Implementing liquid lubrication in micro-electro-mechanical systems

Abstract

Even though there are many types of micro-electro-mechanical devices (MEMS), few commercialized MEMS devices can achieve functions requiring rotation. This is a result of a lack of methods providing efficient lubrication for the micro sliding contacts. Liquid hydrodynamic bearings which are commonly used in the macro sliding contacts could still be efficient in micro sliding contacts if appropriate bearing design and lubricants are applied. This thesis reports on the research focused on miniaturizing liquid hydrodynamic bearings thereby improving the performance of the micro sliding contacts in MEMS devices. A finite volume model considering mass-conservation and cavitation has been developed to investigate the performance of the micro pocketed parallel thrust bearings. Simulation results show that the micro thrust bearing performance is largely affected by cavitation. It also shows that micro pocketed parallel thrust bearings with appropriate pocket shapes can generate sufficient load support and operate in the low friction coefficient full-film lubrication regime when low viscosity lubricant is supplied (e.g. hexadecane). Micro fabrication process has been developed to manufacture the micro pocketed parallel thrust bearings. The manufactured micro bearings have been examined both on a MEMS tribometer and on a prototype micro energy harvesting turbine. Test results show that low friction coefficient, torque and power loss can be achieved in a wide rotational speed range from the micro bearings with proper design and lubricated by low viscosity and low surface tension liquid (e.g. hexadecane). Effects of friction modifiers and partial surface textures on the micro liquid hydrodynamic bearings performance have also been investigated. Results indicate that both methods are still efficient in friction reduction. As a conclusion, miniaturized liquid hydrodynamic bearings with appropriate bearing design and lubricants are efficient to achieve low friction coefficient and wear rate for the micro sliding contacts in a wide rotational speed range.