A New Theory of ZDDP FIlm Formation

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MolaKule

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A New Theory of ZDDP Tribofilm Formation by Molekule A new theory of tribofilm formation by the ZDDP additive has been put forth in a recent industry paper and it appears to have substantial experimental support[1]. Here, I will attempt to distill the paper for the general BITOG reader and define certain words and phrases as we progress. Subject Content: Short History of ZDDP, Prior Theories of ZDDP film Formation, The New Theory and Verification of ZDDP Tribofilm Formation, Summation. Keywords: Tribofilms, thermal films, mechanochemistry, stress-promoted thermal activation, anti-wear, tribochemistry, flash temperature, Short History of ZDDP First used as a lubricant anti-oxidant, it was later found that ZDDP had a welcome additional benefit as an Anti-Wear (AW) agent. ZDDP has been in use in various forms for the past 60 years in all types of lubricants. During that time, many theories have been presented as to how this film forms and how it reacts with surfaces in motion and under loads. ZDDP, dialkyl-dithiophosphate or diaryl-dithiophosphate, is a molecule consisting of phosphorus, sulfur, and zinc atoms. Structurally, this molecule has one phosphorus atom on each end with four sulfur atoms linked to a single zinc atom in the middle. On rubbing surfaces, a thick film of amorphous zinc phosphate is found. ZDDP films are rough and form circular or elongated pads which are 200 nm thick. The ZDDP film is believed to control wear by limiting direct contact of two surfaces in motion, preventing adhesion and transient contact stresses. (amorphous – a non-crystalline solid). When surfaces rub together in a lubricant containing ZDDP, films are generated quickly at temperatures less than or equal to 25C. These are what we call, “Tribofilms.” At bulk oil temperatures above 150C, ZDDP will form films on surfaces even in the absence of motion or near contact. These films are what we call, “Thermal Films.” Prior Theories of ZDDP film Formation We will in this section discuss the four main and prior theories of ZDDP film formation: flash temperature rise, pressure, triboemission, and surface catalysis. Flash temperature theory: When solid surfaces come in contact, heat is generated which results in local and transient temperature rise. These are called “flash temperatures.” Flash temperatures are most prevalent in transmission wet and dry clutches, for example. However, ZDDP films have been shown to form at very low sliding speeds and low temperatures as noted above. High Pressure film forming: In high pressure contact areas such as gear teeth and other non-conforming contacts, pressures can be very high and it has been proposed that high pressures can produce “cross-linking” in the phosphate linkage network. However, once again, high pressure studies have shown no changes in ZDDP films up to 21 GPa. Triboemission: This topic involves the emission of energetic particles such as photons, electrons, ions, and even X-rays during localized stresses and deformations. It is believed that the emission of these energetic particles gives rise to the reaction of ZDDP films on surfaces. But this theory cannot be confirmed in lubricated contacts. Surface Catalysis: This theory promoted the catalytic reaction of various iron (ferrous) species with the phosphorus and sulfur atoms, resulting in ferrous sulfates and ferrous phosphates in the formation of ZDDP films. Many studies have been executed which show various chemical species in the film, but it cannot be determined if this is the “cause” of film formation, or simply the result of asperity mixing. The New Theory and Verification of ZDDP Tribofilm Formation: This theory is called, “Stress-Promoted Thermal Activation,” and is the main topic of this paper. This theory involves the modern developed field of, “mechanochemistry.” Mechonochemistry is where mechanical forces, such as shear-stresses, raises the activation energy of a chemical mix or a set of chemical species to promote a reactant-to-product result. Mechanochemistry is also defined as the, “mechanical activation of covalent bonds”. (Activation energy - the minimum energy which must be available to a chemical system with potential reactants to result in a chemical reaction; Covalent chemical bonds - the sharing of a pair of valence electrons by two atoms, Such bonds lead to stable molecules; Valence electrons are the electrons in the outer shell of an atom. The valence electrons are the ones involved in forming bonds to adjacent atoms). Since shear stresses are always present in mechanical systems, it makes sense to examine more closely the thermal activation of chemical systems by shear stresses. If applied shear stresses are high enough, then ZDDP film formation could occur even in the full film lubrication regime, and in the absence of high pressures. Through the experimental method of using a track-on-ball machine, the authors verified that shear stress of ZDDP solutions forms a tribofilm and the rate of film formation depends on the magnitude of the shear stress in agreement with the shear-stress thermal activation model. For Extreme Pressure additives such as predominately P/S additives, or for zinc-free AW additives such as polymer esters, it is doubtful that shear-stress thermal activation is the cause of film forming. Summary: The results of this experiment could lead to a better understanding of film formation by not only ZDDP, but by other additives in the Performance Package. If indeed shear stress related activation is the primary causation of various films, then improved AW and friction reducing additives could be “fine-tuned” in the molecular sense, and improved for the Low Viscosity engine and transmission oils. 1. Spikes and Zhang, On the Mechanism of ZDDP Antiwear Film Formation, Tribology Letters, 63:24, 2016.
 
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This paper (or a version of it with the same concept) was presented at STLE. As I recall there were quite a few dissenting comments in the end, however I believe that their premise is solid. Interestingly at the presentation the discussion of synergistic effects of other additives (such as FM's, dispersants and detergents) were brought up and it's nice to see that in the paper this is discussed in relationship to the MoDTC/ZDDP synergies that exist:
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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 [56], ceramics [57] and DLCs [58]. 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 [59]. 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 [60] 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.
The interesting part that I took away is how these films build more on each other than they do in reaction to a particular surface material (in the correct conditions they appear indifferent to the nature of the substrate). This would indicate that under the right conditions, tribofilms can form not only directly at the surface, but possibly on top other reactive species (as MoS2 forms on top of ZDDP films). I'm interested to see the impact of Magnesium sulphonate detergents on film formation reactions in comparison to the more traditional calcium ones.
 

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Originally Posted By: Solarent
...I'm interested to see the impact of Magnesium sulphonate detergents on film formation reactions in comparison to the more traditional calcium ones.
I think we have only "scratched the surface" (pun intended) regarding film formation of the various chemical species within additives. I too would like to see more research in detergent/AW/FM interactions to settle the so-called "AW/FM-film-reduction-by-detergents" question. However, I am more interested in the salicylates than the sulfonates. smile
 
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Will the ideas presented in the paper change the notion that engine wear occurs primarily during warm-up, because it is believed that it takes engine heat and high oil temperature to activate AW chemistry?
 
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Originally Posted By: userfriendly
Will the ideas presented in the paper change the notion that engine wear occurs primarily during warm-up, because it is believed that it takes engine heat and high oil temperature to activate AW chemistry?
In theory it could. Because they postulate that it is the shear forces that contribute to the development of the tribofilms, the pure fact that the engine is running should be sufficient to begin the process of the development of the sacrificial wear layer. However this isn't just happening in a vacuum and so I believe that temperature still has a role to play. Some experiments I have seen with various AW/EP chemistry that are "temperature dependent" also transition with different loads as well (the idea that higher loads contribute to the localized temperature increases at the asperity interface). This has been the prevailing opinion on the development of many kinds of tribofilms. With this new observation, it appears that shear stresses play a more significant factor and maybe it's a combination of both. Formed tribofilms do stay on metal surfaces even after shutdown (this is the main marketing claim of products like Castrol Magnatec) the question of preventing wear at startup is a challenging one because those films may not replenish themselves fast enough and those are only effective in protecting against abrasive wear and as I have mentioned several times in the past abrasive wear is not the only wear mechanism that you need to protect your engine from. I don't think that start up wear is as bad in modern engines as it was made out to be 20 years ago.
 
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Engine wear, especially during start up should be reduced further with DI engines, as the fuel can be injected late into the compression stroke. This will reduce the exposure of wet fuel on cylinder walls. Gasoline is a good solvent, perfect for washing off tribofilms. Old engine designs ran forever with very little wear when fuelled with LPG or CNG, because in part, the reduction of start up wear from wet fuel. Will the findings published in this and other papers change engine metallurgy and/or the finish on metallic surfaces? I'm also thinking of stop-start technology, when plain bearings may operate in boundary lubrication.
 
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What about the volatility difference, between primary and secondary ZDDP. I've heard that the otc add is the more volatile primary type. Is that accurate?
 

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Originally Posted By: Solarent
...Some experiments I have seen with various AW/EP chemistry that are "temperature dependent" also transition with different loads as well (the idea that higher loads contribute to the localized temperature increases at the asperity interface). This has been the prevailing opinion on the development of many kinds of tribofilms. With this new observation, it appears that shear stresses play a more significant factor and maybe it's a combination of both.
I agree, and recall the theory is called, Stress-Promoted Thermal Activation. Shear stresses in liquid films do raise localized temperatures in the stressed region so I don't think one can necessarily "de-couple" localized temperature increases from the shear-stress. Localized shear-stresses could raise the entrained lubricant temp to 25C or above very quickly to form the film, even as you say, for cold starts.
 
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MolaKule

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Originally Posted By: userfriendly
While looking for an answer to your question Ohle, I came upon this article, http://www.machinerylubrication.com/Read/29031/extreme-pressure-additives which got me thinking about the effect of bulk oil and machinery surface temperature would have on AW and EP additives and flashpoint.
That is one confusing article as they tend to mix the topic of AW additives with EP additives.
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Sulfur-phosphorus EP additives have a high-temperature limit of approximately 95 degrees C. This restricts the temperature range in which these oils can be used.
203F is not a temperature limit with modern additives.
Quote:
6. Sulfur-phosphorus EP additives are somewhat corrosive to yellow metals, particularly at temperatures higher than 60 degrees C. Worm gearsets frequently contain phosphor-bronze materials, and it is for this reason that gear oils using sulfur-phosphorus EP additives may not provide satisfactory service in worm gear drives.
Modern formulations have for a long time now contained metal deactivators and corrosion inhibitors that prevent this.
Quote:
7. Depending upon the amount used, sulfur-phosphorus EP additives may not be compatible with oils containing zinc anti-wear (AW) additives. This is why it is not recommended to mix AW gear oils with EP gear oils.
Again, ZDDP and Boron are added for a reason and they ARE part of modern gear formulations. ZDDP is added for two reasons: 1. as a low temp AW additive and activates before the S/P chemistry takes effect, 2. ZDDP is also used as an anti-oxidant. Boron is also a low and high temp AW AND EP additive.
 
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interesting but a "bit" over me!! possibly related my tranny-transaxle combo in a 2001 jetta O2J 5 spd manual was barely shifting at 95,000 miles. changed several times with OE juice + the last time with amsoils recommended lube, then i drained it + put in Redline MT-90, it shifted a little better right off but by 5,000 more miles is was almost like new!!! wondering whats in that MT-90 that gets better with use!!!
 
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