Numerical Simulation And Experimental Study On Adhesion Behaviour Between Nano-Probe And Graphene

Graphene is becoming one of the special material that will change the world profoundly with excellent mechanical, electrical, optical, and thermal performance. Graphene is also an important option for solid lubricating purpose as the material has highest strength people had ever measured. Atomic force microscope(AFM) is an important instrument for material research on nanometer scale, and is a common tool in performance studying of graphen. Study on the contacting and frictional behaviour decipline between AFM tip and graphene is of great significance on understanding the interaction of interface, applying AFM testing graphene and other two-dimensional material. The array properties of tip surface atoms and graphene atoms were analysed to choice the valid simulating method for the interesting on interaction force and contact area. FEM modeling was made to simulate the interaction between AFM tip and graphene based on continuum mechanics theory, and adhesion experiments were conducted on silicon substrate.The main contents in the article include:①The mechanical parameters of single-layer carbon atom film was calculated based on the mechanical parameters of carbon nanotube and the geometrical relationship between graphene and carbon nanotubes, that is, graphene is the expanded configuration of carbon nanotube. And then FEM models were developed for graphene by using equivalent elastic shells which have same continuum mechanical property with graphene sheets instead of real graphene.②The interactional energy and forces were calculated among graphene, tip, and substrate according to the adhesion energy of interfacials, interatomic potentials, and atom density of different materials.③Coefficient of from 0 to 1 was chosen as influence factor of substrate surface to multiply the interaction force related with substrate, charactering the effects of substrate roughness on interfacial forces. The two number of 1 and 0 corresponds to ideal flat and extremely rough statue of substrate surface respectively.④Three types of FEM models with infinite friction, zero friction, and limited friction between neighbouring graphene layers were developed to study the two critical friction configuration on condition that exact friction between graphene layers is unclear.⑤The models were simulated by dynamic explicit method accompanied with intermittent tip feeding to dissipate energy during tip’s suspending in order to get static solutions. The difficulties of convergence of static calculating models for nonlinear interaction were avoided by this strategy.⑥Adhesion force and contact area were calculated applying standard and explicit FEM solving methods while the number of graphene layers increase from 1 to 4. The relationship between adhesion force and contact area was studied with different intensity of substrate effect as graphene thickness varying from 1 layer to 4 layers, by calculating the adhesion force and contact area with different influence factors of substrate surface and numbers of graphene layers.⑦Graphene of few layers was made by mechanical exfoliation and adhesion experiments were conducted on silicon substrate. The results from simulation and experiments were compared with each other after experiments finished.The main study results are as following:①The results from all of the models with infinite friction, zero friction, and limited friction between graphene layers show the same adhesion force changing trend, that is, the amplitude of adhesion force variation gets smaller and smaller as the number of layers increasing from 1 to 4. The pull-off forces of graphene sheets with over two layers are very close on value. Adhesion force will reach a stable value, the adhesion force of bulk graphite, with graphene sheet thickness increase continually according the trend appeared in simulations, that is consistent with real situation.②The intense of interaction between graphene and substrate surface affects the relationship between adhesion force and contact area heavily. Adhesion force and contact area varies consistently when the interaction between graphene and substrate is strong enough, that is, the thicker graphene has bigger contact area than the thinner ones under equivalent adhesion force. But the status get inversed when the interaction between graphene and substrate become weak enough. Thicker grpahene has smaller contact area compared to the thinner ones with adhesion load of same value. Furthermore, there is no transition range was found on the value of substrate surface effect to balance the two opposed force-contact area varying trend corresponding to strong and weak influence of substrate surface. The varying trend gets inverted jump on the critical value of substrate surface effect factor.③Friction dependence phenomena arises when the interaction between substrate and graphene gets weak enough, because of the bigger contact area of thinner graphene under adhesion load of near a same amount. The friction force keeping deduced as the number of layers increasing, and reach a stable value at last, that is, the friction force of bulk graphite.④The similarities and differences of simulation and experiments results for graphene sheet with 1 layer or two more layers suggest different impact of tip roughness on monolayer and multilayer graphene. Thinner graphene sheets conform to rough surface well due to lower bending stiffness and get a more compact contaction with tip surface. This together with the bigger contact area from greater stretch deformation for thinner graphene intensify the thickness dependence of friction perfomance.⑤Aging effect should exist on theory in contaction between tip and thin graphene sheet for tips with surface of no ideal smoothing, since the different conformation ability to other roughness induced by different bending stiffness of thin and thick graphene. That means tip and thinner graphene would get a more compact contaction if the contact time was prolongated.⑥The adhesion and tribology properties of two-dimension material of nano scale show apparent difference from that of macro scale materials. van der Waals adhesion force of long range acts as an important role to affect even decide the friction performance of two-dimension material on nano scale.The study managed to modeling the system of graphene, tip, and substrate with continuum mechanics theory. The method can be extended to mechanical studies of other two-dimension material, providing another important approach besides molecular dynamics(MD). The results of simulation of the study offers theory basis for adhesion and frictional experiments between tip of nano scale and grphene sheet.