Influence Mechanism Of Water Adsorption And Oxidation On Friction And Wear Of MoS₂

Molybdenum disulfide（MoS₂）is a superior solid lubricant.However,its lubricating performance is highly environment dependent,and deteriorates severely under humid and oxidizing atmospheres.Researches on MoS₂to date mostly focus on the macro scale,and rarely explore its mechanism at micro scale.Therefore,this thesis presents a mechanistic study on the effect of water absorption and oxidization on friction and wear of MoS₂atomic steps and plane structures.Based on observed microscopic mechanisms,a strategy was proposed to improve the macroscopic friction and wear performance of MoS₂in atmospheric conditions.The main conclusions and contributions of this thesis are as follows:（1）The effect of different atmospheres on the friction and wear of MoS₂atomic steps and plane structures under micro scale was studied.A smaller friction force and a lower critical load of wear initiation at the atomic step were characterized in air compared to the cases in vacuum.In contrast,the friction force and adhesion force on base plane surface are both larger in air;however,the wear behavior is not significantly different in those two environments.Furthermore,the wear behaviors of the step with different structures（Armchair and Zigzag）depending on applied load are similar due to the possible reason of the pre-oxidation of the step in air.Finally,no obvious difference of the critical contact stresses for wear initiation at the atomic step edges was observed for the screatch tests against a diamond probe and a silicon probe in air,indicating a same wear mechanism of MoS₂atomic steps for those two probes.（2）The effects of water adsorption and oxidation on the friction and wear of MoS₂at the microscale were investigated.With the increase of humidity,the friction force decreases at the step edge but increases on the base plane surface.The possible reason is that the original oxidized step edge enable to adsorb more water,which plays a more lubricating role.Differently,the hydrophobic plane surface adsorbs less water molecules,and then the condensation of a small volume liquid bridge at the contact interface is enhanced at higher humidity,resulting in stonger interfacial adhesion and capillary action.The microscopic wear behaviors of steps and planes are almost consistant at all humidity conditions.This is possiblly due to the contributions of adsorbed water film to the MoS₂wear are suppressed under the high load conditions for the MoS₂wear initiation.Furthermore,the friction force increases and the critical load of MoS₂wear initiation decreases after oxidation in thermal treatment.The replacement of S atom by O atom after oxidation leads to more significant uneven charge distribution,which increases the energy barrier for the probe to slide across the treated MoS₂surface.The reduction in the critical wear load is due to the formation of S vacancy and MoS₂-_XO_Xsolid solution phase.（3）A method based on the removal of oxide layer on the step edge to improve the friction and wear performance of MoS₂in atmospheric environment was proposed.The macroscopic experiments results show that adding 0.5 mg of sodium carbonate to the 10 mg MoS₂can further reduce the friction coefficient of MoS₂coating in the atmospheric environment.This is because the addition of sodium carbonate in atmospheric environment can partially inhibit the oxidation of MoS₂step edge.However,addition of sodium carbonate will affect the shear properties of MoS₂interlayers.Due to no water adsorption and further oxidation in vacuum environment,the addition of sodium carbonate will slightly increase the friction of the MoS₂coating.