Lubricated MEMS: effect of boundary slippage and texturing

Abstract

SummaryMany types of micro-electro-mechanical-system (MEMS) based products are currently employed in a variety of applications. Recently, there has been an increase in the demand for higher reliability of MEMS which incorporate moving parts for each intended application. This is because the reliability of MEMS containing moving parts is poor and has a limited lifetime. Applying a lubricant to these systems to avoid wear hampers the movement due to the adhesive/surface forces, leading to stiction. By modifying the contacting surfaces, one is able to enhance the behavior of surfaces in a controlled way and thus alter the flow pattern in the liquid lubricating film for an enhanced performance. In this thesis, the concept of complex slip surface (CSS) as an artificial (deterministic) boundary slip surface is introduced. The thesis examines the exploitation of the artificial boundary slip to improve the performance of liquid lubricated-MEMS, with the emphasis on increasing the load support and reducing the coefficient of friction. Therefore, it is of great importance to get a clear view of the concept of the artificial boundary slip with respect to the performance of lubricated-MEMS.A main principle of fluid film lubrication, as well as a touchstone of the Reynolds equation, is that there is no boundary slip of the liquid lubricant along the two solid surfaces. As a result, lubrication with boundary slip cannot be analyzed by the classical Reynolds equation, which specifically excludes the possibility of slip. The aim of the present work is to build a modified form of the Reynolds equation in which boundary slip is allowed to occur on both of the opposing surfaces. Two different models of boundary slip are discussed, namely: the two-component slip model and the critical shear stress model. The first model assumes that boundary slip will occur when the shear stress at the surface reaches a critical value, and, once the slip begins, that it takes place at a constant slip length. This model is adopted to incorporate some possible slip directions, as well as slip velocities directly.The second model, the critical shear stress model, is based on the assumption that there is a critical shear stress on the liquid-solid interface. No slip occurs at the interface if the surface shear stress is less than the critical shear stress, but the slip takes place if the shear stress reaches the point of critical shear stress. The modified Reynolds equation with the critical shear stress model that was developed in the current work is based on the assumption that a slip is treated to occur both at the stationary and the moving surface. The model is converted to a finite volume form and solved by tri-diagonal-matrix-algorithm (TDMA) combined with alternating-direction-implicit (ADI) scheme. In this way, the model is able to incorporate the influence of boundary slip on the lubrication performance of MEMS, while keeping the computational time within a reasonable range. The model is validated through experimental work published in the lit...