Compréhension des mécanismes d'action de nouveaux additifs de lubrification faiblement polluants : couplage expérience/simulation numérique – LOWPOLUB

One of the main research orientations of the 'Tribochimie, Nanolubrifiants, and Modélisation' team of the LTDS is the development of suitable solutions in order to reduce the environmental risks related to the massive use of lubrication additives in the engine. Around these problems two research axes exist, led by two young assistant professors. The first one focuses on the improvement of the comprehension of the mechanisms of lubrication of the additives currently used. This will allow to optimize their action. The second axis constitutes 'a more original' way since it is based on the development of new nanoparticles based additives. So far, these two approaches were mainly implemented via friction tests combined to post-mortem analyses of surfaces, wear particles, or lubricant. They were carried out via traditional techniques of characterization such as TEM, XPS or Raman spectroscopy. If encouraging results have been obtained and if first scenarios on the action mechanisms of the additives have been proposed, their validations require however to directly probe the behaviour of the interfacial material during the tribological stresses. That needs a new methodology using competences of each of the protagonists of the project. This methodology is based on the combination of experimental methods (in-situ tribology and experimental simulation of tribological tests) and a numerical approach (Computational Chemistry). The coupling of these two methods should allow the in-situ characterization of the interfacial products. The implementation of this methodology requires significant experimental and theoretical developments. The coupling of tribological tests and structural characterization through of in-situ Raman tribology (in collaboration with the Laboratory of Sciences of the Earth (LST) of the 'Ecole Normale Supérieure' of Lyon (ENS)) constitutes a first important point of this project. The idea is to place a 'portable tribometer 'on the Raman spectrometer of the ENS in order to follow in real time the evolution of the interfacial products, on a molecular scale, during the friction test. It is also possible to carry out, still in collaboration with the LST and via a diamond anvils cell, Raman spectroscopy under very high pressure (up to 30 GPa) experiments. Similar studies can also be performed on an EXAFS line of the European Synchrotron Radiation Facility (ESRF) especially dedicated to the in-situ study of the structural modifications of materials under high pressures. However, these tests do not introduce shearing. Therefore, we propose to carry out experiments in the multi-anvils press of the National Laboratory of Argonne which has the advantage to be placed on a dedicated line (13B) of the APS (Advanced Photon Source) synchrotron. This would allow to test simultaneously material in pressure and shearing but also to carry out in-situ X-rays diffraction experiments. From a theoretical point of view, the objective is to simulate friction tests by Molecular Dynamic and/or Quantum Chemistry. This would permit to have information on the evolution of the structure of the material under simplified tribological stresses. In addition to the pressure and the temperature, the shearing and the environment can be considered in these theoretical simulations. This approach can be applied to many tribological systems (nanoparticles and additives of lubrication) and would be perfectly complementary to experiments carried out in diamond anvils cell. The main idea is to develop the use of this type of tools in the LTDS. It is planned within the framework of this project to buy the Molecular Dynamic software from the Pr. Miyamoto laboratory with which we collaborate already. Collaboration will be continued in order to validate the development of other software associating the techniques of Molecular Dynamics and Quantum Chemistry with very short computing times. Such software seems to be very interesting to tackle the tribochemical problems since the exchanges of electrons can be followed. Finally, this approach will allow a considerable advance in the comprehension of the various phenomena which occur in a tribological contact, either with the nanotubes or with many other types of materials (molecules in dispersion). This methodology should allow in the long term to propose new additives of lubrication for the formulation of the future lubricants.