| The working clearance of mesoscale MEMS device is small, and running speed is high. Compared with the traditional machinery, the friction problem appears to be more prominent. The lack of the effective lubrication seriously hinders the development and application of MEMS devices. In this paper,the magnetic field controlled lubrication performance of magnetic fluids with low apparent viscosity is investigated on the mesoscale MEMS device.Based on the corresponding relationship between the velocity condition of friction pair and the viscosity of lubricant and the magnetoviscous effects of magnetic fluids, the magnetic control lubrication model of magnetic fluids was proposed to improve the lubrication effect of mesoscale MEMS device.According to the magnetoviscous effects of magnetic fluids, the chemical coprecipitation method was adoped to prepare Fe3O4 magnetic particles with good magnetic property. Then the magnetic particles, the surfactant and base liquid were used to synthesize four kinds of magnetic fluids with low apparent viscosity, namely water-based, kerosene-based, di-esters based and hydrocarbyl-based magnetic fluid. The magnetic properties, stability and viscosity of the preparared magnetic fluids were tested and the experimental condition of the magnetic field strength was determined.Based on the actual working conditions of mesoscale MEMS device and the experimental requirements, the lubrication system of the pin-on-disc MEMS friction and wear test rig was modified. According to the magnetic control lubrication theory, a new structure of the permanent magnet with magnetic pole was designed. Combined with the results of electromagnetic simulation of ANSYS, the shape and size of the magnetic pole needed for the experiments were obtained so that the strength of the applied magnetic field was ensured to meet the test requirements.The lubrication experiments of mesoscale MEMS device by magnetic field controlled magnetic fluids were conducted on the modified MEMS friction and wear test rig. The friction coefficient and the oil film thickness were the key parameters to discuss the friction mechanism on the surface of the MEMS device and the lubrication performance of magnetic fluids. The SEM wear morphology of the surface and the simulation results of lubricating oil film with ANSYS Workbench were helped to investigate the magnetic field controlled lubrication mechanism of magnetic fluid on MEMS surface. Finally, the magnetic fluid lubrication model was adopted to propose the optimization scheme of magnetic field strength for mesoscale MEMS device. |
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