An ab initio study on the effects of Na passivation on friction reduction of an iron oxide surface

The presence of sodium-rich layers on iron oxide surfaces plays an important role in the functionality of glassy lubricants used in harsh working conditions of metal formation. However, the underlying low-friction mechanism of the sodium layer on iron oxide surfaces at the atomic level is not well understood. In this work, Na adsorption on the most stable surface of Fe2O3 (0001) is studied by density functional theory. The most stable adsorption configuration and the modifications induced by the adsorption on the structural as well as the electronic properties of the surface are discussed. By constructing the potential energy surface, we can quantitatively compare the sliding behaviors of two sodium passivated oxide layers with that of clean surfaces. The determination of energy corrugations, sliding paths, static lateral forces, and shear strengths has suggested a significantly lower friction in the Na-passivated system compared to that of the clean surface. The effects of a load on the friction are also investigated. The results indicate that sodium passivation in glass lubricants can help to prevent the direct contact of two oxide surfaces and thereby maintain a low friction and hence wear reduction at high pressures.