Ab initio study on physical and chemical interactions at borates and iron oxide interface at high temperature
Ab initio molecular dynamics (AIMD) simulations were carried out to study the interactions of sodium tetraborate (Na2B4O7) and boron oxide (B2O3) on an iron oxide surface Fe2O3(0 0 0 1) at 1073 K. The results demonstrate that high temperature transforms Na2B4O7 structure by activating sodium mobility and decomposing BO4 complexes into BO3 and non-bridging oxygen. These products engage in chemical reactions with the Fe2O3 surface by forming O-Na ionic and Fe-O-B covalent bonds, which make Na2B4O7 adhere extremely well on the surface. Density profile and charge analysis show that sodium cations play a critical role at the interface by forming O-Na-O stacking and acting as a bridge to connect the negatively charged areas of the O-rich surface and the oxygen of the borate. On the contrary, the molten B2O3 remains intact with no chemical reactions which accounts for its poor adhesion on the Fe2O3 surface. Furthermore, the Blue Moon method was applied to predict bond dissociation and charge analysis was used to rationalize the differences in binding mechanisms at the lubricant-steel interfaces. The above simulations provide a theoretical interpretation for the lubricity of sodium borate and boron oxide at elevated temperature.