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Mechanochemistry of zinc dialkyldithiophosphate on steel surfaces under elastohydrodynamic lubrication conditions

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Title: Mechanochemistry of zinc dialkyldithiophosphate on steel surfaces under elastohydrodynamic lubrication conditions
Authors: Zhang, J
Ewen, JP
Ueda, M
Wong, JSS
Spikes, HA
Item Type: Journal Article
Abstract: Zinc dialkyldithiophosphate (ZDDP) is added to engine lubricants to reduce wear and ensure reliable operation. ZDDP reacts under rubbing conditions to form protective zinc/iron phosphate tribofilms on steel surfaces. Recently, it has been demonstrated that this process can be promoted by applied stresses in lubricated contacts, as well as temperature, and is thus mechanochemical in origin. In this study, a tribology test rig capable of applying very high loads has been developed to generate ZDDP tribofilms under full-film elastohydrodynamic lubrication (EHL) conditions in steel/steel ball-on-disk contacts. This provides a well-defined temperature and stress environment with negligible direct asperity contact in which to study mechanochemical processes. ZDDPs with branched primary and secondary alkyl substituents have been studied in three base oils, two with high EHL friction and one with low EHL friction. In the high EHL friction base oils, the tribofilm growth rate increases exponentially with shear stress and temperature for both ZDDPs, as predicted by a stress augmented thermal activation model. Conversely, under otherwise identical conditions, negligible ZDDP tribofilm formation takes place in the low EHL friction base oil. This confirms that the ZDDP reaction is driven by macroscopic shear stress rather than hydrostatic pressure. The secondary ZDDP forms tribofilms considerably faster than the primary ZDDP under equivalent conditions, suggesting that the initial decomposition reaction is the rate determining step for tribofilm formation. The rate of tribofilm growth is independent of ZDDP concentration over the range studied, indicating that this process follows zero-order kinetics. Under full-film EHL conditions, ZDDP tribofilm formation is promoted by macroscopic shear stress applied through the base oil molecules, which induces asymmetric stress on adsorbed ZDDP molecules to promote their decomposition and initiate rapid phosphate polymerisation.
Issue Date: 5-Feb-2020
Date of Acceptance: 8-Jan-2020
URI: http://hdl.handle.net/10044/1/75857
DOI: 10.1021/acsami.9b20059
ISSN: 1944-8244
Publisher: American Chemical Society (ACS)
Start Page: 6662
End Page: 6676
Journal / Book Title: ACS Applied Materials & Interfaces
Volume: 12
Issue: 5
Copyright Statement: © 2020 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.9b20059
Sponsor/Funder: Engineering and Physical Sciences Research Council
Engineering & Physical Science Research Council (EPSRC)
Funder's Grant Number: EP/P030211/1
Keywords: Science & Technology
Technology
Nanoscience & Nanotechnology
Materials Science, Multidisciplinary
Science & Technology - Other Topics
Materials Science
Mechanochemistry
tribology
wear
additives
lubricants
zinc dialkyldithiophosphate
tribofilm
FRICTION-ENHANCING PROPERTIES
NANOMECHANICAL PROPERTIES
IN-SITU
PART I
THERMAL-DECOMPOSITION
FILMS
MECHANISMS
TRIBOLOGY
TEMPERATURE
ACTIVATION
Mechanochemistry
additives
lubricants
tribofilm
tribology
wear
zinc dialkyldithiophosphate
03 Chemical Sciences
09 Engineering
Nanoscience & Nanotechnology
Publication Status: Published
Article Number: acsami.9b20059
Online Publication Date: 2020-01-08
Appears in Collections:Mechanical Engineering
Faculty of Natural Sciences
Faculty of Engineering