Transition from Thermal to Athermal Friction under Cryogenic Conditions

Atomic scale frictional forces encountered as a function of temperature for the contact of a ${\mathrm{Si}}_3{\mathrm{N}}_4$ probe tip and the basal plane of ${\mathrm{MoS}}_2$ have been measured with atomic force microcopy over the temperature range 100–500 K. Friction is observed to increase exponentially with decreasing temperature from 500 to 220 K. An Arrhenius analysis of the temperature dependent friction over this range yields an effective activation energy of $\sim 0.3\mathrm{eV}$ for the thermally activated stick-slip motion of the probe tip on this surface. As temperature is reduced further below 220 K, a distinct transition to a largely athermal behavior is detected and is shown to result from the onset of interfacial wear, entailing an alternative energy dissipation pathway.

Figure 1

(a) Lateral force image of ${\mathrm{MoS}}_2$ at 260 K. Scan size: $5\mathrm{nm}\times 5\mathrm{nm}$. Applied load: 4 nN. (b) Lateral force loops acquired at 200 K (black) and 260 K (red) reflecting the resolved stick-slip motion of the tip across the surface. Although the stick-slip motion of the tip is still resolved at 200 K, the friction loop reflects a significant increase in friction.

Figure 2

(a) Kinetic friction force, averaged over sliding distance and direction, measured on ${\mathrm{MoS}}_2$ at 0 nN applied load as a function of surface temperature. (Inset) Average kinetic friction as a function of applied load at 200 K. (b) Microscopic friction coefficients derived from the plots of friction versus applied load (0–20 nN) data as a function of temperature. (Inset) Associated adhesion data (with standard deviations) as a function of sample temperature. (c) An Arrhenius analysis of normalized friction data in (a) over the temperature range 220–500 K. (d) An Arrhenius analysis of the friction coefficient data in (b) in the temperature range 220–500 K. Both analyses render an activation energy of $\sim 0.3\mathrm{eV}$.

Figure 3

STM images of (a) atomically flat ${\mathrm{MoS}}_2$ and (b) lightly sputtered ${\mathrm{MoS}}_2$ surfaces. Scan size, $300\times 300{\mathrm{nm}}^2$; bias, $-0.2\mathrm{V}$; tunneling current, 0.2 nA. The height of the step running across the flat surface is 0.6 nm; maximum feature heights on the sputtered surface are only 0.4 nm. (c) Average kinetic friction forces measured as a function of temperature on the sputtered surface exhibit a marked difference in temperature dependence from that displayed in Fig. 2a, underscoring the influence of atomic scale wear on such friction measurements.