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In this study, we are interested in the anti-wear properties of zinc dialkyl phosphate additive (ZP) in comparison with ‘classical’ zinc dialkyldithiophosphate (ZDDP). Friction tests were performed on a reciprocating tribometer using both ball-on-flat and cylinder-on-flat configurations under a Hertzian contact pressure of 0.9GPa. Experiments were carried out as a function of temperature (25 and 100°C), sliding speed (25, 50 and 100mm/s) and additives concentrations. Ball wear scar diameters as well as friction coefficient were measured. In order to better understand the anti-wear mechanisms of these additives, friction tests were followed by surface analyses such as AES (Auger Electron Spectroscopy) and XPS (X-Ray Photoelectron Spectroscopy). Transmission Electron Microscopy (TEM) observations of the ZDDP and ZP tribofilms were also carried out to visualise the generated layers. The anti-wear capability of ZP molecule is discussed. KeywordsAnti-wear additives–Boundary lubrication–Wear–AES–XPS–TEM (EDS)
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Zinc Dialkyl Phosphate (ZP) as an Anti-Wear Additive:
Comparison with ZDDP
Paule Njiwa Clotilde Minfray Thierry Le Mogne
´atrice Vacher Jean-Michel Martin
Shigeki Matsui Masaru Mishina
Received: 21 February 2011 / Accepted: 1 July 2011 / Published online: 12 July 2011
Springer Science+Business Media, LLC 2011
Abstract In this study, we are interested in the anti-wear
properties of zinc dialkyl phosphate additive (ZP)in
comparison with ‘classical’ zinc dialkyldithiophosphate
(ZDDP). Friction tests were performed on a reciprocating
tribometer using both ball-on-flat and cylinder-on-flat con-
figurations under a Hertzian contact pressure of 0.9 GPa.
Experiments were carried out as a function of temperature
(25 and 100 C), sliding speed (25, 50 and 100 mm/s) and
additives concentrations. Ball wear scar diameters as well as
friction coefficient were measured. In order to better
understand the anti-wear mechanisms of these additives,
friction tests were followed by surface analyses such as AES
(Auger Electron Spectroscopy) and XPS (X-Ray Photo-
electron Spectroscopy). Transmission Electron Microscopy
(TEM) observations of the ZDDP and ZP tribofilms were
also carried out to visualise the generated layers. The anti-
wear capability of ZP molecule is discussed.
Keywords Anti-wear additives Boundary lubrication
1 Introduction
Zinc dialkyldithiophosphate (ZDDP) is a well-known
additive used in lubricating oils because of its
multifunctional anti-wear (AW), extreme pressure (EP) and
antioxidant properties. Nowadays, the harmful effect of
ZDDP molecule on catalytic converter limits its use as an
anti-wear additive for Internal Combustion Engine (ICE)
oil. New lubricants with good tribological performances,
i.e., exhibiting low friction and low wear, are needed
regarding environmental limitations (Norm euros VI). The
idea is to reduce the levels of phosphorus and sulphur,
specific elements contained in the ZDDP molecule, at the
origin of the damage of catalytic converters.
Two options are currently investigated:
the development of systems completely different from
ZDDP molecules using for example nanoparticles
the development of additives with chemical composi-
tion close to ZDDP. The objective is to have the best
anti-wear protection while limiting the content in
phosphorus and sulphur [36] in the molecule.
This work focuses on the second option. The under-
standing of ZDDP anti-wear mechanism is important
before going further with modified molecules. A sum up of
ZDDP tribochemistry of tribofilm generation is so reported
in the following.
The literature provides three main mechanisms
explaining the decomposition of ZDDP additive molecule
within the lubricant under test conditions. This degradation
can be thermal [79], hydrolytic [10] or oxidative [11]
thanks to hydroperoxides and peroxidic radicals present in
the lubricant. Martin [12] estimates that the decomposition
of ZDDP is mainly thermo oxidative when temperature
exceeds 100 C.
During this degradation process, ZDDP molecules and
their decomposition products are adsorbed physically or
chemically on metal surfaces. The deposited film is called
P. Njiwa (&)C. Minfray T. Le Mogne B. Vacher
J.-M. Martin
´de Lyon, Ecole Centrale de Lyon, LTDS,
UMR5513, Ecully, France
S. Matsui M. Mishina
JX Nippon Oil & Energy Corporation, Lubricant Research
Laboratory, Tokyo, Japan
Tribol Lett (2011) 44:19–30
DOI 10.1007/s11249-011-9822-6
‘thermal film’, and it is further modified under rubbing
conditions to generate a protective layer called ‘tribofilm’.
Basically, ZDDP anti-wear additive decreases the wear
in a contact running under mixed or boundary lubrication
conditions thanks to this tribofilm generation (50–100 nm
thick) on rubbing surfaces [1315]. Surface analyses such
as the X-ray Photoelectron Spectroscopy (XPS) showed
that the bulk ZDDP tribofilms are mainly composed of a
mixed zinc and iron short chain (ortho or pyro) phosphate
glass with iron sulphides precipitates [16,17]. The phos-
phate chains are longer [1719] on the top of the tribofilm
than in its bulk [20]. Recently, Zhou et al. [21] assumed the
presence of ultrapolyphosphate in the outer layer. The
ZDDP thermal films have similar composition to ZDDP
tribofilms, but consisting mainly of a thinner outer layer of
polyphosphate (&10 nm thick) grading to pyro- or ortho-
phosphate in the bulk [7].
The structural evolution of tribofilm material (i.e. zinc
polyphosphate) during tribological solicitation is important
to clarify for a better understanding of ZDDP anti-wear
mechanism. To investigate structural modification of the
material thanks to the effect of hydrostatic pressure, zinc
polyphosphate was compressed in Diamond Anvil Cell
coupled with in situ Raman or EXAFS analyses [22,23].
Simulations by quantum chemistry were also carried out
[24]. Results suggest a change of coordination number of
metallic cation and no polymerisation of phosphate chains.
The effect of phosphate glass parameters on their
mechanical properties (influence of metallic cations nature,
presence of hydroxyl group on phosphate molecules etc)
was also investigated [25,26].
To insure a strong adhesion of the tribofilm to the sub-
strate and to ‘digest’ iron oxide wear particles, a tribo-
chemical mechanism was reported in literature [12]
proposing a reaction between zinc metaphosphate (repre-
sentative of the top of the thermal film) and iron oxide
(representative of the native iron oxide layer). During this
reaction, a shortening of phosphate chain length is pro-
posed and was confirmed by friction test on metaphosphate
glass [27].
Recent studies [5,28] showed that the use of zinc
orthophosphate powders (crystalline grains of a few
microns diameter) is an interesting alternative for anti-wear
organic additives. Moreover, this material is free from
sulphur and is very close in composition to the main part of
the final ZDDP tribofilm. Furthermore, it was showed that
the use of zinc orthophosphate powders as an anti-wear
additive has the advantage of being effective at the first
cycles of friction and at 25 C[5].
In our study, the additive is close to zinc orthophosphate
powder in terms of chemical composition and it has the
advantage of being soluble in the base oil thanks to alkyl
groups in the molecule. Then, the use of zinc phosphate
additive (ZP) is expected to facilitate the formation of a
phosphate base tribofilm even at ‘low’ temperature and low
concentration because it can avoid the detrimental induc-
tion period. Actually, the thermal degradation, which is
necessary for the activation of ZDDP molecule through the
generation of degradation products, is not necessary using
directly ZP additive since it is already the final tribofilm
material which is directly introduced in the contact.
The aim of this work is so to compare the anti-wear
property of ZP additive in comparison with zinc dithio-
phosphate (ZDDP).
The first part will relate to the comparison of the anti-
wear behaviour of these two molecules at 25 and 100 C.
The second part will deal with the effect of the sliding
speed on wear at 25 C. Finally, a study of the behaviour of
ZP tribochemical reaction will be carried out by coupling
friction tests with surfaces analyses (AES) at different
experiment durations. The role of the concentration is
2 Materials and Methods
2.1 Lubricants
Three lubricants were tested:
A mineral base oil of group III noted BO in the
A mixture of mineral base oil BO and zinc di-2-ethyl-
hexyl dithiophosphate additive (containing 800 ppm of
phosphorus) noted ZDDP in the following (Fig. 1a).
A mixture of mineral base oil BO and zinc di-2-ethyl-
hexyl orthophosphate (containing 800 ppm of phos-
phorus) noted ZP in the following (Fig. 1b). It is
reminded that the ZP molecule does not contain any
2.2 Materials
The balls used were 12.7 mm in radius and 30 nm (Ra
roughness. The cylinders employed were 6 mm in length
and 6 mm in diameter. The rectangular flats measured
10 9892mm
. All these specimens are made of AISI
52100 steel. This iron alloy contains 97 wt% of Fe, 1.45
R = Alkyl group (2-ethyl-hexyl)
(a) Zinc dialkyl dithiophosphate (ZDDP)(b) Zinc dialkyl phosphate ( ZP)
Fig. 1 ZDDP and ZP molecules
20 Tribol Lett (2011) 44:19–30
wt% of Cr, 1.04 wt% of C, 0.35 wt% of Mn and 0.23 wt%
of Si. The cylinder and flat specimens were polished using
diamond slurry with, respectively, 3 and 1 lm grains. The
roughness after polishing of cylinder (Ra
) and flat (Ra
are, respectively, 50 and 12 nm.
2.3 Methods
2.3.1 Tribological Parameters
Friction experiments were carried out using a home-made
reciprocating cylinder (or ball)-on-flat tribometer [29].
First, experiments were performed in the ball-on-flat con-
figuration to characterise wear behaviour of lubricants.
Second, the cylinder-on-flat configuration was used to
perform XPS surface analyses because the wear track is
larger than the size of the XPS probe. The tribofilms gen-
erated under cylinder-on-flat configuration were homoge-
neous all over the track and of same morphology (patchy) as
tribofilms generated under ball-on-flat configuration. It was
so considered that tribofilms obtained in both cases were
similar in composition (confirmed by XPS analyses not
shown here) and morphology. Because the perfect align-
ment of a cylinder on a flat is difficult, ball-on-flat config-
uration was more convenient for wear measurements.
The influence of temperature and sliding speed on
tribofilm formation was investigated. We choose temper-
atures similar to those encountered in an Internal Com-
bustion Engine such as starting in ambient condition
(25 C) and in steady-state operation (100 C).
The ball slides reciprocally on a fixed flat with a fre-
quency of 7 Hz and a stroke length of 7 mm. The applied
load for each test is 50 N corresponding to a maximum
Hertzian pressure of 928 MPa. For the cylinder-on-flat test,
the load was adjusted to obtain the same maximum
Hertzian pressure as for the ball-on-flat experiment. The
tests were repeated at least twice for each lubricant. The
friction coefficient was measured all over the test. In the
following, the average of all friction coefficient values
measured for one test is reported. Standard deviation is
calculated from the different repeated test values. The wear
scar diameter on the ball was measured by optical
microscopy. The wear tracks obtained were homogenous.
The EHL film thickness and lambda ratio are calculated
using the Hamrock Dowson formula [30] regarding various
sliding speeds and values are reported in Table 1.
2.3.2 Surface Analyses
Before any analyse, samples were degreased by rinsing in
n-heptane several times in ultrasonic bath.
The tribofilms formed on the flat (using the cylinder-
on-flat configuration) were analysed by Auger Electron
Spectroscopy (AES) and X-Ray Photoelectron spectros-
copy (XPS). Surface analyses were performed under a
pressure of 10
Pa in the analytical chamber. These
techniques provide very surface sensitive information. The
depth sensibility is different from one element to another
but it is considered to be less than or equal to 10 nm.
The AES analyses were performed using a FEG electron
gun 1000 (Thermo Scientific) -5 keV. The electron spot
size is about 1 lm, and the lateral resolution is also about
1lm. For XPS analyses, a monochromator X-ray AlKa
source was used. The X-ray probe size (rectangular) is
around 1300 lm
. The emission angle is 90with respect
to the horizontal of the sample. The detection is made by
the ESCALAB 220i (Thermo Scientific) spectrometer. The
spectrometer is calibrated in energy to the 4f7/2 electronic
level of gold (Binding energy: 84.0 eV). In a typical XPS
analysis, a survey scan is carried out first in order to
identify the different elements present in the sample. Then,
high-resolution spectra of selected peaks (characteristics of
each element) are performed. The deconvolution of these
peaks allowed an identification of the different chemical
species. Acquisition conditions for the survey spectra were
as the following: pass energy of 100 eV, dwell time of
500 ms and step size of 1.0 eV. Concerning acquisition
parameters for high-resolution spectra, they were slightly
different: pass energy of 20 eV, dwell time of 500 ms and
step size of 0.1 eV. The binding energy of carbon (C1s *
at 284.8 eV) is used as a reference for any charge
Table 1 Physical properties and rheological parameters of the
mineral base oil (BO) in different test conditions
Mineral base oil group III
Sulphur content: \0.03%w
Viscosity index [120
Density at 15 C:0.835 g/cm
Temperature (C)
25 100
Dynamic viscosity (Pa.s)
3.02. E-02 3.36.E-03
Film thickness: h
(nm) k
25 17.6 0.4 4.05 0.09
50 32.6 0.75 7.48 0.17
100 51.5 1.18 11.8 0.27
Film thickness: calculation carried out from Hamrock relation [30]
Ratio between the lubricant film thickness and the composite sur-
faces roughnesses (a)
qBall (Ra
=30 nm) and Flat (Ra
=12 nm)
Tribol Lett (2011) 44:19–30 21
Special attention has been paid for fitting P
and Zn
photopeaks. CasaXPS [31]
software was used for performing the curve fitting proce-
dures on AES and XPS spectra. For XPS, a Shirley back-
ground was used and the Lorentzian/Gaussian ratio (L/G)
was fixed at 60%.
2.3.3 Transmission Electron Microscopy (TEM)
We used a JEOL 2010F TEM operating with 200 kV
accelerating voltage and equipped with an Energy Dis-
persive X-ray spectrometer (EDX). The cross sections of
the near-surface regions of the flat were obtained by the
Focused Ion Beam (FIB) method. Before milling, platinum
and tungsten layers were deposited on the worn track to
preserve the surface from damage due to nano-machining
with Ga
ion beam.
3 Results
3.1 Friction and Wear
First, let us examine the friction and anti-wear perfor-
mances at low temperature (25 C). The average friction
coefficients measured during tests with ZDDP and ZP
lubricants at 25 C are shown in Table 2. Figure 2illus-
trates the curve of friction coefficient versus time for the
three different lubricants (BO,ZDDP and ZP)at25C
and 100 mm/s.
Wear scar diameters were measured on the balls for the
three lubricants: BO,ZDDP and ZP, and the tests were
carried out at 25 C (room temperature) with sliding speeds
of 100, 50 and 25 mm/s, respectively.
At 25 C and 100 mm/s, friction coefficient (Table 2)
obtained with ZDDP lubricant (0.119 ±0.001) is
approximately equal to the one obtained with ZP lubricant
(0.117 ±0.001). Concerning the anti-wear efficiency at
100 mm/s, Fig. 3and Table 3show the ball wear track
diameters obtained after ball-on-flat test. As we can see, at
room temperature, there is no significant difference
between the anti-wear properties for the three lubricants.
This is attributed to a predominant EHL/mixed lubrication
regime at this temperature (see elevated film thickness and
lambda ratio in Table 1). To increase contact severity, we
divided the sliding speed by two (50 mm/s) but the sliding
distance (360 m) was kept the same as for the experiment
Table 2 Friction coefficient for both the lubricant (ZDDP and ZP)at
25 and 100 C with sliding speed of 25 and 100 mm/s
Friction coefficient
Temperature (C) 25 100
Sliding speed
25 100 100
BO 0.138 ±0.000 0.095 ±0.005 0.16 ±0.02
ZP 0.116 ±0.006 0.117 ±0.001 0.105 ±0.004
ZDDP 0.121 ±0.002 0.119 ±0.000 0.085 ±0.009
Fig. 2 Friction coefficient curves as a function of time at 25 C with
sliding speed of 100 mm/s under 0.9 GPa of Hertzian pressure for
BO,ZDDP and ZP lubricants
Fig. 3 Ball wear track diameters after ball-on-flat tests at 25 C with
sliding speed of 25, 50 and 100 mm/s under 0.9 GPa of Hertzian
maximum pressure for lubricants BO,ZDDP and ZP
Table 3 Ball wear track diameters after ball-on-flat test at 25 C
with sliding speed of 25, 50 and 100 mm/s under 0.9 GPa of Hertzian
maximum pressure for lubricants BO,ZDDP and ZP
Wear track diameter on the ball (lm)
Temperature (25C)
Sliding speed (mm/s) 100 50 25
BO 428 ±48 452 ±213 813 ±22
ZDDP 409 ±21 407 ±61 448 ±90
ZP 375 ±39 321 ±10 344 ±38
22 Tribol Lett (2011) 44:19–30
at 100 mm/s. Figure 3and Table 3also present the wear
results obtained at 50 mm/s. They indicate a slightly better
anti-wear effect for ZP lubricant. We further increased the
severity by using a sliding speed of 25 mm/s (always with
the same sliding distance of the ball). Figure 3and Table 3
illustrate too the wear results obtained for ZDDP and ZP
lubricants at 25 C and 25 mm/s. It can be noticed that
wear is much higher with BO (813 ±22 lm) than with
ZDDP (448 ±90 lm) and ZP (344 ±38 lm) lubricants
due to the occurrence of the boundary regime. Concerning
the friction, there is no noticeable difference in friction
coefficient (*0.12) between ZDDP and ZP lubricants.
However, this small sliding speed allowed us to discrimi-
nate clearly anti-wear performances with the ZDDP and
ZP additives at room temperature. The overall results
indicate a much better anti-wear performance for the ZP
lubricant at low temperature and low speed in the boundary
regime. Furthermore, optical observations of flat wear
tracks for ZDDP and ZP lubricants (25 mm/s—25 C)
presented in Fig. 4suggest also a better anti-wear behav-
iour of ZP additive as the number of tribofilm pads is
higher for ZP than for ZDDP lubricant.
At 100 C and 100 mm/s of sliding speed, compared
with BO (685 ±168 lm), wear on the balls for ZDDP
(342 ±8lm) and ZP (418 ±59 lm) lubricants drasti-
cally decreases (Table 4). The friction coefficients obtained
with the three lubricants are summarised in Table 2. The
optical images of flat wear tracks obtained with ZDDP and
ZP lubricants at 100 mm/s are represented in Fig. 4. They
show the presence of coloured patchy tribofilms typical of
anti-wear action of this kind of P-containing additives.
Looking at calculated EHL film thickness (11.8 nm) and
lambda ratio (0.27) at 100 C, we can assume that the
lubrication regime at 100 mm/s sliding speed is predomi-
nantly boundary. The anti-wear performances of ZP
(418 ±59 lm) and ZDDP (342 ±8lm) lubricants are
close, although ZDDP molecule exhibits a slightly better
anti-wear behaviour Fig. 5.
3.2 AES and XPS Analyses
Let us examine surface chemistry of tribofilm at low
temperature (25 C) and low sliding speed (25 mm/s)
where ZP was found much better than ZDDP (Fig. 3).
Phosphorus, sulphur (detected only for ZDDP) and zinc are
found in AES spectra performed on ZDDP and ZP tribo-
films (Fig. 6). Iron is detected in the case of ZDDP only.
Oxygen is mainly in oxide form (peak O
*512 eV)
in the ZDDP tribofilm and in a phosphate form (peak
*507 eV) for the ZP case. ZDDP tribofilm at room
temperature consists of a mixture of zinc and iron
Sliding speed (mm/s) 25 100
Temperatures 25 °C 100 °C
Fig. 4 Optical images of flat
wear tracks (Ball-on-Flat
configuration) obtained at 25
and 100 C with sliding speed
of 25 and 100 mm/s under
0.9 GPa of Hertzian maximum
pressure with ZDDP and ZP
Table 4 Ball wear track diameters after ball-on-flat tests at 100 C
with sliding speed of 100 mm/s under 0.9 GPa of Hertzian maximum
pressure for lubricants BO,ZDDP and ZP
Lubricants Wear track diameter on the ball (lm)
Temperature (100 C)
Sliding speed (100 mm/s)
BO 685 ±168
ZDDP 342 ±8
ZP 418 ±59
Tribol Lett (2011) 44:19–30 23
phosphate with probably iron oxide and metallic sulphides
[32]. On the other hand, the ZP tribofilm is made of zinc
phosphate only.
XPS spectra carried out on ZDDP and ZP tribofilms
obtained at room temperature and 25 mm/s (Fig. 7) also
display phosphorus, sulphur (with ZDDP) and zinc. The
peaks from ZP and ZDDP tribofilms show two con-
tributions indicating that oxygen is involved mainly in
phosphate form (531.6 eV (P–O) and 533.2 eV (P–O–P))
and another contribution is attributed to oxide form
(529.6 eV) [17,32]. However, this last contribution is
found in very small amount and is negligible considering
the fact that it is close to the detection limit.
Finally, the ZDDP and ZP tribofilms also consists
mainly of a mixture of zinc and iron phosphate, with sul-
phide (162.3 eV) in case of ZDDP tribofilms. Iron oxide is
also detected but AES and XPS results are not in total
agreement in the case of ZDDP tribofilm. A strong iron
oxide contribution was clearly found on the AES analysis
but this was not so obvious on the XPS analyses. As the
analysed area with AES technique (&1lm
) is much
smaller than with XPS (&1300 lm
), this difference is
attributed to the local tribofilm heterogeneity.
Auger spectra of ZDDP and ZP tribofilms obtained at
100 mm/s and at 100 C are shown in Fig. 8. The char-
acteristic elements of the additives are detected in both ZP
and ZDDP tribofilms: phosphorus, sulphur (detected only
for ZDDP) and zinc. Oxygen is clearly in the phosphate
chemical form (peak O
*506 eV). No iron is detected
at the top of the two tribofilms. The results indicate that
both ZDDP and ZP tribofilms are made of zinc phosphate
(probably with some metallic sulphides for ZDDP).
XPS analyses were performed in the same tribofilm loca-
tions as for AES analyses. The advantage of XPS is to provide
semi-quantitative elementary analysis. Figure 9shows the
general survey (SG) and O1s spectra of ZDDP and ZP
tribofilms obtained at 100 Cand100mm/s.TheO
from ZDDP tribofilm shows two contributions indicating
that oxygen is involved mainly in phosphate form (531.6 eV
(P–O) and 533.2 eV (P–O–P)) and another contribution
attributed to oxide form (529.6 eV). However, this last con-
tribution is found in very small amount (about 1.6 at % cf.
Table 5) and is negligible considering the fact that it is close to
the detection limit. Two small contributions of oxygen linked
to carbon are also detected at 531.6 eV (C=O) and 533.2
(C–O) at same positions as phosphate peaks. Taking into
account semi-quantification of corresponding carbon peak
deconvolution, these two contributions are expected to be of
few atomic percentages. The O
peak from ZP tribofilm
shows the contributions of oxygen in phosphate form
(531.6 eV (P–O) and 533.2 eV (P–O–P)) with no oxide form.
binding energy from ZDDP additive corresponds to
metallic sulphides (ZnS, FeS, FeS
)[16,17] and is detected
only in ZDDP tribofilm. The Table 5shows the quantification
of component detected on ZDDP and ZP tribofilms.
0 200 400 600 800 1000
Kinetic Energy (eV)
ZDDP Oxide
440 460 480 500 520 540
Kinetic Energy (eV)
Fig. 6 Auger Spectra of ZDDP
and ZP tribofilms obtained at
25 C with a sliding speed of
25 mm/s under 0.9 GPa of
Hertzian maximum pressure
Fig. 5 Ball wear track diameters after ball-on-flat tests at 100 C
with sliding speed of 100 mm/s under 0.9 GPa of Hertzian maximum
pressure for lubricants BO,ZDDP and ZP
24 Tribol Lett (2011) 44:19–30
The overall results of AES and XPS studies clearly show
that ZDDP and ZP tribofilms formed at 100 C and
100 mm/s consists of a zinc phosphate (with sulphide
(162.3 eV) in the case of the ZDDP).
Figure 10 shows the TEM images of FIB cross sections
for ZDDP and ZP tribofilms obtained at 100 C and
100 mm/s. The tribofilms formed on steel is about 60 nm
thick for both additives. The EDS spectra carried out on
both ZDDP and ZP tribofilms confirm the elemental
composition previously obtained by AES.
3.3 Wear Behaviour of ZDDP and ZP Molecule
for Various Sliding Distances
Some additional tribological experiments (ball-on-flat)
were performed at various sliding distance to study wear
a1 b1
(a) XPS spectra: SG (a1) and O1s (a2) of ZDDP (b) XPS spectra: SG (a1) and O1s (a2) of ZP
Binding Energy (eV)
536 534 532 530 528 536 534 532 530 528
Binding Energy (eV)
1000 800 600 400 200 0
Binding Energy (eV)
1000 800 600 400 200 0
Binding Energy (eV)
Fig. 7 XPS spectra of ZDDP
and ZP tribofilms obtained at
25 C with a sliding speed of
25 mm/s under 0.9 GPa of
Hertzian maximum pressure
0 200 400 600 800 1000
Kinetic Energy (eV)
440 460 480 500 520 540
Kinetic energy (eV)
506 eV
Fig. 8 Auger Spectra of ZDDP and ZP tribofilms obtained at 100 C with a sliding speed of 100 mm/s under 0.9 GPa of Hertzian maximum
Tribol Lett (2011) 44:19–30 25
behaviour of ZDDP and ZP molecules during the tribofilm
formation. The study was carried at 25 C—25 mm/s
(Fig. 11a) in the low-temperature regime where ZP was
found more efficient and at 100 C and 100 mm/s
(Fig. 11b). Figure 11 presents wear results obtained at
various sliding distance for the two additives at 25 C—
25 mm/s and at 100 C—100 mm/s. Figure 11a presents
wear results obtained at various test durations (4, 20, 120
and 240 min) corresponding, respectively, to different
sliding distances (6, 30, 180 and 360 m) for the two
additives at 25 C and 25 mm/s. For ZP and ZDDP
additives, the wear obtained at the beginning of the
experiment (sliding distance =6 m) is very small and
close to the calculated Hertzian diameter. It is the same for
ZP even after 360 m of sliding. However, for ZDDP,we
observe a wear increase after 360 m of sliding. The Auger
analyses of the wear scar show the presence of additive
elements after 6 m of sliding in ZDDP and ZP tribofilms
(Fig. 12), although iron is detected in each case. After
360 m of sliding, iron is detected in the ZDDP tribofilm
but not for ZP. Moreover, friction tests were performed for
1–60 min at 100 mm/s of sliding speed and 100 C.
Experiment durations were adjusted in order to have same
sliding distances (6 and 360 m) as for the experiment at
1000 800 600 400 200 0
1000 800 600 400 200 0
Binding Energy (eV)Binding Energy (eV)
C1s P2p
536 534 532 530 528
Binding Energy (eV)
O1s P-O
Binding Energy (eV)
(a) XPS spectra: SG (a1) and O1 (a2) of
(b) XPS spectra: SG (b1) and O1s (b2)
spectrum of ZP
536 534 532 530 528
Fig. 9 XPS spectra of ZDDP
and ZP tribofilms obtained at
100 C with a sliding speed of
100 mm/s under 0.9 GPa of
Hertzian maximum pressure
Table 5 XPS quantification (%
at) of ZDDP and ZP tribofilms
obtained at 100 C with a
sliding speed of 100 mm/s
under 0.9 GPa of Hertzian
maximum pressure
Name Position
(±0.2 eV)
FWHM % at conc ZDDP
% at conc
ZP tribofilm
C1s (C–H) 284.8 1.3 10.7 9.1
C1s (C–O) 286.4 2.7 2.0
C1s (C=O) 288.5 1.0 0.6
O1s (P–O, C–O, C=O) 531.6 1.4 34.8 38.7
O1s (P–O–P) 533.2 8.7 10.6
O1s (oxide) 529.6 1.6
P2p3/2 (phosphate) 133.8 1.7 19.5 22.0
S2p3/2 (zinc, iron) sulphur 162.3 2.5 4.6
Zn2p3/2 (zinc phosphate) 1022.6 1.6 16.5 17.0
26 Tribol Lett (2011) 44:19–30
25 mm/s. For ZP and ZDDP additives, the wear obtained
at the beginning of the experiment (sliding dis-
tance =6 m) is small and close to Hertzian diameter. This
wear increases slightly for ZDDP after 360 m of sliding.
For ZP additives, same feature is found at the beginning of
tests but wear is found to increase a little more after 360 m
of sliding.
3.4 Effects of ZP and ZDDP Concentrations
We also focused on the effect of ZDDP and ZP concen-
trations on their anti-wear efficiency at room temperature
and 25 mm/s of sliding speed for 240 min. Several dilu-
tions were made by mixing a certain volume of base oil
with a volume of ZDDP or ZP lubricants (25 and 75% of
Cu Cu
Cu Cu
Cu Cu
Fig. 10 TEM observations (a
et b
) and EDX spectra (a
and b
) of the FIB cross section of ZDDP (a) and ZP (b) tribofilms obtained at 100 C
with a sliding speed of 100 mm/s under 0.9 GPa of Hertzian maximum pressure
Fig. 11 Ball wear track
diameters after ball-on-flat tests
at 25 and 100 C with different
sliding distance for ZDDP and
ZP lubricants
Tribol Lett (2011) 44:19–30 27
base oil corresponding, respectively, to 600 and 200 ppm
of P, respectively). Figure 13 shows wear scar diameters on
balls at the three concentrations and at room temperature.
As can be seen for the most dilute solution (75% BO
200 ppm of P), the wear scar diameter obtained with
ZDDP lubricant (*358 ±0.1 lm) is significantly higher
than with ZP lubricant (*321 ±3.0 lm). The comparison
of ZDDP and ZP lubricants at low concentration gives
evidence for the better anti-wear property of ZP at room
4 Discussion
At 100 C and 100 mm/s for 60 min, both ZP and ZDDP
molecules exhibit anti-wear capabilities (ZDDP molecule
is slightly better) as well as similar tribofilms compositions.
The absence of sulphur in ZP molecule does not inhibit the
tribofilm formation. The origin of anti-wear capabilities of
such phosphorus-based additives is probably the same for
the two molecules. It is important to notice that we did not
check the efficiency of these additives in the EP regime.
Different mechanisms are proposed in literature related to
specific tribochemical reaction pathways [12,19] or spe-
cific tribofilm material modification under solicitations
In the first case (tribochemical reactions), it is proposed
that ZDDP molecule and its degradation products react
under boundary conditions with native iron oxide of steel
surfaces to form mixed zinc and iron phosphate glass [12].
Thanks to this tribochemical pathway, the tribofilm adheres
well on metal surfaces. Additionally, any iron oxide par-
ticle trapped in the contact will lose its abrasive character
when being digested in the phosphate tribofilm. In case of
more severe lubrication conditions (extreme pressure), the
tribofilm could not stay in the contact and a different
tribochemical reaction occurs between iron metal and
sulphur species. Metallic sulphides are generated in the
contact. This last reaction is explaining the extreme pres-
sure capabilities of ZDDP molecule.
Concerning the second mechanism at the origin of anti-
wear capabilities of such material, it is related to interfacial
material modification (zinc phosphate) under solicitations.
A change of zinc atoms coordination number is proposed
under hydrostatic pressure [24] and could so contribute to
the modification of tribofilm mechanical properties under
solicitations. The effect of shearing was not investigated.
In our experimental conditions, as the tribofilms gener-
ated with both additives are made of phosphate materials,
same kind of anti-wear mechanisms can be proposed for
both molecules. The tribochemical reaction of polyphos-
phate with native iron oxide could explain the adhesion of
the tribofilm on the substrate and the loss of abrasive
character of wear particles. Although our experiments do
Fig. 12 Auger spectra obtained
on ZDDP and ZP tribofilms
after 6 and 360 m of sliding at
25 C with a sliding speed of
25 mm/s under 0.9 GPa of
Hertzian maximum pressure
Fig. 13 Ball wear track diameters after ball-on-flat tests at 25 C
with sliding speed of 25 mm/s under 0.9 GPa of Hertzian maximum
pressures for lubricant with different additives concentration (200 and
600 ppm of phosphorus)
28 Tribol Lett (2011) 44:19–30
not allow any conclusion about structural changes during
the tribological solicitations, same kind of modifications
are expected for ZP and ZDDP tribofilms. Concerning
extreme pressure conditions, no ZP activity is expected as
there is no sulphur in the molecule.
At 25 C and 25 mm/s for 360 m of sliding, tribological
tests results show that ZP molecule exhibits a better anti-
wear behaviour than ZDDP. At lower concentration
(200 ppm of phosphorus—25 C—25 mm/s—240 min),
ZP additive shows an even better anti-wear behaviour than
ZDDP. As it was proposed first (§1), the ZP molecule is
closed in composition to tribofilm final material, so ZP
molecule does not need to follow the thermo-oxidative
degradation pathway of ZDDP molecule, avoiding high
wear rate during the detrimental induction period of ZDDP.
This makes the ZP molecule more reactive and more effi-
cient than ZDDP at 25 C, 25 mm/s and 360 m of sliding.
To conclude, ZP molecule has an advantage compared
to ZDDP in terms of anti-wear capabilities at 25 C. At
100 C, it is ZDDP molecule that exhibits a better anti-
wear behaviour. A mixture of both molecules is so an
interesting option to get a good compromise in terms of
anti-wear capabilities in the range of 25 and 100 C with
the same amount of phosphorus and with a small amount of
sulphur atoms in the lubricant.
5 Conclusion
The comparison of ZP anti-wear performance with ZDDP
allows us to conclude that
although the anti-wear efficiency of these two mole-
cules is found at 100 mm/s of sliding speed and 100 C,
ZDDP exhibits a slightly better anti-wear behaviour
than ZP at this temperature. For both additives,
tribofilms are mainly made of zinc and iron phosphate.
at room temperature and 25 mm/s of sliding speed, ZP
is a better anti-wear additive
at room temperature and 25 mm/s of sliding speed, ZP
is able to protect steel surfaces from wear even at
200 ppm of phosphorus. In these conditions, ZDDP is
not so active.
Taking into account all these data, we show that ZP is
an interesting anti-wear additive for the lubrication of
Internal Combustion Engines at ambient temperature,
which is the characteristic of cold engine start. In steady-
state conditions, ZDDP molecule is more efficient than ZP.
The combination of both additives (keeping a small amount
of P) is a good option to optimise the anti-wear capabilities
of engine lubricants. The efficiency of these additives in
the EP regime was not studied. However, the loss of
extreme pressure properties and antioxidant properties with
ZP molecule is expected and requires the addition of other
molecules in the lubricant completely formulated.
Acknowledgments The authors would like to thank the ANR for
the support in the ANR-07-JCJC-0060 LOWPOLUB project.
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... By comparison with the O 1 s peaks of APP at 531.1 eV (P = O) and 533.0 eV (P-O-P and P-O -NH 4 + ) inFig. 1c,[32] three new bonding energies of Fe/Zn-LDH@APP at 531.8 eV, 531.3 eV and 531.2 eV should be assigned to the HO -Fe 3+ , HO -Zn 2+ and P-O -Zn 2+ bonding, respectively,[32] indicating the generation of Fe/Zn-LDH and successful occurrence of the ion exchange reaction between the NH 4 + in APP and Zn 2+ . ...
... By comparison with the O 1 s peaks of APP at 531.1 eV (P = O) and 533.0 eV (P-O-P and P-O -NH 4 + ) inFig. 1c,[32] three new bonding energies of Fe/Zn-LDH@APP at 531.8 eV, 531.3 eV and 531.2 eV should be assigned to the HO -Fe 3+ , HO -Zn 2+ and P-O -Zn 2+ bonding, respectively,[32] indicating the generation of Fe/Zn-LDH and successful occurrence of the ion exchange reaction between the NH 4 + in APP and Zn 2+ . ...
Increasing power density and excess heat production in integrated electronic devices create the strong demand for polymeric thermal management materials with excellent thermal stability, flame retardancy and thermal conductivity. To this end, high-performance ionic liquid-wrapped boron nitride nanosheets ([email protected]) were firstly simultaneously exfoliated and flame-retardant functionalized via one-step ball milling process based on the strong mechanochemical action. Epoxy (EP)-based layered films with highly in-plane oriented [email protected] were then fabricated by a novel, effective and solvent-free cyclic layer-by-layer (CLbL) blade-casting method. Arising from the highly flat oriented structure and rich flame-retardant functional groups of [email protected], as well as the high compatibility between filler and matrix, the as-fabricated EP/[email protected] films exhibited high anisotropic thermal conductivity (K∥ of 8.3 and K⊥ of 0.8 W m⁻¹ K⁻¹), outstanding thermal stability and flame retardancy with a dramatic decrease in PHRR (104.2 W/g) and THR (8.1 kJ/g) corresponding to reductions of 72.9% and 75.7% compared with neat EP, respectively. Additionally, the flat oriented structure and strong interfacial interaction also endow EP/[email protected] films with high flexibility and excellent mechanical properties. Therefore, the high-effective CLbL casting method and the obtained high-performance EP-based film exhibit a significant potential application in high power flexible electrical devices and thermal management products.
... En comparant avec l'huile de base et les autres lubrifiants contenant le MoDTC, l'usure la plus faible est observée dans le test où le ZDDP est ajouté au lubrifiant. La baisse de l'usure avec l'utilisation du ZDDP a été observée dans plusieurs travaux et sous différentes conditions de fonctionnement [56][57][58]. On peut également remarquer que la présence du MoDTC, c'est-à-dire le lubrifiant 003A, a un effet réducteur d'usure. ...
Le contact segment-piston-chemise (SPC) est l’une des premières origines des pertes d'énergie mécanique par frottement dans les moteurs à combustion interne. Les performances tribologiques de ce composant dépendent des conditions du fonctionnement, de l'état des surfaces en contact et des propriétés des lubrifiants utilisés. Dans ce contexte, ce travail de thèse a pour objectif d’étudier l'effet conjoint de la texturation des chemises moteur utilisées dans le domaine automobile et des additifs de lubrification en régime limite/mixte sur le comportement tribologique du contact SPC (loi de frottement, formation de tribofilms, usure). Ce travail s'appuie sur la mise en œuvre de techniques expérimentales en tribométrie permettant d'évaluer le frottement et l'usure des échantillons provenant de chemises moteur réelles en présence de deux lubrifiants : une huile de base et une huile formulée, sous différentes conditions de température, pression et durées de fonctionnement. Les additifs dans les huiles formulées permettent de former un tribofilm réducteur de frottement et d’usure. La composition chimique de ce tribofilm est caractérisée par des techniques de spectroscopie (XPS, ToF-SIMS) et sa structure analysée par FIB, MET et EDX, permettant de comprendre les interactions physico-chimiques des additifs avec les surfaces. Les résultats montrent que la texturation des surfaces favorise la formation du tribofilm.Par ailleurs, on propose une approche nouvelle en tribométrie, mettant en œuvre un dispositif préexistant du laboratoire (tribomètre oscillant) et basée sur l’analyse de la réponse libre d'un oscillateur frottant. Cette technique non invasive permet d’appréhender les lois de frottement de manière originale en distinguant notamment les contributions dépendantes et indépendantes de la vitesse de glissement. La nouveauté de l’approche introduite consiste au suivi des réponses libres tout au long de la formation du tribofilm ainsi qu’au suivi des énergies dissipées en fonction des contributions susdites au frottement. Les résultats montrent que la diminution du coefficient de frottement induite par le tribofilm résulte de la diminution drastique de la seule contribution au frottement indépendante de la vitesse de glissement.
... O--C bonds peaks appeared at 532.1 eV, related to lubricant oxidation. O-P signals can shift depending on both the size of the polyphosphate chain and type of metal bonded to phosphate groups [61,[64][65][66]. Signals located at 532.2 eV and 533.3 eV are related to oxygen bonded to phosphate and sulfur groups, respectively [67]. ...
This work uses synchrotron radiation XANES and XPS measurements to investigate the structure of ZDDP tribofilms formed from lubricants contaminated with anhydrous and hydrated ethanol (5%wt). The tribofilms were formed at 40oC from PAO8 lubricant containing only ZDDP additive onto carbon steel specimens using sliding tests, cylinder-on-flat configuration (line contact) under boundary lubrication. Ethanol increased friction as well as surface damage, detected by an increase in relative porous area. Angle-resolved measurements (15o, 45o, 90o) helped detecting differences along the thickness of the tribofilm. The amount of long-chain phosphates apparently reduced with ethanol, leading to reduced wear performance. The sulphides in the tribofilms were richer in Fe in the presence of ethanol, also indicative of more severe wear.
... 42,43 Three different binding energies of 529.9, 531.8 and 533.4 eV can be attributed to Fe 2 O 3 and polyphosphates of P-O, P-O-P formations for the O1s peaks in Table 3, respectively. 44,45 Combined with S2p and Fe2p peaks at binding energies of 710.5 and 168 eV show the metal sulfides such as FeS 2 compounds on the surfaces. 43,46 Besides, the peak Fe2p at 707.3 AE 1 eV in a low atomic ratio shows the bulk Fe. 47 When the MoS 2 derived tribofilm on wear scar was focused on, there were two peaks at Mo3d scan with 233.1 and 229.8 eV binding energies. ...
Additives are important components to reduce friction and wear of tribological systems. Zinc dithiophosphate (ZDDP), the most commonly used anti-wear additive prevents the tribological system from the friction and wear loss without any commercially available antagonist additive. However, its environmentally harmful content enforces to lubricating oil manufacturers to replace it with an alternative one. Therefore, in this study, the H 3 BO 3 þ MoS 2 combination was tested in the boundary lubrication regime in a tribometer with a pin on flat reciprocating test module to be a possible alternative anti-wear additive against ZDDP. The friction and wear behavior of Boric acid (H 3 BO 3) þ Molybdenum Disulfide (MoS 2), ZDDP, MoS 2 , and H 3 BO 3 evaluated in terms of friction coefficient (COF), wear rate, surface energy and tribo-chemical analysis of tribofilms. SEM/EDX and XPS were employed to explore tribofilm formations on the wear scars. According to the results, the addition of MoS 2 into H 3 BO 3 presented a similar tribological performance to ZDDP by forming a strength tribofilm on the surface containing B 2 O 3 , MoO 3, and metal sulfate.
... 11,12 However, their incessant use is prohibited because of their susceptibility to attenuating the efficacy of exhaust emission catalytic converters, thus enhancing air pollution. 13 From an environmental perspective, various standards have been put forth to control sulphated ash, sulphur and phosphorus (SAPS) contents in an additive. 14 Nanolubricants 15,16 are well appreciated for their dexterous antiwear behavior but their agglomeration poses a great problem for the stability of their dispersions. ...
Full-text available
Some tetrahydropyrazolopyridines (THPP-H) with the methoxy (THPP-OMe) and methyl (THPP-Me) substituents were synthesized by a one-pot multi-component reaction. NMR spectroscopy (¹H and ¹³C) was used to authenticate the synthesis. According to the results of tribological tests ASTM D4172, and ASTM D5183 on a four-ball tester in paraffin oil (PO) at a concentration of 0.25% w/v, their relative tribo-activity along with a reference additive, zinc dialkyldithiophosphate (ZDDP) could be figured out as mentioned below-THPP-OMe > THPP-Me > THPP-H > ZDDP. The calculation of frictional power loss from the coefficient of friction data of the tested additives supports the given order. As is apparent from AFM and SEM micrographs of the wear scar surface for plain oil with and without different tetrahydropyrazopyridines, surface evenness endorses the above trend. Proof for strong adsorption of the synthesized additives is provided by EDX analysis of the steel ball surface after performing the tribological test, where nitrogen and oxygen are vividly seen as heteroatoms. XPS studies reveal the composition of the in situ formed tribofilm. The moieties containing carbon bonded to oxygen/nitrogen as decomposed products of the additive together with oxides of iron in +II or +III oxidation states are perceptible in the tribofilm, the tribofilm interferes with the proximity of the surfaces keeping them far apart. Consequently, friction and wear are remarkably reduced. Findings from Density Functional Theory (DFT) calculations are in full agreement with the results obtained from tribological experiments.
... Lubrication materials are the essentially key materials to improve the functionality of new energy vehicles, aerospace, marine ships and intelligent machineries [1][2][3] , and their further development. Much of research work on problems associated with lubricating materials has been carried out, which results in abundant simulation or test data [4][5][6][7] . However, these individual data are currently still not being effectively utilized, and difficulty in evaluating performance of lubricating materials in various industrial applications is still widely remaining as a major problem. ...
Full-text available
Long developing period and cumbersome evaluation for the lubricating materials performance seriously jeopardize the successful development and application of any database system in tribological field. Such major setback can be solved effectively by implementing approaches with high throughput calculation. However, it often involves with vast number of output files, which are computed on the basis of first principle computation, having different data format from that of their experimental counterparts. Commonly, the input, storage and management of first principle calculation files and their individually test counterparts, implementing fast query and display in the database, adding to the use of physical parameters, as predicted with the performance estimated by first principle approach, may solve such setbacks. Investigation is thus performed for establishing database website specifically for lubricating materials, which satisfies both data: (i) as calculated on the basis of first principles and (ii) as obtained by practical experiment. It further explores preliminarily the likely relationship between calculated physical parameters of lubricating oil and its respectively tribological and anti-oxidative performance as predicted by lubricant machine learning model. Success of the method facilitates in instructing the obtainment of optimal design, preparation and application for any new lubricating material so that accomplishment of high performance is possible.
This paper focuses on the performance evaluation of three greases for mechanical transmissions using a fretting tribometer under boundary lubrication conditions. A procedure built at Cetim allowed to contribute to the understanding of their ability to protect materials against fretting. In addition to the requirements of ASTM D4170 (loss mass measurement), the thrust ball bearings races were analyzed to assess the load distribution depending on their angular and lower or upper position in the column. After only 5 h, with a load of 2450 N and a frequency of 30 Hz at room temperature, a significant difference in tribological behavior between these greases is reported. The lowest mass loss was obtained with the grease used in the food industry. The thrust ball bearing in the lower position is more worn than the one at the top of the column.
Inspired by the in-situ coprecipitation technology, Fe/Zn-layered double hydroxides (Fe/Zn-LDH) doped ammonium polyphosphate (APP) was prepared for high fire-safety epoxy resins (EP). The investigation on the chemical composition and micromorphology of Fe/[email protected] confirmed the uniform deposition of Fe/Zn-LDH on the surface of APP. The results showed that EP containing 4 wt% Fe/[email protected] exhibited the highest LOI of 29.4% and passed the UL-94 V0 level. Additionally, compared to neat EP, the peak of heat release rate, peak of smoke production rate, and the fire growth rate decreased by 66.4%, 48.4%, and 80.7%. The greatly enhanced flame retardancy of EP was contributed to the highly synergistic charring catalysis via abundant interfacial contact sites. Moreover, the mechanical performance of EP/Fe/[email protected] were slightly affected due to the improved interfacial interaction of Fe/[email protected] Generally, this work exploited a tactful strategy to construct LDH doped APP hybrid material, and presented the potential application in industry.
Medium chain chlorinated paraffin (MCCP) and zinc dialkyl dithiophosphate (ZDDP) are widely used as extreme pressure (EP) additives in metalworking fluids (MWFs). Currently, sulfur-based EP additives are found to be more environmentally friendly and biodegradable. Although naphthenic base oil (NBO) is suitable to be used in MWFs due to its low pour point and better emulsified ability, there is a lack of study on the sulfur additives in this type of oil. In this study, two organo disulfides compounds namely dibenzyl disulfide (DBDS) and dioctyl disulfide (DODS) have been synthesized as EP additives. These DBDS and DODS demonstrate an excellent EP performance, non-corrosive, ashless, and good anti-wear (AW) properties in NBO. The DBDS shows 1961 N (EP value) and 6 mm² (AW scar area) in NBO, which is superior to DODS, MCCP and ZDDP. This is due to the formation of sulfur-based tribofilm on the interface that acts as a protective sulfur layer thus, enhancing the property of EP.
Zinc dialkyldithiophosphate (ZDDP)-based antiwear additives are crucial in automotive lubricants, where its effectiveness in reducing wear of ferrous alloys is well established. However, prior studies of light-weight aluminum-based alloys reveal that ZDDP is not as effective an anti-wear agent on Al-based surfaces for reasons that remain under debate. Here we use in situ atomic force microscopy (AFM) to study nanoscale ZDDP-derived tribofilms at the sliding interface between an alumina microcolloid probe and substrates comprised of either aluminum (with native oxide) or aluminum oxide (single crystal sapphire). The experiments reveal that ZDDP tribofilms form on both substrates, supporting the idea that tribofilm formation crucially involves thermally-activated, stress-assisted chemical reactions, and does not require cation exchange from wear debris originating from the substrate.
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Recent studies on a variety of metal phosphates (MP) have revealed that MPs tend to be soft at ambient pressure if the coordination on the metal cation is low and the degree of hydration or hydrogenation is high, while they are stiff otherwise. In addition, the softer MPs were found to stiffen dramatically more quickly with increasing pressure than the stiffer MPs. Here we review these findings and support their relevance with new results on the mechanical properties of tribofilms aged in air of relative humidity, which were produced from commercial, zinc phosphate-containing lubricant packages via heating and rubbing. We find that the films can soften quite substantially after having been exposed to humidity, as to be expected from the studies of bulk MPs. Moreover, when the hydrated films are exposed to high loads, the force-distance withdrawal curve becomes identical to that of unaged, non-hydrated films. A straightforward explanation of this observation is that large pressure reverses the hydration of the tribofilms.
Zinc dialkylphosphate (ZP), a sulfur free analogue of Zinc dialkyldithiophosphate (ZDTP) has been synthesized and tested for engine oil application. The sulfur free engine oil based on ZP showed excellent TBN retention and engine cleanliness in several engine tests which were operated at low and high temperatures. A prototype low phosphorus (0.05%) and low sulfated ash (0.5%) engine oil exhibited even better longer drain performance than a fully synthetic engine oil containing 0.1% P and 1.1% sulfated ash. Thus, zinc dialkylphosphate can be a promising candidate as a ZDTP substitute for future catalyst system compatible engine oil.
Zinc phosphate glass is considered to be the main constituent of tribofilms generated under boundary lubrication with zinc dialkyldithiophosphate (ZDDP), a well-known antiwear additive. The reaction occurring during friction between zinc phosphate glasses and steel native iron oxide layer is investigated by both an experimental approach and by Molecular Dynamics simulations (MD). The importance of this “tribochemical” reaction in the general ZDDP antiwear process is discussed.
X-ray photoelectron spectroscopy using a synchrotron source (SR-XPS) with variable photon energy has been used to non-destructively elucidate the variations in surface chemistry from ~5 nm to ~10 nm into the tribofilm derived from zinc dialkyldithiophosphate (ZDDP) in a mineral oil under boundary lubrication conditions. The elemental ratio of P/Zn and “bridging” oxygen (BO)/“non-bridging” oxygen (NBO) decrease as a function of distance from the top surface of the film, suggesting a decrease of the polyphosphate chain-length into the film, as shown in many recent XPS and XANES studies. More importantly, the measured P/Zn ratio of ~3, the BO/NBO ratios of >0.5, the P 2p spectra, and the absence of other balancing cations such as iron, show the first strong evidence for an ultrapolyphosphate (such as ZnP4O11), organophosphates along with other Zn polyphosphates. The existence of ultraphosphates and/or organophosphates in this film appears to be the long-awaited answer to the apparent deficiency of cations in these antiwear films.
This paper reviews research into the mechanisms of action of the lubricating oil additive, zinc dialkyldithophosphate (ZDDP). The development of the use and research into ZDDP is first charted historically, starting with the additive's first introduction in engine oils in the late 1930s. Then our current state of knowledge of each of the main facets of ZDDP behaviour both in solution and at metal surfaces is identified and discussed. It is concluded that we now know a great deal about the properties and morphology of ZDDP antiwear films but still relatively little about the reaction pathways that lead to ZDDP film formation or about the kinetics of ZDDP film generation and removal.
This is a review of the additive, zinc dialkyldithiophosphate (ZDDP), which is found commonly in lubricating oil where it plays a role as both an antioxidant and an antiwear additive. This zinc complex is highly effective but its mechanisms of action have not been definitively reported. This review covers work pertaining to the characterisation and mechanisms of action of ZDDP and includes studies carried out by sophisticated instrumentation as well as laboratory studies. There are some references to the nature of the antiwear films generated by ZDDP and the usefulness of its derivatives.
Zinc dialkyldithiophosphates (zddp's) have been used in internal combustion engine oils for over 30 years as antiwear and antioxidant additives. In this paper, their mode of action is investigated. It was concluded, using various analytical techniques, that zddp's decompose in oil solution predominantly by a hydrolytic mechanism, ultimately to zinc polyphosphate and a mixture of alkyl sulphides, which are the precursors of the antiwear action of zddp
A wear machine in which samples were observed through the countersurface as they were sliding in lubricating engine oils was used to obtain insight into the antiwear action of zinc dialkyldithiophosphates (ZDDPs). Good antiwear activity was associated with the formation of a friction-polymer-like material in the oil at the leading edge of contacts and with the formation of a thick film on the wear surface. Poor boundary lubrication action was associated with the formation of lesser amounts of friction polymer and with bare metal surfaces. Steady state appearances usually developed within a few centimetres of sliding, with the progression of events depending more upon distance slid than velocity (range 0.03-4.3 mm s-1). The observed interaction between the friction polymer and the surface film and the rapidity of their formation put the antiwear activity of ZDDPs into a new perspective.
The aim of this study is to combine a diamond anvil cell with in-situ Raman spectroscopy to simulate and analyze the effect of pure pressure on the length of phosphate chains in an antiwear film formed in a tribological contact. In-situ Raman spectra of Zn2P2O7 glass, alpha-Zn-3(PO4)(2), and gamma-Zn2P2O7 crystals submitted to high hydrostatic pressure up to 20 GPa were recorded. Evolution of Raman spectra as a function of pressure was studied in the characteristic high frequency range of PO4 tetrahedra molecular resonance (650-1300 cm(-1)). When exposed to high pressure, the structure of the sample becomes less ordered. Phase transitions in alpha-Zn-3(PO4)(2) structure are observed during compression from ambient pressure to 3 GPa. The length of the phosphate chains is conserved up to 20 GPa when samples are subjected to hydrostatic pressure. (c) 2007 American Institute of Physics.