| Nowadays,nanomaterials have a wide range of applications in many fields,such as nano-coatings,lubricants,micro-electromechanical systems,nano-robots,etc.The friction properties of nano-materials are one of the focuses of current research.Micronano-scale two-dimensional crystals have different friction properties from traditional materials,such as the classic stick-slip model,the correlation with the contact area,velocity dependence,and its temperature dependence.Existing research mainly focuses on the friction characteristics of homogeneous two-dimensional materials.However,recent progress has found that there are many heterogeneous two-dimensional crystals in two-dimensional crystals,and these heterogeneous crystals have specific physical and mechanical properties.They have important applications in new superconducting,photovoltaic and other devices.Due to the abnormal structure of incommensurate torsion,van der Waals heterojunction,in-plane defects,etc.,heterogeneous crystals will have abnormal sliding energy barriers,periodicity,hardness,and orientation during the friction process.These characteristics are the deterministic factors on their frictional behaviors,consequently,research on the friction properties of heterogeneous twodimensional crystals is also urgently needed.In this thesis,molecular dynamics simulations are performed using LAMMPS package to investigate frictional properties of few layer graphene,factors including the number of layers and the size of graphene,the dependence of friction on the scanning direction is also explored.Next,we choose a specific type of inhomogeneous van der Waals heterostructure,few layer nanoporous graphene as a model system to explore their frictional properties.The following results are obtained through simulation:(1)For perfect graphene,when the surface area reaches a certain vale,changing the area has negligible effect on the friction response.When the AFM tip scans over the graphene layer along different directions,the AFM tip and graphene form different contact orientation,and the stick-slip periodicity of the obtained friction curve is related to the that of the graphene lattice;due to the combined effect of the lattice spacing and the silicon tip geometry,the friction response are found to be related to the scanning direction.(2)For heterogeneous graphene materials embedded with nanopores,the embedded nanopores increase the friction signal,while the friction force curve maintains the stick-slip characteristics.The size of the nanopore plays a key role in the change of the maximum friction force,and the value varies with the size of the hole.The strong out-of-plane and in-plane deformation around the pores is the main reason for the formation of wrinkles and generates resistance to the tip during the sliding process.When the silicon tip scans toward the center of the nanopore,negative friction can be observed.This phenomenon is caused by the asymmetric change of the in-plane strain when the silicon tip is pressed in.The interlayer friction characteristics of embedded graphene nanopores are further studied,and it is found that as the number of graphene layers increases,the influence of nanopores on the friction signal decreases,which is similar to the interlayer friction in an ideal graphene layer.The graphene embedded in the middle layer,the higher the graphene,the more obvious the impact of nanopores on friction. |
