Lubrication of High Sliding Silicon Micromachines

Abstract

A major challenge in silicon devices based on micro-electro-mechanical systems (MEMS) is the provision of effective lubrication for sliding parts. This greatly limits the development and exploitation of MEMS devices, with current designs avoiding sliding contacts where possible. This thesis describes research aimed at lubricating high sliding MEMS devices. A micro-scaled tribometer has been constructed to obtain measurements of friction between two sliding silicon surfaces. This work focuses on lubricating MEMS with liquids, a self-replenishing lubrication method which had been dismissed previously as they were assumed to carry too much viscous drag. The major finding is that ferromagnetic fluids make excellent lubricants for sliding MEMS surfaces. These fluids provide low friction at high speeds, and reduce the boundary friction at low speeds when the hydrodynamic film is absent. The properties of such fluids allow the liquid to be contained in the presence of a magnetic field, meaning that only a small, localised amount is required. Low viscosity liquids were also shown to provide acceptably low friction at high speeds. These results agreed reasonably well with theory. Friction modifier (FM) additives were added to low viscosity liquids in order to reduce boundary friction by forming boundary films, when no hydrodynamic film is generated at low speeds. Drag has also been shown to be insignificant. A study of the wear of silicon surfaces under prolonged sliding was conducted. Previous studies have focussed on dry coatings and apparently untreated surfaces. In this thesis, the effects of different surface preparations, the use of low viscosity liquids and vapour phase lubrication on wear have been studied. This thesis concludes that it is feasible to use liquids to lubricate sliding MEMS. High sliding MEMS is possible and practicable in future if self-replenishing methods, such as those studied in this work, are employed in real devices.