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Macroscale superlubricity and polymorphism of long-chain n-alcohols

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Title: Macroscale superlubricity and polymorphism of long-chain n-alcohols
Authors: Reddyhoff, T
Ewen, J
Deshpande, P
Frogley, M
Welch, M
Montgomery, W
Item Type: Journal Article
Abstract: Simple n-alcohols, such as 1-dodecanol, show anomalous film-forming and friction behaviors under elastohydrodynamic lubrication (EHL) conditions, as found inside bearings and gears. Using tribometer, diamond anvil cell (DAC), and differential scanning calorimetry (DSC) experiments, we show that liquid 1-dodecanol undergoes a pressure-induced solidification when entrained into EHL contacts. Different solid polymorphs are formed inside the contact depending on the temperature and pressure conditions. Surprisingly, at a moderate temperature and pressure, 1-dodecanol forms a polymorph that exhibits robust macroscale superlubricity. The DAC and DSC experiments show that superlubricity is facilitated by the formation of lamellar, hydrogen-bonded structures of hexagonally close-packed molecules, which promote interlayer sliding. This novel superlubricity mechanism is similar to that proposed for the two-dimensional materials commonly employed as solid lubricants, but it also enables the practical advantages of liquid lubricants to be maintained. When the pressure is increased, 1-dodecanol undergoes a polymorphic transformation into a phase that gives a higher friction. The DAC and DSC experiments indicate that the high-friction polymorph is an orthorhombic crystal. The polymorphic transformation pressure coincides with the onset of a dimple formation in the EHL films, revealing that the anomalous film shapes are caused by the formation of rigid orthorhombic crystals inside the contact. This is the first demonstration of a macroscale superlubricity in an EHL contact lubricated by a nonaqueous liquid that arises from bulk effects rather than tribochemical transformations at the surfaces. Since the superlubricity observed here results from phase transformations, it is continuously self-replenishing and is insensitive to surface chemistry and topology. This discovery creates the possibility of implementing superlubricity in a wide range of machine components, which would result in enormous improvements in efficiency and durability.
Issue Date: 24-Feb-2021
Date of Acceptance: 2-Feb-2021
URI: http://hdl.handle.net/10044/1/87616
DOI: 10.1021/acsami.0c21918
ISSN: 1944-8244
Publisher: American Chemical Society
Start Page: 9239
End Page: 9251
Journal / Book Title: ACS Applied Materials and Interfaces
Volume: 13
Issue: 7
Copyright Statement: © 2021 American Chemical Society. This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Applied Materials and Interfaces, after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acsami.0c21918
Sponsor/Funder: Royal Academy Of Engineering
Funder's Grant Number: RF\201920\19\269
Keywords: Science & Technology
Nanoscience & Nanotechnology
Materials Science, Multidisciplinary
Science & Technology - Other Topics
Materials Science
elastohydrodynamic lubrication
elastohydrodynamic lubrication
Nanoscience & Nanotechnology
03 Chemical Sciences
09 Engineering
Publication Status: Published
Online Publication Date: 2021-02-10
Appears in Collections:Mechanical Engineering
Earth Science and Engineering