Impact of Substrate Corrugation on the Sliding Friction Levels of Adsorbed Films

We report a quartz crystal microbalance (QCM) study of sliding friction for solid xenon monolayers at 77 K on Cu(111), Ni(111), $\mathrm{\text{graphene}}/\mathrm{Ni}(111)$, and ${\mathrm{C}}_{60}$ substrates. Simulations have predicted a strong dependence of phononic friction coefficient ($\eta$) on surface corrugation in systems with similar lattice spacing, $\eta \propto U_0^2$, but this has never before been shown experimentally. In order to make direct comparisons with theory, substrates with similar lattice spacing but varying amplitudes of surface corrugation were studied. QCM data reveal friction levels proportional to $U_0^2$, validating current theoretical and numerical predictions. Measurements of $\mathrm{Xe}/{\mathrm{C}}_{60}$ are also included for comparison purposes.

Figure 1

Xenon mass uptake and quality factor shift (raw data) for Cu(111) (triangles), Ni(111) (squares), and $\mathrm{\text{graphene}}/\mathrm{Ni}(111)$ (circles).

Figure 2

The slip times for xenon on Cu(111) (triangles), Ni(111) (squares), and $\mathrm{\text{graphene}}/\mathrm{Ni}(111)$ (circles) vs coverage. The coverage (corrected for slip effects) is obtained by solving the right expression in Eq. (2) for $(\delta f_{\mathrm{film}})$ and then substituting the value into the left expression in Eq. (2). The frequency shift of a monolayer of xenon is calculated from the xenon mass and spacings (coverage) given in Table . Compressed monolayer coverage is $6\mathrm{\text{atoms}}/{\mathrm{nm}}^2$, corresponding to $1.31\times {10}^{-12}\mathrm{ng}/{\mathrm{nm}}^2$.

Figure 3

(a) Xenon mass uptake and quality factor shift (raw data) for ${\mathrm{C}}_{60}/\mathrm{Ag}(111)$. (b) Slip time for xenon sliding on ${\mathrm{C}}_{60}/\mathrm{Ag}(111)$. Note that the slip time is zero for coverages less than $0.01\mathrm{\text{atoms}}/{\mathrm{\text{angstrom}}}^2$.

Figure 4

Slip time vs surface corrugation. The corrugation value for $\mathrm{Xe}/\mathrm{Ag}(111)$ is estimated here as 1.7 meV. The dashed lines around the ${\mathrm{C}}_{60}$ represent the uncertainty in slip time. The corrugation of $\mathrm{Xe}/{\mathrm{C}}_{60}$ is unknown.