Film Forming and Friction Properties of Sulphur-Free Antiwear Additives Zinc Dialkylphosphates (ZDPs)
Author(s)
Hoshino, Koji
Type
Thesis
Abstract
ZDPs (zinc dialkylphosphates) are sulphur-free analogues of the conventional antiwear
additives ZDDPs (zinc dialkyldithiophosphates). Recently, the application of ZDPs in
practical use has been investigated as an alternative of ZDDPs. The key benefit of ZDPs is
that they do not contain sulphur which may damage some catalysts in after-treatment exhaust
systems. Utilizing ZDPs instead of ZDDPs contributes to decrease in sulphur content in
engine oils. In addition, ZDP-formulated oils show better base number retention than ZDDPs,
which could contribute to prolongation of engine oil life. This is because ZDPs do not
generate sulphuric acid in their decomposition process, while ZDDPs generate sulphuric acid
which consumes detergents. However there is still little known about the mechanism of action
of ZDPs as antiwear additives. Therefore, the purpose of this study is to clarify how ZDP
antiwear additives work by investigating the film forming and friction properties of ZDPs.
The main technique employed was MTM-SLIM (mini traction machine - spacer layer
interferometry). This machine can measure friction properties under various rubbing
conditions and monitor film formation by additives during rubbing tests.
This study has shown that ZDPs form stable reaction films by rubbing, which protect metal
surfaces from wear. This is fundamentally similar to the mechanism for antiwear performance
of ZDDPs. However, some differences between ZDPs and ZDDPs are seen in their filmforming
properties, which may affect their antiwear performances. For example, the filmforming
rates of ZDPs are slower than those of ZDDPs at the initial stage. Stabilized films
formed by ZDPs are thinner than films formed by ZDDPs. By contrast, ZDPs show similar
friction properties to ZDDPs. In the boundary and mixed lubrication regions, there is no
noticeable difference in friction between stabilized films formed by ZDPs and ZDDPs. In
addition, these properties are largely affected by various factors such as rubbing conditions,
concentrations, and alkyl structures of the additives. For example, ZDPs having linear alkyl
chains show slower film formation and lower boundary friction than ZDPs having branched
alkyl chains. Moreover, other additives such as dispersants and detergents largely affect the
properties of ZDPs. Generally, highly polar additives appear to retard the film formation of
ZDPs. The mechanisms for the above features are also discussed in this thesis.
additives ZDDPs (zinc dialkyldithiophosphates). Recently, the application of ZDPs in
practical use has been investigated as an alternative of ZDDPs. The key benefit of ZDPs is
that they do not contain sulphur which may damage some catalysts in after-treatment exhaust
systems. Utilizing ZDPs instead of ZDDPs contributes to decrease in sulphur content in
engine oils. In addition, ZDP-formulated oils show better base number retention than ZDDPs,
which could contribute to prolongation of engine oil life. This is because ZDPs do not
generate sulphuric acid in their decomposition process, while ZDDPs generate sulphuric acid
which consumes detergents. However there is still little known about the mechanism of action
of ZDPs as antiwear additives. Therefore, the purpose of this study is to clarify how ZDP
antiwear additives work by investigating the film forming and friction properties of ZDPs.
The main technique employed was MTM-SLIM (mini traction machine - spacer layer
interferometry). This machine can measure friction properties under various rubbing
conditions and monitor film formation by additives during rubbing tests.
This study has shown that ZDPs form stable reaction films by rubbing, which protect metal
surfaces from wear. This is fundamentally similar to the mechanism for antiwear performance
of ZDDPs. However, some differences between ZDPs and ZDDPs are seen in their filmforming
properties, which may affect their antiwear performances. For example, the filmforming
rates of ZDPs are slower than those of ZDDPs at the initial stage. Stabilized films
formed by ZDPs are thinner than films formed by ZDDPs. By contrast, ZDPs show similar
friction properties to ZDDPs. In the boundary and mixed lubrication regions, there is no
noticeable difference in friction between stabilized films formed by ZDPs and ZDDPs. In
addition, these properties are largely affected by various factors such as rubbing conditions,
concentrations, and alkyl structures of the additives. For example, ZDPs having linear alkyl
chains show slower film formation and lower boundary friction than ZDPs having branched
alkyl chains. Moreover, other additives such as dispersants and detergents largely affect the
properties of ZDPs. Generally, highly polar additives appear to retard the film formation of
ZDPs. The mechanisms for the above features are also discussed in this thesis.
Date Issued
2010-10
Date Awarded
2011-03
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Spikes, Hugh
Sponsor
JX Nippon Oil & Energy Corporation
Creator
Hoshino, Koji
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)