MEMS Lubrication: An Atomistic Perspective of a Bound + Mobile Lubricant
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MEMS Lubrication: An Atomistic Perspective of a Bound + Mobile Lubricant Douglas L Irving, and Donald W Brenner Materials Science and Engineering, North Carolina State University, Box 7907, Raleigh, NC, 27695-7907 ABSTRACT The adhesive pressure needed to separate two ocatdecyltrichlorosilane (ODTS) coated surfaces both with and without the addition of tricresyl phosphate (TCP) as a function of separation rate is characterized using molecular dynamics simulation. The simulations predict that when TCP is added between surfaces the adhesive pressure needed for separation is reduced compared to the system containing ODTS only. Both the adhesive pressure and the break up of the TCP layers exhibit a separation rate dependence that appears unrelated to the rate of diffusion of TCP on the ODTS. The ability of the TCP to remain localized to defected areas of the ODTS layer upon normal separation of the contact is also characterized. It is found that the TCP remains localized to defect sites and, thus, effectively coats the damaged area. INTRODUCTION Monolayer lubricants, such as self-assembled monolayers (SAMs), have proven to be very successful in the protection of microelectricalmechanical systems (MEMS) from stiction related failure that often occurs during processing of the device. This success is related to the ability of the SAM to transform the surface chemistry from hydrophilic to hydrophobic while at the same time introducing a low self-adhesion. Unfortunately, protection against failure due to in-use friction is marginal, which limits many devices to “single shot” type applications. Further complicating the issue of lubrication is the limited frictional protection only occurs within a narrow range of temperatures and environments. It is, therefore, of interest to explore new lubrication strategies that extend the lifetime of devices in which reciprocating contact is needed and that broaden the environmental conditions under which the lubricant can be used. To overcome the shortcomings of the above lubricant a new “bound + mobile” scheme for MEMS, similar to that used in hard drives, has been studied [1-4]. It was first shown by Eapen et al. that a combination of chemisorbed and physisorbed Fomblin Zdol extends the lifetimes of a MEMS device [1]. This extension was attributed to the ability of the physisorbed layers to diffuse to defected areas and protect the uncovered surface. Although this scheme extends the lifetime, it does not necessarily extend the conditions under which the device can be used. Environmental insensitivity of the lubricant could possibly be achieved by incorporation of one or more mobile species with the traditional bound lubricant. In this direction, a combination of a bound octadecyltrichlorosilane (ODTS) with a mobile tricresyl phosphate (TCP) has been studied both experimentally [4] and computationally [2, 3]. Neeyakorn et al. showed that 3-5
monolayers of TCP readily absorb onto an ODTS covered surface, while at the same time maintaining a reasonable slippage as measured b
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