ZDDP Tribofilm: Durability and Chemistry

ZDDP Tribofilm
The image is re-used from Lubricants 2013, 1(4), 132-148; doi:10.3390/lubricants1040132.

The classical lubrication theory suggests the use of oil to reduce wear by the development of a thin separating lubricating film. However, the trends in the industry dictate miniaturization of the mechanical devices with a concurrent increase in the load carrying capacity. This in turn brings the surfaces to a closer contact and increases wear. Anti-wear additives are widely used to prevent excessive wear of the materials in these situations.

One of the most effective anti-wear additives used in engine oil is Zinc Dialkyl Dithiophosphate (ZDDP). It is well known, that ZDDP generates a tribofilm on the surface of the material and protects them from wear. While being effective, it is also environmentally hazardous and the new regulations restrict the use of ZDDP in oils and therefore, there is an urgent need for the substitute. In order to create the new anti-wear additive, it is necessary to understand the mechanisms of the effective performance of the ZDDP and particularly to answer the question why ZDDP is wear resistant?

Researchers from University of Leeds, UK, and SKF Research and Engineering Centre, The Netherlands, recently reported a detailed study of the ZDDP tribofilm generation and removal. The researchers developed a novel experimental method and investigated the durability of ZDDP tribofilm at various conditions by measuring the tribofilm thickness.

“This is interesting in a sense that the amount of tribofilm removed at different stages of tribofilm build-up can be seen. We also have shown the effect of physical parameters such as temperature and load on the film removal. This is so important for understanding the new mechanism involved in wear protection” – Ali Ghanbarzadeh, one of the leading researchers commented.

The results of the research shine light on the wear mechanisms of the ZDDP tribofilm and will be very useful in the further development of environmentally friendly anti-wear additives.

Further details can be found in the original research article: http://dx.doi.org/10.1016/j.apsusc.2017.01.178, Parsaeian, P, Ghanbarzadeh, A, Van Eijk, MCP et al. (3 more authors) (2017) A New Insight into the Interfacial Mechanisms Involved in the Formation of Tribofilm by Zinc Dialkyl Dithiophosphate. Applied Surface Science, 403. pp. 472-486. ISSN 0169-4332  .

 

Aydar Akchurin
About Aydar Akchurin 35 Articles
Editor-in-Chief, PhD (Tribology), Researcher at ASML, Eindhoven, the Netherlands. Expertise in modeling of lubrication, friction and wear.

16 Comments

  1. There is no “urgent need for [a] substitute” for (environmentally-unfriendly) ZDDP, but an urgent need to stop further research efforts into dead technologies.

    What the University of Leeds and SKF have done is developed a novel approach to viewing the removal of the ZDDP tribofilm with normal operation (already well-established knowledge), and provide additional glaring evidence of the failure of AW additive technology as a whole and of ZDDP (metallophosphates) in particular.

    • Rick,
      thanks for your comment. This research is a part of our systematic research on finding the mechanism in which ZDDP act as antiwear. We observed that it’s not as simple as forming a protective layer and thus preventing the direct asperity contact. the wear in this case is a mechano-chemical wear process even when the tribofilm is protecting the surfaces. For this purpose, we need to show how the chemistry of the film is evolving in time and also in depth of the tribofilm. Furthermore, this research had some interesting findings, ZDDP is not only protecting the surface but it is actually wearing the surface based on different tribochemical phenomena. We believe that this findings will help us finding a way to reduce wear in boundary lubrication by designing and using desired amount of anti-wear additives.

      • “We believe that this findings will help us finding a way to reduce wear in boundary lubrication by designing and using desired amount of anti-wear additives.”

        Exactly my earlier point, Ali.

        Designing new amounts of dead technology AW additives doesn’t make AW technology viable. You are fighting a futile battle that cannot be won in that way.

        Additionally,the graphic above is a fantasy. The bearing surface is in fact full of asperities (equal to or greater than the film depth). If ISN can produce Ra values in the single-digit nm range, why would tribology endeavor to find new ways to deposit 50 – 150 nm tribofilms on the surface?

        Further still, be careful in the “mechano-chemical” nomenclature for what is happening on the surface with use of ZDDPs. I suggest that the phenomenon observed is actually chemical erosion of the surface, leading to the genesis of additional (new and deeper) asperities.

  2. I think the AW technology is not useless. It is proved to reduce wear and at the same time it can also be used to control friction. As for example in power transmission units. It is desired to have a controlled (possibly highest) friction, while keeping wear low. In this case, AW technology is the best in my understanding, since due to high friction, wear is inevitable. But instead of wearing off the base material, you can grow tribofilm and wear it, with minimum base material removal. So knowing these mechanisms can help a lot in optimizing such systems.

    • Aydar, the friction patterns in transmissions are macro friction surfaces. These have nothing to do with ISN and are unaffected by ISN.

      AW is more than useless, it is harmful to the system. The idea that you only wear the sacrificial tribolayer and not the underlying metal is fantasy too.

      AW/EP is a dead technology. The means/technology now exists to perfect the metal surfaces and coat them in the world’s best tribofilm… graphene!

  3. But how do you control friction? I know, you can minimize friction and wear simultaneously with graphene, but how do you control it? How to make sure, that the friction is high and wear is low?

  4. This is in gears, but what about Continuum Variable Transmissions: https://www.youtube.com/watch?v=8VYPsrOyIdw. The power is transmitted through a belt rubbing against the pulley. The friction between the pulley and the belt has to be maximized, while the wear has to be minimized. So when there is tribofilm on the surface, it will be worn at high rate due to mechanical action, but it will also be formed due to chemical reaction. Depending on the balance between those two, there will be a certain base material removal, but it will be less, than it would be in the case of absence of tribofilm. This way the wear is minimized, while the friction can still be high.

    • Aydar, CVT transmissions come in two varieties. There are the planetary gear CVTs and the belt/pulley ones. The one you have chosen above is a JATCO belt/pulley one.

      I urge you to Google “Dodge CVT transmission problems” and read about the reports of profound failure of the ATF technology used with these units.

      As an aside, we have used ISN technology in the planetary gear units with phenomenal results.

  5. It may have problems, but for this particular belt/pulley system, there is a need to decrease wear with increase of friction. And this is not the only system requiring this type of optimization. In this case, the tribofilm is necessary in my understanding and ZDDP then is one of the best, although hazardous.

  6. 🙂 I know there are some cars operating with belt/pulley type of CVT, Nissan on some models, so I guess they perform alright. But anyway, for controlling friction and wear, tribofilm growth may be a good concept.

  7. For a push belt CVT, a complex , and well balanced tribofilm is the key for efficient power transfer. The composition and morphology of this tribofilm has to be exactly right, to balance moderate wear behaviour and high friction coefficient. It takes a very delicate combination of a variety of lubricant additives, amongst which AW/EP and a couple of other additives to get the tribofilm into the correct composition and morphology to reduce wear whilst maintain high friction coefficient.

    Don’t understand the argument Rick has put up with that technology of AW/EP is dead.

    • “Don’t understand the argument Rick has put up with that technology of AW/EP is dead.”

      Because the asperities always win in the end.

      Wear and failure due to asperities can never be eliminated or even effectively controlled by AW/EP efforts; the asperities always win that battle and negate the tribological efforts employed against them. The videos above are proof of this point. Asperity removal is the only lasting solution.

      True engineering doesn’t provide answers to problems that involve “complex” and “very delicate” solutions that result in inevitable profound failure of the system.

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