Structure and mechanical properties of ZDTP films in oil

Lubrication at the Frontier / D. Dowson et al. (Editors) © 1999 Elsevier Science B.V. All rights reserved.

39

Structure and mechanical properties of Z D T P films in oil A. Tonck a, S. Bec a, J.M. Georges a, R.C. Coyb, J.C. Bell b and G.W. Roperb aLaboratoire de Tribologie et Dynamique des Systrmes, UMR CNRS 5513, Ecole Centrale de Lyon, B.P. 163, 69131 Ecully Cedex, France bLubricants and Technology Dept., Shell Research and Technology Centre, Thornton P.O. Box 1, Chester CH1 3SH, England

For many years, zinc dialkyldithiophosphate (ZDTP) lubricant additives have been extensively used for their anti-oxidant and anti-wear properties in rubbing systems but due to environmental considerations, there is now a need to develop new anti-wear additives with different chemistry but comparable anti-wear efficiency. As the anti-wear action of ZDTP additives is associated to the formation of a protective solid film onto friction surfaces under boundary lubrication conditions, the knowledge of the anti-wear film behaviour is a key point. The anti-wear film composition and the mechanisms of film formation have been extensively studied through chemical investigations. In this work, we describe the structure and the mechanical properties of anti-wear films generated on a Reciprocating Amsler machine from a simple zinc dialkyldithiophosphate (ZDTP) solution. The originality of this work is the mechanical characterisation of films as obtained from the friction test, without any solvent washing. The rheological characterisation of the ZDTP films in oil is obtained at low pressure from sphere-plane experiments with a Surface Force Apparatus (SFA). The mechanical properties of the ZDTP film with and without solvent washing with n-heptane are obtained from nanoindentation experiments coupled with topographic imaging procedures performed with the same surface force apparatus after replacing the sphere by a diamond indenter. The results obtained on the unwashed films both in sphere-plane experiments and in indentation experiments show that the solvent washing damages the structure of the original anti-wear film. The anti-wear film is formed by a thin layer of sulphides and oxides on which some islands of inorganic phosphates are present. This solid part of the anti-wear film appears to be covered with a viscoelastic overlayer of ZDTP degradation precipitates which is removed when the film is washed with a solvent. The evidence for this layer came originally from analytical studies (SEM and SIMS analysis) and is now supported by our mechanical tests. Combining the results of these mechanical characterisations with analytical results, we obtain a schematic picture of the "full" anti-wear ZDTP film structure and mechanical properties.

1. INTRODUCTION In the field of internal combustion engines, the complex contact conditions of the valve train system require specific lubricant performances as it operates under elastohydrodynamic conditions greatly influenced by asperity contacts between the opposing surfaces due to the counterformal contacts between cams and followers [1]. In some regions, because of the very low oil film thickness, the lubricant cannot prevent contacts between the

rubbing surfaces. Contact prevention is obtained by using zinc dialkyldithiophosphate (ZDTP) lubricant additives, which are able to produce protective solid films onto friction surfaces under boundary lubrication conditions. Among the required properties for these films, they need to have a good wear resistance which is related to sufficiently high shear strength and adequate rate of film formation but they also need to have sufficiently low shear strength to ensure that the shear plane is located inside the protective film. The efficiency of the film

40

then results from the equilibrium between film wear and film formation rates, associated with appropriate mechanical properties. The film composition and the mechanisms of film formation have been extensively studied through chemical investigations [2-4], depending on the wear conditions. Analytical techniques have been used to determine the nature of the ZDTP film which is mainly composed of amorphous phosphates [5], but may also contains sulfides and oxides [6]. The tribochemical reaction between polyphosphate glasses and metal oxides has also been studied [7]. On the other hand, the inhomogeneous nature of the tribochemical films and their low thickness (a few tens of nanometers) make it difficult to measure their elastic and plastic properties, because of the scales involved. This explains why there are few experimental data about the mechanical properties of friction films in the literature. Recently, we have developed a method based on nanoindentation experiments coupled with topographic surface imaging to measure the hardness and the Young's modulus of oxide and ZDTP films [8]. In more recent works, scanning probe microscopy techniques have been used to characterise the film topography and frictional behaviour [9]. Indications about the hardness and wear resistance of phosphate films derived from different ZDTP solutions are also given in the literature [10, 11]. In all these studies, the mechanical properties are determined after washing the film with a solvent, which is necessary to obtain reliable AFM imaging [9]. The aim of this work is to measure the rheological properties of a "full" unwashed ZDTP film and to relate them to the layered film structure. The rheology of the ZDTP film in oil is measured at low pressure from sphere/plane squeeze experiments. The mechanical properties of the layers that constitute the ZDTP film are measured with nanoindentation experiments, using a Berkovitch diamond tip.

2. E X P E R I M E N T A L

2.1. Apparatus The Ecole Centrale de Lyon surface force apparatus (SFA) used in these experiments has been described in previous publications [12, 13]. 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 normal and tangential forces. 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 are 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 [14]. All the experiments are conducted at room temperature (23°C). 2.2. Sample preparation The ZDTP films were generated with a reciprocating Amsler tribometer [1] designed to simulate the contact conditions of the cam/follower system in an internal combustion engine valve train. The rotating disc and the specimen (flat block, 8 mm x 8 mm size, 4 mm thick) are made in throughhardened EN31 steel. Special care is taken with respect to the roughness of the blocks. The blocks were originally surface ground to a nominal surface roughness of Ra = 0.07 ~tm. However, for the work at Ecole Centrale de Lyon, the blocks were polished prior to running in the Reciprocating Amsler rig. The polished roughness was approximately Ra = 0.01 ~tm. The films were generated at a normal load of 400 N (mean contact pressure of 0.36 GPa), speed of 600 rpm and fluid temperature of 100°C for 5 hours. The lubricant is a ZDTP solution (commercial secondary alkyl ZDTP additive at 0.1% weight of phosphorus in a highly refined base oil). The rubbing area on the polished block is typically 5 mm long in the sliding direction. Previous analyses have shown that the composition in the centre of the wear track is reasonably uniform, while the composition within 1 mm of the ends of the wear track can vary significantly. The mechanical test measurements 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. The blocks are preserved in the base oil (containing predominantly paraffinic hydrocarbons, with very low concentration of polar compounds)

41

immediately after the Reciprocating Amsler test and they are immersed again, when they are not in use. 3. RESULTS 3.1. Anti-wear film structure From previous analytical studies, it is possible to construct an hypothetical film model, that is consistent with the microscopy data. In the presence of a ZDTP solution, initially a thin film ( ~ 20 nm) of sulphide is formed as a result of high temperatures and pressures during asperity contacts. This is rapidly coated by a protective polyphosphate film ( ~ 4 0 nm), although some sulphide film remains. Further analysis (SEM, ToFSIMS) using cold stage to avoid washing with solvent have shown the evidence of a layered structure of phosphates and metal sulphides and oxides, with a substantial organic content on top of the mainly inorganic phosphate layer, originating from the alkyl groups of the ZDTP [ 15, 16]. This organic layer, not detectable by conventional surface analysis, is removed by solvent cleaning. Following this, figure 1 shows a schematic structure of the "full" ZDTP anti-wear film, where the effects of surface roughness are ignored.

¢5

~lvent wash

?

Figure 1. Structure of the full anti-wear film formed by a simple ZDTP solution according to analytical studies : the ferrous substrate is covered by a thin layer of sulphides and oxides with some islands of inorganic phosphates and alkyl phosphate precipitates. Washing the film with an alkane solvent removes the oil solution and the alkyl phosphate precipitates.

Further information about the thickness of each layer is presented in the following sections, using the SFA measurements. Other analytical techniques have also been applied to study in more detail the chemical structure of the phosphate containing layers [ 17-18]. 3.2. Sphere/plane squeeze experiments in oil The aim is to characterise the rheology of the full ZDTP film as obtained from the Amsler friction test, without any cleaning. The specimen is mounted on the SFA as taken from the storage base oil. Excess of base oil is simply removed by placing the side of the specimen on absorbing paper, which allows the surface to be always preserved by an oil film (thickness > 10 ~tm). The sphere can be moved toward and away from the plane with a normal movement (Oz direction). The speed of Oz displacement is the sum of a steady ramp, which gives a constant approach speed of 0.5 nm/s, and a small amplitude oscillatory motion

varying from D=0.3 nm to 0.08 nm RMS, with a pulsation co= 232 rad/s. The mechanical transfer function of the contact is related to its dissipative and conservative or elastic response and is recorded simultaneously with the quasi-static normal force and with the sphere/plane capacitance. The zero of the distance scale is obtained from the extrapolation of the inverse of the capacitance measurements. It corresponds to the electrical contact between the two solid surfaces. For calibration, a preliminary experiment is carried out with the storage base oil squeezed between two cobalt surfaces. The total thickness of the immobile layer on the two surfaces is found to be 2LH(base oil)= 3.4 nm. It is comparable to the distance of first repulsive contact and corresponds to the presence of an homogeneous layer symmetrically adsorbed on each surface [ 13]. Figure 2 shows the quasi-static normal force Fz versus distance D, between the cobalt sphere and the Amsler block covered with the ZDTP anti-wear film. For a distance range of about 900 nm, the normal force profile is not reversible, which indicates a plastic behaviour of the film. The thickness value of the immobile layer, LH = 50 nm, is large compared to that of the adsorbed layer thickness onto the cobalt sphere (LH(baseoil)= 1.7 nm). It corresponds to the thickness of the immobile layer near the Amsler block surface. The distance of first repulsive contact,

42

L = 900 nm, very large compared to the immobile layer thickness, LH = 50 nm, corresponds to the presence of an heterogeneous and discontinuous surface layer on top of the Amsler block, with a poor surface coverage (LH/L<
1000 Z-.t

LH(base oil)

800 ~

Bas~2:;'.__~

600 400 Q

Z

200 L,h~=

50 n m

,-.q

0

'

0

,

,o; mi

r

400 600 800 2()0 Sphere-plane distance, D (nm)

1000

Figure 2. Normal force versus sphere-plane distance when the base oil is squeezed between a cobalt sphere and an unwashed Amsler block covered with a full anti-wear ZDTP film.

The low mechanical properties of the alkyl phosphate precipitates overlayer can be estimated with the mean pressure applied during the test. Using the Derjaguin approximation (the solid substrates are not deformed), the mean pressure H(D) is given by: n(D) =

1 dF z 2rcR dD

(1)

where R is the sphere radius. At a sphere/plane distance D = 450 nm (which is about one half of the layer thickness) the pressure is very low and equal to 1-I(D) = 0.1 MPa, the contact radius is about 44 ~tm and geometrical considerations give a large apparent contact area of about 6000 ~tm2. This area is greatly overestimated as the layer is highly discontinuous and the calculated pressure is then underestimated.

3.3. Tip/plane experiments On the SFA, the sphere is then replaced by a diamond tip to perform indentation experiments. A

lot of tests were performed in an area which is hundreds of micrometer large. The method used to perform nanoindentation experiments with the SFA has already been published in detail [20]. In these experiments, a trigonal diamond tip with an angle of 115.12 ° between edges (Berkovitch) is used. The indentation tests are performed in controlled displacement mode. The standard set-up includes the continuous quasistatic 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 includes the simultaneous measurements of the rheological behaviour (dissipative and conservative or elastic contributions) of the tested surface, thanks to simultaneous small sinusoidal motions (about 0.2 nm RMS). Using the Z feedback in the constant force mode and the tangential displacement of the indenter, the surface topography is imaged before and after the indentation test, with the same diamond tip. This is made practically possible because of the partial elastic recovery which makes the indent open during the unloading. For this scanning procedure, a normal load of 0.5 ~tN is used. On the ZDTP films, nanoindentation experiments are carried out on the "full" film in oil (like in sphere/plane experiments) and on the film washed with n-heptane.

a) Experiments in oil Because of the patchy nature of the ZDTP films, it is necessary to map globally the surface prior to the indentation tests to locate the hills and the valleys on the film. To do that, a non destructive method has been developed to obtain a topographic survey of a large area of the ZDTP films [8]. Successive landings and acquisitions of the corresponding altitude and co-ordinates every 0.5 ~tm on an area of l lx18 ~tm2 produce the desired image. These landings are performed in constant normal force mode, and in between, the displacement is carried out in constant displacement mode. On the unwashed ZDTP film, two successive topographic surveys of the same area are performed and compared to a survey performed on a solvent washed ZDTP film. For the unwashed film, the typically measured height variation is about 1200 nm between the valleys and the hills, which is about ten times larger than the height variation in the case

43

of the solvent washed film (figure 3). Furthermore, whereas the surveys are reproducible on the washed film, the two successive surveys on the unwashed film are very different, which indicates that the surface film is modified by the tip contact. The layer of alkyl phosphate precipitates appears therefore mobile under the tip. Moreover, imaging the surface topography by tip scanning is not possible at this step, because the image instabilities are too great.

is comparable to that obtained on an island of the film after solvent washing.

Test 1

Test 3 Test 13

Test on washed specimen 2000 Z 1500 1000

Unswashed film, first survey X (~tm)

Solvent washed film (n-heptane) X (~tm) I ~I i!i!~iii~ili~:iii::ii ! 74148

0

5

0

5

10

@~~"ii:!51 H....I..~ Z(nm)

0 . . . . . Y (pm) 0 5 10 Toposurf

Figure 3. Topographic surveys on a "full" ZDTP film (without solvent washing) and on a solvent washed ZDTP film. The surface of the unwashed film appears to be covered by very few but very high islands. On the contrary, a lot of large but not very high islands cover the solvent washed film.

To confirm the mobility of the layer of alkyl phosphate precipitates, more than fifteen successive indentation tests are conducted in a 10 ~tm x 10 ~tm fresh area, 100 ~tm away from the surveys, after cleaning of the tip. The first indentation curve (test 1) and two of the following curves obtained (tests 3 and 13) are presented figure 4. The origin of the displacement scale is the point of contact with the film where there is a detectable force. For test 1, representative of a fresh area, the film has extremely low indentation resistance over a few hundreds of nanometers depth from its surface, then it has higher resistance for larger depth. In a further test, test 3 for example, which is situated within the zone of influence of previous indentation, the layer of alkyl phosphate precipitates has been partially displaced. When the number of tests in that area is high (13 for example), the indentation curve profile

~ 0

600

m

5

500

L,_c.~ -5-400 200 Normal displacement, Z (nm)

Z 0

0

Figure 4. Successive indentation curves on a ZDTP film in oil. Comparison with the curve obtained on a solvent washed film (n-heptane).

These experiments prove that, in oil, the layer of alkyl phosphate precipitates is detected by indentation tests. It is a soft layer, mobile under the tip, with a thickness of a few hundreds of nanometers. This thickness measurement is in good agreement with that of the sphere/plane squeeze experiments. Indentation tests remove the layer of alkyl phosphate precipitates in the proximity of the tip, probably through a shear flow. This procedure is a soft "mechanical" cleaning of the Amsler block. After this mechanical cleaning, the mechanical properties of the ZDTP film are found to be similar to those of the ZDTP film after solvent washing. To go further in the quantification of the properties of the layer of alkyl phosphate precipitates, the apparent hardness value is estimated using the beginning of the indentation test 1 and using an indentation analysis [20]. It is only a rough estimation of the hardness, because for these discontinuous surface films, the actual contact area is not known and certainly not given simply by geometrical considerations. Nevertheless, hardness values of the layer of alkyl phosphate precipitates are found to be between a few MPa and a few tens of MPa. These values are 1 or 2 orders of magnitude larger than the estimated Derjaguin pressure in the sphere/plane squeeze experiment, which is an average value over a larger area on a discontinuous layer (the contact area is overestimated).

44

b) Experiments after removal of the alkyl phosphate precipitates overlayer The solvent-resistant part of the ZDTP film is constituted by the polyphosphate layer over the sulphide/oxide layer. Two sets of experiments are carried out: (i) experiments after n-heptane washing, on high and low areas of the film and (ii) experiments after soft mechanical removal of the alkyl phosphate layer with the diamond tip. On such areas, it is possible to scan the film to obtain the surface topography. So each test includes a preliminary image of the surface topography to choose precisely the indentation point and the load (depending on the local roughness of the film), indentation with simultaneous recording of the quasi-static normal force (Fz) and the normal and tangential stiffnesses (Kz and Kx) versus normal displacement (Z) and topographic image of the residual indent to measure the actual contact area and quantify pile-up effect. Figure 5 shows examples of topographic images of indents (a) on an island of the solvent washed ZDTP film and (b) on the ZDTP film after soft mechanical cleaning.

and somewhat softer at the surface compared to the solvent washed film. In both cases, plastic pile-up around the indents increases the contact areas. The increases of contact area are respectively 40% for the solvent washed film and 20% for the mechanically cleaned. Hardness depth profiles are obtained for a mechanically cleaned film (figure 6a, b, c), for high and low areas of a solvent washed film (figure 6d, e) and on a reference unworn steel block (figure 6f). The method used to calculate the hardness is described in the literature [ 17]. The initial hardness Ho of the unwashed film is equal to about 1 GPa. Then, the hardness increases with the penetration depth, which indicates that the ZDTP film accommodates the indentation applied pressure by hardness increase. It is interesting to note that Ho value is close to the Amsler test pressure, 0.36 GPa, at which the film is generated. We can assume that the asperity contact generates a local pressure higher than the mean pressure and that the polyphosphate film acts as a pressure sensor.

e

6 ~z

Image size • 1.25 ~tm x 1.25 ~tm, indentation load" 2 mN b

d a .....

~..

.......... .:-~:...=2: ................. c ........ii...........:;::;;~::~'~'~'~"~,~,,.~.~_~ +"+'+'++"+'*+ " +'+'+'+ * " "+'+""" " " ++"+'+"+'* * , • , • , , , ,

I

-1:~0

a) Solvent washed ZDTP film

b) Mechanically cleaned

Pile-up • + 40 % of contact area Z D T P film Pile-up • + 20 % of contact area

Figure 5. Topographic images of residual indents (a) on a solvent washed ZDTP anti-wear film (b) on a ZDTP film after soft mechanical cleaning by approaching the diamond tip.

The surface topographic images of the indented film after soft mechanical cleaning are less stable than those on the solvent washed film. This indicates that the mechanically cleaned film is heterogeneous

I

I

-120

+

I

I

+

A+ + . . . . . . . .

I

t

~'ml~.~..,_lml,.j

+

I

o. °000%

I

-90 -60 -30 Normal displacement, Z (nm)

,

4

¢D

2

~Z

0

I

0

Figure 6. Hardness versus normal displacement : a, b, c : after removal of the soft alkyl phosphate precipitates by mechanical cleaning, d, e : respectively on high and low areas of a solvent washed film, f: reference unworn steel block.

Elastic properties are also measured with the indentation test. But the elastic properties of the films are much more delicate to extract due to both the substrate's effect and the film behaviour. Elastic behaviour is obtained by stiffness measurements, which are global measurements. To extract the

45

properties of each layer of the film, a simple model has been developed. The experimental stiffness versus normal displacement Z curve is identified with the elastic response of a structure composed of one or two homogeneous elastic layers on a substrate (semi-infinite elastic half space). This structure is modelled by springs associated in series and indented by a rigid cylindrical punch. For such a system, the calculated stiffness depends on the reduced Young's modulus of the substrate (measured with separate indentation test), on the contact area (calculated from the normal displacement) and depends also on four unknown parameters which are the reduced Young's modulus and the thickness of each layer. For each test, these values are adjusted to obtain a good fit between the measured stiffness curve and the calculated one. This procedure provides the structure (one or two layers), the thickness and the reduced Young's modulus of each layer that constitutes the solid part of the solvent resistant ZDTP film. For small penetration depths, typically a few nanometers, it is necessary to consider a purely elastic contact (Hertzian description) with a parabolic tip to obtain a good agreement between the beginning of the two curves. This is obtained by introducing two additional parameters: the tip top radius and a tip shape coefficient (which describes the depth from which the geometry becomes a pyramidal one). They are found to be rather constant for a given tip. This independent elastic modelling validates the value of the Young's modulus of the film at its surface. This simple model correctly describes the behaviour of some model system such as gold layers onto a silicon substrate [20]. In the case of the ZDTP films, deviations may be observed at a critical pressure or at a critical depth from which the experimentally measured stiffness may be found to exceed the theoretical one or to become smaller. When the measured stiffness is found to exceed the theoretical one, this may be due to a change in the surface properties due to the applied pressure and related to the observed 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, Ho the stiffness curve is 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>Ho (ie when the film accommodates the applied pressure through hardness increase), the film modulus El* is proportional to the hardness (the ratio Ef*/H remains constant). It gives the following equation : •

Ef

•

= El0

H

Ho

(2)

Ef0* is the reduced Young's modulus value, when the applied pressure is equal to or lower than Ho. By introducing this effect in our modelling and by adjusting the value of the threshold pressure, we are able, to fit correctly the whole stiffness curve. On the other hand, a measured stiffness smaller than the theoretical one is found in some tests. It can be interpreted in terms of film flow around the tip, for example due to a poor adhesion of the layer concerned. The results are summarised in table 1. They show two types of behaviours : (i) after an initial hardness Ho, the film resists to the indentation by increasing its hardness and its elastic modulus according the relation (2) ; (ii) the film does not resist to the pressure increase and flows. The sulphide/oxide layer properties are the same for the two cleaning procedures, solvent washing and mechanical cleaning. They are not affected by the solvent cleaning, and are comparable to those of a ferrous oxide film formed from friction with steel specimens in n-dodecane [8]. On the other hand, the polyphosphate layer is damaged by the solvent washing. In the case of mechanical cleaning, the polyphosphate layer is heterogeneous with significant variation in its elastic properties coupled with thickness variations (three ranges of values, labelled as type # 1, # 2 and # 3 in table 1). It also presents a complex behaviour related either to pressure accommodation, which results in increasing the film's stiffness from Ho, or related to film flow or poor adhesion.

46

Table 1 Thickness and elastic properties of the solid layers that constitutes the ZDTP film. The values are obtained from the stiffness measurements after removal of the alkyl phosphate precipitates overlayer by solvent cleaning or mechanical cleaning. Reduced Film flow Thickness modulus Initial hardness (poor Nature of the layer (nm) (GPa) (GPa) adhesion) Sulphide/oxide Mechanical cleaning

Alkane cleaning (n-heptane)

___80

90

4.7

20 - 30 70- 100 140

15 27- 30 40

1 - 1.5 1- 2 1- 2

Sulphide/oxide

_<50

90

4.7

Polyphosphate*

50 - 90

30 - 40

2

Polyphosphate type # 1 Polyphosphate type # 2 Polyphosphate type # 3

Possible* Possible*

• When film flow is observed, the layer does not accommodate the applied pressure. * On the solvent washed specimen, the polyphosphate layer is not present on the low areas of the film (valleys).

4. CONCLUSIONS From all the results, the structure and mechanical properties of the full ZDTP anti-wear film and of the film damaged by the n-heptane washing is obtained. In the case of the full film, a relatively thick (less than l~tm) viscous overlayer of alkyl phosphate precipitates covers the solid surface. This layer is mobile and does not resist to a liquid flow or a mechanical approach. This behaviour contrasts with the one of the two superposed solid layers of oxidesulphide and polyphosphate layers. These layers are not homogeneously distributed over the surface. The hardness Ho of the layer at the beginning of the indentation is not far from the mean applied pressure in the Amsler test. This suggests that the layer accommodates the real contact pressure in the tribotest, and in this case, the polyphosphate layer acts as a pressure sensor. Mechanical measurements indicate that the alkyl phosphates layer behaves as a viscous polymer which, after compaction and shearing, is transformed into solid polyphosphate polymer, which might or might not adhere to the sulphide/oxide layer. If the polymer layer does not slip, during the compression process the polymer

mesh is reduced, and therefore, its elastic modulus increases and it also flows plastically [ 14]. Solvent cleaning removes the alkyl phosphate layer and only the thicker part of the polyphosphate film with the higher elastic modulus (type # 2 and # 3 in table 1) remains and forms the high areas of the ZDTP film. The sulphide/oxide layer is not covered by the polyphosphate layer in the low areas of the film (valleys). This remark is important, because it can indicate that, during the Amsler friction test, the polyphosphate film, formed by mechanical compaction and shearing of the alkyl phosphate films, can flow into the valleys. But such films have poor adhesion to the substrate and can be removed with a solvent, in the same manner as the alkyl phosphate layer does. This also indicates that there is some link between the alkyl phosphate layer and the polyphosphate layer. The characteristics of the full ZDTP film resemble those of ideal surface protective coatings, which are constructed to ensure gradual changes in mechanical properties between the substrate, bonding layers and the outer surfaces to maximise their integrity. In conclusion, this study extends our understanding of the basic mechanisms of ZDTP. It has been demonstrated that these films have good wear resistance, which is related not only to a

47

sufficiently high shear strength to maintain its integrity and an adequate rate of film formation, but also to a sufficiently low shear strength in order to ensure that the shear plane is located inside the protective film. The film has a varying structure and changing properties with depth. The outermost organic layer of ZDTP degradation precipitates serves as precursors to the more solid like inorganic polyphosphates and is mobile and heterogeneous. The polyphosphate film has a hardness comparable to the mean applied pressure in the rubbing contact and this part of the film has a solid nature. The properties of the layered film are also sensitive to the applied pressure and hence can adapt to the varying contact conditions. Because of the high surface viscosity, coming principally from the alkyl phosphate precipitates and the polyphosphates, films can flow and fill the roughness valleys of the steel surfaces and hence provide an EHL capability. The exceptional wear resisting properties of ZDTP films thus arise from their unique ability to respond to a wide range of imposed conditions and provide appropriate levels of resistance to contact between the metal surfaces. As the severity of loading increases, the resistive forces within the film also increase as the sheared region penetrates down to the deeper layers of the film. Thus ZDTP antiwear films can truly be described as smart materials.

Acknowledgements The authors thank Shell Research Limited for financial support and permission to publish.

4. 5.

6. 7. 8.

9. 10.

11. 12. 13. 14.

15.

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