Lubrication of Microelectromechanical Devices Using Liquids of Different Viscosities

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.