The temperature dependence of the kinetic friction between a highly oriented pyrolytic graphite surface and a silicon nitride probe tip has been evaluated through atomic force microscopy measurements performed under an ultra-high vacuum environment over the temperature range 140–750 K. As temperature increases from 140 to 400 K a sharp decrease in friction is observed. A relatively weaker dependence on temperature is observed in the friction measured between 400 and 750 K. Collectively, these results obtained from a fundamental interface are consistent with an activated mechanism of energy dissipation during sliding.

Thermally Activated Friction

Polytetrafluoroethylene (PTFE) is widely regarded as an excellent candidate for solid lubrication in vacuum. However, it is often precluded from many prac- tical applications due to its intrinsically high wear rate. Over the past decade, it has been discovered that small loading fractions of alumina nanofillers can increase the wear resistance of PTFE by three to four orders of mag- nitude. This dramatic increase in wear resistance has in turn prompted numerous tribological studies to examine the robustness of this performance. In this study, the wear and friction behavior of unfilled PTFE and PTFE and alumina nanocomposites were evaluated under a broad range of vacuum environments from 760 to 4 9 10 -6 Torr. The nanocomposites of PTFE/alumina showed a dramatic increase in wear of over two orders of magnitude at the highest vacuum conditions. There appears to be an optimal vacuum environment around 1-10 Torr, in which these samples achieved the lowest wear rates of approximately 2.5 9 10 -7 mm 3 /(Nm).

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In Vacuo Tribological Behavior of Polytetrafluoroethylene (PTFE) and Alumina Nanocomposites: The Importance of Water for Ultralow Wear

Atomic scale frictional forces encountered as a function of temperature for the contact of a Si3N4 probe tip and the basal plane of MoS2 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 approximately 0.3 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.

Transition from thermal to athermal friction under cryogenic conditions.

Studies to explore the nature of friction, and in particular thermally activated friction in macroscopic tribology, have lead to a series of experiments on thin coatings of molybdenum disulfide. Coatings of predominately molybdenum disulfide were selected for these experiments; five different coatings were used: MoS2/Ni, MoS2/Ti, MoS2/Sb2O3, MoS2/C/Sb2O3, and MoS2/Au/Sb2O3. The temperatures were varied over a range from −80 °C to 180 °C. The friction coefficients tended to increase with decreasing temperature. Activation energies were estimated to be between 2 and 10 kJ/mol from data fitting with an Arrhenius function. Subsequent room temperature wear rate measurements of these films under dry nitrogen conditions at ambient temperature demonstrated that the steady-state wear behavior of these coatings varied dramatically over a range of K = 7 × 10−6 to 2 × 10−8 mm3/(Nm). It was further shown that an inverse relationship between wear rate and the sensitivity of friction coefficient with temperature exists. The highest wear-rate coatings showed nearly athermal friction behavior, while the most wear resistant coatings showed thermally activated behavior. Finally, it is hypothesized that thermally activated behavior in macroscopic tribology is reserved for systems with stable interfaces and ultra-low wear, and athermal behavior is characteristic to systems experiencing gross wear.

A Possible Link Between Macroscopic Wear and Temperature Dependent Friction Behaviors of MoS2 Coatings

Wetting transitions and their closely related cousins of capillary condensation and filling are found all around us. This article provides an overview of the key ideas and issues related to these phenomena as they have emerged in the past few decades. Both continuous and first order wetting transitions and their analogues for condensation and filling are covered, and some new work on two-dimensional wetting associated with prewetting is presented. A glimpse into the promising future of this field ends the presentation.

Wetting, Capillary Condensation and More

We have constructed a sliding block cryotribometer designed to measure friction at cryogenic temperatures and UHV conditions. An optical cryostat and high speed video was used to monitor the motion of the block. The temperature range of the measurements was 4–460 K. Both the static and dynamic friction coefficients were determined for a steel ball on stainless steel, sapphire, and PTFE (Teflon) plates, and for a sapphire ball on a sapphire plate. For all material combinations, no systematic variation of friction with temperature was observed in the temperature range 4–200 K.

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Temperature dependence of friction under cryogenic conditions in vacuum

We have developed a cryogenic pulsed laser deposition (PLD) system to deposit lithium films onto a quartz crystal microbalance (QCM) at 4 K. Adsorption isotherms of 4He on lithium were measured in the temperature range between 1.42 K and 2.5 K. The isotherms are qualitatively different from isotherms on strong substrates such as gold and weak substrates such as cesium. There is no evidence of the formation of solid-like layers of helium, and the helium coverage is approximately linear in the pressure over a wide range. By measuring the low coverage slope of the isotherms, the binding energy of helium to lithium was found to be approximately −13.6 K. For lithium substrates less than approximately 100 layers thick, the chemical potential at which the superfluid transition was observed was surprisingly sensitive to the details of lithium deposition.

J. E. Rutledge

Papers

Temperature dependence of friction under cryogenic conditions in vacuum

Helium Adsorption on Lithium Substrates