The development of micro- and nano-scale devices with moving parts continues to progress rapidly, but the issue of tribological failure remains critical. The high surface-to-volume ratio at small scales requires that nano-scale adhesion and friction be characterized and reduced. Silicon, the main material currently used in micro- and nanodevices, suffers from poor tribological properties. Possible replacement materials include is ultrananocrystalline diamond (UNCD), and diamond-like carbon (DLC). We present the first measurements of nano-scale adhesion and friction of the tribologically relevant underside of UNCD. This surface is far less adhesive than a silicon reference sample. Furthermore, UNCD can be processed to render it chemically identical to single crystal diamond, minimizing the work of adhesion to the van der Waals limit and strongly reducing friction as well. With DLC, we also measure low friction and adhesion, and have characterize their dependence on relative humidity. As the humidity varies from <5% to 60%, friction exhibits a monotonic and reversible change, increasing with higher humidity. In particular, the interfacial shear strength increases by ~40% between our two extremes in humidity. However, we observe absolutely no change in the adhesion force and correspondingly, no change in the work of adhesion. Dopants in DLC are known to modify the surface energy and bonding structure of these films. We will discuss how dopants (specifically, fluorine and silicon) affect nanoscale friction and adhesion.
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