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Micro-Nano Tribology

Lubrication of Microelectromechanical Devices Using Liquids of Different Viscosities

[+] Author and Article Information
I. S. Y. Ku, T. Reddyhoff, R. Wayte

Tribology Group Department of Mechanical Engineering,  Imperial College London, London SW7 2AZ, United Kingdom

J. H. Choo

Materials Group Department of Mechanical Engineering,  The National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore

A. S. Holmes

Optical and Semiconductor Devices Group Department of Electrical and Electronic Engineering,  Imperial College London, London SW7 2AZ, United Kingdom

H. A. Spikes

Tribology Group Department of Mechanical Engineering,  Imperial College London, London SW7 2AZ, United Kingdom

J. Tribol 134(1), 012002 (Mar 06, 2012) (7 pages) doi:10.1115/1.4005819 History: Received May 10, 2010; Revised January 09, 2012; Published March 05, 2012; Online March 06, 2012

Lubrication of contacting and sliding surfaces in MEMS (microelectromechanical systems) is particularly challenging because of the predominance of surface forces at the microscale. The current paper explores the possibility of using liquid lubrication in this application. Measurements of friction and lubricant film thickness have been made for liquid lubricants of different viscosities, including low viscosity silicone oil, hexadecane, squalane, and water. Testing was carried out using a newly developed MEMS tribometer in which a rotating silicon disk is loaded against a stationary silicon disk. Two different test setups were used: one where both disks are flat, and the other where the stationary disk is structured as in a thrust pad bearing. In all tests the disks were fully submerged in the lubricant. With the flat-on-patterned disk combination, the variation of friction with rotation speed was found to follow classical Stribeck curves for all the lubricants tested. The friction at high speeds also decreased with increasing normal load, in accordance with hydrodynamic lubrication theory. For the least viscous lubricants, it was found that the hydrodynamic friction coefficients remained relatively low even at higher speeds. In particular, for water the friction coefficient for water was around 0.1 at 10,000 rpm. However, boundary friction was found to be unacceptably high at low speeds where there was insufficient lubricant entrainment. The experimental results have been compared with a finite difference solution of Reynolds equation and reasonable agreement is seen between theory and experiment. The results indicate that liquid lubrication is potentially an effective means of lubricating MEMS components with high levels of sliding.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Photograph of the microtribometer

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Figure 2

Close up of the two specimens in contact. The white dashed box indicates the position of the liquid container for the fully hydrodynamic setup.

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Figure 3

Photographs showing silicon test platforms; two alternative inner platform designs are shown. The outer platform has a footprint of 25 × 25 mm2 .

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Figure 4

(a) Schematic of the light lever mechanism (viewed from beneath test rig). (b) Schematic of the normal load measurement system (elevation view).

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Figure 5

(a) Photograph of a silicon wafer with various designs of inner platform and (b) SEM image of a patterned lower test specimen. (a) Full speed range. (b) A close up of the low speed region highlighting the high friction in the mixed/boundary region.

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Figure 6

A plot of friction coefficients versus speed for the flat-on-flat tests. (a) Full speed range. (b) A close up of the low speed region highlighting the high friction in the mixed/boundary region.

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Figure 7

A plot of friction coefficient against speed for the flat/patterned tests

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Figure 8

Friction coefficient versus speed for patterned pad lubricated with squalane

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Figure 9

Friction coefficient versus speed for contact lubricated with squalane, showing loaded and unloaded conditions

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Figure 10

Speed of motor shaft versus current supplied to motor, for four contact conditions

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Figure 11

Schematic diagram of bearing

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Figure 12

Output from finite difference code for contact lubricated with squalane, rotating with an angular speed of 10,000 rpm, under an initial load of 0.05 N

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Figure 13

A plot of film thickness against speed for the results obtained in experiment and in theory. Contact lubricated with squalane, with an initial load of 0.2 N.

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