Ab initio investigation of tribochemistry mechanisms in solid and boundary lubrication

Restuccia1, S. Lohelè2, G. Levita3, S. Kajita3,4 and M. C. Righi1,3

1.Department of Physics, Informatics and Mathematics, University of Modena and Reggio Emilia, Italy

2.Total Research Center Solaize, France

3.CNR-Institute of Nanoscience,

4.Toyota Central R&D Labs., Nagakute, Japan

Tribologically-induced chemical modifications of surfaces interacting with lubricant additives or other molecules present in the environment surrounding the sliding media can substantially change the adhesion and friction of materials in contact, therefore it is highly desirable to understand how they take place. However, tribochemical reactions are difficult to monitor in real-time by experiments, which leaves a gap in the atomistic understanding required for their control.

We investigate the tribochemistry of some of the most common solid lubricants, namely molybdenum disulfide, graphene/graphite and carbon-based films, the performances of which are highly affected by humidity. We apply ab initio molecular dynamics to monitor the chemical reactions involving water molecules activated at the sliding interface. Combining the results of the dynamic simulations with thermodynamic analysis based on static first principles calculations, we elucidate important differences in the tribochemistry of the considered layered materials.[1] We explain the effects of Si dopants in enhancing the hydrophilic character of carbon-based films and discuss its impact on the sliding properties.[2,3]

As second issue, we investigate the tribochemistry of sulfur-, phosphorus-containing additives and graphene at iron interfaces.[4,5,6] The results point at the important role of metal passivation in reducing the adhesion and shear strength of the interface. We generalize the result by establishing a connection between the tribological and the electronic properties of interfaces. This adds a new piece of information for the ultimate understanding of the fundamental nature of frictional forces.

[1] P. Restuccia, G. Levita and M. C. Righi Understanding the tribochemistry of graphene and molybdenum disulfide interacting with water by ab initio molecular dynamics. To be published.

[2] S. Kajita and M. C. Righi Insights into the tribochemistry of silicon-doped carbon based films by ab initio analysis of water/surface interactions, Tribology Letters, 61,17 (2016).

[3] S. Kajita and M. C. Righi A fundamental mechanism for carbon-film lubricity identified by means of ab initio molecular dynamics,Carbon 103, 193 (2016).

[4] M. C. Righi, S. Loehle’, M. I. de Barros Bouchet, D. Philippon and J. M. Martin Trimethyl-phosphite dissociative adsorption on iron by combined first-principle calculations and XPS experiments, RSC Advances 5, 101162 (2015).

[5] M. I. de Barros Bouchet, M. C. Righi, D. Philippon, S. Mambingo-Doumbe, T. Le-Mogne, J. M. Martin and A. Bouffet A comparative study on the functionality of S- and P-based lubricant additives by combined first principles and experimental analysis, RSC Advances 6, 47753 (2016).

[6] P. Restuccia and M. C. Righi Tribochemistry of graphene on iron and its possible role in lubrication of steel. Carbon 106, 118 (2016).