8:30 AM - *CP05.06.02
Tribochemistry of Lubricant Materials by Ab Initio, QM/MM and High Throughput Approaches
M. Clelia Righi1
University of Modena and Reggio Emilia1
We apply state-of-the-art methods used in materials design and computational physics/chemistry, such as multiscale quntum mechanics – molecular mechanics (QM/MM) dynamics simulations and high throughput calculations, for the first time to tribology. These innovative approaches allowed us to unravel fundamental mechanisms of friction and describe tribochemistry mechanisms in important solid lubricants. In particular:
i) We developed a computational protocol to calculate from first principles the work of adhesion, and the ideal hear strength of solid interfaces in a high throughput way. By screening a wide series of interactions including metallic, covalent, and physical bonds, we derive a connection between the intrinsic tribological properties and the electronic properties solid interfaces. In particular, we show that the adhesion and frictional forces are dictated by the electronic charge redistribution occurring due to the relative displacements of the two surfaces in contact. This suggests unconventional ways of measuring friction by recording the evolution of the interfacial electronic charge during sliding. Finally, we explain that the key mechanism to reduce adhesive friction is to inhibit the charge flow at the interface and provide examples of this mechanism in common lubricant additives. 
ii) We adopted a quantum Mechanics/Molecular Mechanics (QM/MM) approach to simulate a tribological interface. This method makes computationally feasible to effectively simulate systems with the size required to properly model chemical reactions occurring in tribological conditions. The comparison with ab initio (Car-Parrinello) and classical (using the ReaxFF force field) molecular dynamics calculations highlights the advantages of this novel approach. Using QM/MM, we accurately describe the tribochemistry processes reproducing the dissociation paths and rates of water molecules interacting with graphene ribbons within sliding graphene layers, a system which is relevant for technological applications. Our simulations provide useful insights to understand the effects of humidity in graphitic systems, which I will discuss in comparison with MoS2, another important solid lubricant, where humidity has opposite effects.
 P. Restuccia, G. Levita, M. Wolloch, G. Losi, G. Fatti, M. Ferrario and M. C. Righi Ideal adhesive and shear strengths of solid interfaces: A high troughput ab initio approach In printing, Computational Materials Science.
 M. Wolloch, G. Levita, P. Restuccia, and M. C. Righi, Ideal adhesive and shear strengths of solid interfaces: A high throughput ab initio approach Phys. Rev. Lett. 121, 026804 (2018).
 P. Restuccia, M. Ferrario and M. C. Righi Quantum Mechanics/Molecular Mechanics (QM/MM) applied to tribology: real-time monitoring of tribochemical reactions of water at graphene edges, Submitted.
 G. Levita, and M. C. Righi, Effects of Water Intercalation and Tribochemistry on MoS2 Lubricity: An Ab Initio Molecular Dynamics Investigation, ChemPhysChem 18, 1475 (2017).