Effect Of Surface Charge On Micro-nano Scale Friction And Wear In Aqueous Environment

In recent years,micro/nano electromechanical systems(MEMS/NEMS)are widely used in many fields,due to the decreasing dimension and increasing surface-to-volume ratio of MEMS/NEMS,surface friction has become an important factor that restricts its development.Theoretical and experimental studies have shown that surface charge on the solid-liquid interface will form an electric double layer(EDL)in aqueous environment,and the force between the EDLs is believed to affect the micro/nano friction,but the studies focused on the case where the friction pair is of the same material,few investigates the friction pair made of different materials,moreover,the study on the effect of surface charge on micro/nano wear of the surface isn’t sufficient.Considering the above problems,the present study comprehensively analyzes the effect of surface charge on friction and wear of the friction pairs made of different materials in aqueous environment.The AFM probe-sample electrostatic force theoretical model is first established,the corresponding calculation formulas are deduced,the surface charge densities at the solidliquid interface of Si O2、Al2O3、Si3N4 are measured and the mechanisms of origin of surface charge are analyzed.The effects of surface charge on electrostatic force between EDLs are studied.Finally,the formula of van der Waals force is deduced based on the probe-sample model.The friction force formula is modified by considering the influence of the electrostatic force and van der Waals force.Then,a method for converting the AFM voltage signal into friction force is established by analyzing the force changes of the AFM probe during the scanning of the sample surface.By measuring the frictional coefficient between Si3N4/mica surfaces in air,the result of which is compared with previous studies,the accuracy of this method is verified.Based on the friction force conversion method,the friction coefficients of Si O2 probe/Al2O3 surface,Si3N4 probe/Si O2 surface,and Si3N4 probe/Al2O3 surface in different p H solutions are measured by AFM operated in contact mode.Combining the theoretical calculation results of electrostatic force,the study investigates the effects of the electrostatic force between asymmetric EDLs on the friction coefficient between two surfaces.Finally,the wear experiments of mica and Mg O in solutions with different p H values are conducted by using AFM contact mode,the depth of wear scars are measured in-situ.The mechanism of the generation of wear on mica and Mg O surface and the effect of surface charge on wear depth are investigated.In summary,the surface charge density at the solid-liquid interface is measured,the electrostatic force between the EDLs is theoretically calculated,and the method for converting the AFM voltage signal into friction force is established.The effects of the electrostatic force on friction and surface wear are studied by using AFM contact mode.