Graphene-based Friction Scale Effect Study

The surface friction properties of graphene at the micro/nano scale are important for its application as a solid lubricant in micro/nano-electromechanical system components.In this paper,atomic force microscopy and a homemade Si O₂ microsphere probe through a 3D micromachining platform are used to study the changes in morphological height,friction and adhesion of graphene under scale effects.The experimental results are as follows:When the graphene morphology was scanned at different scales,the height measurement of graphene increased with the increase of the tip size.At the same time,the change of graphene sample morphology has less effect on this pattern.The increase of the tip size has less effect on the measurement of the graphene-substrate-graphene spacing width and its height.The difference in graphene thickness measurements at different scales,as illustrated by the forces on the needle sample contact,was due to an increase in the tip radius,an increase in the van der Waals force between the tip-sample and an increase in the normal displacement of the tip.The surface friction properties of graphene under the scale effect were further investigated.The graphene surface friction increased with the increasing radius of curvature of the tip at the same constant load.Under dynamic loading conditions,positive and negative friction coefficient transitions were observed on the graphene surface at both submicro scale and micro scale.When the tip diameter was 400 nm,720nm,and 5μm,the change in the range of load reduction has little effect on the applied normal load corresponding to the turning point of the positive and negative friction coefficients,but increasing the graphene thickness leads to an increase,which showed a strong dependence on thickness.When the tip diameter was increased to 10μm,the applied load corresponding to the turning point varied with different unloading ranges,while its regularity was not obvious at different graphene thicknesses.The adhesive force values at different scales also increased with the increase of the radius of curvature of the tip,which was analyzed by establishing a tip-sample contact model,and the increase of the tip radius made the increase of the van der Waals and capillary forces,leading to the increase of the adhesive force.The stick-slip curves on the graphene surface remained smooth at the nanoscale,while the sloping phenomenon,i.e.,friction enhancement,occurred at both the submicro and micro scales,and the friction enhancement effect decreased with the increase of graphene thickness.The theoretical analysis by the improved PT model also concluded that as the tip size increased,the maximum lateral force increased and the tip-sample action potential would increase.So the dependence of the applied load on the graphene thickness corresponding to the negative friction coefficient and the turning point of the positive and negative friction coefficient was attributed to the large tip-sample forces and the relationship between the graphene out-of-plane stiffness and its thickness.