Tribology of self-assembled monolayers

Tribological properties of materials and interfaces on the micron scale have not been investigated until recently. With the development of microelectromechanical systems (MEMS) further understanding of frictional processes on the micron scale is needed. MEMS are a billion dollar a year industry and significant growth is seen in the future. To ensure growth, frictional properties of MEMS need to be understood. To date, several technologies have been investigated with the goal of minimizing frictional forces experience in MEMS. All have shown utility that falls short of the needs of the MEMS community. One recent development is the utility of self-assembled monolayers (SAMs) as friction reducing coatings for MEMS. SAMs are formed from the reaction of siloxanes with silica surfaces. The atomic force microscope (AFM) has permitted microscopic studies of friction. However, due to problem with calibration of the AFM only qualitative or comparative results can be obtained. Only by studying two SAMs that have been formed adjacent to one another on a surface can we obtain quantitative or comparative friction results. We have developed a comparative technique for studying friction of SAMs using a photolithographic patterning technique. We present a validation process that verifies the reliability of the method, results that compare frictional properties of various straight chain SAMs to silica and results of frictional studies of novel SAM moieties. Results indicate that the frictional properties of SAMs are derived from a combination of several sources. The significant contributors to friction of SAMs are, dispersion interactions, dipole-dipole interactions and conformational lability of the monolayer