LUBRICATION OF HIGH-SLIDING MICROMACHINES

Lead Research Organisation: Imperial College London
Department Name: Dept of Mechanical Engineering

Abstract

This project seeks to develop novel and effective ways of lubricating high-sliding MEMS. It is a collaboration between two research groups at Imperial College London, one specialising in MEMS and the other in tribology.One of the most important technological advances in the last 15 years has been the development of micro-electromechanical systems or MEMS. MEMS devices can be made cheaply and in very large numbers using silicon chip fabrication methods and they are now being used in a wide range of machines, from inkjet printers to motor cars. At present, one of the main factors that limits the use of MEMS is the friction and wear of the moving parts, since it turns out to be much more difficult to obtain low friction coefficients in micro-scale contacts than in macro-scale ones. This means that all of the MEMS devices currently in use are ones that require little or no sliding contact. However there is great interest in building micromachines with large amounts of sliding such as tiny gas turbine engines and generators. Such devices could be used to power mobile phones and other portable equipment such as survival gear, but the problem is how to lubricate them effectively.At present, most MEMS are either not lubricated at all or are lubricated by depositing a monolayer of organic molecules on their surfaces prior to use. However in high-sliding MEMS this layer is damaged and lost during sliding, resulting in high friction and wear. The applicants believe that successful lubrication of high-sliding MEMS will require some means of replenishing the lubricant film during prolonged MEMS operation. Therefore, in this project three novel ways of establishing a replenishing lubricant film in MEMS will be explored: vapour phase replenishment, the use of a thin, mobile liquid film and the operation of MEMS partially or wholly immersed in low viscosity liquid. This last possibility is the most controversial since it has generally been assumed up to now that the viscous drag forces created in an immersed MEMS device would be too large. However calculations by the applicants suggest that this should not be the case for some larger devices, in which case low viscosity lubricants containing additives may be an attractive option. If MEMS devices can be operated within a liquid environment then it makes possible not just lubrication by formation of a liquid film between rubbing parts - hydrodynamic lubrication - but also lubrication by additives dissolved in the liquid which adsorb and react with the rubbing surfaces - boundary lubrication.The project will first construct a MEMS tribometer in which friction can be monitored and wear measured using lubricated MEMS components. This instrument will then be used in conjunction with a second, millimeter-scale tribometer to test and develop the above three methods of lubricating MEMS.

Publications

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I Ku (2010) Lubrication of MEMS devices using liquids of different viscosities in ASME Transactions Journal of Tribology

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Ku I (2010) A novel tribometer for the measurement of friction in MEMS in Tribology International

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Reddyhoff T (2010) Friction Modifier Behaviour in Lubricated MEMS Devices in Tribology Letters

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Wang P (2016) Development of hydrodynamic micro-bearings in Journal of Physics: Conference Series

 
Description This project has developed novel and effective ways of lubricating high-sliding MEMS. It has been a collaboration between two research groups at Imperial College London, one specialising in MEMS and the other in tribology.

One of the most important technological advances in the last 15 years has been the development of micro-electromechanical systems or MEMS. MEMS devices can be made cheaply and in very large numbers using silicon chip fabrication methods and they are now being used in a wide range of machines, from inkjet printers to motor cars. At present, one of the main factors that limits the use of MEMS is the friction and wear of the moving parts, since it turns out to be much more difficult to obtain low friction coefficients in micro-scale contacts than in macro-scale ones. This means that all of the MEMS devices currently in use are ones that require little or no sliding contact. However there is great interest in building micromachines with large amounts of sliding such as tiny gas turbine engines and generators. Such devices could be used to power mobile phones and other portable equipment such as survival gear, but the problem is how to lubricate them effectively.

Currently, most MEMS are either not lubricated at all or are lubricated by depositing a monolayer of organic molecules on their surfaces prior to use. However in high-sliding MEMS this layer is damaged and lost during sliding, resulting in high friction and wear. The applicants believe that successful lubrication of high-sliding MEMS requires some means of replenishing the lubricant film during prolonged MEMS operation. Therefore, in this project three novel ways of establishing a replenishing lubricant film in MEMS have been explored: vapour phase replenishment, the use of low viscosity liquid lubricants with and without additives and the use of magnetic fluids. The possibility of using liquid lubricants has tended to be discounted by MEMS designers since it was believed that the viscous drag forces created in an immersed MEMS device would be too large. However the current project has shown that this is not necessarily the case, and that low friction can be obtained using low viscosity liquids.

A MEMS-based instrument has been designed in which friction, wear and lubricant film thickness can be measured between sliding, silicon MEMS components. This is the first tribometer that allows quantitative measurement of friction and wear under conditions representative of MEMS. It also the first MEMS tribometer able to study high sliding contacts in liquid lubricated conditions

It has been found that high speed sliding MEMS components can be successfully lubricated, and give low friction and wear, using a suitable low viscosity liquid with dissolved additives. At high speeds the liquid can provide a thick, separating fluid film while at low speeds the additives form adsorbed protective films on the silicon surfaces. Lubricant film thickness measurements have also been validated using a fluid film numerical model.

The project has demonstrated that there are effective methods to lubricate high sliding MEMS based on the principle of lubricating film replenishment.
Exploitation Route Development of lubricated MEMs
Sectors Aerospace, Defence and Marine,Electronics,Environment,Healthcare,Manufacturing, including Industrial Biotechology,Transport