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.
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.
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