| Boundary lubrication exists in general at all kinds of frictional interfaces, so improving the performance of boundary lubrication could increase the wear life of friction pairs and decrease the frictional loss. As a great potential technique for enhancing boundary lubrication, potential-controlled friction(PCF) has been increasingly well-known among the tribologists, who have interest in developing smart surfaces and interfaces. At present, great progress has been achieved in the PCF study for aqueous lubricants. However, because of the low electrical conductivity of mineral oils, little effort has been made to explore the PCF effect for lubricating oils. And thus the PCF study is needed for oils in terms of lubricant material selection and mechanism investigation. Therefore, this thesis has made a systematical study on experiment and mechanism of the PCF phenomena for oils, especially, the organic ester-based lubricants.Firstly, based on the previous research results of PCF, a criterion was proposed for selecting lubricating materials suitable for PCF research, and three kinds of ester-based lubricants, which might possess the PCF effect, were optimally selected according to the criterion. Also, an electrode arrangement model, in a form of radiation, was designed and achieved. This model corresponded to a way, in which the electrical potential of electrode surface was controlled, and which is proved to be reasonable through the measurement using the double potentiostat method.Then, the PCF phenomena were experimentally observed for the ester with the addition of an ionic surfactant, verifying the criterion of lubricating material selection proposed in the study. The friction and wear properties of the lubricated steel surfaces, as a function of the surface potential, was also investigated to find a very good lubricity for the steel surfaces at +0.3 V. And, the adsorption behavior of additive ions was detected at different electrical potentials using a combination of several characterizing techniques, indicating the competitive adsorption of additive ions and ester molecules on the frictional surface. The results from the tribological tests and adsorption studies at different electrical potentials, as presented above, showed that electrostatic interaction at the metal-lubricant interface caused the PCF phenomena for the ester-based lubricant.Further, the effect of ionic liquids(ILs) on the lubricity of esters was studied at different electrical potentials. The relationship was obtained between the surface potential and friction property, wear property of steel surface, or adsorption behavior of additive ions, which demonstrated that the lubricity of ionic liquids for steel surfaces was largely improved at ?0.6 V. For the ester solutions of ionic liquids, their PCF effect resulted from the modulation of adsorption behavior of cations and anions via surface potential, which was similar to that found for the surfactant solution in ester. So, this similarity confirmed the mechanism of PCF for ester-based lubricants, as proposed above.Finally, the influence of nitriding or surface texturing of metal surface on PCF was experimentally investigated. The experiment results provided a potential technique, which might combine PCF with nitriding or surface texturing to improve the tribological performance of metal surfaces. Moreover, an electrostatic adsorption model was developed for PCF. The effect of surface potential on the free energy of ion adsorption was considered in this model, and it could explain the PCF phenomena for ester-based lubricants, proving the PCF model to be valid. |
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