On the Mechanism of ZDDP Antiwear Film Formation

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Title: On the Mechanism of ZDDP Antiwear Film Formation
Authors: Zhang, J
Spikes, HA
Item Type: Journal Article
Abstract: © 2016, The Author(s).Zinc dialkyldithiophosphate additives are used to control wear and inhibit oxidation in almost all engine oils as well as many other types of lubricant. They limit wear primarily by forming a thick, protective, phosphate glass-based tribofilm on rubbing surfaces. This film formation can occur at low temperatures and is relatively indifferent to the chemical nature of the substrate. There has been considerable debate as to what drives ZDDP tribofilm formation, why it occurs only on surfaces that experience sliding and whether film formation is controlled primarily by temperature, pressure, triboemission or some other factor. This paper describes a novel approach to the problem by studying the formation of ZDDP films in full film EHD conditions from two lubricants having very different EHD friction properties. This shows that ZDDP film formation does not require solid–solid rubbing contact but is driven simply by applied shear stress, in accord with a stress-promoted thermal activation model. The shear stress present in a high-pressure contact can reduce the thermal activation energy for ZDDP by at least half, greatly increasing the reaction rate. This mechanism explains the origins of many practically important features of ZDDP films; their topography, their thickness and the conditions under which they form. The insights that this study provides should prove valuable both in optimising ZDDP structure and in modelling ZDDP antiwear behaviour. The findings also highlight the importance of mechanochemistry to the behaviour of lubricant additives in general.
Issue Date: 29-Jun-2016
Date of Acceptance: 13-Jun-2016
ISSN: 1573-2711
Publisher: Springer Verlag
Journal / Book Title: Tribology Letters
Volume: 63
Copyright Statement: © The Author(s) 2016. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (, which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Sponsor/Funder: Engineering & Physical Science Research Council (EPSRC)
Funder's Grant Number: EP/G026114/1
Keywords: Mechanical Engineering & Transports
0912 Materials Engineering
0913 Mechanical Engineering
Publication Status: Published
Article Number: 24
Appears in Collections:Faculty of Engineering
Mechanical Engineering
Faculty of Natural Sciences

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