[Paper Review] Super Low Traction under EHD and Mixed Lubrication Regimes

Philippe Vergne|ArXiv.org|Apr 13, 2007

Force Microscopy Techniques and Applications7 citations

TL;DR

This paper investigates super low traction in elastohydrodynamic (EHD) and mixed lubrication regimes, focusing on the limited potential for friction reduction despite advances in superlubricity. It finds that typical EHD friction coefficients (3–6%) are already very low, diminishing the expected benefit of superlubricity in macro-scale applications, which may explain the scarcity of recent studies in this area.

ABSTRACT

After the pioneered experimental works on superlubricity by Martin et al. on MoS2 [1], Hirano et al. on tungsten and silicon [2] and the further confirmation by Dienwiebel et al. on graphite [3], many groups around the word investigated the occurrence of near frictionless sliding contacts. This large mobilization of tribologists, material sciences specialists and physicists has lead to emerging solutions involving new materials and coatings, the most promising being carbon based like graphite, diamond, carbon composites or diamond-like-carbons. Some of them are currently used in practical applications. The situation is different especially in EHL: the highest friction coefficients are close to 10% when traction fluids are involved, i.e. fluids that have especially designed to transmit the highest friction, and they vary within 3-6% for the rest of lubricants. The range of variation is consequently very narrow and these typical values are really low compared to those obtained in dry contacts: as a consequence the gain expected from a super low traction regime in lubrication will be probably more limited, especially in the case of experiments conducted at the meso or macro scales. This weak perspective could be one explanation on the relatively low number of articles in recent literature dealing with lubricated superlubricity in the above conditions.

Motivation & Objective

• To assess the feasibility and potential benefit of superlubricity in elastohydrodynamic (EHD) and mixed lubrication regimes.
• To evaluate whether super low traction regimes offer significant advantages over existing low-friction lubricants in practical applications.
• To analyze the reasons behind the limited number of recent studies on lubricated superlubricity in EHD and mixed regimes.
• To compare experimental friction coefficients in EHD lubrication with those in dry or boundary contact regimes.
• To determine the practical implications of superlubricity when baseline EHD friction is already minimal.

Proposed method

• Reviewing experimental literature on superlubricity, including studies on MoS2, tungsten, silicon, and graphite.
• Analyzing reported friction coefficients in EHD and mixed lubrication regimes, particularly for traction fluids and conventional lubricants.
• Comparing the range of friction coefficients (3–6%) in EHD systems with those in dry or boundary contact conditions.
• Evaluating the theoretical and practical implications of achieving super low traction in macro-scale systems.
• Assessing the role of material selection (e.g., carbon-based coatings) in achieving low friction in lubricated contacts.
• Using data from published studies to infer the limited performance gain expected from superlubricity in EHD conditions.

Experimental results

Research questions

• RQ1 What is the range of friction coefficients observed in EHD and mixed lubrication regimes with conventional and traction fluids?
• RQ2 How does the baseline friction in EHD systems compare to that in dry or boundary contact regimes?
• RQ3 Why is there a relative scarcity of recent studies on lubricated superlubricity in EHD and mixed regimes?
• RQ4 What is the expected practical benefit of achieving super low traction in macro-scale EHD systems?
• RQ5 To what extent can superlubricity provide meaningful friction reduction when EHD systems already exhibit very low friction?

Key findings

• Friction coefficients in EHD lubrication regimes typically range from 3% to 6%, with traction fluids reaching up to 10%.
• These values are already significantly lower than those observed in dry or boundary contact regimes.
• The narrow variation range of EHD friction coefficients suggests limited room for further improvement via superlubricity.
• The minimal expected gain from superlubricity in EHD systems may explain the limited number of recent studies in this area.
• Despite promising results in dry or boundary contact systems, superlubricity offers less practical advantage in macro-scale EHD applications.
• Carbon-based materials such as graphite, diamond, and DLC show promise in superlubricity but face limited impact in EHD contexts due to already low baseline friction.