On the ionic liquid films ‘pinned’ by core–shell structured Fe3O4@carbon nanoparticles and their tribological properties

Abstract

A strongly ‘pinned’ ionic liquid (IL, [BMIM][PF₆]) film on a silicon (Si) surface via carbon capsuled Fe₃O₄ core–shell (Fe₃O₄@C) nanoparticles is achieved, revealing excellent friction-reducing ability at a high load. The adhesion force is measured to be ∼198 nN at the Fe₃O₄@C-Si interface by the Fe₃O₄@C colloidal AFM tip, which is stronger than that at both Fe₃O₄@C-Fe₃O₄@C (∼60 nN) and IL-Si (∼10 nN) interfaces, indicating a strong ‘normal pin-force’ towards the Si substrate. The resulting strengthened force enables the formation of lateral IL networks via the dipole–dipole attractions among Fe₃O₄ cores. The observed blue shift of the characteristic band related to the IL anion in the ATR-FTIR spectra confirmed the enhanced interaction. The N–Si, P–O chemical bonds formed as a result of the IL interactions with the Si substrate confirmed by XPS spectroscopy suggested that the IL lay on the Si plane. This orientation is favorable for Fe₃O₄@C nanoparticles to exert ‘normal pin-force’ and press the IL film strongly onto surfaces. The IL ions/clusters are thus anchored by these Fe₃O₄@C ‘pins’ onto the substrate to form a dense film, resulting in a smaller interaction size parameter, which is responsible for the reduced friction coefficient μ.