Tribochemical investigation of ZDDP tribofilm
File(s)
Author(s)
Ueda, Mao
Type
Thesis
Abstract
The current trend for using lower-viscosity lubricants with the aim of improving fuel
efficiency of mechanical systems means that machine components are required to operate
for longer periods in thin oil film, mixed and boundary lubrication conditions, where the
risk of surface damage is increased. For this reason, the role of tribofilms generated from
the antiwear additive zinc dialkyldithiophosphate (ZDDP) in providing surface protection
has become increasingly important. However, the properties, performance and the
mechanisms of tribofilm formation are not fully understood. Therefore, this thesis aims
to further understand the tribochemical behaviour of ZDDPs. Several inter-connected
areas of research are described in this thesis. These all investigate the formation of
tribofilms by ZDDP and the impact of tribofilm formation on wear. Taken together they
contribute to our understanding of the mechanisms of tribological behaviour of ZDDPs
and should assist in the design of lubricants and rubbing components.
Firstly, the evolution of ZDDP tribofilm properties, in particular, tribofilm durability and
the origins of this durability, are examined. It is found that ZDDP tribofilms undergo a
structural transformation during rubbing from a predominantly amorphous structure to
one that is nanocrystalline, resulting in the tribofilm becoming much stronger and more
durable.
Secondly, the reaction mechanisms of tribofilm formation on various non-ferrous metal
and non-metallic materials are studied, both by ion-implanting various alloying elements
into steel surfaces, and by using non-metallic rubbing materials. It is found that a
potentially important factor in the formation of ZDDP tribofilms is the presence and
concentration of ferrous and/or non-ferrous metal atoms at the surface. Non-ferrous
metals may act as adsorption sites for ZDDP in a similar manner to Fe in steel, enabling
formation of ZDDP tribofilms.
Thirdly, to further understand the antiwear performance of ZDDP, its impact on the wear
of a-C:H DLC in the presence of molybdenum dialkyldithiocarbamate (MoDTC), which
is known to be problematic, is investigated. It is well known that MoDTC can produce
very high wear of DLC and that this can be mitigated by the presence of ZDDP in the
lubricant. From this research it is shown that ZDDPs reduce DLC wear in the presence of
MoDTC mainly by forming thick antiwear tribofilms and reducing the ratio of
MoO3/MoS2 in the tribofilm.
Fourthly, although ZDDP is very effective for reducing most types of wear, it is known
that ZDDP can promote one particular form of wear, micropitting wear. In this research a
new method of studying micropitting that enables both micropitting and tribofilm
formation to be studied in parallel is developed which is key to assessing the impact of
lubricant chemistry on micropitting. This is used to show that the influence of ZDDP on
micropitting originates from its tendency to prevent running-in of the contacting surfaces.
The mechanism by which a widely-used black oxide coating limits micropitting is
explored and it is shown that this relatively soft coating provides adequate running-in
even in the presence of ZDDP-containing oils. Finally, the impact of friction on
micropitting is studied by isolating its effects from those of running-in, by controlling
separately the formation of ZDDP and MoDTC tribofilms. Results show that friction has
a very significant impact on micropitting.
efficiency of mechanical systems means that machine components are required to operate
for longer periods in thin oil film, mixed and boundary lubrication conditions, where the
risk of surface damage is increased. For this reason, the role of tribofilms generated from
the antiwear additive zinc dialkyldithiophosphate (ZDDP) in providing surface protection
has become increasingly important. However, the properties, performance and the
mechanisms of tribofilm formation are not fully understood. Therefore, this thesis aims
to further understand the tribochemical behaviour of ZDDPs. Several inter-connected
areas of research are described in this thesis. These all investigate the formation of
tribofilms by ZDDP and the impact of tribofilm formation on wear. Taken together they
contribute to our understanding of the mechanisms of tribological behaviour of ZDDPs
and should assist in the design of lubricants and rubbing components.
Firstly, the evolution of ZDDP tribofilm properties, in particular, tribofilm durability and
the origins of this durability, are examined. It is found that ZDDP tribofilms undergo a
structural transformation during rubbing from a predominantly amorphous structure to
one that is nanocrystalline, resulting in the tribofilm becoming much stronger and more
durable.
Secondly, the reaction mechanisms of tribofilm formation on various non-ferrous metal
and non-metallic materials are studied, both by ion-implanting various alloying elements
into steel surfaces, and by using non-metallic rubbing materials. It is found that a
potentially important factor in the formation of ZDDP tribofilms is the presence and
concentration of ferrous and/or non-ferrous metal atoms at the surface. Non-ferrous
metals may act as adsorption sites for ZDDP in a similar manner to Fe in steel, enabling
formation of ZDDP tribofilms.
Thirdly, to further understand the antiwear performance of ZDDP, its impact on the wear
of a-C:H DLC in the presence of molybdenum dialkyldithiocarbamate (MoDTC), which
is known to be problematic, is investigated. It is well known that MoDTC can produce
very high wear of DLC and that this can be mitigated by the presence of ZDDP in the
lubricant. From this research it is shown that ZDDPs reduce DLC wear in the presence of
MoDTC mainly by forming thick antiwear tribofilms and reducing the ratio of
MoO3/MoS2 in the tribofilm.
Fourthly, although ZDDP is very effective for reducing most types of wear, it is known
that ZDDP can promote one particular form of wear, micropitting wear. In this research a
new method of studying micropitting that enables both micropitting and tribofilm
formation to be studied in parallel is developed which is key to assessing the impact of
lubricant chemistry on micropitting. This is used to show that the influence of ZDDP on
micropitting originates from its tendency to prevent running-in of the contacting surfaces.
The mechanism by which a widely-used black oxide coating limits micropitting is
explored and it is shown that this relatively soft coating provides adequate running-in
even in the presence of ZDDP-containing oils. Finally, the impact of friction on
micropitting is studied by isolating its effects from those of running-in, by controlling
separately the formation of ZDDP and MoDTC tribofilms. Results show that friction has
a very significant impact on micropitting.
Version
Open Access
Date Issued
2021-09
Date Awarded
2022-02
Copyright Statement
Creative Commons Attribution Non-Commercial No Derivatives licence
Advisor
Spikes, Hugh
Kadiric, Amir
Sponsor
Shell Lubricants Japan K.K.
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)