Density functional theory calculations of chemical interactions of lubricant additives sodium pyrophosphate (Na4P2O7) and orthophosphate (Na3PO4) on nascent iron Fe(110) and iron oxide Fe2O3(0001) surfaces have been carried out. Comparisons of adsorption behaviors of the two lubricant additives on different surfaces have been implemented on the basis of the thermodynamics of adsorption and electronic structure analyses. The results indicate that sodium phosphates chemically adsorb on iron and iron oxide surfaces by forming Fe–O bonds and stick on the surfaces through Fe–O–P linkages. The stronger binding of Na3PO4 than that of Na4P2O7 on both Fe(110) and Fe2O3(0001) surfaces is consistent with its better antiwear performance observed by the experiments. It is found that Fe–O bonds formed during the phosphate adsorption are stable covalent bonds that are as strong as P–O bonds of the Fe–O–P linkages. However, the binding of Fe–O–P on Fe surface is caused by a donor–acceptor mechanism; in contrast, the donation/back-donation interaction mechanism on the Fe2O3 surface. This study provides an in-depth understanding of the early stage of the tribochemical reaction between polyphosphates and metal/oxide surfaces.

Silicate glasses are potential materials for lubrication at elevated temperature because of their exceptional thermal stability, low cost, and environmentally friendly properties. Although the frictional behaviors and characterizations of silicate glass tribofilms have been studied by a number of experiments, the formation mechanisms as well as the chemical insight into the iron–silicate tribofilm still remain unclear. In the present study, the adsorption and depolymerization of the sodium sorosilicate cluster Na6Si2O7 on a Fe(110) surface have been studied using both first-principles molecular dynamics and density functional theory. Comparisons of adsorption processes and electronic structure of some typical configurations at different temperatures have been carried out. The results strongly suggest that the silicate cluster chemically adsorbs on the Fe(110) surface by forming multiple Fe–nonbridging oxygen (NBO) bonds. The iron surface plays an important role in the dissociation of the NBO by significantly reducing the strength of Si–NBO bonds. Electronic structure calculation reveals that the charge transfer between the lubricant and the iron surface during the adsorption may lead to a flow of alkali ions and a layering process in the tribofilm. Depolymerization is observed at higher temperature and has larger activation energy compared to Si–NBO dissociations, indicating that the process is more difficult. Temperature is also an important factor that contributes to the dissociation of NBOs as well as the depolymerization of the lubricant, both of which can have several impacts on the lubricity of the tribofilm. This study provides a detailed understanding of the chemical reaction of sodium polysilicate on the iron surface at high temperature.

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We have developed a reactive force field (ReaxFF), which is able to reproduce accurately the physical and chemical properties of a comprehensive Fe/Na/P/O system. This ReaxFF was trained systematically using a large number of quantum data of relative energy, heat of formation, partial charges, bulk modulus, and crystal cell parameters of binary, ternary, and quaternary oxides using a robust parallel and multiparameter optimization of genetic algorithm (GA) to achieve the global optimization. The results indicated a substantial improvement upon previous ReaxFFs for systems of Fe x O y , Na x O y , and P x O y crystals. Moreover, an excellent prediction of molecular, electronic, and chemical properties of inorganic alkali polyphosphate (IAP) was found at low and elevated temperatures. An application of this developed ReaxFF in thin film lubrication of IAP confined between hematite surfaces showed a good agreement with experiments which showed that sodium played a vital role at IAP−hematite interfaces. The tribological performance of the sliding interface has been improved due to the formation of a hierarchical structure of the tribofilm.