Origin of Ultralow Friction and Wear in Ultrananocrystalline Diamond

The impressively low friction and wear of diamond in humid environments is debated to originate from either the stability of the passivated diamond surface or sliding-induced graphitization/rehybridization of carbon. We find ultralow friction and wear for ultrananocrystalline diamond surfaces even in dry environments, and observe negligible rehybridization except for a modest, submonolayer amount under the most severe conditions (high load, low humidity). This supports the passivation hypothesis, and establishes a new regime of exceptionally low friction and wear for diamond.

Figure 1

Friction data from four wear tracks made with either 1.0 N (solid line) or 0.1 N (dashed line) load, and 1.0% (black line) or 50% (gray line) RH. (inset) Zoom of the last 1000 cycles showing low, steady-state friction. The two dashed curves fall on top of one another.

Figure 2

(a) X-PEEM image at 289 eV inside the $HD$ track. (b) C $1s$ spectra, offset for clarity. The top spectrum (black line) is a reference taken far from the wear track, and the bottom spectrum (gray line) is from the region drawn in (a). Inset: magnified plot of the spectra from (b) and a spectrum from track $LW$, in the $1s\to {\pi }^{*}$ region.

Figure 3

A comparison of NEXAFS spectra. Top: experimental data from the most heavily worn region of the $HD$ wear track (solid black line) and a simulation for one monolayer of graphite on UNCD (solid gray line). Bottom: spectrum from hydrogenated amorphous carbon (dashed black line) and subtraction of an unworn UNCD spectrum from the $HD$ track spectrum (dashed gray line).