Research On Construction Of Ultra-low Frictional Interface And Frictional Characteristic Of Molybdenum Disulfide

The failure of mechanical components caused by friction and wear at the contact interface will result in a large amount of energy consumption and economic loss.Molybdenum disulfide(MoS₂)possesses high mechanical strength,chemical stability and weak interlayer van der Waals interaction,as one of the commonly used solid lubricants.Constructing an ultra-low frictional interface at high humidity is essential for realizing the application of two-dimensional materials in ultra-low friction.In this thesis,the heterogeneous frictional interface involving MoS₂ and self-assembled monolayers(SAMs)was designed to obtain an ultra-low friction coefficient of MoS₂.The main contents of this thesis include:The factors that affect the nanofriction on MoS₂,such as thickness,substrate and anisotropy were comprehensively investigated via the calibrated AFM.The nanofriction of MoS₂ depends on its thicknes.The thicker the nanosheet,the smaller the out-of-plane deformation and the lower the nanofriction.Furthermore,the stiffness of the substrate and its bonding strength with MoS₂ will affect the out-of-plane deformation of molybdenum disulfide.The friction anisotropy of MoS₂ strongly correlates with the thickness:the nanofriction on the thick MoS₂ exhibits a significant anisotropy.The nanofrictional along the zigzag lattice direction is less than that along the armchair lattice direction.This is because the deformation of thick MoS₂ is small,and the frictional anisotropy airses from energy dissipation in different directions.The out-of-plane deformation of thin MoS₂ is large under the push of the AFM probe,reducing the differences in nanofriction along different lattice directions,and weakening the friction anisotropy.The friction characteristics of MoS₂ under high humidity and high temperature was investigated.The nanofriction on MoS₂ increased slightly with the increase of relative humidity.It is difficult for AFM tip and MoS₂ to form a meniscus under low humidity,due to the hydrophobicity of MoS₂.The meniscus forms between molybdenum disulfide and Si probe,and the nanofriction on MoS₂ increases slightly under high humidity.Therefore,the relative humidity affects the nanofriction on MoS₂ weakly.The nanofriction on MoS₂ gradually decreases with the temperature increasing.On the one hand,the water molecules and the meniscus force between the tip and the MoS₂ decreases with the increase of temperature.On the other hand,the increase of temperature provides more energy for the AFM tip to overcome the energy barrier between the MoS₂ lattices,namely“thermal lubrication”.In addition,the thicker the nanosheets,the lower the thermal conductivity of MoS₂,of which the nanofriction is less affected by temperature.The impact of surface and microstructure on the friction force on MoS₂ under different environments was investigated.The increase of friction force on MoS₂ in deionized water is very slow,undergoing physical adsorption and then chemical adsorption.The MoS₂ surface under UV/Ozone undergoes a short period of physical adsorption,then chemical adsorption,and finally complete aging(lubrication failure).When exposed to UV/Ozone for a short time,the nanofriction on MoS₂ increases slightly due to physical adsorption.When the active oxygen atoms adsorb on MoS₂ surface,the MoS₂ surface becomes hydrophilic.However,there are few structural defects on the MoS₂ surface.As the aging time increases,the structural defects increase.The number of active sites increases,and oxidation reactions occur on the MoS₂ surface.When MoS₂ is etched layer by layer,the friction force on MoS₂ increases rapidly under high temperature.Once MoS₂ is completely oxidized,the lubrication of MoS₂ fails.Construct the perfluorodecyltrichlorosilane self-assembled monolayer/molybdenum disulfide interface.The assembling of perfluorodecyltrichlorosilane monolayers(FDTS SAMs)on the atomic force microscope(AFM)tip enables the FDTS SAMs/MoS₂interface to obtain an ultra-low friction coefficient.On one hand,the hydrophobicities of FDTS SAMs and MoS₂ result in low meniscus forces at the frictional interface,reducing the friction force.On the other hand,FDTS SAMs and MoS₂ incommensurately contact,which reduces the frictional energy dissipation.Due to their hydrophobicities,the FDTS SAMs/MoS₂ interface still maintains an ultra-low friction coefficient under high relative humidity.