Microsoft Word - S_Bec_ZDTP_MoDTC_Tribology_Letters.doc
Synergistic effects of MoDTC and ZDTP on frictional behaviour 
of tribofilms at the nanometer scale 
S. Bec1*, A. Tonck1, J.M. Georges1 and G.W. Roper2 
1Laboratoire de Tribologie et Dynamique des Systèmes, UMR CNRS 5513, 
Ecole Centrale de Lyon, 36 av. Guy de Collongue, 69134 Ecully Cedex, France. 
2Lubricants Technology Dept., Shell Global Solutions, Shell Research and Technology 
Centre, Thornton, P. O. Box 1, Chester CH1 3SH, UK. 
*To whom correspondence should be addressed 
Abstract 
The layered structure and the rheological properties of anti-wear films generated in a 
rolling/sliding contact from lubricants containing zinc dialkyldithiophosphate (ZDTP) and/or 
molybdenum dialkyldithiocarbamate (MoDTC) additives have been studied by dynamic 
nanoindentation experiments coupled with a simple modelling of the stiffness measurements. 
Local nano-friction experiments were conducted with the same device in order to determine 
the evolution of the friction coefficient as a function of the applied pressure for the different 
lubricant formulations. For the MoDTC film, the applied pressure in the friction test remains 
low (<0.5 GPa) and the apparent friction coefficient is high (µ>0.4). For the tribofilms 
containing MoDTC together with ZDTP, which permits the applied pressure to increase up to 
a few GPa through some accommodation process, a very low friction domain appears 
(0.01<µ<0.05), located a few nanometers below the surface of the tribofilm. This low friction 
coefficient is attributed to the presence of MoS2 planes sliding over each other in a favourable 
configuration obtained when the pressure is sufficiently high, which is made possible by the 
presence of ZDTP. 
Keywords : ZDTP, MoDTC, tribofilm structure, nanoindentation, mechanical properties, 
nanofriction, low friction. 
1. Introduction 
In addition to zinc dialkyldithiophosphate (ZDTP) additives, extensively used for their 
exceptional antioxidant and anti-wear properties under boundary conditions in automotive 
engines, lubricating oils contain several additives, among which there are detergent and 
dispersant additives whose main role is to keep oil insoluble contaminants and degradation 
products in suspension, at elevated temperature for the detergent additives, and at low 
temperatures for the dispersant ones. Organo molybdenum compounds such as molybdenum 
dithiocarbamate (MoDTC) are also used as friction modifiers for energy saving. However, 
when used together in formulated oils, additives interact in various ways resulting either in 
synergies or in adverse effects affecting the oil performance regarding anti-wear and friction 
behaviour, and modifying the characteristics of the protective surface films generated during 
friction (tribofilms). A lot of investigations have been conducted to evaluate the performances 
of additive mixtures and to determine the composition of associated tribofilms. Several 
factors were identified as playing a role: additive structure [1, 2], additives concentration [3-
6], base oil nature [7, 8], …, or combinations of these parameters. A detailed review on 
published information on that topic was written by Willermet [9]. Non-chemical parameters 
such as characteristics of the solid antagonists (hardness, roughness) or test conditions (load, 
temperature, sliding speed) [3, 10] also might influence the additive interactions. 
Among this variety of additive interactions, we will focus on that between ZDTP and 
MoDTC, extensively studied through chemical investigations. All published works agree 
upon the fact that friction and anti-wear performances of oils are improved when ZDTP and 
MoDTC are used together. The formation of molybdenum disulphide (MoS2) on the rubbing 
surfaces has been evidenced by several authors [11, 12]. Using UHV friction tests, coupled 
with high-resolution TEM observation of wear debris and spectroscopic studies, Grossiord et 
al. has given evidence for the mechanism of single MoS2 sheet lubrication [13]. 
The aim of this paper is to enlarge the knowledge of the local mechanical and frictional 
properties of anti-wear tribofilms to those of films obtained from lubricants containing 
different additives (ZDTP, MoDTC, detergent/dispersant) or mixtures of additives, in order to 
explore the ZDTP/MoDTC synergy on a mechanical point of view. The only published results 
on that topic are the recent papers from Ye et al. who performed AFM observations and 
nanoindentation measurements on ZDTP and ZDTP + MoDTC tribofilms [14, 15]. 
In the present study, nanoindentation tests with continuous stiffness measurements were 
performed on unwashed and solvent-washed tribofilms to determine their mechanical 
properties. The frictional behaviour of the tribofilms was investigated through local 
nanofriction experiments, conducted with the same device. The evolution of the friction 
coefficient as a function of the applied pressure for the different lubricant formulations 
leading to different tribofilms has been determined. 
2. Preliminary results obtained on ZDTP anti-wear tribofilms 
The structure and the rheological properties of anti-wear films from a zinc 
dialkyldithiophosphate (ZDTP) solution generated in a rolling/sliding contact, simulating 
engine valve train conditions, have been studied in detail with analytical and surface force 
tools and the results have been published by the authors in a previous paper [16]. As preamble 
to the present paper, only the main points are summarised here. 
The ZDTP solution was a commercial secondary alkyl ZDTP additive at 0.1% weight of 
phosphorus in a highly refined base oil. The ZDTP anti-wear films have a complex structure 
that has been determined by extensive use of surface analytical techniques. It has been shown 
that the ZDTP films consisted of at least three non-homogeneous layers: on the steel surface, 
there is a sulphide/oxide layer, which is almost completely covered by a protective phosphate 
layer, with the addition of a viscous overlayer of ZDTP degradation precipitates (alkyl 
phosphate precipitates). This latter layer was removed when the film was washed with an 
alkane solvent. Therefore, the properties of the ZDTP films have been studied both before and 
after solvent washing with n-heptane. First, sphere/plane squeeze experiments were 
performed with a surface force apparatus (SFA) on unwashed films, showing that the 
overlayer of alkyl phosphate precipitates was heterogeneous and discontinuous, with a 
thickness of about 900 nm. Second, the mechanical properties were obtained from 
nanoindentation experiments, performed after replacing the sphere by a diamond tip, and 
coupled with in-situ topographic imaging procedures to measure the contact area. From the 
indentation experiments, the properties of the films were determined from normal stiffness 
measurements and through the application of a rheological film model. On the unwashed 
specimens, the viscous layer of alkyl phosphate precipitates was detected by the indentation 
tests. It is a very soft layer, mobile under the diamond tip, with a thickness of a few hundred 
of nanometers, which was in good agreement with that of sphere/plane experiments. It was 
also shown that indentation experiments removed this overlayer in the proximity of the tip, 
probably through a shear flow mechanism. This procedure can be compared to a soft 
"mechanical" sweep and the mechanical properties of ZDTP tribofilms after such a cleaning 
were found to be similar to those of solvent washed specimens. The solvent washed 
tribofilms, comprising sulphide and phosphate layers, exhibited an elastoplastic behaviour 
and, during the loading stage of the indentation, the hardness and the Young's modulus of the 
phosphate layer increased from their initial values of about 2 GPa for the hardness and 
between 30 and 40 GPa for the Young's modulus. In particular, the initial hardness of the 
polyphosphate layer at the beginning of the indentation tests was close to the mean applied 
pressure during the films generation. This suggested that the layer accommodated the contact 
pressure in the tribotest or during the loading stage of the indentation, and could thus be 
regarded as a final and local pressure sensor. The characteristics of the full ZDTP films ensure 
gradual changes in mechanical properties between the substrate, bonding layers and outer 
layers with the viscous overlayer serving as the tribofilm's precursor. The properties of these 
layered films can thus adapt to a wide range of imposed conditions and provide appropriate 
level of resistance to contact between the metal surfaces. As the severity of loading increases, 
so too do the resistive forces within the film. This ensures that the shear plane remains located 
inside the ZDTP protective film, which explains the exceptional efficiency of ZDTP films as 
anti-wear films.  
3. Experimental 
3.1. Tribofilms 
The tribofilms were generated at Shell Research and Technology Centre, Thornton, U.K., 
with a reciprocating Amsler machine [17] designed to simulate the contact conditions of the 
cam/follower system in an internal combustion engine valve train. A flat block specimen 
(8 mm x 8 mm size, 4 mm thick) has a reciprocating motion in loaded contact with a rotating 
disc. The block and the disc were made in through-hardened EN31 steel. Special care was 
taken with the roughness of the blocks which were polished until the average roughness was 
Ra = 0.01 µm. The movement of the block was driven by a crank linked to the motion of the 
disc axis through a gearbox. The block motion was approximately sinusoidal and at the same 
frequency as the disc rotation. Load was applied to the contact by a spring arrangement, 
acting through a roller bearing. The surface in contact with the loading bearing (rear surface 
of the reciprocating element) was curved to permit self-alignment between the block and the 
disc. The films were generated at a normal load of 400 N (mean contact pressure of 0.36 
GPa), speed of 600 rev/min., block temperature of approximately 100°C for 5 hours. The 
lubricants consisted of a highly refined base oil with different commercial additives (details of 
the oil formulation are not relevant to the present work): 
- MoDTC solution, 
- ZDTP + MoDTC solution, 
- ZDTP + MoDTC + detergent/dispersant solution ("full formulation"). 
The rubbing area on the polished block was typically 5 mm long in the sliding direction. 
Previous analyses have shown that the composition in the centre of the wear track was 
reasonably uniform, while the composition within 1 mm of the ends of the wear track could 
vary significantly. The mechanical measurements on the films with the Surface Force 
Apparatus have been performed in the central area of the wear track. An additional unworn 
and polished block was used to obtain reference values for the EN31 steel substrate. 
To preserve the film structures, the blocks were stored in the base oil (containing 
predominantly paraffinic hydrocarbons, with very low concentration of polar compounds) 
immediately after production of the films in the reciprocating Amsler tests and they were 
immersed again, when not in use. 
3.2. Surface Force Apparatus 
The Ecole Centrale de Lyon Surface Force Apparatus (SFA) used in these experiments has 
been described in previous publications [18, 19]. The general principle is that a macroscopic 
spherical body or a diamond tip can be moved toward and away from a planar one (the ZDTP 
specimen) using the expansion and the vibration of a piezoelectric crystal, along the three 
directions, Ox, Oy (parallel to the plane surface) and Oz (normal to the plane surface). The 
plane specimen is supported by double cantilever sensors, measuring quasi-static normal and 
tangential forces (respectively Fz and Fx). Each of these is equipped with a capacitive sensor. 
The sensor's high resolution allows a very low compliance to be used for the force 
measurement (up to 2 x 10-6 m/N). Three capacitive sensors were designed to measure relative 
displacements in the three directions between the supports of the two solids, with a resolution 
of 0.01 nm in each direction. Each sensor capacitance was determined by incorporating it in 
an LC oscillator operating in the range 5 - 12 MHz [20]. 
3.3. Tests methodology 
All the experiments were conducted at room temperature. Preliminary results obtained on 
anti-wear films from a ZDTP solutions have shown that n-heptane washing damages the film 
[16]. That is why the blocks were tested first as obtained from the Amsler friction test, 
without any cleaning and second after washing with n-heptane. The unwashed specimens 
were mounted on the SFA as taken from the storage base oil. Excess of base oil was simply 
removed by placing the side of the specimen on absorbing paper, which allowed the surface to 
be always preserved by an oil film (thickness > 10 µm). 
Nanoindentation tests 
The aim of these tests was to determine the elastoplastic properties of the tribofilms (hardness 
and Young's modulus) and their “mechanical” structure (number of layers and estimation of 
the thickness of each layer that constitutes the film). The method used to perform 
nanoindentation experiment with the SFA has already been published in detail [21]. Specific 
procedures have been developed for the characterisation of ZDTP tribofilms and have been 
described in previous papers [16, 22]. In this study, the determination of the near surface 
mechanical properties (first nanometers) was obtained through a specific tip shape calibration, 
performed on a gold film deposited by magnetron sputtering onto a silicon substrate. This 
film was very smooth (peak to valley roughness around 1 nm, measured on a scan length of 
1 µm) and its hardness was constant versus depth from the surface and until the penetration 
depth equals the gold film's thickness [21]. 
For the nanoindentation experiments, a trigonal diamond tip with an angle of 115.12° between 
edges (Berkovitch type) was used. The indentation tests were performed in controlled 
displacement mode. The standard set-up included the continuous quasi-static measurements 
of the resulting normal force Fz versus the normal displacement Z, at a slow penetration 
speed, generally 0.1 to 0.5 nm/s. It also included the simultaneous measurements of the 
rheological behaviour (dissipative and conservative or elastic contributions) of the tested 
surface, thanks to simultaneous small sinusoidal motions at a frequency of 37 Hz, with an 
amplitude of about 0.2 nm RMS. Furthermore, using the Z feedback in the constant force 
mode and the tangential displacement of the indenter, the surface topography was imaged 
before and after the indentation test, with the same diamond tip. This was made practically 
possible because of the partial elastic recovery during the unloading cycle and hence the 
geometry of tip and indent were different which was necessary to permit resolution of the 
indent. For this scanning procedure, a constant normal load of 0.5 µN was typically used. 
Such in-situ imaging procedure enables the operator to choose precisely the location of the 
indentation test on the surface and, after the test, to quantify the plastic pile-up around the 
indent and thus to measure the actual contact area. 
Rheological film model 
The elastic properties of the films were very difficult to extract from the indentation tests 
because of both the influence of the substrate and of the film structure itself. They were 
obtained through the stiffness measurements, which are global (film+substrate) 
measurements. To extract the properties of each layer of the film, a simple model has been 
developed, and its main features are described as follows. The experimental stiffness versus 
normal displacement curve was identified with the elastic response of a structure composed of 
one or two homogeneous elastic layers on a substrate (semi-infinite elastic half space) 
indented by a rigid cylindrical punch of radius a. For such a system, modelled by two springs 
connected in series [23], the calculated global stiffness (Kz) depends on the reduced Young's 
modulus of the substrate (Es* with Es*=Es/(1-νs2)), measured on an unworn steel block, over 
the contact radius (a) and depends also on four unknown parameters which are the reduced 
Young's modulus (Ef*, Ef*=Ef/(1-νf2)) and the thickness (t) of each layer. For each test, their 
values were adjusted to obtain a good fit between the measured stiffness curve and the 
calculated one. This procedure provided the structure (one or two layers), the thickness and 
the reduced Young's modulus of each layer that constituted the tribofilms. Details are given in 
a previous paper [16]. Following this model, the global stiffness of a single layer system is 
given by:  
22K t a
a E aEz f s
π π * *
 (1) 
This simple model describes perfectly the behaviour of model systems such as gold layers on 
a silicon substrate [21]. In the case of tribofilms, deviations may be observed at a critical 
pressure or at a critical depth from which the experimentally measured stiffness may be found 
to exceed significantly the theoretical one. This is interpreted as a change in the surface 
properties due to the applied pressure and appears to be related to a measured hardness 
increase. Indeed, as the applied pressure can reach values much larger than the initial hardness 
value of the surface, the resulting plastic flow may induce a small volume reduction and 
molecular rearrangements which could be sufficient to induce a noticeable change in the 
mechanical properties. From a threshold pressure value, H0, the stiffness curve was then 
influenced both by the substrate's elasticity and by the change in mechanical properties. This 
pressure dependence can be introduced in the model by writing that in the deformed volume 
of material, when H>H0 (i.e. when the film accommodates the applied pressure through 
hardness increase), the film modulus Ef* is proportional to the hardness (the ratio Ef*/H 
remains constant). It gives the following equation: 
EE =  (2) 
Ef0* is the reduced Young's modulus value, when the applied pressure is equal to or lower 
than the threshold pressure H0. When necessary, by introducing this effect in our modelling 
and by adjusting the value of the threshold pressure, we were able to fit correctly the whole 
stiffness curve. An example of such fit is given on figure 1. The evolution of the film modulus 
Ef* versus plastic depth can also be extracted from equation 1 using the experimentally 
measured global (film+substrate) stiffness values Kz and the film's thickness, t, independently 
of equation 2. This permits a check on whether it is proportional to the hardness as assumed in 
equation 2. In the example shown figure 2, the calculated Young's modulus of the film (from 
equation 1 with a film thickness t=25 nm) is found to be proportional to the measured 
hardness with a mean ratio Ef*/H=16.5, in good agreement with the ratio 
Ef0*/H0=17/1.05=16.2 obtained from the stiffness fit. 
Full formulation (ZDTP+MoDTC+detergent/dispersant)
Solvent washed tribofilm
0 10 20 30 40 50
Penetration depth (nm)
Measured stiffness
Calculated stiffness, t=25 nm, Efo*=17 GPa,
without pressure accommodation
Calculated stiffness, t=25 nm, Efo*=17 GPa,
with pressure accommodation, Ho=1.05 GPa
Figure 1: Example of application of the rheological film model: measured and calculated 
global stiffness for a tribofilm obtained from the full formulation (MoDTC + ZDTP + 
detergent/dispersant). A good fit between the measured and the calculated values is obtained 
with a single layer system (thickness t=25 nm and reduced Young's modulus Ef0*=17 GPa) 
and a pressure accommodation effect from a threshold pressure H0=1.05 GPa. 
t = 25 nmEf0* = 17 GPa H0 = 1.05 GPa
Full formulation (ZDTP+MoDTC+detergent/dispersant)
Solvent washed tribofilm
0 10 20 30 40 50 60
Plastic depth (nm)
Reduced Young's modulus of the tribofilm, Ef*
Hardness of the tribofilm, H
t = 25 nmEf0* = 17 GPa H0 = 1.05 GPa
Full formulation (ZDTP+MoDTC+detergent/dispersant)
Solvent washed tribofilm
0 10 20 30 40 50 60
Plastic depth (nm)
Reduced Young's modulus of the tribofilm, Ef*
Hardness of the tribofilm, H
Figure 2: Example of evolution of film's reduced Young's modulus and hardness versus 
plastic depth, for a tribofilm obtained from the "full formulation" (MoDTC + ZDTP + 
detergent/dispersant). The Young's modulus of the film is calculated using equation 1 with the 
measured stiffness values and using only the film's thickness determined from the fit shown 
figure 1 (t = 25 nm). 
Nanofriction experiments 
Nanofriction experiments were conducted on the blocks by moving the diamond tip along Ox 
direction (parallel to the surface) at low speed (2 to 5 nm/s) along a distance of 0.5 µm. The 
objective of these tests was to determine how the friction coefficient varies as a function of 
the applied pressure. The tests were conducted at monitored increasing depth. During the 
tests, the normal, Fz, and the tangential, Fx, forces were recorded, which allowed us to 
calculate the apparent friction coefficient µ=Fx/Fz (see example figure 3). 
Full formulation (ZDTP+MoDTC+detergent/dispersant)
Solvent washed tribofilm
0 20 40 60 80 100 120 140
Time (s)
µ=Fx/Fz
0 5 10 15 20 25
Penetration depth (nm)
Indentation test
Nanofriction test Smaller contact area
Figure 3: Procedure used for the nanofriction tests. The diamond tip is oriented edge first and 
the nanofriction tests are conducted at monitored increasing depth. During the test, the 
normal (Fz) and tangential (Fx) forces are recorded. The friction coefficient µ=Fx/Fz is 
calculated. 
Large pile-up was observed in the case of nanofriction with the diamond tip oriented face 
first, which may induce large uncertainty in the calculation of the contact area. That is why 
the nanofriction tests were conducted edge first. In these conditions, the estimation of the 
applied pressure at a given depth was obtained using low load nanoindentation tests, made in 
the near proximity of the nanofriction tests. Assuming that, at a given depth, the hardness of 
the tribofilm should be the same for the friction test and for the near indentation test, the 
contact area, and then the applied pressure, were obtained from the difference between the 
normal force measured for the two tests at the same depth (see insert on figure 3). Using the 
in-situ imaging procedure, figure 4 shows an example of an image of the surface of a 
tribofilm after such a nanofriction test. 
100 nm
Beginning
of the testBeginning of the wear
Direction of friction
100 nm
Beginning
of the testBeginning of the wear
Direction of friction  
Figure 4: Typical image of the surface 
of a tribofilm after a nanofriction 
experiment. The image is obtained with 
the in-situ imaging procedure. 
4. Results 
The first part presents the mechanical properties of the different tribofilms, determined from 
the nanoindentation experiments. Their structure, one or two layers, and their thickness were 
deduced from the use of our rheological film model. 
The results concerning the frictional behaviour of the tribofilms are given in a second part. 
4.1. Structure and mechanical properties of the tribofilms 
MoDTC tribofilms 
The tribofilm obtained from base oil + MoDTC has been tested without washing and after 
washing with n-heptane. Even on the solvent washed block, it was not possible to make any 
local topographic image nor line scanning preliminary to the indentations tests, revealing that 
the film was very soft and was easily damaged by the diamond tip. Representative hardness 
curves obtained on the MoDTC tribofilms are shown on figure 5. 
MoDTC tribofilm
0 40 80 120 160
Plastic depth (nm)
Unwashed tribofilm
Solvent-washed tribofilm
Figure 5: Typical hardness curves obtained on the MoDTC tribofilms. Open symbols 
correspond to hardness curves obtained on the unwashed film. Black symbols correspond to 
hardness curves obtained on the solvent-washed film. 
Very low mechanical properties were measured on the unwashed MoDTC tribofilm. The 
surface hardness ranged from 0.02 to 0.1 GPa indicating the presence of a very soft overlayer 
covering the tribofilm. 
After washing with n-heptane, the indentation tests showed that this overlayer has been 
removed by the washing procedure. The remaining tribofilm was a soft homogeneous layer, 
whose hardness was typically in the range 0.4 - 0.5 GPa at the beginning of the tests. 
Adhesion to the diamond tip was detected at the end of the unloading part of the tests. The 
film thickness and the structure (number of layers) have been obtained from the stiffness 
measurements performed during the experiments using the rheological film model. 
The film appeared to be homogeneous in its thickness, and for most of the tests, its elastic 
behaviour corresponded to that of a single layer, with constant properties versus depth. The 
thickness of the film was found to be between 30 and 75 nm. The reduced Young's modulus 
was typically equal to 7 – 8 GPa. 
ZDTP + MoDTC tribofilms 
From optical observation, the ZDTP + MoDTC unwashed film was very thin. This was 
confirmed by the indentation tests. Prior to any contact, a very soft layer, 60 to 120 nm thick, 
was detected at the surface of the unwashed film. 
Indentation tests conducted after scanning or imaging the surface of the unwashed film 
("mechanical sweep") showed that the film was spatially heterogeneous. Its thickness and its 
mechanical properties varied depending on the test location: 
- In some places, only a very thin layer (a few nanometers thick) with a reduced Young's 
modulus of 50 GPa covered the work-hardened steel substrate (tests A and B on figure 6). 
- A thicker layer (15 to 30 nm) with a reduced Young's modulus of 50 – 80 GPa was found 
in other places (tests C and D on figure 6), sometimes with accommodation pressure 
effect (threshold pressure H0 = 4.8 GPa). Such layer behaves like the sulphide-oxide layer 
of the ZDTP tribofilm [16]. 
- Elsewhere, the structure of the tribofilm was more complex, with a soft layer covering a 
stiffer one. For example, test E on figure 6 corresponds to a soft layer, 12 nm thick, with 
properties comparable to those of the MoDTC tribofilm (hardness of 0.2 GPa and reduced 
Young's modulus of 5 GPa) which covers a stiffer layer, 18 nm thick, with a reduced 
Young's modulus of 50 GPa. 
This heterogeneity was confirmed by the indentation tests conducted on the solvent-washed 
ZDTP + MoDTC tribofilm, where, at least, three different types of film were identified: 
- In some places, the film behaved like a one layer system, able to accommodate the 
pressure (pressure threshold 2.8 GPa). Its thickness was between 35 nm and 150 nm. The 
surface hardness was about 2 – 3 GPa and the reduced Young's modulus was about 
55 - 65 GPa. 
- In other places, the film behaved like a bilayered structure: a surface layer, about 25 nm 
thick, with properties comparable to those of the MoDTC tribofilm (hardness of 
0.3 - 0.4 GPa, reduced Young's modulus of 8 GPa), covers a stiffer layer, 150 nm thick, 
with a reduced Young's modulus of about 80 GPa. 
- Elsewhere, the surface film was between 3 and 15 nm thick, with properties comparable to 
the lower properties measured on the ZDTP tribofilm (hardness about 1 – 1.5 GPa and 
reduced Young's modulus about 10 GPa). For some tests, this surface film was able to 
accommodate the pressure, with a pressure threshold of 1 – 1.5 GPa. It covers a stiffer 
layer, 10 to 55 nm thick, with a reduced Young's modulus varying from 60 to 110 GPa. 
ZDTP+MoDTC tribofilm
Unwashed block
0 20 40 60 80 100 120
Plastic depth (nm)
Prior to any contact
After imaging - test A
After imaging - test B
After imaging - test C
After imaging - test D
After imaging - test E
Figure 6: Representative hardness curves obtained on the unwashed ZDTP + MoDTC 
tribofilm, prior to any contact and after the imaging procedure. The film is spatially 
heterogeneous in thickness and in mechanical properties. 
ZDTP + MoDTC + detergent/dispersant tribofilms ("full formulation" tribofilms) 
Nanoindentation tests performed in fresh areas, prior to any contact showed that, at the 
surface of the unwashed tribofilm, there was a very soft layer, mobile under the diamond tip, 
with an apparent thickness of a few hundreds of nanometers. 
Representative hardness curves obtained on the unwashed block near these initial contacts are 
shown figure 7. Contrary to the ZDTP + MoDTC tribofilm, the film was found to be spatially 
homogeneous. Only its thickness was found to vary, depending on the tested area. A very thin 
softer layer was detected at the surface of the tribofilm, which did not resist to imaging nor 
scanning, except if the normal load was very low (lower than 0.3 µN). This layer had a 
hardness value (about 0.3 – 0.4 GPa) comparable to the hardness value of the MoDTC 
tribofilm. The observed large hardness increase when the load increased also indicated that 
the tribofilm had a great capability to accommodate the applied pressure. This result was 
confirmed by the interpretation of the stiffness measurements using the rheological model, 
which also showed that the tribofilm had a complex structure. At its surface, there was first a 
layer with a thickness of only a few nanometers (2 nm to 7 nm) and a reduced Young's 
modulus of 10 - 15 GPa. Then, there was a second layer (thickness between 20 nm and 140 
nm) with a higher reduced Young modulus of 65 – 80 GPa. 
A similar tribofilm was tested after n-heptane washing. It also had a great ability to 
accommodate the applied pressure. From the stiffness measurements, on most places, the film 
was found to behave like a film constituted by two layers. The surface layer was thin (5 to 25 
nm) with a reduced Young's modulus value in the range 15 – 20 GPa. The thickness of the 
underlayer was found to vary between 0 (no underlayer, example of figures 1 and 2) and 100 
nanometers and its elastic modulus was in the range 110 - 120 GPa. 
ZDTP + MoDTC + detergent/dispersant
Unwashed tribofilm
0 10 20 30 40 50 60
Plastic depth (nm)
First test, prior to any contact
Without preliminary scanning
After scanning or imaging
Figure 7: Representative hardness curves obtained on the unwashed ZDTP + MoDTC + 
detergent/dispersant tribofilm ("full formulation"), prior to any contact and in the region near 
the first contacts, either without preliminary surface scanning or after scanning/imaging 
procedure. 
Figure 8 compares representative hardness curves for all tested tribofilms. For the ZDTP + 
MoDTC tribofilms, three curves are plotted because of the variety of obtained results 
revealing the spatial heterogeneity of this tribofilm. A representative hardness curve for the 
ZDTP tribofilm tested in the same conditions in a previous study [16] has been added for 
comparison. 
0 5 10 15 20 25 30 35 40
Total penetration depth (nm)
ZDTP, solvent washed MoDTC, solvent washed
ZDTP + MoDTC, unwashed (2 tests) ZDTP + MoDTC, solvent washed
Full formulation, unwashed Full formulation, solvent washed
Figure 8: Comparison of the hardness curves obtained on the different tribofilms. The 
hardness curve obtained for a ZDTP anti-wear tribofilm obtained from a previous study is 
plotted for comparison. 
4.2.  Nanofriction experiments 
Nanofriction experiments were conducted on the three preceding tribofilms and also on a 
ZDTP tribofilm and on a ZDTP + detergent/dispersant tribofilm. In order to simplify the 
following graphs, only one representative curve was plotted for each tribofilm (or two when it 
was necessary to illustrate the dispersion when it was significant). 
Figure 9 shows the evolution of the friction force versus the normal force for the tested 
tribofilms. For a given formulation, there was very little difference between the results 
obtained on unwashed and on solvent washed tribofilms at low load, indicating that the 
solvent washing does not seem to affect the frictional behaviour of the tribofilm. This agrees 
with the idea that the soft viscous overlayer is supposed to serve as precursor for the tribofilm 
rather than that it plays a mechanical role during friction. 
0 3 6 9 12 15
Normal force, Fz (µN)
ZDTP, solvent washed ZDTP + Det/Disp, solvent washed
MoDTC, solvent washed ZDTP + MoDTC, solvent washed
ZDTP + MoDTC, solvent washed ZDTP + MoDTC, unwashed
Full formulation, unwashed Full formulation, solvent washed
Figure 9: Friction force (Fx) versus normal force (Fz) during nanofriction tests with 
increasing penetration depth for different tribofilms. 
It is also worth noting that the heterogeneity in mechanical properties found on the ZDTP + 
MoDTC tribofilm also exists in the frictional properties. For this tribofilm, the friction force 
at low normal loads may be comparable either to the friction force obtained for the ZDTP 
tribofilm or to the friction force obtained for the "full formulation" tribofilm. 
Under the present testing conditions, it can be observed that the lower friction forces were 
obtained for films containing MoDTC together with ZDTP. The higher were obtained for the 
tribofilm from MoDTC alone. 
Figure 10 shows the evolution of the friction coefficient versus mean pressure. The existence 
of low friction coefficient values (0.01<µ<0.05) appears to be related both to the presence of 
MoDTC additive in the initial lubricant and to the ability for the tribofilm to reach sufficiently 
high pressure values (1.5 – 3 GPa) during the friction test. Thus, the MoDTC tribofilm, which 
is not able to resist to the contact pressure by increasing its mechanical properties seems to be 
ineffective in reducing friction, contrary to the tribofilms containing ZDTP and MoDTC 
together, which are able to accommodate the contact pressure by increasing their mechanical 
properties. Nevertheless, both behaviours (high or low friction) were observed for the ZDTP 
+ MoDTC tribofilms. This is certainly due to the spatial heterogeneity of these tribofilms, 
which behave on some places like ZDTP tribofilms, or elsewhere like "full formulation" 
tribofilms. It was also observed that tribofilms formed without MoDTC were ineffective in 
reducing friction even if high contact pressures were reached during the friction tests. 
0 1 2 3 4 5 6
Mean pressure P (GPa)
ZDTP, solvent washed ZDTP + Det/Disp, solvent washed
MoDTC, solvent washed ZDTP + MoDTC, solvent washed
ZDTP + MoDTC, solvent washed ZDTP + MoDTC, unwashed
Full formulation, unwashed Full formulation, solvent washed
Figure 10: Apparent friction coefficient versus mean pressure for the different tested 
tribofilms. 
When the evolution of the friction coefficient is plotted versus penetration depth (figure 11), it 
appears that, when it existed, the low friction coefficient domain was detected a few 
nanometers below the surface of the tribofilm. It also shows that, for the full formulation, the 
low friction domain was deeper for the unwashed tribofilm than for the solvent washed one. 
The unwashed tribofilm appears to be covered by a surface layer with rather bad frictional 
properties, which can be removed by solvent washing or by "mechanical" sweep (low load 
scanning procedures for example). 
0 2 4 6 8 10 12 14 16 18 20
Penetration depth (nm)
ZDTP, solvent washed ZDTP + Det/Disp, solvent washed
MoDTC, solvent washed ZDTP + MoDTC, solvent washed
ZDTP + MoDTC, solvent washed ZDTP + MoDTC, unwashed
Full formulation, unwashed Full formulation, solvent washed
Figure 11: Apparent friction coefficient versus penetration depth for the different tested 
tribofilms. 
5. Discussion 
Because of the inhomogeneous and patchy nature of anti-wear tribofilms and of their low 
thickness, very few results are published concerning their mechanical properties [24-28]. 
Moreover, the differences in sample preparation and the diversity of used techniques and 
experimental procedures render delicate the comparison of the obtained results. For example, 
the Young’s modulus values given by Aktary et al. for a ZDTP tribofilm [28] are significantly 
higher that those we measured but one explanation can be that they did not take into account 
the substrate’s elasticity in their calculations, contrary to what is done in the current study. Or 
if we attempt to compare our results with those recently published by Ye et al. on ZDTP and 
ZDTP + MoDTC tribofilms [14, 15], this reveals significant differences. For example, Ye et 
al. found that both tribofilms possess the same hardness and modulus depth distributions, 
corresponding to continuously and functionally graded materials, when in the present work, 
the hardness curves for similar tribofilms did not coincide and the use of our rheological film 
model allowed us to describe the tribofilms as layered materials with properties adaptable to 
contact conditions. The hardness and modulus values, respectively 10 GPa at a contact depth 
of 30 nm and 215 GPa at a depth of 20 nm, that they reported are also significantly higher 
than those we measured and also higher than those given by Aktary et al. This could be due to 
differences in sample preparation and also certainly to the use of different methods and 
assumptions for the treatment of the nanoindentation data. 
Concerning the frictional behaviour of the tribofilms, the presented nanofriction tests were 
conducted in unlubricated conditions, at very low speed (2 to 5 nm/s) and the measured 
nanofriction coefficients corresponded to the friction between the diamond tip and the 
tribofilm (over its steel substrate). That is why it also seems difficult to compare our values to 
macroscopic friction coefficient values obtained on classical tribometers. The latter are 
representative of steel on steel contact in the presence of a tribofilm and are averaged over the 
whole contact surface. However, our local values are not far from the end of test Amsler 
macroscopic friction coefficient values published by Pidduck and Smith [25] for ZDTP, 
ZDTP + detergent/dispersant and ZDTP + friction modifier tribofilms. Moreover, these 
macroscopic values were found to be proportional, with a factor 0.7, to micro-friction 
coefficient values measured with Lateral Force Microscopy by the same authors, making 
them suggest that there may be a link between macro and micro-frictional behaviour of 
smooth regions of anti-wear tribofilms. Unfortunately, no tribofilm obtained from friction 
modifier alone were tested in this study, with which we could compare our results. 
Nevertheless, macroscopic friction coefficient values, in the range 0.10 – 0.14, measured on 
an alternative ball on plane tribometer were reported by Muraki and Wada [6] for oil 
containing MoDTC alone. They conclude that such lubricant was ineffective in reducing 
friction, contrary to the oil containing MoDTC together with ZDTP. More recently, similar 
high macroscopic friction coefficient values (in the range 0.095 – 0.2) were measured by 
Unnikrishnan et al. for oil containing MoDTC alone [29]. On the other hand, Grossiord et al. 
reported very low steady-state friction coefficient (0.04) measured for base oil + MoDTC 
during SRV friction tests, and a lower steady-state value (0.02) for friction tests in a UHV 
tribometer, carried out by sliding a macroscopic hemispherical steel pin again a flat covered 
by a MoDTC tribofilm [13]. From tests carried out in a high frequency reciprocating rig, 
Graham et al. [30] also reported that, in the absence of ZDTP, MoTDC additives were 
effective in reducing friction at a combination of high additive concentration and high 
temperature (up to 0.4% wt. and 200°C). Such diversity of results, certainly partly due to the 
various tests conditions, makes unreasonable a comparison between the very high 
nanofriction coefficient measured on the MoDTC tribofilm under the present testing 
conditions and those published values. As, regarding the literature, the formation of MoS2 was 
well established for MoDTC containing lubricants, the question is how can we explain such 
high friction coefficient during the nanofriction tests ? Or what caused the very low friction 
observed when ZDTP was used together with MoDTC ? From figure 10, the low friction 
coefficient values (0.01<µ<0.05) were observed for the MoDTC containing lubricants when 
the contact pressure was in the range 1.5 – 3 GPa (the question of the spatial heterogeneity of 
the ZDTP + MoDTC tribofilm will be discussed latter). These high pressures were measured 
for tribofilms able to increase their mechanical properties, thus accommodating the contact 
conditions, which was demonstrated to be the case for ZDTP anti-wear tribofilms [16]. On the 
other hand, high pressures were not reached for the soft MoDTC tribofilm. Thus, the easy 
sliding of the MoS2 sheets could result from a favourable orientation induced by sufficiently 
high contact pressure values. The ability of MoS2 sheets to orient in a favourable direction 
was reported by Grossiord et al. [31] and Martin et al. [32], who recently investigated 
tribochemical interactions between ZDTP, MoDTC and OCB (overbased detergent calcium 
borate) additives. Using high resolution TEM observations of wear debris, coupled with wear 
scar micro-spot XPS analysis, they observed perfectly oriented MoS2 sheets, with their basal 
plane parallel to the flaky wear fragments. Such "mechanical" interpretation of the role of the 
contact pressure agrees with previous work of Muraki et al. who studied the effect of roller 
hardness on the rolling sliding characteristics of MoDTC in the presence of ZDTP and 
concluded that the friction reduction effect increased with higher degree of roller hardness 
[10]. Yamamoto also reported that a necessary condition for improving the friction and wear 
characteristics of a lubricant was the formation of surface films composed of iron phosphates 
with high hardness and Mo-S compounds [11]. Concerning the spatial heterogeneity of the 
ZDTP + MoDTC tribofilms, it can be worth noting that using high resolution TEM 
observations of wear debris collected after friction tests, coupled with AES and XPS studies 
of rubbing surfaces, Grossiord et al. described the ZDTP + MoDTC tribofilm as being 
composed of a mixture of glassy zinc phosphate zones containing molybdenum, and carbon-
rich zones containing zinc and highly-dispersed MoS2 single sheets [13, 33]. 
The observation that, during the nanofriction tests, the low friction domain was located a few 
nanometers below the surface also corroborates this interpretation. As the nanofriction tests 
were conducted at increasing depth, the sufficiently high pressures were obtained after a few 
nanometers penetration depth inside the MoS2 containing layer (with properties similar to the 
MoDTC tribofilm), thanks to the presence of the underneath resisting anti-wear layer, whose 
characteristics are similar to those of the phosphate layer of the ZDTP tribofilm. 
Finally, combining the results obtained from the nanoindentation and nanofriction 
experiments, we can propose a possible schematic description of the anti-wear tribofilms 
obtained from the "full formulation" oil. Some assumptions are also made on what happened 
during nanofriction tests on such tribofilms (see figure 12 on which for convenient drawing, 
as the Berkovitch diamond tip is not sharp, it was represented by a flat punch). 
A soft layer containing non-oriented MoS2 sheets is present at the surface of the tribofilm 
(layer (a) in figure 12). This layer, 0 to 25 nm thick, has mechanical properties comparable 
with those of the MoDTC tribofilm (0.3 – 0.5 GPa for the hardness and 3 – 10 GPa for the 
reduced Young's modulus). Its friction coefficient is rather high. This layer is easily damaged 
or removed by the diamond tip during imaging or line-scanning procedures. When the contact 
pressure is sufficiently high, friction induces a favourable orientation of the MoS2 sheets, over 
a thickness of 1 or 2 nanometers (layer (b) in figure 12), resulting in very low friction 
coefficient values which combine with the anti-wear efficiency of the tribofilm. Under this 
layer, there is then an anti-wear layer (layer (c) in figure 12), with properties similar to those 
of the polyphosphate layer of the ZDTP tribofilm. Then, just over the substrate (noted (e)in 
figure 12), there is a bonding layer (layer (d) in figure 12) with high mechanical properties 
(oxides, sulfides). 
Figure 12: Possible schematic description of the anti-wear tribofilm obtained from the "full 
formulation" and orientation of the MoS2 planes of the outer layer resulting from a 
nanofriction tests (for convenient drawing, as the Berkovitch diamond tip is not sharp, it was 
represented by a flat punch). The thickness of each layer is arbitrary drawn as it varies 
significantly depending on the tested area (from zero when the layer is not present to a few 
tens of nanometers). 
(a) Soft layer containing non-oriented MoS2 sheets, with mechanical properties comparable 
to those of the MoDTC tribofilm, 
(b) Layer of favourably frictionally oriented MoS2 sheets with a typical thickness of 1 or 
2 nm, 
(c) Layer with properties similar to those of the polyphosphate layer of the ZDTP tribofilm, 
(d) Bonding layer with high mechanical properties (oxides, sulfides), 
(e) Steel substrate. 
6.  Conclusions 
Thanks to the combined used of (i) nanoindentation experiments with continuous stiffness 
measurements coupled with imaging procedures, (ii) a specifically developed rheological film 
model and (iii) nanofriction tests, synergistic effects of ZDTP and MoDTC on frictional 
behaviour of anti-wear tribofilms have been evidenced from mechanical considerations. One 
original feature of this study lies in the characterisation of unwashed anti-wear tribofilms with 
their full structure preserved. 
The structure and nanomechanical properties (hardness and reduced Young's modulus) of 
tribofilms formed with different mixtures of additives (ZDTP, MoDTC, detergent/dispersant) 
were first determined. 
Concerning the occurrence of very low friction (0.01<µ<0.05), the contact pressure was found 
to be a critical parameter. The low friction coefficient values were attributed to a favourable 
orientation of MoS2 sheets present in the outer layer of the tribofilms formed from MoDTC 
containing lubricants. Such a favourable orientation occurred only if sufficiently high contact 
pressure was reached. These high contact pressures were attained when ZDTP was used as oil 
additive together with MoDTC because one of the main characteristics of ZDTP additives is 
to form protective anti-wear tribofilms under boundary lubrication, with varying structure and 
properties with depth, among which is an amazing ability to increase their mechanical 
properties, thus accommodating the contact conditions. 
A possible schematic description of the tribofilms containing both ZDTP and MoDTC was 
deduced and a mechanism was proposed to account for the mechanical synergy that occurs 
during nanofriction tests on such tribofilms. 
Aknowledgement 
The authors thank Shell Research Limited for financial support and permission to publish. 
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  The layered structure and the rheological properties of anti-wear films,
generated in a rolling/sliding contact from lubricants containing zinc
dialkyldithiophosphate (ZDTP) and/or molybdenum dialkyldithiocarbamate (MoDTC)
additives, have been studied by dynamic nanoindentation experiments coupled
with a simple modelling of the stiffness measurements. Local nano-friction
experiments were conducted with the same device in order to determine the
evolution of the friction coefficient as a function of the applied pressure for
the different lubricant formulations. For the MoDTC film, the applied pressure
in the friction test remains low (<0.5 GPa) and the apparent friction
coefficient is high ($\mu$ > 0.4). For the tribofilms containing MoDTC together
with ZDTP, which permits the applied pressure to increase up to a few GPa
through some accommodation process, a very low friction domain appears (0.01 <
$\mu$ < 0.05), located a few nanometers below the surface of the tribofilm.
This low friction coefficient is attributed to the presence of MoS2 planes
sliding over each other in a favourable configuration obtained when the
pressure is sufficiently high, which is made possible by the presence of ZDTP.

Introduction 
In addition to zinc dialkyldithiophosphate (ZDTP) additives, extensively used for their 
exceptional antioxidant and anti-wear properties under boundary conditions in automotive 
engines, lubricating oils contain several additives, among which there are detergent and 
dispersant additives whose main role is to keep oil insoluble contaminants and degradation 
products in suspension, at elevated temperature for the detergent additives, and at low 
temperatures for the dispersant ones. Organo molybdenum compounds such as molybdenum 
dithiocarbamate (MoDTC) are also used as friction modifiers for energy saving. However, 
when used together in formulated oils, additives interact in various ways resulting either in 
synergies or in adverse effects affecting the oil performance regarding anti-wear and friction 
behaviour, and modifying the characteristics of the protective surface films generated during 
friction (tribofilms). A lot of investigations have been conducted to evaluate the performances 
of additive mixtures and to determine the composition of associated tribofilms. Several 
factors were identified as playing a role: additive structure [1, 2], additives concentration [3-
6], base oil nature [7, 8], …, or combinations of these parameters. A detailed review on 
published information on that topic was written by Willermet [9]. Non-chemical parameters 
such as characteristics of the solid antagonists (hardness, roughness) or test conditions (load, 
temperature, sliding speed) [3, 10] also might influence the additive interactions. 
Among this variety of additive interactions, we will focus on that between ZDTP and 
MoDTC, extensively studied through chemical investigations. All published works agree 
upon the fact that friction and anti-wear performances of oils are improved when ZDTP and 
MoDTC are used together. The formation of molybdenum disulphide (MoS2) on the rubbing 
surfaces has been evidenced by several authors [11, 12]. Using UHV friction tests, coupled 
with high-resolution TEM observation of wear debris and spectroscopic studies, Grossiord et 
al. has given evidence for the mechanism of single MoS2 sheet lubrication [13]. 
The aim of this paper is to enlarge the knowledge of the local mechanical and frictional 
properties of anti-wear tribofilms to those of films obtained from lubricants containing 
different additives (ZDTP, MoDTC, detergent/dispersant) or mixtures of additives, in order to 
explore the ZDTP/MoDTC synergy on a mechanical point of view. The only published results 
on that topic are the recent papers from Ye et al. who performed AFM observations and 
nanoindentation measurements on ZDTP and ZDTP + MoDTC tribofilms [14, 15]. 
In the present study, nanoindentation tests with continuous stiffness measurements were 
performed on unwashed and solvent-washed tribofilms to determine their mechanical 
properties. The frictional behaviour of the tribofilms was investigated through local 
nanofriction experiments, conducted with the same device. The evolution of the friction 
coefficient as a function of the applied pressure for the different lubricant formulations 
leading to different tribofilms has been determined. 
2. Preliminary results obtained on ZDTP anti-wear tribofilms 
The structure and the rheological properties of anti-wear films from a zinc 
dialkyldithiophosphate (ZDTP) solution generated in a rolling/sliding contact, simulating 
engine valve train conditions, have been studied in detail with analytical and surface force 
tools and the results have been published by the authors in a previous paper [16]. As preamble 
to the present paper, only the main points are summarised here. 
The ZDTP solution was a commercial secondary alkyl ZDTP additive at 0.1% weight of 
phosphorus in a highly refined base oil. The ZDTP anti-wear films have a complex structure 
that has been determined by extensive use of surface analytical techniques. It has been shown 
that the ZDTP films consisted of at least three non-homogeneous layers: on the steel surface, 
there is a sulphide/oxide layer, which is almost completely covered by a protective phosphate 
layer, with the addition of a viscous overlayer of ZDTP degradation precipitates (alkyl 
phosphate precipitates). This latter layer was removed when the film was washed with an 
alkane solvent. Therefore, the properties of the ZDTP films have been studied both before and 
after solvent washing with n-heptane. First, sphere/plane squeeze experiments were 
performed with a surface force apparatus (SFA) on unwashed films, showing that the 
overlayer of alkyl phosphate precipitates was heterogeneous and discontinuous, with a 
thickness of about 900 nm. Second, the mechanical properties were obtained from 
nanoindentation experiments, performed after replacing the sphere by a diamond tip, and 
coupled with in-situ topographic imaging procedures to measure the contact area. From the 
indentation experiments, the properties of the films were determined from normal stiffness 
measurements and through the application of a rheological film model. On the unwashed 
specimens, the viscous layer of alkyl phosphate precipitates was detected by the indentation 
tests. It is a very soft layer, mobile under the diamond tip, with a thickness of a few hundred 
of nanometers, which was in good agreement with that of sphere/plane experiments. It was 
also shown that indentation experiments removed this overlayer in the proximity of the tip, 
probably through a shear flow mechanism. This procedure can be compared to a soft 
"mechanical" sweep and the mechanical properties of ZDTP tribofilms after such a cleaning 
were found to be similar to those of solvent washed specimens. The solvent washed 
tribofilms, comprising sulphide and phosphate layers, exhibited an elastoplastic behaviour 
and, during the loading stage of the indentation, the hardness and the Young's modulus of the 
phosphate layer increased from their initial values of about 2 GPa for the hardness and 
between 30 and 40 GPa for the Young's modulus. In particular, the initial hardness of the 
polyphosphate layer at the beginning of the indentation tests was close to the mean applied 
pressure during the films generation. This suggested that the layer accommodated the contact 
pressure in the tribotest or during the loading stage of the indentation, and could thus be 
regarded as a final and local pressure sensor. The characteristics of the full ZDTP films ensure 
gradual changes in mechanical properties between the substrate, bonding layers and outer 
layers with the viscous overlayer serving as the tribofilm's precursor. The properties of these 
layered films can thus adapt to a wide range of imposed conditions and provide appropriate 
level of resistance to contact between the metal surfaces. As the severity of loading increases, 
so too do the resistive forces within the film. This ensures that the shear plane remains located 
inside the ZDTP protective film, which explains the exceptional efficiency of ZDTP films as 
anti-wear films.  
3. Experimental 
3.1. Tribofilms 
The tribofilms were generated at Shell Research and Technology Centre, Thornton, U.K., 
with a reciprocating Amsler machine [17] designed to simulate the contact conditions of the 
cam/follower system in an internal combustion engine valve train. A flat block specimen 
(8 mm x 8 mm size, 4 mm thick) has a reciprocating motion in loaded contact with a rotating 
disc. The block and the disc were made in through-hardened EN31 steel. Special care was 
taken with the roughness of the blocks which were polished until the average roughness was 
Ra = 0.01 µm. The movement of the block was driven by a crank linked to the motion of the 
disc axis through a gearbox. The block motion was approximately sinusoidal and at the same 
frequency as the disc rotation. Load was applied to the contact by a spring arrangement, 
acting through a roller bearing. The surface in contact with the loading bearing (rear surface 
of the reciprocating element) was curved to permit self-alignment between the block and the 
disc. The films were generated at a normal load of 400 N (mean contact pressure of 0.36 
GPa), speed of 600 rev/min., block temperature of approximately 100°C for 5 hours. The 
lubricants consisted of a highly refined base oil with different commercial additives (details of 
the oil formulation are not relevant to the present work): 
- MoDTC solution, 
- ZDTP + MoDTC solution, 
- ZDTP + MoDTC + detergent/dispersant solution ("full formulation"). 
The rubbing area on the polished block was typically 5 mm long in the sliding direction. 
Previous analyses have shown that the composition in the centre of the wear track was 
reasonably uniform, while the composition within 1 mm of the ends of the wear track could 
vary significantly. The mechanical measurements on the films with the Surface Force 
Apparatus have been performed in the central area of the wear track. An additional unworn 
and polished block was used to obtain reference values for the EN31 steel substrate. 
To preserve the film structures, the blocks were stored in the base oil (containing 
predominantly paraffinic hydrocarbons, with very low concentration of polar compounds) 
immediately after production of the films in the reciprocating Amsler tests and they were 
immersed again, when not in use. 
3.2. Surface Force Apparatus 
The Ecole Centrale de Lyon Surface Force Apparatus (SFA) used in these experiments has 
been described in previous publications [18, 19]. The general principle is that a macroscopic 
spherical body or a diamond tip can be moved toward and away from a planar one (the ZDTP 
specimen) using the expansion and the vibration of a piezoelectric crystal, along the three 
directions, Ox, Oy (parallel to the plane surface) and Oz (normal to the plane surface). The 
plane specimen is supported by double cantilever sensors, measuring quasi-static normal and 
tangential forces (respectively Fz and Fx). Each of these is equipped with a capacitive sensor. 
The sensor's high resolution allows a very low compliance to be used for the force 
measurement (up to 2 x 10-6 m/N). Three capacitive sensors were designed to measure relative 
displacements in the three directions between the supports of the two solids, with a resolution 
of 0.01 nm in each direction. Each sensor capacitance was determined by incorporating it in 
an LC oscillator operating in the range 5 - 12 MHz [20]. 
3.3. Tests methodology 
All the experiments were conducted at room temperature. Preliminary results obtained on 
anti-wear films from a ZDTP solutions have shown that n-heptane washing damages the film 
[16]. That is why the blocks were tested first as obtained from the Amsler friction test, 
without any cleaning and second after washing with n-heptane. The unwashed specimens 
were mounted on the SFA as taken from the storage base oil. Excess of base oil was simply 
removed by placing the side of the specimen on absorbing paper, which allowed the surface to 
be always preserved by an oil film (thickness > 10 µm). 
Nanoindentation tests 
The aim of these tests was to determine the elastoplastic properties of the tribofilms (hardness 
and Young's modulus) and their “mechanical” structure (number of layers and estimation of 
the thickness of each layer that constitutes the film). The method used to perform 
nanoindentation experiment with the SFA has already been published in detail [21]. Specific 
procedures have been developed for the characterisation of ZDTP tribofilms and have been 
described in previous papers [16, 22]. In this study, the determination of the near surface 
mechanical properties (first nanometers) was obtained through a specific tip shape calibration, 
performed on a gold film deposited by magnetron sputtering onto a silicon substrate. This 
film was very smooth (peak to valley roughness around 1 nm, measured on a scan length of 
1 µm) and its hardness was constant versus depth from the surface and until the penetration 
depth equals the gold film's thickness [21]. 
For the nanoindentation experiments, a trigonal diamond tip with an angle of 115.12° between 
edges (Berkovitch type) was used. The indentation tests were performed in controlled 
displacement mode. The standard set-up included the continuous quasi-static measurements 
of the resulting normal force Fz versus the normal displacement Z, at a slow penetration 
speed, generally 0.1 to 0.5 nm/s. It also included the simultaneous measurements of the 
rheological behaviour (dissipative and conservative or elastic contributions) of the tested 
surface, thanks to simultaneous small sinusoidal motions at a frequency of 37 Hz, with an 
amplitude of about 0.2 nm RMS. Furthermore, using the Z feedback in the constant force 
mode and the tangential displacement of the indenter, the surface topography was imaged 
before and after the indentation test, with the same diamond tip. This was made practically 
possible because of the partial elastic recovery during the unloading cycle and hence the 
geometry of tip and indent were different which was necessary to permit resolution of the 
indent. For this scanning procedure, a constant normal load of 0.5 µN was typically used. 
Such in-situ imaging procedure enables the operator to choose precisely the location of the 
indentation test on the surface and, after the test, to quantify the plastic pile-up around the 
indent and thus to measure the actual contact area. 
Rheological film model 
The elastic properties of the films were very difficult to extract from the indentation tests 
because of both the influence of the substrate and of the film structure itself. They were 
obtained through the stiffness measurements, which are global (film+substrate) 
measurements. To extract the properties of each layer of the film, a simple model has been 
developed, and its main features are described as follows. The experimental stiffness versus 
normal displacement curve was identified with the elastic response of a structure composed of 
one or two homogeneous elastic layers on a substrate (semi-infinite elastic half space) 
indented by a rigid cylindrical punch of radius a. For such a system, modelled by two springs 
connected in series [23], the calculated global stiffness (Kz) depends on the reduced Young's 
modulus of the substrate (Es* with Es*=Es/(1-νs2)), measured on an unworn steel block, over 
the contact radius (a) and depends also on four unknown parameters which are the reduced 
Young's modulus (Ef*, Ef*=Ef/(1-νf2)) and the thickness (t) of each layer. For each test, their 
values were adjusted to obtain a good fit between the measured stiffness curve and the 
calculated one. This procedure provided the structure (one or two layers), the thickness and 
the reduced Young's modulus of each layer that constituted the tribofilms. Details are given in 
a previous paper [16]. Following this model, the global stiffness of a single layer system is 
given by:  
22K t a
a E aEz f s
π π * *
 (1) 
This simple model describes perfectly the behaviour of model systems such as gold layers on 
a silicon substrate [21]. In the case of tribofilms, deviations may be observed at a critical 
pressure or at a critical depth from which the experimentally measured stiffness may be found 
to exceed significantly the theoretical one. This is interpreted as a change in the surface 
properties due to the applied pressure and appears to be related to a measured hardness 
increase. Indeed, as the applied pressure can reach values much larger than the initial hardness 
value of the surface, the resulting plastic flow may induce a small volume reduction and 
molecular rearrangements which could be sufficient to induce a noticeable change in the 
mechanical properties. From a threshold pressure value, H0, the stiffness curve was then 
influenced both by the substrate's elasticity and by the change in mechanical properties. This 
pressure dependence can be introduced in the model by writing that in the deformed volume 
of material, when H>H0 (i.e. when the film accommodates the applied pressure through 
hardness increase), the film modulus Ef* is proportional to the hardness (the ratio Ef*/H 
remains constant). It gives the following equation: 
EE =  (2) 
Ef0* is the reduced Young's modulus value, when the applied pressure is equal to or lower 
than the threshold pressure H0. When necessary, by introducing this effect in our modelling 
and by adjusting the value of the threshold pressure, we were able to fit correctly the whole 
stiffness curve. An example of such fit is given on figure 1. The evolution of the film modulus 
Ef* versus plastic depth can also be extracted from equation 1 using the experimentally 
measured global (film+substrate) stiffness values Kz and the film's thickness, t, independently 
of equation 2. This permits a check on whether it is proportional to the hardness as assumed in 
equation 2. In the example shown figure 2, the calculated Young's modulus of the film (from 
equation 1 with a film thickness t=25 nm) is found to be proportional to the measured 
hardness with a mean ratio Ef*/H=16.5, in good agreement with the ratio 
Ef0*/H0=17/1.05=16.2 obtained from the stiffness fit. 
Full formulation (ZDTP+MoDTC+detergent/dispersant)
Solvent washed tribofilm
0 10 20 30 40 50
Penetration depth (nm)
Measured stiffness
Calculated stiffness, t=25 nm, Efo*=17 GPa,
without pressure accommodation
Calculated stiffness, t=25 nm, Efo*=17 GPa,
with pressure accommodation, Ho=1.05 GPa
Figure 1: Example of application of the rheological film model: measured and calculated 
global stiffness for a tribofilm obtained from the full formulation (MoDTC + ZDTP + 
detergent/dispersant). A good fit between the measured and the calculated values is obtained 
with a single layer system (thickness t=25 nm and reduced Young's modulus Ef0*=17 GPa) 
and a pressure accommodation effect from a threshold pressure H0=1.05 GPa. 
t = 25 nmEf0* = 17 GPa H0 = 1.05 GPa
Full formulation (ZDTP+MoDTC+detergent/dispersant)
Solvent washed tribofilm
0 10 20 30 40 50 60
Plastic depth (nm)
Reduced Young's modulus of the tribofilm, Ef*
Hardness of the tribofilm, H
t = 25 nmEf0* = 17 GPa H0 = 1.05 GPa
Full formulation (ZDTP+MoDTC+detergent/dispersant)
Solvent washed tribofilm
0 10 20 30 40 50 60
Plastic depth (nm)
Reduced Young's modulus of the tribofilm, Ef*
Hardness of the tribofilm, H
Figure 2: Example of evolution of film's reduced Young's modulus and hardness versus 
plastic depth, for a tribofilm obtained from the "full formulation" (MoDTC + ZDTP + 
detergent/dispersant). The Young's modulus of the film is calculated using equation 1 with the 
measured stiffness values and using only the film's thickness determined from the fit shown 
figure 1 (t = 25 nm). 
Nanofriction experiments 
Nanofriction experiments were conducted on the blocks by moving the diamond tip along Ox 
direction (parallel to the surface) at low speed (2 to 5 nm/s) along a distance of 0.5 µm. The 
objective of these tests was to determine how the friction coefficient varies as a function of 
the applied pressure. The tests were conducted at monitored increasing depth. During the 
tests, the normal, Fz, and the tangential, Fx, forces were recorded, which allowed us to 
calculate the apparent friction coefficient µ=Fx/Fz (see example figure 3). 
Full formulation (ZDTP+MoDTC+detergent/dispersant)
Solvent washed tribofilm
0 20 40 60 80 100 120 140
Time (s)
µ=Fx/Fz
0 5 10 15 20 25
Penetration depth (nm)
Indentation test
Nanofriction test Smaller contact area
Figure 3: Procedure used for the nanofriction tests. The diamond tip is oriented edge first and 
the nanofriction tests are conducted at monitored increasing depth. During the test, the 
normal (Fz) and tangential (Fx) forces are recorded. The friction coefficient µ=Fx/Fz is 
calculated. 
Large pile-up was observed in the case of nanofriction with the diamond tip oriented face 
first, which may induce large uncertainty in the calculation of the contact area. That is why 
the nanofriction tests were conducted edge first. In these conditions, the estimation of the 
applied pressure at a given depth was obtained using low load nanoindentation tests, made in 
the near proximity of the nanofriction tests. Assuming that, at a given depth, the hardness of 
the tribofilm should be the same for the friction test and for the near indentation test, the 
contact area, and then the applied pressure, were obtained from the difference between the 
normal force measured for the two tests at the same depth (see insert on figure 3). Using the 
in-situ imaging procedure, figure 4 shows an example of an image of the surface of a 
tribofilm after such a nanofriction test. 
100 nm
Beginning
of the testBeginning of the wear
Direction of friction
100 nm
Beginning
of the testBeginning of the wear
Direction of friction  
Figure 4: Typical image of the surface 
of a tribofilm after a nanofriction 
experiment. The image is obtained with 
the in-situ imaging procedure. 
4. Results 
The first part presents the mechanical properties of the different tribofilms, determined from 
the nanoindentation experiments. Their structure, one or two layers, and their thickness were 
deduced from the use of our rheological film model. 
The results concerning the frictional behaviour of the tribofilms are given in a second part. 
4.1. Structure and mechanical properties of the tribofilms 
MoDTC tribofilms 
The tribofilm obtained from base oil + MoDTC has been tested without washing and after 
washing with n-heptane. Even on the solvent washed block, it was not possible to make any 
local topographic image nor line scanning preliminary to the indentations tests, revealing that 
the film was very soft and was easily damaged by the diamond tip. Representative hardness 
curves obtained on the MoDTC tribofilms are shown on figure 5. 
MoDTC tribofilm
0 40 80 120 160
Plastic depth (nm)
Unwashed tribofilm
Solvent-washed tribofilm
Figure 5: Typical hardness curves obtained on the MoDTC tribofilms. Open symbols 
correspond to hardness curves obtained on the unwashed film. Black symbols correspond to 
hardness curves obtained on the solvent-washed film. 
Very low mechanical properties were measured on the unwashed MoDTC tribofilm. The 
surface hardness ranged from 0.02 to 0.1 GPa indicating the presence of a very soft overlayer 
covering the tribofilm. 
After washing with n-heptane, the indentation tests showed that this overlayer has been 
removed by the washing procedure. The remaining tribofilm was a soft homogeneous layer, 
whose hardness was typically in the range 0.4 - 0.5 GPa at the beginning of the tests. 
Adhesion to the diamond tip was detected at the end of the unloading part of the tests. The 
film thickness and the structure (number of layers) have been obtained from the stiffness 
measurements performed during the experiments using the rheological film model. 
The film appeared to be homogeneous in its thickness, and for most of the tests, its elastic 
behaviour corresponded to that of a single layer, with constant properties versus depth. The 
thickness of the film was found to be between 30 and 75 nm. The reduced Young's modulus 
was typically equal to 7 – 8 GPa. 
ZDTP + MoDTC tribofilms 
From optical observation, the ZDTP + MoDTC unwashed film was very thin. This was 
confirmed by the indentation tests. Prior to any contact, a very soft layer, 60 to 120 nm thick, 
was detected at the surface of the unwashed film. 
Indentation tests conducted after scanning or imaging the surface of the unwashed film 
("mechanical sweep") showed that the film was spatially heterogeneous. Its thickness and its 
mechanical properties varied depending on the test location: 
- In some places, only a very thin layer (a few nanometers thick) with a reduced Young's 
modulus of 50 GPa covered the work-hardened steel substrate (tests A and B on figure 6). 
- A thicker layer (15 to 30 nm) with a reduced Young's modulus of 50 – 80 GPa was found 
in other places (tests C and D on figure 6), sometimes with accommodation pressure 
effect (threshold pressure H0 = 4.8 GPa). Such layer behaves like the sulphide-oxide layer 
of the ZDTP tribofilm [16]. 
- Elsewhere, the structure of the tribofilm was more complex, with a soft layer covering a 
stiffer one. For example, test E on figure 6 corresponds to a soft layer, 12 nm thick, with 
properties comparable to those of the MoDTC tribofilm (hardness of 0.2 GPa and reduced 
Young's modulus of 5 GPa) which covers a stiffer layer, 18 nm thick, with a reduced 
Young's modulus of 50 GPa. 
This heterogeneity was confirmed by the indentation tests conducted on the solvent-washed 
ZDTP + MoDTC tribofilm, where, at least, three different types of film were identified: 
- In some places, the film behaved like a one layer system, able to accommodate the 
pressure (pressure threshold 2.8 GPa). Its thickness was between 35 nm and 150 nm. The 
surface hardness was about 2 – 3 GPa and the reduced Young's modulus was about 
55 - 65 GPa. 
- In other places, the film behaved like a bilayered structure: a surface layer, about 25 nm 
thick, with properties comparable to those of the MoDTC tribofilm (hardness of 
0.3 - 0.4 GPa, reduced Young's modulus of 8 GPa), covers a stiffer layer, 150 nm thick, 
with a reduced Young's modulus of about 80 GPa. 
- Elsewhere, the surface film was between 3 and 15 nm thick, with properties comparable to 
the lower properties measured on the ZDTP tribofilm (hardness about 1 – 1.5 GPa and 
reduced Young's modulus about 10 GPa). For some tests, this surface film was able to 
accommodate the pressure, with a pressure threshold of 1 – 1.5 GPa. It covers a stiffer 
layer, 10 to 55 nm thick, with a reduced Young's modulus varying from 60 to 110 GPa. 
ZDTP+MoDTC tribofilm
Unwashed block
0 20 40 60 80 100 120
Plastic depth (nm)
Prior to any contact
After imaging - test A
After imaging - test B
After imaging - test C
After imaging - test D
After imaging - test E
Figure 6: Representative hardness curves obtained on the unwashed ZDTP + MoDTC 
tribofilm, prior to any contact and after the imaging procedure. The film is spatially 
heterogeneous in thickness and in mechanical properties. 
ZDTP + MoDTC + detergent/dispersant tribofilms ("full formulation" tribofilms) 
Nanoindentation tests performed in fresh areas, prior to any contact showed that, at the 
surface of the unwashed tribofilm, there was a very soft layer, mobile under the diamond tip, 
with an apparent thickness of a few hundreds of nanometers. 
Representative hardness curves obtained on the unwashed block near these initial contacts are 
shown figure 7. Contrary to the ZDTP + MoDTC tribofilm, the film was found to be spatially 
homogeneous. Only its thickness was found to vary, depending on the tested area. A very thin 
softer layer was detected at the surface of the tribofilm, which did not resist to imaging nor 
scanning, except if the normal load was very low (lower than 0.3 µN). This layer had a 
hardness value (about 0.3 – 0.4 GPa) comparable to the hardness value of the MoDTC 
tribofilm. The observed large hardness increase when the load increased also indicated that 
the tribofilm had a great capability to accommodate the applied pressure. This result was 
confirmed by the interpretation of the stiffness measurements using the rheological model, 
which also showed that the tribofilm had a complex structure. At its surface, there was first a 
layer with a thickness of only a few nanometers (2 nm to 7 nm) and a reduced Young's 
modulus of 10 - 15 GPa. Then, there was a second layer (thickness between 20 nm and 140 
nm) with a higher reduced Young modulus of 65 – 80 GPa. 
A similar tribofilm was tested after n-heptane washing. It also had a great ability to 
accommodate the applied pressure. From the stiffness measurements, on most places, the film 
was found to behave like a film constituted by two layers. The surface layer was thin (5 to 25 
nm) with a reduced Young's modulus value in the range 15 – 20 GPa. The thickness of the 
underlayer was found to vary between 0 (no underlayer, example of figures 1 and 2) and 100 
nanometers and its elastic modulus was in the range 110 - 120 GPa. 
ZDTP + MoDTC + detergent/dispersant
Unwashed tribofilm
0 10 20 30 40 50 60
Plastic depth (nm)
First test, prior to any contact
Without preliminary scanning
After scanning or imaging
Figure 7: Representative hardness curves obtained on the unwashed ZDTP + MoDTC + 
detergent/dispersant tribofilm ("full formulation"), prior to any contact and in the region near 
the first contacts, either without preliminary surface scanning or after scanning/imaging 
procedure. 
Figure 8 compares representative hardness curves for all tested tribofilms. For the ZDTP + 
MoDTC tribofilms, three curves are plotted because of the variety of obtained results 
revealing the spatial heterogeneity of this tribofilm. A representative hardness curve for the 
ZDTP tribofilm tested in the same conditions in a previous study [16] has been added for 
comparison. 
0 5 10 15 20 25 30 35 40
Total penetration depth (nm)
ZDTP, solvent washed MoDTC, solvent washed
ZDTP + MoDTC, unwashed (2 tests) ZDTP + MoDTC, solvent washed
Full formulation, unwashed Full formulation, solvent washed
Figure 8: Comparison of the hardness curves obtained on the different tribofilms. The 
hardness curve obtained for a ZDTP anti-wear tribofilm obtained from a previous study is 
plotted for comparison. 
4.2.  Nanofriction experiments 
Nanofriction experiments were conducted on the three preceding tribofilms and also on a 
ZDTP tribofilm and on a ZDTP + detergent/dispersant tribofilm. In order to simplify the 
following graphs, only one representative curve was plotted for each tribofilm (or two when it 
was necessary to illustrate the dispersion when it was significant). 
Figure 9 shows the evolution of the friction force versus the normal force for the tested 
tribofilms. For a given formulation, there was very little difference between the results 
obtained on unwashed and on solvent washed tribofilms at low load, indicating that the 
solvent washing does not seem to affect the frictional behaviour of the tribofilm. This agrees 
with the idea that the soft viscous overlayer is supposed to serve as precursor for the tribofilm 
rather than that it plays a mechanical role during friction. 
0 3 6 9 12 15
Normal force, Fz (µN)
ZDTP, solvent washed ZDTP + Det/Disp, solvent washed
MoDTC, solvent washed ZDTP + MoDTC, solvent washed
ZDTP + MoDTC, solvent washed ZDTP + MoDTC, unwashed
Full formulation, unwashed Full formulation, solvent washed
Figure 9: Friction force (Fx) versus normal force (Fz) during nanofriction tests with 
increasing penetration depth for different tribofilms. 
It is also worth noting that the heterogeneity in mechanical properties found on the ZDTP + 
MoDTC tribofilm also exists in the frictional properties. For this tribofilm, the friction force 
at low normal loads may be comparable either to the friction force obtained for the ZDTP 
tribofilm or to the friction force obtained for the "full formulation" tribofilm. 
Under the present testing conditions, it can be observed that the lower friction forces were 
obtained for films containing MoDTC together with ZDTP. The higher were obtained for the 
tribofilm from MoDTC alone. 
Figure 10 shows the evolution of the friction coefficient versus mean pressure. The existence 
of low friction coefficient values (0.01<µ<0.05) appears to be related both to the presence of 
MoDTC additive in the initial lubricant and to the ability for the tribofilm to reach sufficiently 
high pressure values (1.5 – 3 GPa) during the friction test. Thus, the MoDTC tribofilm, which 
is not able to resist to the contact pressure by increasing its mechanical properties seems to be 
ineffective in reducing friction, contrary to the tribofilms containing ZDTP and MoDTC 
together, which are able to accommodate the contact pressure by increasing their mechanical 
properties. Nevertheless, both behaviours (high or low friction) were observed for the ZDTP 
+ MoDTC tribofilms. This is certainly due to the spatial heterogeneity of these tribofilms, 
which behave on some places like ZDTP tribofilms, or elsewhere like "full formulation" 
tribofilms. It was also observed that tribofilms formed without MoDTC were ineffective in 
reducing friction even if high contact pressures were reached during the friction tests. 
0 1 2 3 4 5 6
Mean pressure P (GPa)
ZDTP, solvent washed ZDTP + Det/Disp, solvent washed
MoDTC, solvent washed ZDTP + MoDTC, solvent washed
ZDTP + MoDTC, solvent washed ZDTP + MoDTC, unwashed
Full formulation, unwashed Full formulation, solvent washed
Figure 10: Apparent friction coefficient versus mean pressure for the different tested 
tribofilms. 
When the evolution of the friction coefficient is plotted versus penetration depth (figure 11), it 
appears that, when it existed, the low friction coefficient domain was detected a few 
nanometers below the surface of the tribofilm. It also shows that, for the full formulation, the 
low friction domain was deeper for the unwashed tribofilm than for the solvent washed one. 
The unwashed tribofilm appears to be covered by a surface layer with rather bad frictional 
properties, which can be removed by solvent washing or by "mechanical" sweep (low load 
scanning procedures for example). 
0 2 4 6 8 10 12 14 16 18 20
Penetration depth (nm)
ZDTP, solvent washed ZDTP + Det/Disp, solvent washed
MoDTC, solvent washed ZDTP + MoDTC, solvent washed
ZDTP + MoDTC, solvent washed ZDTP + MoDTC, unwashed
Full formulation, unwashed Full formulation, solvent washed
Figure 11: Apparent friction coefficient versus penetration depth for the different tested 
tribofilms. 
5. Discussion 
Because of the inhomogeneous and patchy nature of anti-wear tribofilms and of their low 
thickness, very few results are published concerning their mechanical properties [24-28]. 
Moreover, the differences in sample preparation and the diversity of used techniques and 
experimental procedures render delicate the comparison of the obtained results. For example, 
the Young’s modulus values given by Aktary et al. for a ZDTP tribofilm [28] are significantly 
higher that those we measured but one explanation can be that they did not take into account 
the substrate’s elasticity in their calculations, contrary to what is done in the current study. Or 
if we attempt to compare our results with those recently published by Ye et al. on ZDTP and 
ZDTP + MoDTC tribofilms [14, 15], this reveals significant differences. For example, Ye et 
al. found that both tribofilms possess the same hardness and modulus depth distributions, 
corresponding to continuously and functionally graded materials, when in the present work, 
the hardness curves for similar tribofilms did not coincide and the use of our rheological film 
model allowed us to describe the tribofilms as layered materials with properties adaptable to 
contact conditions. The hardness and modulus values, respectively 10 GPa at a contact depth 
of 30 nm and 215 GPa at a depth of 20 nm, that they reported are also significantly higher 
than those we measured and also higher than those given by Aktary et al. This could be due to 
differences in sample preparation and also certainly to the use of different methods and 
assumptions for the treatment of the nanoindentation data. 
Concerning the frictional behaviour of the tribofilms, the presented nanofriction tests were 
conducted in unlubricated conditions, at very low speed (2 to 5 nm/s) and the measured 
nanofriction coefficients corresponded to the friction between the diamond tip and the 
tribofilm (over its steel substrate). That is why it also seems difficult to compare our values to 
macroscopic friction coefficient values obtained on classical tribometers. The latter are 
representative of steel on steel contact in the presence of a tribofilm and are averaged over the 
whole contact surface. However, our local values are not far from the end of test Amsler 
macroscopic friction coefficient values published by Pidduck and Smith [25] for ZDTP, 
ZDTP + detergent/dispersant and ZDTP + friction modifier tribofilms. Moreover, these 
macroscopic values were found to be proportional, with a factor 0.7, to micro-friction 
coefficient values measured with Lateral Force Microscopy by the same authors, making 
them suggest that there may be a link between macro and micro-frictional behaviour of 
smooth regions of anti-wear tribofilms. Unfortunately, no tribofilm obtained from friction 
modifier alone were tested in this study, with which we could compare our results. 
Nevertheless, macroscopic friction coefficient values, in the range 0.10 – 0.14, measured on 
an alternative ball on plane tribometer were reported by Muraki and Wada [6] for oil 
containing MoDTC alone. They conclude that such lubricant was ineffective in reducing 
friction, contrary to the oil containing MoDTC together with ZDTP. More recently, similar 
high macroscopic friction coefficient values (in the range 0.095 – 0.2) were measured by 
Unnikrishnan et al. for oil containing MoDTC alone [29]. On the other hand, Grossiord et al. 
reported very low steady-state friction coefficient (0.04) measured for base oil + MoDTC 
during SRV friction tests, and a lower steady-state value (0.02) for friction tests in a UHV 
tribometer, carried out by sliding a macroscopic hemispherical steel pin again a flat covered 
by a MoDTC tribofilm [13]. From tests carried out in a high frequency reciprocating rig, 
Graham et al. [30] also reported that, in the absence of ZDTP, MoTDC additives were 
effective in reducing friction at a combination of high additive concentration and high 
temperature (up to 0.4% wt. and 200°C). Such diversity of results, certainly partly due to the 
various tests conditions, makes unreasonable a comparison between the very high 
nanofriction coefficient measured on the MoDTC tribofilm under the present testing 
conditions and those published values. As, regarding the literature, the formation of MoS2 was 
well established for MoDTC containing lubricants, the question is how can we explain such 
high friction coefficient during the nanofriction tests ? Or what caused the very low friction 
observed when ZDTP was used together with MoDTC ? From figure 10, the low friction 
coefficient values (0.01<µ<0.05) were observed for the MoDTC containing lubricants when 
the contact pressure was in the range 1.5 – 3 GPa (the question of the spatial heterogeneity of 
the ZDTP + MoDTC tribofilm will be discussed latter). These high pressures were measured 
for tribofilms able to increase their mechanical properties, thus accommodating the contact 
conditions, which was demonstrated to be the case for ZDTP anti-wear tribofilms [16]. On the 
other hand, high pressures were not reached for the soft MoDTC tribofilm. Thus, the easy 
sliding of the MoS2 sheets could result from a favourable orientation induced by sufficiently 
high contact pressure values. The ability of MoS2 sheets to orient in a favourable direction 
was reported by Grossiord et al. [31] and Martin et al. [32], who recently investigated 
tribochemical interactions between ZDTP, MoDTC and OCB (overbased detergent calcium 
borate) additives. Using high resolution TEM observations of wear debris, coupled with wear 
scar micro-spot XPS analysis, they observed perfectly oriented MoS2 sheets, with their basal 
plane parallel to the flaky wear fragments. Such "mechanical" interpretation of the role of the 
contact pressure agrees with previous work of Muraki et al. who studied the effect of roller 
hardness on the rolling sliding characteristics of MoDTC in the presence of ZDTP and 
concluded that the friction reduction effect increased with higher degree of roller hardness 
[10]. Yamamoto also reported that a necessary condition for improving the friction and wear 
characteristics of a lubricant was the formation of surface films composed of iron phosphates 
with high hardness and Mo-S compounds [11]. Concerning the spatial heterogeneity of the 
ZDTP + MoDTC tribofilms, it can be worth noting that using high resolution TEM 
observations of wear debris collected after friction tests, coupled with AES and XPS studies 
of rubbing surfaces, Grossiord et al. described the ZDTP + MoDTC tribofilm as being 
composed of a mixture of glassy zinc phosphate zones containing molybdenum, and carbon-
rich zones containing zinc and highly-dispersed MoS2 single sheets [13, 33]. 
The observation that, during the nanofriction tests, the low friction domain was located a few 
nanometers below the surface also corroborates this interpretation. As the nanofriction tests 
were conducted at increasing depth, the sufficiently high pressures were obtained after a few 
nanometers penetration depth inside the MoS2 containing layer (with properties similar to the 
MoDTC tribofilm), thanks to the presence of the underneath resisting anti-wear layer, whose 
characteristics are similar to those of the phosphate layer of the ZDTP tribofilm. 
Finally, combining the results obtained from the nanoindentation and nanofriction 
experiments, we can propose a possible schematic description of the anti-wear tribofilms 
obtained from the "full formulation" oil. Some assumptions are also made on what happened 
during nanofriction tests on such tribofilms (see figure 12 on which for convenient drawing, 
as the Berkovitch diamond tip is not sharp, it was represented by a flat punch). 
A soft layer containing non-oriented MoS2 sheets is present at the surface of the tribofilm 
(layer (a) in figure 12). This layer, 0 to 25 nm thick, has mechanical properties comparable 
with those of the MoDTC tribofilm (0.3 – 0.5 GPa for the hardness and 3 – 10 GPa for the 
reduced Young's modulus). Its friction coefficient is rather high. This layer is easily damaged 
or removed by the diamond tip during imaging or line-scanning procedures. When the contact 
pressure is sufficiently high, friction induces a favourable orientation of the MoS2 sheets, over 
a thickness of 1 or 2 nanometers (layer (b) in figure 12), resulting in very low friction 
coefficient values which combine with the anti-wear efficiency of the tribofilm. Under this 
layer, there is then an anti-wear layer (layer (c) in figure 12), with properties similar to those 
of the polyphosphate layer of the ZDTP tribofilm. Then, just over the substrate (noted (e)in 
figure 12), there is a bonding layer (layer (d) in figure 12) with high mechanical properties 
(oxides, sulfides). 
Figure 12: Possible schematic description of the anti-wear tribofilm obtained from the "full 
formulation" and orientation of the MoS2 planes of the outer layer resulting from a 
nanofriction tests (for convenient drawing, as the Berkovitch diamond tip is not sharp, it was 
represented by a flat punch). The thickness of each layer is arbitrary drawn as it varies 
significantly depending on the tested area (from zero when the layer is not present to a few 
tens of nanometers). 
(a) Soft layer containing non-oriented MoS2 sheets, with mechanical properties comparable 
to those of the MoDTC tribofilm, 
(b) Layer of favourably frictionally oriented MoS2 sheets with a typical thickness of 1 or 
2 nm, 
(c) Layer with properties similar to those of the polyphosphate layer of the ZDTP tribofilm, 
(d) Bonding layer with high mechanical properties (oxides, sulfides), 
(e) Steel substrate. 
6.  Conclusions 
Thanks to the combined used of (i) nanoindentation experiments with continuous stiffness 
measurements coupled with imaging procedures, (ii) a specifically developed rheological film 
model and (iii) nanofriction tests, synergistic effects of ZDTP and MoDTC on frictional 
behaviour of anti-wear tribofilms have been evidenced from mechanical considerations. One 
original feature of this study lies in the characterisation of unwashed anti-wear tribofilms with 
their full structure preserved. 
The structure and nanomechanical properties (hardness and reduced Young's modulus) of 
tribofilms formed with different mixtures of additives (ZDTP, MoDTC, detergent/dispersant) 
were first determined. 
Concerning the occurrence of very low friction (0.01<µ<0.05), the contact pressure was found 
to be a critical parameter. The low friction coefficient values were attributed to a favourable 
orientation of MoS2 sheets present in the outer layer of the tribofilms formed from MoDTC 
containing lubricants. Such a favourable orientation occurred only if sufficiently high contact 
pressure was reached. These high contact pressures were attained when ZDTP was used as oil 
additive together with MoDTC because one of the main characteristics of ZDTP additives is 
to form protective anti-wear tribofilms under boundary lubrication, with varying structure and 
properties with depth, among which is an amazing ability to increase their mechanical 
properties, thus accommodating the contact conditions. 
A possible schematic description of the tribofilms containing both ZDTP and MoDTC was 
deduced and a mechanism was proposed to account for the mechanical synergy that occurs 
during nanofriction tests on such tribofilms. 
Aknowledgement 
The authors thank Shell Research Limited for financial support and permission to publish. 
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