Temperature Dependence of Friction at the Nanoscale: When the Unexpected Turns Normal

Abstract

The dynamics of frictional motion have been studied for hundreds of years, yet many key aspects of these important processes are not understood. The main challenge in predicting frictional response is the complexity of highly non-equilibrium processes going on in any tribological contact. This includes the continuous detachment and reattachment of multiple microscopic junctions at the sliding interface, the kinetics of which are controlled by the interface temperature. Our experiments reveal a non-monotonic enhancement of dry nanoscale friction at cryogenic temperatures for different material classes. We propose a model that reproduces the experimental observations and shows that the peak in temperature dependence of friction emerges from two competing processes acting at the interface: the thermally activated formation as well as the rupturing of an ensemble of atomic contacts. Our experiments and simulations provide a direct link between the temperature and velocity dependencies of friction, thus offering a new conceptual framework to describe the dynamics of dry nanoscale friction.

Analytical Models for Atomic Friction

“…An interpretation of the friction peak was proposed by Barel et al [30] who argued that, in addition to thermal activation from the energy barrier, there also exist thermally activated formation and rupturing of multiple contacts, the competition between which can explain the force peak. An enhanced PT model incorporating the effect of multiple contacts was successfully applied to phenomenologically verify this mechanism [30,31]. This effect cannot be captured by a single contact PT model such as that used in this article, so no force peak is observed in the results presented here.…”

Effect of Temperature on the Friction and Wear of PTFE by Atomic-Level Simulation

“…Trends for the corresponding adhesive force between the surfaces were less clear. Other findings by the same authors [23] showed that in non-polymeric systems, temperature-independent friction below 220 K was related to the onset of wear process. Other sources [26,27] suggested that for polymeric systems, nanoscale friction depends strongly on the relaxation dynamics of the polymer chains close to the glass transition temperature.…”

“…A logical extension of this hypothesis suggests a reduction in friction with increasing temperature. Additionally, in studies ranging from macro-to nanoscale tribometry to numerical simulations [19,20,22], peak-like enhancement of nanoscale friction at cryogenic temperatures has been demonstrated for different classes of hard materials, including amorphous, crystalline and layered surfaces [23].…”

“…Primarily, two theoretical models have been proposed in the literature to explain the aforementioned variations in mean friction force as a function of temperatures: (1) the Prandtl-Tomlinson model [29,30] and (2) the mechanokinetic model which describes friction through a thermally activated rupture and formation of molecular contacts [23,25]. In the former, there is pronounced stick-slip behavior in the mean friction force over a large temperature range.…”

Current trends in the physics of nanoscale friction

“…Simulations performed within the Prandtl-Tomlinson (PT) model reveal that temperature can affect the slip length resulting in a nonmonotonic temperature dependence of friction [86]. Simulations best representing the experimental conditions show that this dependence emerges from two competing processes acting at the interface: the thermally activated formation and the rupturing of an ensemble of atomic contacts [85,87]. In addition, a new competing mechanisms due to athermal instability inherent in AFM measurement has been proposed [88].…”