPa.The rise in friction coefficient is caused by the lack of continuous lubricating film,and the decline in hardness results from the inverse hall-petch relationship.Under the condition of equal weight,both bigger SLG and bilayer graphene(BLG)can lead to a rougher surface and be conducive to lubrication,and BLG is likely to reduce the hardness relatively.(2)In experimental analysis,SEM figures Show the same details as the simulation results,such as surface morphology and the embedded state of graphene;XRD analysis proves the preferred growth of Al(111)plane in composite coatings;the friction coefficient of composite coatings prepared under the graphene concentration of 0.1g/L is higher than that of pure Al coatings;the direct reinforcement of graphene is more obvious,which can be verified by comparing the hardness of composite coatings with pure Al coatings under the Same grain size.(3)The compact Al/grephene films are gained through MD simulations of magnetron sputtering process.The diffusion of deposited atoms on polycrystalline Al substrate is surveyed based on the calculation of diffusion energy barriers.The high Ehrlich-Schwoebel barrier and steering effect result in the volmer-weber growth mode.Enrichment of graphene flakes can also bring about an increase of the local thickness.Most graphene flakes in composite films appear to be parallel to the film surface.The influence trend of graphene on composite films is consistent with that on electroplating coatings,but these films contains less f.c.c.atoms,has a flatter surface,shows a lower friction coefficient and a higher hardness.(4)The increased number of layers can decrease the friction coefficient of graphene lying on Al surface.However,only the value measured on SLG here is significantly larger than the data in the reference.As the embedding depth of SLG increases,the friction coefficient rises from 0.045 to 0.835.When the distance between graphene flakes is long enough,high friction coefficient will be measured along this distance.Graphene embedded on Al surface is beneficial to the overall hardness too,which will rise with the increasing number of layers.For the vertical embedded state,SLG is able to improve the hardness,while multilayer graphene will lead to a drop in hardness.