PerspectiveApplied Physics

Why Thick Can Be Slick

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Science  02 Apr 2010:
Vol. 328, Issue 5974, pp. 52-53
DOI: 10.1126/science.1188086

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A main impediment to the development and commercialization of micro- and nanoelectromechanical systems has been the difficulty in effectively lubricating the small-scale moving parts. Greasing the sliding interfaces in the usual way, with liquids, fails because they are readily squeezed out of the contact region. The resulting friction and wear have catastrophic consequences for the functionality of the device. To keep the lubricant within the contact region, molecules can be functionalized so that they self-assemble into a protective and strongly adhering coating on the surface. But problems still remain, primarily insufficient antiwear performance and the difficulty of scaling up the coating process to the size of a wafer (1). An alternative strategy based on solid lubricants such as graphite or layered MoS2 is already being used for macroscopic systems (2), but thickness becomes a variable in ways that would not be seen with liquid lubricants. On page 76 of this issue, Lee et al. (3) unravel a systematic trend of friction with the thickness of the solid lubricant that, surprisingly, is a general effect independent of the material.