Tribofilms and friction control

By Mark Devlin, Contributing Editor | TLT Lubrication Fundamentals July 2026

Let’s explore what happens when surfaces are touching.

I have a confession to make. Even though I have a doctorate degree in chemistry, I struggled through organic chemistry in graduate school, and some of it is still a mystery to me. In fact, one of my professors promised I would pass his class if I promised to be a physical chemist. (I needed a B- to pass, so I do know enough organic chemistry to be dangerous.) Often while looking at an organic chemistry problem I would remember the classic cartoon where equations were shown on one side of a blackboard, and on the other side there was a solution. In between there was the phrase “A Miracle Occurs,” and the professor in the cartoon commented that more details were needed. When working on organic chemistry problem sets, I would often think a miracle has to occur to have the reactants on one side of the page become the product on the other side of the page. 

The next step in our lubrication fundamentals journey is to explore what happens when surfaces are touching. This involves the formation of tribofilms and how friction is related to the structure of these tribofilms. In past articles I have discussed how temperature, pressure and shear affect viscosity and how this relates to the packing of molecules in a contact zone. In these past discussions I tacitly assumed that the energy added to the lubricant from temperature, pressure and shear was not high enough to break any bonds in the molecules in the lubricant. We now have to think about how temperature, pressure and shear cause bond breaking and reformation. Do not worry, I will go slowly, and I hope to give you a little insight into the miracle that occurs when tribochemistry occurs in a contact zone. Remember, I only know enough organic chemistry to be dangerous.

In addition to environmental conditions, the surface in the contact zone affects the arrangement of molecules and reactions between them. Initially, we will imagine an iron (Fe) oxide surface (similar to steel) that is not smooth, since real surfaces are not smooth. The Fe oxide surface has some charge to it that can attract molecules in the fluid to the surface allowing molecules to pack in such a way to make chemical reactions easier to occur. This is essentially how some catalysts work. The Fe oxide surface can also provide extra electrons to change the arrangement of bonds in molecules when reactions occur. Finally, there are peaks (called asperities) and valleys on the surface, which affect the micro-environment (pressure, temperature and shear) experienced by the molecules trapped near the surface. 

Each atom in a molecule is sharing electrons with other atoms in order to be in a low energy state. There may be other ways for atoms to share electrons that are more stable than the initial molecular arrangement. However, the atoms need to move around in order to get to those more stable states. This movement of atoms requires energy. (My organic chemist friends will see why I struggled with organic chemistry because I think about atoms moving when in fact electrons are what are moved around in organic chemical reactions.) Heat (measured by an increase in temperature) causes atoms in molecules to oscillate back and forth, and if enough heat is added to the system, atoms may overcome the energy needed to find a more stable state. Pressure changes the packing of molecules and in so doing some atoms may be in close enough proximity to find a new binding arrangement that is more stable than their existing arrangement. 

Chemists often overlook shear as an energy source because we learn about shear on the macroscopic scale while learning about rheology rather than the atomic scale while learning about chemical reactions. However, rheological concepts help us understand shear effects on chemical reactions. If one surface is stationary and another surface is moving, the various layers of a fluid between the surfaces slide against one another. Now imagine the fluid consists of molecules that are attached to both surfaces. As one surface moves versus the other the bonds between atoms in each molecule are stretched, and these atoms may find a more stable state so bonds will break and reform. Of course, temperature changes, pressure changes and shear forces are all occurring at the same time in a contact zone. The miracle required to understand tribochemistry is figuring out how temperature, pressure and shear contribute to tribofilm formation and the structure of the tribofilm. 

Luckily many very smart tribologists have investigated the formation and properties of tribofilms formed by zinc dithiophosphate (ZDDP), the most common antiwear additive in engine oils.1-6 The actual tribofilm formation process is rather complex, so I won’t try to summarize that here. However, the following atoms exist in the contact zone: Zn, S and P atoms from ZDDP, and Fe and O from our idealized steel surface. Once temperature, pressure and shear perform their miracle, polymeric layers of P, S and O are formed with Zn or Fe intercalated between the surfaces. Tribofilms do not grow uniformly across the surface because surfaces are not uniform. At the tops of asperities, gaps between surfaces are small and shear is greatest at these points. In addition, the tallest asperities will be the first points of contact and most of the load between surfaces will be concentrated at these asperities increasing pressure. Lastly, as asperities slide against each other friction will generate heat. I told you it would take a miracle to understand this. Don’t worry, the formation and structure of tribofilms is a very active area of research in the field of tribology since there are many sophisticated techniques available to study surfaces at the molecular and atomic scale.7-11

The tribofilm formed on a surface is in a low energy state. Additional shear forces can be applied to the tribofilm if we want to move one surface against another. We can then invoke the picture we imagined in discussing rheology. The layers containing P, S and O move relative to the intercalated layers of Zn and Fe. The force required to move these layers is friction. The frictional force required depends upon the strength of the bonds between tribofilm layers. To change the strength of the intermolecular interactions between tribofilm layers we can imagine changing the metals in the tribofilms. In fact, this has already been suggested based on molecular modeling calculations.6 There are other sources of metals in lubricants, including additives called detergents that can contain calcium (Ca) and magnesium (Mg), and these metals can be incorporated into tribofilms.12 (Detergents are primarily used to control deposits and oil degradation.) When Ca and Mg are incorporated into tribofilms, friction does vary and is related to the amount of Ca, Mg and Zn in the tribofilm.13

Other metal containing additives can be intentionally added to lubricants such as molybdenum (Mo) compounds and inorganic nanoparticles to reduce friction.14-17 Mo compounds by themselves can form Mo disulfide (MoS2) films on surfaces to reduce friction or Mo can be incorporated into ZDDP tribofilms. In the case of MoS2 films, the S in these films can be part of the molecular structure of the Mo-containing compound in the case of Mo dithiocarbamate (MoDTC) or can be from ZDDP or other S sources in the lubricant. The layers of MoS2 formed on the surface can slide across one another resulting in a reduction in friction. The cores of inorganic nanoparticles contain a high concentration of metal. The mechanism by which nanoparticles incorporate metal into a tribofilm film is by being peeled open under pressure releasing the metal which is then added to the film. The addition of metals from inorganic nanoparticles creates tribofilms with different frictional properties. For example, oil soluble Cerium (Ce) oxide and oil soluble Zn sulfide nanoparticles reduce friction by forming tribofilms containing Ce and Zn.18 

So far, our conversations in the current TLT Lubrication Fundamentals series have focused on how lubricants can help improve fuel efficiency by lowering viscosity and reducing friction. In this  article I focused on tribofilm formation, but there has been no discussion of the chemistry or physical mechanism for the behavior of traditional organic “friction modifiers.” This has been intentional since the phrase “friction modifier” (and for that matter “antiwear,” “dispersant” or “antioxidant”) describes the function of the lubricant additive and not the chemistry. In fact, studies describing ZDDP tribofilm formation often track changes in frictional properties of lubricating systems so ZDDP could be called a “friction modifier.”1-5 The next TLT Lubrication Fundamentals article will discuss how non-metallic molecules such as traditional surfactants and polymers control friction. 

Homework this month is a bit more sophisticated and is not required to understand the next TLT Lubrication Fundamentals article. In a series of TLT Cutting Edge articles, Wilfred T. Tysoe and Nicholas D. Spencer give more details describing the formation of tribofilms and the role of shear. For those that want to know more details about the “mysteries” of chemistry in a rubbing contact, this homework assignment will help: Ref. 19 and 20.

REFERENCES
1. Spikes, H. A. (2004), “The history and mechanism of ZDDP,” Tribology Letters, 17 (3).
2. Taylor, L., Glovnea, R., Ribeaud, M. and Spikes, H.A. (2000), “The nature and properties of antiwear additive films,” presented at the International Tribology Conference, October 2000, Nagasaki, Japan.
3. Taylor, L., Spikes, H. A. and Camenzind, H. (2000), “Film-forming properties of zinc-based and ashless antiwear additives,” SAE 2000-01-2030.
4. Zhang, J. and Spikes, H. A. (2016), “On the mechanism of ZDDP antiwear film formation,” Tribology Letters, 63 (24).
5. Gosvani, N. N., Bares, J. A., Mangolini, F., Konicek, A. R., Yablon, D. G. and Carpick, R. W. (2015), “Mechanisms for antiwear tribofilm growth revealed in situ by single-asperity sliding contacts,” Science, 348, pp. 102-106.
6. Mosey, N. J., Muser, M. H. and Woo, T. K. (2005), “Molecular mechanism for the functionality of lubricant additives,” Science, 307, pp. 1612-1615.
7. Cann, P. M., Hutchinson, J. and Spikes, H. A. (1996), “The development of a spacer layer imaging method (SLIM) for mapping elastohydrodynamic contacts,” Tribology Transactions, 36, pp. 915-921.
8. Oechsner, H. (1984), “Secondary neutral mass spectrometry (SNMS) and its application to depth profile and interface analysis,” Topics in Current Physics, 37, Springer Verlag Berlin, pp. 63-85.
9. Yablon, D. G., Kalamaras, P. H., Deckman, D. E. and Webster, M. N. (2006), “Atomic force microscopy and Raman spectroscopy investigation of additive interactions responsible for anti-wear film formation in lubricated contact,” Tribology Transactions, 49, pp.108-116.
10. Uy, D., Simko, S. J., O’Neill, A. E., Jensen, R. K., Gangopadhyay, A. K. and Carter III, R. O. (2006), “Raman characterization of anti-wear films formed from fresh and aged engine oils,” SAE 2006-01-1099.
11. Guevremont, J. M., Garelick, K. and Devlin, M. T. (2012), “Adsorption of various ZDDP’s on steel: Study of the initiation of tribofilm formation using quartz crystal microbalance with dissipation monitoring (QCM-D),” presented at the 67th STLE Annual Meeting, May 2012, St. Louis, Mo.
12. Rudnick, L. R. (2022), Lubricant Additives: Chemistry and Applications, Third Edition, CRC Press.
13. Devlin, M. T. (2018), “Common properties of lubricants that affect vehicle fuel efficiency: A North American historical perspective,” Lubricants, 6 (3), p. 68.
14. Spikes, H. A. (2015), “Friction modifier additives,” Tribology Letters, 60 (5).
15. Tang, Z. and Li, S. (2014), “A review of recent developments of friction modifiers for liquid lubricants (2007–present),” Current Opinion in Solid State and Materials Science, 18, pp. 119-139.
16. Wong, V. W. and Tung, S. C. (2016), “Overview of automotive engine friction and reduction trends—effects of surface, material, and lubricant-additive technologies,” Friction, 4 (1), pp. 1-28.
17. Tung, S. C. and McMillian, M. L. (2004), “Automotive tribology overview of current advances and challenges for the future,” Tribology International, 37, pp. 517-536.
18. Devlin, M. T., Aradi, A. A, Guevremont, J. M., Jao, T-C, Abdelsayed, V. and El-Shall, M. S. (2008), “Friction and film-formation properties of oil-soluble inorganic nanoparticles,” SAE 2008-01-2460.
19. Tysoe, W. T. and Spencer, N. D. (2015), “Reaction to rubbing,” TLT, 71 (8), pp. 84-86. Available at www.stle.org/files/TLTArchives/2015/08_August/Cutting_Edge.aspx.
20. Tysoe, W. T. and Spencer, N. D. (2019), “Controlling shear induced tribochemistry,” TLT, 75 (6), pp. 100-102. Available at www.stle.org/files/TLTArchives/2019/06_June/Cutting_Edge.aspx.

Mark Devlin is a retired chemist and STLE Fellow living in Richmond, Va. You can reach him at markdstle@gmail.com.