Materials Science

Slippery When Wet

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Science  04 Jul 2008:
Vol. 321, Issue 5885, pp. 16-17
DOI: 10.1126/science.321.5885.16d

Diamond has low friction and wear, particularly in humid environments, but the cause of this behavior is an issue of debate. One idea is that the bonds rehybridize to an ordered sp2 form, which is consistent with graphite being the thermodynamically stable allotrope at room temperature and pressure; graphite is also an excellent lubricant because of its layered structure. An alternative idea is that the surface becomes passivated, which is consistent with data that show lower wear and friction for diamond in hydrous or H2 atmospheres compared to experiments in vacuum. To explore this question, Konicek et al. created films of ultrananocrystalline diamond (UNCD), which has an extremely smooth surface and shares many of the properties of large-grained or single-crystal diamond films. Spheres coated with UNCD were rubbed against the films, either at high or low loading and high or low humidity, and the wear tracks were measured and compared with the unworn areas. The most significant wear damage occurred under high loading/low humidity conditions, which also exhibited an initially higher friction coefficient (though all four systems showed similar steady-state values). A number of techniques failed to reveal the presence of graphitic bonding, indicating that rehybridization effects were negligible and that it is rapid passivation of dangling bonds that is responsible for the low friction and wear of diamond. — MSL

Phys. Rev. Lett. 100, 235502 (2008).

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