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ORIGINAL PAPER
Zinc Dialkyl Phosphate (ZP) as an Anti-Wear Additive:
Comparison with ZDDP
Paule Njiwa •Clotilde Minfray •Thierry Le Mogne •
Be
´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
Wear AES XPS TEM (EDS)
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
[1,2].
•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 [3–6] 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 [7–9], 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
Universite
´de Lyon, Ecole Centrale de Lyon, LTDS,
UMR5513, Ecully, France
e-mail: paule.njiwa@ec-lyon.fr
S. Matsui M. Mishina
JX Nippon Oil & Energy Corporation, Lubricant Research
Laboratory, Tokyo, Japan
123
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 [13–15]. 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 [17–19] 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
investigated.
2 Materials and Methods
2.1 Lubricants
Three lubricants were tested:
•A mineral base oil of group III noted BO in the
following.
•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
sulphur.
2.2 Materials
The balls used were 12.7 mm in radius and 30 nm (Ra
B
)in
roughness. The cylinders employed were 6 mm in length
and 6 mm in diameter. The rectangular flats measured
10 9892mm
3
. All these specimens are made of AISI
52100 steel. This iron alloy contains 97 wt% of Fe, 1.45
OR
P
OR
S
S
RO
P
RO
S
S
Zn
OR
P
OR
O
O
RO
P
RO
O
O
Zn
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
123
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
C
) and flat (Ra
F
)
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
-7
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
2
. 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
correction.
Table 1 Physical properties and rheological parameters of the
mineral base oil (BO) in different test conditions
Sliding
speed
(mm/s)
Mineral base oil group III
Sulphur content: \0.03%w
Viscosity index [120
Density at 15 C:0.835 g/cm
3
Temperature (C)
25 100
Dynamic viscosity (Pa.s)
3.02. E-02 3.36.E-03
Film thickness: h
a
(nm)
k
b
h
a
(nm) k
b
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
a
Film thickness: calculation carried out from Hamrock relation [30]
b
Ratio between the lubricant film thickness and the composite sur-
faces roughnesses (a)
a¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Ra2
BþRa2
F
qBall (Ra
B
=30 nm) and Flat (Ra
F
=12 nm)
Tribol Lett (2011) 44:19–30 21
123
Special attention has been paid for fitting P
2p
,S
2p
(for
ZDDP), O
1s
,Fe
2p3
and Zn
2p3/2
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
(mm/s)
25 100 100
Lubricants
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
Lubricants
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
123
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
KLL
*512 eV)
in the ZDDP tribofilm and in a phosphate form (peak
O
KLL
*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
Tribfilms
ZDDP
ZP
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
lubricants
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
123
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
O
1s
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
2
) is much
smaller than with XPS (&1300 lm
2
), 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
KLL
*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
1s
peak
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
1s
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.
S
2p
binding energy from ZDDP additive corresponds to
metallic sulphides (ZnS, FeS, FeS
2
…)[16,17] and is detected
only in ZDDP tribofilm. The Table 5shows the quantification
of component detected on ZDDP and ZP tribofilms.
Phosphate
0 200 400 600 800 1000
ZP
Zn
LMM
SLMM
PLMM
CKLL
Fe
LMM
Kinetic Energy (eV)
ZDDP Oxide
440 460 480 500 520 540
ZP
511eV
507eV
Kinetic Energy (eV)
ZDDP
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
123
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
a2
(a) XPS spectra: SG (a1) and O1s (a2) of ZDDP (b) XPS spectra: SG (a1) and O1s (a2) of ZP
Oxide
P-O
Binding Energy (eV)
P-O-P
O1s
536 534 532 530 528 536 534 532 530 528
P-O
Oxide
Binding Energy (eV)
O1s
P-O-P
1000 800 600 400 200 0
Binding Energy (eV)
1000 800 600 400 200 0
Binding Energy (eV)
ZP
P2p
C1s
O1s
Zn2p
b2
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
Phosphate
0 200 400 600 800 1000
Kinetic Energy (eV)
ZP
PLMM
Fe
LMM
CKLL
ZDDP
SLMM
Zn
LMM
440 460 480 500 520 540
Kinetic energy (eV)
506 eV
ZDDP
ZP
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
pressure
Tribol Lett (2011) 44:19–30 25
123
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
a1b1
1000 800 600 400 200 0
1000 800 600 400 200 0
Binding Energy (eV)Binding Energy (eV)
ZP
C1s P2p
O1s
Zn2p
a2b2
536 534 532 530 528
P-O
P-O-P
Binding Energy (eV)
Oxide
O1s P-O
Binding Energy (eV)
P-O-P
O1s
(a) XPS spectra: SG (a1) and O1 (a2) of
ZDDP
(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
tribofilm
% 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
123
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
PZn
SFe
Fe
Fe
O
Zn
Zn
Cu Cu
Cu
PZn
SFe
Fe
Fe
O
Zn
Zn
Cu Cu
Cu
PZn
SFe
Fe
Fe
O
Zn
Zn
Cu Cu
Cu
P
Zn
Fe
Fe
Fe
O
Zn
Zn
Cu
Cu
Cu
Ga
Ga
Ga
C
P
Zn
Fe
Fe
Fe
O
Zn
Zn
Cu
Cu
Cu
Ga
Ga
Ga
C
P
Zn
Fe
Fe
Fe
O
Zn
Zn
Cu
Cu
Cu
Ga
Ga
Ga
C
a
1
b
1
a
2
b
2
(a)
ZDDP
(b)
ZP
Fig. 10 TEM observations (a
1
et b
1
) and EDX spectra (a
2
and b
2
) 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
123
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
temperature.
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
[24].
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
123
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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