Film Forming Properties Of Nanoscale Lubricant Films And Microbubbles Emergence Behavior Under External Electric Fields

The study on thin film lubrication under an external electric field (EEF) is a frontier field of the lubrication research, and it is of considerable academic and engineering values to charged lubrication systems and micro/nano electromechanical devices, etc. This dissertation focuses on the properties of nanoscale lubricant films under an EEF in terms of the film forming properties and the microbubble emergence behavior in thin lubricant films. The following conclusions are drawn:Firstly, the film forming properties of oil based lubricants, emulsion and water based lubricants under EEFs are investigated. It is proposed that the difference in the response of cations to EEFs determines the discrepancy of the film forming properties among ionic liquids with various side chain lengths. The relationships between the film forming properties of oil-in-water emulsions and the EEF strength, the emulsifier concentration and the oil phase concentration are studied. Moreover, an oil destabilization model in the inlet region is constructed to analyze the effects of the deformation and breakup of oil droplets under EEFs on the film forming properties theoretically. Furthermore, the film forming properties of surfactant solutions with positive and negative charged head-groups and various surfactant concentrations are compared, and the effect of the interaction between surfactant molecules and charged solid surfaces on the film forming properties is also analyzed.Secondly, an interesting phenomenon is found that the fluidity of nanoconfined lubricants in the contact region becomes very weak and disappears eventually when an EEF of larger than the threshold intensity is applied. It is proposed that the solid-like transition of liquid molecules near charged confined solid surfaces is greatly intensified by the EEF, resulting in the reduction in the fluidity of lubricant films in the contact region.Thirdly, another interesting phenomenon that many microbubbles emerge in thin lubricant films under EEFs is studied. It is found that microbubbles emerge more intensively after a dielectric layer is coated onto the electrode while other experimental conditions remain unchanged. The relationships between the electric current, interfacial damage on the contact pairs and the emergence of microbubbles are discussed. It is proposed that overheating and electrochemical reactions contribute collectively to the occurrence of microbubbles in thin lubricant films under EEFs, and overheating plays a predominant role. A theoretical model describing the microbubble growth and motion characteristics in thin lubricant films is constructed, and the experimental relationships between the growth, motion characteristics of microbubbles and lubricant physicochemical properties can be well predicted with the proposed model.Finally, a bearing electrocorrosion tester is constructed to investigate the electrodamage behavior on the lubricated surfaces in bearings under the action of weak electrical currents as low as ~ 1 mA. It is demonstrated that the collapse of microbubbles contributes to the formation of craters on lubricated surfaces.