Part of the complexity of the tribological phenomena resides in the wide number of chemical and physical processes that occur at the sliding buried interface, which is very hard to characterize experimentally. First principles simulations are particularly suited to elucidate tribological mechanism at the molecular level because they can accurately describe both the electronic and ionic degrees of freedom, which are important to model tribochemical reactions. The mechanism of function of friction modifiers additives is particularly interesting to investigate. These compounds undergo several chemical transformations driven by the effect of mechanical stresses, temperature and the interactions with the substrates. Molybdenum dithiocarbamates (MoDTCs) and zinc dialkyldithiophosphates (ZDDP) are the most successful organometallic friction modifier additives in the automotive industry. Yet, the mechanism through which they decompose and reorganize into beneficial tribofilms has been the subject of debate until very recently, despite their wide use since decades. In this talk, the intricate functionality of these extreme pressure additives will be investigated thanks to a very effective computational protocol based on Ab Initio Molecular Dybamics and Quantum Mechanics/Molecular Mechanics (QM/MM) simulations. The simulations provided valuable insight on the effect of oxygen in the additive molecules [1], the interactions with the substrates, the decomposition mechanism [2] and allowed a complete characterization of the MoDTC and ZDDP compounds useful to support experimental data [3, 4]. The results obtained in these studies represent an additional step towards the design of better performing and more sustainable lubricating materials and demonstrate the full capability of the computational protocol to accurately describe tribochemical reactions.

The presented results are part of the "Advancing Solid Interface and Lubricants by First Principles Ma terial Design (SLIDE)" project that has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant agreement No. 865633).

References

[1] S. Peeters, P. Restuccia, S. Loehlé, B. Thiebaut, and M. C. Righi, J. Phys. Chem A, 123, 32, 7007 (2019)

[2] S. Peeters, P. Restuccia, S. Loehlé, B. Thiebaut and M. C. Righi, J. Phys. Chem. C, 124, 25 (2020)

[3] G. Losi, S. Peeters, F. Delayens, H. Vezin, S. Loehlé, B. Thiebaut, M. C. Righi, Langmuir, 37, 16, 4836 (2021)
[4] S. Peeters et al. in preparation