Study On The Influence Of Environmental Humidity On Micro-adhesion And Friction Hysteresis

Water molecules and nanoparticles are ubiquitous in the air,and most nano-devices exposed to the ambient air can adsorb water on solid surfaces,the hydrogen-bonding interaction between the surface and the adsorbed water molecules depend not only on the intrinsic properties of material,but also the external working conditions.When the size of solid particles is reduced to the micro-/nano-scales,the influence of water condensation on the interaction(such as adhesion and friction forces)between nanoparticles and substrate becomes significant as a result of the greatly increased specific surface area.This makes the nanoparticles a serious hidden danger in the micro/nanomanufacturing and MEMS operation.The retention or removal of these nanoparticles would easily cause high friction and severe wear on the device,thus greatly reducing the service lifetime.On the other hand,as technology advances,environmental protection and energy conservation have become more and more important,and the purification capacity of the water purification device will be reduced due to the agglomeration of nanoparticles..Therefore,this paper focused on the influence of the structural evolution of the adsorbed water layer on the hysteresis of adhesion and friction behavior for sphere-on-flat geometry,so as to better understand the adsorption and desorption characteristics of substrate surface under different humidity conditions.In this paper,the wear experiment of AFM probe was firstly carried out.The change rule of Si₃N₄AFM probe curvature radius with the number of sliding cycles was characterized by SEM imaging technology and standard sample reverse imaging technology.The error sources of the latter in calibrating the tip curvature radius were compared and clarified,and the curvature radius correction model of tip reverse scanning imaging was set up.Secondly,the adhesion hysteresis of Si₃N₄probe with different curvature radius was tested on the original silicon surface,and the influences of the curvature radius of tip and the stability time of the water structure on the adhesion hysteresis was analyzed.Thus,the hysteresis mechanism of the adhesion behavior between Si₃N₄tip and silicon substrate was revealed.Finally,the influence of tip size and stability time of water structure on friction hysteresis was investigated based on the research of adhesion hysteresis.Moreover,the friction hysteresis between Si₃N₄tip and hydrophilic and hydrophobic materials was compared.Through comparing the similarities and differences between adhesion hysteresis and friction hysteresis,the reasons for the differences were preliminarily revealed.(1)The wear law of Si₃N₄AFM tip with wear times was proved,and the curvature radius correction model based on tip standard sample reverse imaging technology was established,which laid a foundation for accurate measurement of AFM tip curvature radius.The wear law of reciprocating sliding between Si₃N₄AFM tip and silicon nitride surface with cycle times was investigated.The changes of the curvature radius and wear volume of the tip were characterized by SEM characterization and reverse scanning imaging.Due to the influence of the curvature radius of the standard sample itself,the curvature radius of the AFM tip obtained by the standard sample reverse scanning technology was larger than the SEM characterization result.The smaller the curvature radius,the greater the gap.According to the contact geometric relationship between AFM probe and the tip of standard sample,a curvature radius correction model of tip standard sample reverse imaging technology was established,which lays a foundation for the subsequent experiments of adhesion and friction hysteresis under the action of tip with different curvature radius.(2)Humidity-dependent microscopic adhesion hysteresis was discovered and it was also found that the adhesion hysteresis has probe"size effect"As the ambient humidity increased from 0%to 70%and decreased to 0%,the adhesion between the Si₃N₄AFM tip and the original silicon surface showed a serious hysteresis,i.e.the adhesion of the humidity drop was greater than that of the rise stage.Further research found that this hysteresis phenomenon only exists under the condition that the curvature radius of the Si₃N₄AFM probe was less than 150 nm,that is,microscopic adhesion has has the"size effect"of the probe.(3)From the perspective of adsorption and desorption of interfacial water film,the mechanism of microscopic adhesion hysteresis was preliminarily revealed,and it was found that adhesion hysteresis has"time effect"The results of repeated adhesion hysteresis experiments showed that tip wear is not the main factor causing hysteresis.Further research found that the hysteresis was clearly related to the adsorption and desorption of the interfacial adsorbed water film.The desorption process of the interfacial water film was slower than the adsorption process,resulting in the occurrence of hysteresis.With the increase of desorption time,the hysteresis would gradually weaken,and when the desorption time increases to 20 minutes,the hysteresis basically disappeared.(4)The micro friction hysteresis was explored,and the similarities and differences between adhesion hysteresis and friction hysteresis were compared and analyzed.When the curvature radius of the Si₃N₄AFM probe was less than 150 nm,both micro-adhesion and friction had obvious hysteresis,and gradually disappeared with the increase of the curvature radius of the tip or desorption time.The comparative study found that the variation trends of adhesion hysteresis and friction hysteresis were inconsistent,which may be mainly due to the difference of interfacial adsorbed water film structure caused by different movement modes of the tip during friction and adhesion measurement.