Structural response of alkali metal borates at Fe2O3 sliding interface: The effect of alkali cations

Abstract

Structural transformation and chemical reaction induced by sliding motion is an interesting topic in material science and tribochemistry. In this paper, we used density functional theory (DFT) to study the adaptation of molecular and electronic structures at the iron oxide–alkali borate interface. Sodium borates with different sodium concentrations and two other common alkali metal borates of lithium and potassium have been confined between two Fe2O3 surfaces and subjected to the sliding at 1 and 38GPa. The results suggest a significant impact of alkali elements on the energy shape and bonding behavior in the systems. The low energy variation was obtained in the systems with sodium borate at 33% and 50% concentration of Na2O. Meanwhile, high variation energy was encountered with the sodium borate with low Na2O concentration (25% Na2O), lithium borate and potassium borates at 3 GPa. The formation of an “easy-shear” pattern was found in the lowest energy variation system with 50% Na2O at 1 GPa. The high variation energy was closely related to the dissociation of iron atoms from the oxide surface and the subsequent rupture of Fe−O bonds of the surface. Based on the current simulations, the possible interfacial layering has been proposed to rationalize experimental findings on the friction and wear of borate lubricated iron oxide surfaces.