First-Principles Study of the Adsorption and Depolymerization Mechanisms of Sodium Silicate on Iron Surfaces at High Temperature

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 Na6Si2O7on 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.