Experimental Study On The Time Dependence Of Micro-nano Adhesive Contact Based On Atomic Force Microscopy

The adhesion between two solid surfaces is critical to the development of a Micro Electro Mechanical Systems(MEMS).In micro-nano scale,the mechanical system is mainly affected by surface effect rather than inertia effect.Adhesion is one of the main reasons for the failure of MEMS in manufacture and service.With the further miniaturization of MEMS,it is urgent to study how to avoid adhesion failure to improve system reliability.Therefore,it is necessary to carry out a large number of experimental and theoretical studies to explore the adhesion mechanism.Atomic force microscope(AFM)is one of the most important tools for micro-nano adhesion experiments.The experimental parameters have a great influence on the measurement results of adhesion.Among them,contact time often plays a leading role.The contact time dependence of adhesion is very important for understanding its mechanism.Some studies on the time dependence of adhesion have been conducted using AFM.However,the reported experimental results are often inconsistent and sometimes contradictory.In view of this,this paper used AFM to investigate the time dependence of adhesion under different conditions(relative humidity,surface material,surface hydrophilicity,graphene material,etc.).The main work is as follows:(1)The adhesion between the silicon cantilever and different hydrophilic samples was measured at low humidity and the contact time dependence was investigated.The results show that the time dependence of adhesion can be observed on all hydrophilic samples.The dependent adhesive force is divided into three stages with the increase of residence time: first,it increases sharply,then it increases slowly,and finally,it reaches saturation and remains unchanged.The saturation time also varies according to the hydrophilicity of the sample.The more hydrophilic the surface,the longer the saturation time.The water film flow model of the bridge growth can well explain the trend of increasing adhesion.(2)The surface adhesion of different materials and hydrophilic samples was measured in a humid laboratory environment,and the contact time dependence of the adhesion was investigated.The results show that the contact time dependence of adhesion is related to the surface materials.On silicon,optical glass,and sapphire surfaces,the adhesion is timedependent.This is caused by the high viscosity of the water film adsorbed on the surface.The adhesion is independent of residence time on the surfaces of mica,silica,gold film,highly oriented pyrolytic graphite,polymethyl methacrylate,diamond-like film,and graphene.This is due to the low viscosity of the water film on these surfaces,resulting in a very short saturation time.(3)Using hydrophobic and hydrophilic AFM silicon cantilevers in a humid laboratory environment,we measured the adhesion on different hydrophilic silica and graphene surfaces and investigated the relationship between the adhesion and residence time.The results show that graphene can shield the time dependence of the adhesion between the silicon and silicon interface and the instability with repeated contact.Specifically,the adhesion between siliconsilicon interface shows different behavior with the increase of residence time.Depending on the hydrophilicity of the two surfaces,these behaviors include:(1)stepwise increase with residence time,(2)logarithmic increase,and(3)independence.The adhesion between silicon and silicon interface shows different behaviors with residence time depending on the hydrophilicity of the two surfaces:(1)stepwise increase,(2)logarithmic increase,and(3)independent.However,for either hydrophobic or hydrophilic cantilevers,the adhesion between the silicon-graphene interface is independent of residence time,and the effect of repeated contact on the adhesion is negligible.The differences in the time dependence of adhesion and the instability with repeated contact between the two interfaces are attributed to the evolution of the interfacial fluid bridge.(4)The interfacial adhesion of Si-Si and Si-graphene was measured.The evolution of adhesion with relative humidity and the contact time dependence at a certain humidity was investigated.The results show that the adhesion of the two interfaces varies with the evolution of relative humidity.In the process of repeated contact at the same point,the adhesion of the silicon-silicon interface decreases first and then increases slightly with the decrease of relative humidity and then increases with the increase of relative humidity.However,the adhesion of the silicon-graphene interface remains unchanged with the decrease of relative humidity and tends to decrease overall with the increase of relative humidity.In the force-volume mode,the adhesion of the silicon-silicon interface continues to increase with the increase of relative humidity,while the adhesion of the silicon-graphene interface remains constant with the increase of relative humidity.The difference in adhesion behavior between the two interfaces is attributed to the evolution of liquid bridge caused by the change of water film thickness adsorbed on the surface and the change of water film flow.(5)The adhesion of the newly cleaved mica surface was measured in a humid laboratory environment.The effects of repeated contact at the same point,residence time,and scanner speed on the adhesion were investigated.The results show that the adhesive force is relatively stable with the increase of contact times,and the reproducibility is good.With the increase of residence time(0?0.4s),the adhesive strength abnormally decreased,and then remained unchanged(0.4?10s).This abnormal reduction behavior is attributed to the dynamic formation process of the liquid bridge.