Motor oils are usually enriched by chemical additives, which reduce friction and wear at microasperity contacts where hydrodynamic lubrication is no longer effective. In these conditions, known as boundary lubrication conditions, the additives react with the surfaces and modify their properties of adhesion, resistance to wear and friction. Recent experiments of gas phase lubrication, which have been carried out to understand the functionality of organophosphorus additives, revealed that a significant reduction of wear and friction at steel interfaces could be obtained by the tribologically-induced formation of iron-phosphide.[1] To get atomistic insight into this result, we performed an ab initio study of the mechanisms of iron phosphide formation from the dissociative adsorption of trimethylphosphite (TMPi) at the Fe(110) surface. We identify the reaction paths for P release at the surface by means of the nudged elastic band method and elucidated the effects of interfacial phosphorus on the properties of adhesion and resistance to sliding of iron interfaces. This latter analysis was performed for sulfur also, which was found more effective in reducing adhesion and shear strength than phosphorus.In the second part of the talk I will consider graphite and molybdenum disulphide (MoS2), which are well known solid lubricants. We performed density functional theory calculations, including van der Waals interactions, of interlayer adhesion and shear strength. We constructed the potential energy surfaces (PES), which governs interlayer sliding, and described its changes as a function of load. We found marked difference between the two materials and explained them analyzing the load-induced charge displacements.[2,3][1] M. I. De Barros-Bouchet, M.C. Righi, D. Philippon, S. Mambingo-Doumbe, T. Le-Mogne, J.M. Martin, and A. Bouffet, submitted.[2] G. Levita, A. Cavaleiro, E. Molinari, M.C. Righi, T. Polcar, submitted.[3] M. Reguzzoni, A. Fasolino, E. Molinari, M. C. Righi, Phys. Rev. B 86, 245434 (2012).

Ab initio investigation of tribochemical phenomena in solid and boundary lubrication

248th National Meeting (2014)

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