Diffusion of Physisorbed Layers and their Connection to MEMS Effectiveness

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Title: Diffusion of Physisorbed Layers and their Connection to MEMS Effectiveness
Author: Miller, Brendan Paul
Advisors: Prof. Donald W. Brenner, Committee Member
Prof. John E. Rowe, Committee Member
Prof. Jacqueline Krim, Committee Chair
Prof. Thomas P. Pearl, Committee Member
Abstract: The aim of this work is to connect the physics of surface diffusion of a lubricant to Micro-Electro-Mechanical System (MEMS) lubrication. Some hurdles must be overcome in order to make this connection. One must have a way to experimentally measure surface diffusivity. Length scales must be taken into account since the mechanism of lubrication varies from the macro scale to the micro scale and even to the nano scale. Lastly, a theoretical model of lubrication that can conform to MEMS geometry is needed for an accurate prediction. In the work presented here, I have used different techniques including a quartz crystal microbalance (QCM), a macroscopic tribometer, a micro tribometer, and an atomic force microscope (AFM) to measure the friction of a lubricant on surfaces relevant to MEMS. The QCM method is different from the others aforementioned since it measures the atomic scale friction of a sliding layer in a contact-free environment directly related to surface mobility. The other three methods are a way to measure lubrication over three different length scales. A surface lubrication model developed by Prof. Donald Brenner incorporates surface diffusion as a mechanism for lubricating a periodic contact. There is a push and pull between the removal of the lubricant from the periodic contact and the replenishment of the lubricant via surface diffusion. A steady-state center concentration can be computed, which is used to determine whether or not lubrication can occur. This model was fit to magnetic hard disc drives (MHDDs), MEMS, and macroscopic industrial machines, but will work for any system with a periodic contact relying on surface diffusion as the replenishment mechanism. Two groups of lubricants were studied in this work. Tricresyl Phosphate (TCP), which is a known high temperature additive to industrial oils, was selected since it possesses a low vapor pressure and has been extensively studied by our group. All of the above mentioned techniques were used to study TCP. The other group of lubricants studied were alcohols, specifically pentanol, ethanol, and trifluoroethanol (TFE). These lubricants were studied exclusively with the QCM technique. Alcohols have been shown to lubricate a MEMS device indefinitely as long as an environment of the alcohol vapor surrounds the contact. The results shown here can be used to directly predict the effectiveness of a lubricant candidate to a MEMS device. Some extra parameters were determined to affect lubrication including contact stress, adhesion, and wetablitiy. These parameters need to be taken into account for future selection of lubricants.
Date: 2009-12-08
Degree: PhD
Discipline: Physics
URI: http://www.lib.ncsu.edu/resolver/1840.16/5941

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