Nano- and Macroscale Evidence of Thermally Activated Friction

The recent availability of very low wear materials that undergo primarily interfacial sliding has enabled studies that more directly probe the fundamental aspects of friction. This paper reports on variable temperature experiments that were conducted with common polymeric and lamellar solid lubricant materials. Friction coefficient consistently increased with decreased temperature well into the cryogenic temperature regime in a manner consistent with the proposed notion of thermally activated friction. As wear and deformation components increased, friction coefficients became larger and less thermally sensitive. AFM experiments on the nanometer length-scale showed large relative increases in friction coefficient and a transition to athermal behavior when the friction forces became high enough to induce tip wear. Findings from these studies, which span wide length and timescales, consistently suggest that thermally activated barriers to sliding constitute a fundamental component of friction. INTRODUCTION There are many applications where the use of a solid lubricant is needed to survive the operational temperature extremes, yet the effects of changing temperature on the friction coefficients of these materials remain unclear. There are both practical and scientific needs to study the temperature dependent friction coefficients of common solid lubricants. Polymer friction has been widely studied at temperatures above room temperature and the data are consistent with viscoelastic behavior (reduced friction at elevated temperature and reduced speed) [1-6]. The cryogenic friction literature [79], on the other hand, which primarily contains data collected at or below 77K (often submerged in the liquid cryogen), shows reduced friction compared to that collected during ambient testing in open air. These data suggest a cryogenic specific friction reduction mechanism such as increased hardness [8]. Currently, there is a bifurcation in the trends of friction with changing temperature, but neither the cause nor the temperature at which it occurs are known. In addition, the entire aerospace-relevant temperature regime from 77-273K remains essentially unstudied. This paper reports on drysliding tribological studies of solid lubricants over this critical range of temperatures. Several experiments, materials, environments and length-scales are discussed. EXPERIMENTAL Macroscale friction experiments were conducted using two different pin-on-flat tribometers with PTFE-based and MoS2based solid lubricants. Ambient pressure experiments used impinging jets of dry nitrogen (LN2 boil-off) to cool the disk and evaporate ice contamination. The experimental setup is described in detail in McCook et al. [10]. A linear reciprocating tribometer was constructed in a vacuum chamber for operation at pressures below 10 torr; operation in vacuum further reduces potential for ice contamination. The design of the tribometer and uncertainties closely follow those described in Schmitz et al. [11, 12]. The counterface is mounted to a copper block that is directly heated with a PID controlled heater and cooled with flexible copper braids that are chilled with liquid nitrogen. The pin and counterface were thermally insulated from the rest of the system using Polyetheretherketone (PEEK) sample holders. Temperature dependent friction was studied at a more fundamental level using Molecular Dynamics simulation (MD) and Atomic Force Microscopy (AFM) techniques. The MD experiments were conducted on a 4.5 nm cubic PTFE system that is described in Jang et al. [13]. AFM experiments were