Low friction states for thin solid lubricant film of MoS₂

Abstract

Purpose During the operation of Wendelstein 7-X (W7-X), any mechanical disturbance such as stick-slip may cause quenching of superconducting (SC) coils. The friction behavior of MoS2 lubrication (thin film) for narrow support elements between the SC coils in W7-X is rather important, as there is a design requirement for a coefficient of friction (COF) 0.05 between the sliding surfaces to control the stress contribution (from friction). Design/methodology/approach The author has carried out intensive calibrations or verifications using verified models considering previous friction tests on various samples which measured the COF in 4.2 K, 77 K and room temperature conditions (at high vacuum) to simulate the actual working condition. Findings The author has given useful explanations and diagnosis for previous anomalous scattered data. To improve the performance of MoS2, the author has predicted its better COF (0.002 via tuning of the activation volume), which could be a superlubricating state for MoS2 thin films considering the long-term operation requirement W7-X. Originality/value In this paper, the author has adopted Eyring’s approach to predict the low COF (0.002 via tuning of the activation volume), which could be a superlubricating state for MoS2 thin films considering the long-term operation requirement W7-X. Finally, some recent progresses about the possible few-layer MoS2 role in the electromagnetic loads have been provided.

Comments on ‘The abrupt ending of a spinning disk’

“…The latter could occur in [1] during the experiment performed by Cross. Some researchers have modeled this atomic-scale wear behavior (via the transition state theory or absolute reaction approach [11][12][13]) as a thermally activated bond-breaking process. To give a pedagogical example, considering the calculation of the coefficient of friction (F Y /N [1]: temperature-dependent [13]; N the vertical reaction force at the contact point, F Y the horizontal friction force at the contact point [1]) we have the temperature-dependent shearing stress (τ the friction force divided by the corresponding area [13])…”

“…where ΔE a is the activation energy (for a microscopic barrier), V k is the activation volume, k B is the Boltzmann constant, T is the temperature, z  is the temperature-dependent shear strain rate, I 0 is a constant ratio for the shear strain rate and the transition rate and ω 0 is microscopically an attempt frequency [11][12][13]. To estimate the temperature regime during the experiment in [1] considering the steel disk we remind the readers to trace [8].…”