The remarkable lubricant properties of molybdenum dithiocarbamates (MoDTCs) make this class of oil additives well-known in the automotive industry. However, the mechanism of function of these compounds is still not completely understood at the atomistic level. We provide new insights on the dissociation of MoDTC in tribological conditions which are the key to describe the debated mechanism to form MoS₂. Quantum Mechanics / Molecular Mechanics (QM/MM) dynamic simulations allowed us to monitor in real time the tribochemical reactions occurring at the iron interface and revealed that the presence of the iron substrate and the mechanical stresses alter the dissociation path with respect to what is expected for the isolated MoDTC molecules. Moreover, they uncovered the important role of molecular oxidation on the dissociation pattern: the presence of oxygen atoms in the ligand position of MoDTC favor the release of central units of the molecules, containing just Mo and S atoms with the correct stoichiometry to form MoS₂. This work demonstrates how the predictive power of ab initio simulations can be very valuable in order to design new lubricant additives.

The Reynolds lubrication equation (RLE) is widely used to design sliding contacts in mechanical machinery. While providing an excellent description of hydrodynamic lubrication, friction in boundary lubrication regions is usually considered by empirical laws, because continuum theories are expected to fail for lubricant film heightsh₀≪ 10 nm, especially in highly loaded tribosystems with normal pressuresp_n≫ 0.1 GPa. Here, the performance of RLEs is validated by molecular dynamics simulations of pressurized (withp_n= 0.2 to 1 GPa) hexadecane in a gold converging-diverging channel with minimum gap heightsh₀= 1.4 to 9.7 nm. Forp_n≤ 0.4 GPa andh₀≥ 5 nm, agreement with the RLE requires accurate constitutive laws for pressure-dependent density and viscosity. An additional nonlinear wall slip law relating wall slip velocities to local shear stresses extends the RLE’s validity to even the most severe loading conditionp_n= 1 GPa andh₀= 1.4 nm. Our results demonstrate an innovative route for continuum modeling of highly loaded tribological contacts under boundary lubrication.

Among metal β-diketonates, nickel acetylacetonate (Ni(acac)₂) has been widely employed as a precursor for many chemical structures, due to its catalytic properties. Here we investigate, by means of density functional theory calculations, the adsorption and the dissociation of this complex: after an evaluation of structural and electronic properties of Ni(acac)₂, a comparison between different dissociation patterns reveals that the most favorable pattern for the complex adsorbed on iron is different from the one suggested by considering the strength of the bonds in the isolated complex and an attempt to generalize this dissociation model is made in this work. Moreover, the most favorable adsorption configurations turned out to be a long bridge positioning of the nickel atom along with an on top positioning of the oxygen atoms of Ni(acac)₂, while a short bridge positioning is the most favorable for the central metallic unit alone.