Here, we report on the lubricating effects of self-assembled monolayers (SAMs) on MEMS by measuring static and dynamic friction with two polysilicon surface-micromachined devices. The first test structure is used to study friction between laterally sliding surfaces and with the second, friction between vertical sidewalls can be investigated. Both devices are SAM-coated following the sacrificial oxide etch and the microstructures emerge released and dry from the final water rinse. The coefficient of static friction, ps, was found to decrease from 2.1 f 0.8 for the SiOl coating to 0.1 1 f 0.01 and 0.10 k 0.01 for films derived from octadecyltrichloro-silane (OTS) and 1 H, 1 H,2H,2H-perfluorodecyltrichlorosilane (FDTS). Both OTS and FDTS SAM-coated structures exhibit dynamic coefficients of friction, pd, of 0.08 f 0.01. These values were found to be independent of the apparent contact area, and remain unchanged after 1 million impacts at 5.6 pN (17 kPa), indicating that these SAMs continue to act as boundary lubricants despite repeated impacts. Measurements during sliding friction from the sidewall friction testing structure give comparable initial p d values of 0.02 at a contact pressure of 84 MPa. After 15 million wear cycles, p d was found to rise to 0.27. Wear of the contacting surfaces was examined by SEM. Standard deviations in the p data for SAM treatments indicate uniform coating coverage.
Electrophoresis can be employed to deposit a wide variety of materials including MoS2 coatings that exhibit properties comparable to the properties of sputtered MoS2 coatings used as lubricants for vacuum applications. Coatings which display coefficients of friction as low as 0.03 can be deposited from an aqueous suspension containing approximately 2.5 wt %, micrometer-sized MoS2 particles stablized with approximately 500 ppm of a nonionic surfactant. Uniform coatings with appropriate thicknesses can be deposited on nonplanar surfaces in minutes with minimal equipment. The morphology of the as-deposited coatings suggest that MoS2 is electrophoretically active as a result of a net positive surface charge along the basal plane.
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