On the Mechanism of ZDDP Antiwear Film Formation
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Zinc dialkyldithiophosphate additives are used to control wear and inhibit oxidation in almost all engine oils as well as many other types of lubricant. They limit wear primarily by forming a thick, protective, phosphate glass-based tribofilm on rubbing surfaces. This film formation can occur at low temperatures and is relatively indifferent to the chemical nature of the substrate. There has been considerable debate as to what drives ZDDP tribofilm formation, why it occurs only on surfaces that experience sliding and whether film formation is controlled primarily by temperature, pressure, triboemission or some other factor. This paper describes a novel approach to the problem by studying the formation of ZDDP films in full film EHD conditions from two lubricants having very different EHD friction properties. This shows that ZDDP film formation does not require solid–solid rubbing contact but is driven simply by applied shear stress, in accord with a stress-promoted thermal activation model. The shear stress present in a high-pressure contact can reduce the thermal activation energy for ZDDP by at least half, greatly increasing the reaction rate. This mechanism explains the origins of many practically important features of ZDDP films; their topography, their thickness and the conditions under which they form. The insights that this study provides should prove valuable both in optimising ZDDP structure and in modelling ZDDP antiwear behaviour. The findings also highlight the importance of mechanochemistry to the behaviour of lubricant additives in general.
KeywordsZDDP Zinc dialkyldithiophosphate Antiwear Tribochemistry Mechanochemistry Stress-promoted thermal activation Activation energy Reaction rate
Most research has studied ZDDP behaviour on ferrous surfaces, but it has been shown that ZDDPs also form both thermal films and tribofilms on a wide range of materials, including other metals [10, 11, 12, 13], ceramics [14, 15, 16], silicon  and DLC coatings .
Various suggestions have been made as to what drives the formation of ZDDP tribofilms to cause them to form preferentially on rubbing surfaces. Possible factors include flash temperature rise, pressure, triboemission and surface catalysis. When solid surfaces are rubbed together, heat is generated which results in a local and transient temperature rise termed “flash temperature” and it has been proposed that this might cause ZDDP tribofilm formation . The magnitude of this temperature rise is governed by moving heat source theory and depends on the rate of heat generation (the product of friction and sliding speed), the speed of the surfaces with respect to the contact, the thermal properties of the solid surfaces and the contact dimensions . Flash temperature rises may be very substantial at high sliding speeds but are generally quite small at sliding speeds below 0.1 m/s. ZDDPs have been shown to form tribofilms at very low sliding speeds when flash temperature rise is negligible, suggesting that the latter is not a key driver for ZDDP film formation . However, flash temperature calculations are based on the assumption of a Boltzmann energy distribution and it has been suggested that the very intense energy dissipation at asperity conjunctions may lead to non-Boltzmann energy distributions.
It has also been proposed that the very high pressures present in non-conforming contacts such as exist in gears, rolling bearings and the ball-on-flat geometry commonly used in rubbing experiments may promote chemical reaction of ZDDP. Based on quantum chemical simulations, Mosey et al.  proposed that very high pressures can induce cross-linking in a phosphate network leading to a highly connected network with improved mechanical properties. However, the pressures required were far in excess of the yield pressures of most materials and thus unlikely to occur in rubbing contacts. High-pressure infrared studies have also shown no evidence of structural changes in ZDDP up to 21 GPa .
It is well known that when solid surfaces rub together, the resulting plastic deformation and fracture processes that occur at contacting asperities generate localised charged regions that result in the emission of energetic particles including photons, electrons, ions and even X-rays [23, 24, 25]. When a ZDDP-containing lubricant is present, it has been suggested that this “triboemission” is responsible for the reaction of ZDDP to form tribofilms . While triboemission is quite easy to detect in dry rubbing contact, it has not so far proved possible to observe it in lubricated systems, where any emitted particles are presumed to be captured immediately by the lubricant. This makes it difficult to confirm or refute triboemission’s possible impact on tribofilm formation.
2 Stress-Promoted Thermal Activation
Quite recently, a few researchers have suggested that the reaction of ZDDP and other additives to form tribofilms may be driven directly by the shear stress present during rubbing. The concept of physical and chemical reactions being driven by applied stress originated in the last century with the work of Prandtl who developed a stress-promoted thermal activation model of crystal plasticity . In the following years, a very similar principle was applied to develop a model of liquid viscosity by Eyring , to describe rubber friction by Shallamach , to predict the strength of solids by Zhurkov  and, in 1978, to model biochemical reactions by Bell . The work of Zhurkov and Bell in particular has led to the development of the modern field of mechanochemistry, specifically devoted to study of the influence of applied force or shear stress on chemical reactions [32, 33]. Since shear stresses are always present in rubbing contacts, the stress-promoted thermal activation concept has a particular relevance to tribology and its history and application have recently been reviewed by one of the authors .
Figure 4b shows a recent representation of the influence of applied force on activation energy in the context of mechanochemistry. It illustrates the impact of applied force, F o , on the energy profile along the reaction coordinate, ξ, which represents the path the system follows over the potential energy plane from reactant to product. The solid line shows the reaction profile at zero force where R is the reactant, TS is the transition state, and P is the product configuration. The dashed line shows how this is reduced by a factor proportional to the distance over which the force is applied .
Equation 4 thus predicts that the rate of the process is increased due to applied force, f by a factor e NfΔx/RT or applied shear stress, τ by e NτΔv/RT . It should be noted that the above equations represent the simplest form of the stress-promoted thermal activation model and several refinements exist, particularly in the context of mechanochemistry, that take account, for example, of the shape of the energy barrier and the influence of applied force upon this [36, 37].
Gosvami et al.  studied the influence of normal force and temperature on ZDDP film formation on silicon using an atomic-force microscope (AFM) and showed that the kinetics of film formation were consistent with a stress-promoted thermal activation model, while Ghanbarzadeh et al.  employed an expression equivalent to Eq. 4 in a simulation of ZDDP film formation and removal and thus wear. Felts et al.  used an AFM to show that the influence of applied shear stress and rubbing time on the removal of bonded oxygen atoms from graphene and consequent reduction in friction was consistent with the stress-promoted thermal activation model. Recently, Adams et al.  have also shown the rate of decomposition of adsorbed methyl thiolate on copper during rubbing, as measured by the rate of methane emission, is consistent with this model.
As will be discussed later in this paper, the application of this model to explain ZDDP tribofilm formation is appealing since it is able to explain several important features of ZDDP films. Unfortunately, there is a considerable practical problem in proving conclusively that ZDDP film formation is in fact controlled directly by stress-promoted thermal activation since tribofilms generally form in rubbing contacts where all the other possible influencing factors occur in parallel. For example, triboemission is known to result from plastic deformation of contacting asperities, and plastic deformation itself has been modelled using a stress-promoted thermal activation model. A conclusive proof that ZDDP film formation is promoted directly by shear stress and is thus a manifestation of mechanochemistry requires this film formation to take place in the absence of all other possible drivers or that the rate at which the ZDDP film is formed varies with shear stress when all other possible drivers are held constant. This paper describes an experimental study aimed at showing that ZDDP is indeed controlled by shear stress in accord with the stress-promoted thermal activation model.
3 Experimental Approach
In thin-film rubbing contacts, ZDDP film formation is generally assumed to begin at sliding asperity contacts and it has been shown that in steel/steel rolling contacts at modest pressures film formation does not normally occur if the surfaces are separated by a liquid lubricating film sufficient to give the lambda ratio (ratio of hydrodynamic film thickness to composite surface roughness) significantly greater than unity . If, however, ZDDP film formation were driven solely by applied shear stress, then in principle it would occur even in full film lubrication conditions if the fluid shear stress were high enough. If it can be shown that ZDDP film formation does in fact occur when the rubbing surfaces are fully separated by a fluid film, this precludes the possibility of asperity plastic deformation and thus triboemission driving the ZDDP reaction. Furthermore, if it can be shown to occur in low-sliding-speed contacts in full film conditions, this eliminates any possible effect on ZDDP film formation of flash temperature. Finally, if it can be shown to occur only for fluids that give high shear stress and not for ones that have low shear stress at the same pressure, this will indicate that shear stress rather than applied pressure drives the ZDDP film formation.
The experimental approach adopted in this study is thus to test and compare the ability of ZDDP solutions to form tribofilms in elastohydrodynamic (EHD) lubricated conditions in a rolling-sliding, ball-on-disc rig at high entrainment speed (to ensure high EHD film thickness), low sliding speed (to ensure negligible flash temperature) using both high- and low-EHD friction base fluid solvents.
4 Test Methods
The SLIM attachment enables the formation of any tribofilm on the ball to be monitored throughout a test by periodically halting rotation, unloading the ball from the disc and uploading it against a glass window coated with a semi-reflecting film and a spacer layer to capture an optical interference image of the ball track.
In each test, the ball and disc were rubbed together at a fixed entrainment speed, slide-roll ratio (ratio of sliding speed to entrainment speed), applied load and temperature for 4 h while periodically using SLIM to monitor the formation of a tribofilm on the ball. At the end of each test, the film was further explored ex situ using an atomic-force microscope (AFM) and SEM EDX surface analysis.
Conditions for full EHD ZDDP film formation tests
3.0, 3.1 m/s
The WC balls were 19.0 mm diameter, and the balls and discs had an elastic modulus of 630 GPa and Poisson’s ratio 0.21. Root mean square ball roughness, R q , was 10.0 nm and disc roughness 7.0 nm, giving a composite surface roughness of 12.2 nm. Two tests were carried out on each ball and disc by reversing the ball in its holder and using both sides of the disc. Ball and disc were cleaned just prior to use in an ultrasonic bath using toluene and then Analar acetone.
Contact loads used and corresponding Hertz pressures and radii for 19 mm diameter WC ball-on-flat contact
Applied load N
Max. Hertz pressure (GPa)
Mean Hertz pressure (GPa)
Hertz contact radius (μm)
In order to ensure full film EHD conditions during MTM tests, EHD film thickness was measured using ultrathin-film interferometry (PCS Instruments) for both of the test lubricants over the entrainment speed and temperature conditions of interest. These were carried out in nominally pure rolling using a WC ball on sapphire disc at a mean Hertz pressure of 1.12 GPa.
5 Test Fluids
The low EHD friction base oil was a polyalphaolefin (PAO) obtained by mixing two PAOs of viscosity 5.9 cSt and 7.1 cSt at 100 °C to form a blend having same EHD film thickness as the DM2H at the maximum temperature studied of 120 °C. The viscosity of the resulting PAO blend was 33.6 cSt at 40 °C, 6.38 cSt at 100 °C and 4.49 mPas at 120 °C. A PAO blend of higher viscosity than DM2H was needed to give the same EHD film thickness since the former has a lower pressure–viscosity coefficient than the latter. The PAOs used were both hydrogenated oligomers of n-dodec-1-ene molecules, and a typical molecular structure of a trimer is shown in Fig. 6b. Due to their predominantly linear hydrocarbon chain structure, they give relatively low EHD friction.
One commercial mixed ZDDP was studied containing 30 % primary and 70 % secondary alkyl groups. In all tests, it was used at a concentration of 800 ppm phosphorus.
6 Preliminary Tests
Based on these measurements, subsequent ZDDP solution testing was carried out at an entrainment speed of 3.0 m/s for the PAO solution and 3.1 m/s for the DM2H. The speed difference was to ensure identical EHD film thicknesses for the two solutions. Optical interferometry indicates a film thickness of 68 nm at 120 °C for both fluids at 30 N in a WC/sapphire contact. Based on EHD film thickness depending on load to the power −0.073 and on reduced elastic modulus to the power −0.12, in accord with EHD theory , this corresponds to a film thickness of 64 nm at 50 N load and 62 nm at 75 N load in a WC/WC contact. Since the composite surface roughness of the WC ball and disc was 12.2 nm, this implies a minimum lambda ratio (ratio of EHD film thickness to surface roughness) of 5.1; i.e. full EHD film conditions.
For the purposes of this study, the property of primary interest was not the friction coefficient, but the shear stress. The mean shear stress is simply the measured friction divided by the contact area where the latter is, for a ball-on-flat the contact, a circle of area πa 2 where a is the Hertz contact radius.
Based on these results, a slide-roll ratio of 3 % was selected for subsequent ZDDP film formation work. This is high enough for DM2H to be close to the maximum of its EHD friction curve and thus provides a large difference in shear stress between the two fluids, but is low enough to avoid significant frictional heating. Based on flash temperature theory , the mean oil film temperature rise in the contact was calculated to be 3.3 °C for DM2H and 0.7 °C for PAO at 75 N and 120 °C.
7 ZDDP Film Formation Results
This indicates that the difference between ZDDP behaviour in PAO and DM2H seen in Fig. 11 does not result simply from the slightly greater temperature rise of the DM2H solution within the contact.
SEM EDX was used to analyse the films formed at the end of the 4 h rubbing tests at 75 N and 120 °C. This showed the expected presence of P, S and Zn, with a preponderance of P.
The AFM topography image of the thermal film formed after 10 h from ZDDP solution in DM2H had similar morphology to that formed in thick EHD film conditions at 60 °C (Fig. 4a), with a lower roughness of R q = 3.1 nm.
These tests support the hypothesis that ZDDP tribofilm formation in rubbing contact is driven by applied shear stress and is thus a manifestation of mechanochemistry. The finding that a ZDDP tribofilm is formed in thick film conditions when no solid–solid contact occurs suggests that triboemission does not drive ZDDP film formation in this study. The fact that a tribofilm is formed from ZDDP in a high-EHD-friction fluid, but not from ZDDP in a low one at otherwise identical conditions suggests that shear stress rather than pressure is the driving factor.
From the gradient of Fig. 17, the activation volume is calculated to be 0.18 nm3. This value is considerably higher than that calculated by Gosvami et al.  of 0.0038 nm3, but these authors used mean pressure rather than shear stress in their evaluation and also note that their calculated pressures are an upper bound since they do not take account of the influence of the ZDDP film itself on the contact pressure. It should be noted that the activation length, ∆x in Eq. 4, cannot be determined simply by taking the cube root of the activation volume since, as indicated by Eyring , the latter is not a true volume but rather the product of the area of the molecule over which the shear stress acts to impart a force and the activation length in the same plane through which this force acts. If we consider a typical bond length of 0.2 nm, this implies a molecular area of 1 nm2 over which shear stress acts, which is not unrealistic for a dialkyldithiophosphate group. N∆vτ represents the contribution of mechanical force to lowering the molar activation energy. At 75 N, when τ max = 250 MPa, this corresponds to a value of 27 kJ/mole. In practical terms, the effect of this at 250 MPa is to increase the rate of ZDDP reaction by e NΔvτ/RT , i.e. by a factor of almost 4000.
It is of interest to compare this effect of shear stress with the thermal activation energy E, which can be estimated from the three DM2H tests at the same shear stress and different temperatures by plotting log(rate) − NΔvτ/RT against 1/T. This yields E = 53 kJ/mol, suggesting that at the highest applied load the applied shear stress effectively decreases the thermal activation energy by half.
A mechanochemical mechanism of ZDDP tribofilm formation is able to explain several important features of ZDDPs. Firstly, of course, it rationalises the relationship between thermal and tribofilms; suggesting that the latter are simply thermal films in which reaction has been promoted by the applied frictional force to take place at lower temperatures. Any differences between the two, which appear to largely concern their mechanical properties, arise only from the different rates and conditions under which they form and possibly a greater proportion of iron cations emanating from the substrate due to mechanical action, and consequently a different phosphate/polyphosphate ratio in the tribofilm. It also explains why tribofilms form at low temperatures in sliding and rolling-sliding contacts, but not in pure rolling ones where shear stresses within the contact are very low .
This mechanism also provides an explanation as to why ZDDPs form similar tribofilms on many different types of surfaces, ranging from metals to ceramics. This is because the primary driver is the stress generated by rubbing a pair of high elastic modulus and hard surfaces together rather than a chemical reaction between ZDDP molecules and surface-derived material. There may be an initial ZDDP molecule–surface interaction that promotes adhesion of the film to the rubbing surfaces, but the subsequent build-up of a thick film is probably an oligomerisation process as suggested by Jones and Coy , not involving the substrate.
A shear stress-driven process also explains why ZDDPs forms their characteristic rough, pad-like film structure. In thin-film conditions, ZDDP film formation will begin at asperity conjunctions, where the contact pressure and thus the shear stress is highest. But once the film starts to form on asperities, it will continue grow only at these locations since the developing pads will bear a higher and higher proportion of the applied load. The characteristic deep valleys that separate ZDDP tribofilm pads result simply from there being negligible shear stress present in these regions. This is a relevant insight since the roughness of ZDDP films is believed to be the cause of their giving high friction in mixed lubrication conditions .
The fact that shear stress controls ZDDP film formation may also explain a phenomenon observed intermittently by the authors that even in thin-film sliding/rolling conditions, solutions of ZDDP in an API Group 3 or group 4 base oil sometimes fail to form tribofilms. This is reported in  and can usually be resolved by adding a dispersant to the base oil or replacing the Group 3 or 4 oil by a Group 2 one. It might originate from the very low EHD friction produced by some Group 3 and 4 base oils and a consequently low shear stress even in mixed lubrication conditions.
It is of interest to consider where within the ZDDP molecule the application of a mechanical force might deform or stretch a bond sufficiently to promote the rate of formation of a ZDDP tribofilm. This is not straightforward since the reaction sequence via which ZDDP forms a phosphate film is still poorly understood . One possibility is promotion of breakage of the bond between the alkyl (or aryl) group and O (or S after O/S exchange ). It has long been recognised that secondary alkyl ZDDPs show faster tribofilm formation and lower thermal stability than primary ones, suggesting that strength of this bond influences the rate of ZDDP reaction [7, 51, 52]. However, another simpler possibility is that applied stress acts on the ZDDP molecule to stretch S–Zn or O–Zn bonds, thus making these more polar and less covalent. This is likely to considerably increase the nucleophilicity of the resulting thiophosphate or phosphate ion, promoting polymerisation , and also to reduce the solubility of the molecule.
Since shear stress seems to be an important driver for ZDDP reaction, an interesting conjecture is whether the rate of film formation of a ZDDP may be influenced by the architecture of its alkyl groups via the latters’ ability to transmit shear force to the rest of the molecule. In EHD lubrication, branched hydrocarbons tend to give higher EHD friction than linear chain ones since they cannot easily slide past one another during shear  and Hoshino et al.  found that a di-ethylhexyl-based ZDDP gave faster film formation that a di-n-octyl-based one, despite both being primary ZDDPs. It should also be noted that very earliest ZDDP patent was based on cyclic alcohols rather than linear or branched chain ones  and cyclic hydrocarbons form the basis of almost all traction fluids.
Finally, it is of interest to consider whether a similar stress-promoted thermal activation mechanism is likely to be prevalent with other tribofilm-forming lubricant additives. Certainly, it appears quite probable that the reaction of the friction modifier MoDTC to form low-shear-strength MoS2 nanocrystals is stress-promoted. Like ZDDP, MoDTC is relatively indifferent to the nature of the substrate on which it forms MoS2. Thus, it reduces friction not just on steel but also on pre-formed ZDDP tribofilms , ceramics  and DLCs . Graham et al. found that the formation of MoS2 from MoDTC and consequent friction reduction was strongly dependent on the severity of the contact conditions . Thus, it occurred rapidly in reciprocated contact conditions with both rough and smooth surfaces but only with rough surfaces in linear sliding conditions. In rolling-sliding conditions, friction reduction only took place at high applied load, not at low load. All of this suggests that a high shear stress produced by a high pressure at asperity contacts is necessary for MoDTC to react. Indeed, it is conceivable that the “synergy” often noted between MoDTC and ZDDP in which ZDDP appears to improve MoDTC’s friction-reducing capability  may originate from ZDDP forming a rough tribofilm and thus providing regions of enhanced pressure and consequently enhanced shear stress at which MoDTC can react on otherwise smooth surfaces.
It is less likely that tribofilm formation by zinc-free P and P/S-based antiwear additives is driven predominantly by shear stress. In the absence of Zn2+, these additives require the generation of a suitable metal cation, normally Fe2+/Fe3+, by rubbing or corrosion, in order to form thick phosphate-based films and this process is likely to be the rate-determining step. It is noteworthy that these additives tend to form tribofilms much more slowly than ZDDP and their film formation is much more sensitive than ZDDP’s to the composition of the rubbing solids, with large differences in tribofilm formation even between different steels.
As well as explaining previously poorly understood features of ZDDP tribofilm formation, this confirmation this ZDDP reaction is controlled by the stress-promoted thermal activation reaction model has some significant implications in terms of ZDDP molecular design and use. Thus, it should be possible to tune ZDDP reactivity in a less empirical fashion than in the past and even design ZDDPs for different types of lubricated contact. Clearly, considerable further work still needs to be done; in particular to use a test apparatus that can reach high shear stresses with steel/steel contacts and to study and compare a range of single-component ZDDPs of differing structures, unlike the mixed ZDDP employed in this study. It has been suggested that primary and secondary ZDDPs undergo different reaction routes  and this might be expressed in differing shear stress responses. However, the approach outlined in this study, of separating ZDDP reactivity into a thermal activation energy and an activation volume, and thus exploring the effects of both temperature and shear stress on ZDDP tribofilm formation, appears very fruitful.
This paper shows unambiguously that the shear stress experienced by ZDDP solutions controls the rate at which ZDDP forms a tribofilm on rubbing surfaces and that rate of film formation depends on shear stress in agreement with the stress-promoted thermal activation model. It has been shown that the application of a shear stress can effectively reduce the thermal activation barrier to ZDDP film formation by half. The fact that shear stress drives ZDDP film formation explains many features of ZDDP behaviour; why it forms a tribofilm on very different types of surface, why it forms a film only when sliding contact is present, why the films are rough and why they level out at a critical thickness. The finding also highlights the importance of mechanochemistry to tribology. It suggests that the reactions of other lubricant additives, in particular the soluble organomolybdenum friction modifiers, may be also be best understood in terms of the influence of applied frictional forces on the chemical reactions involved.
The finding that ZDDP reaction is driven by the mechanically applied shear forces suggests ways that the reactivity of ZDDP and also that of other additives may be tuned in an informed manner by molecular design. This is of considerable practical significance as antiwear and friction modifier additives are become increasingly important with the introduction of ever lower-viscosity lubricants. The confirmation of the relevance of the stress-promoted thermal activation model should also prove of value in modelling lubricant additive behaviour and thus helps towards the long sought-after goal of reliable models of boundary lubrication.
The authors wish to thank Idemitsu Kosan for supply of the high-traction base fluid DM2H for use in this study and Dr. Ksenija Topolovec-Miklozic for adaptation of Fig. 2.
All data pertinent to this study is made freely available upon request to the following address; firstname.lastname@example.org.
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