High lubricity and electrical responsiveness of solvent-free ionic SiO2 nanofluids

Abstract

Solvent-free ionic nanofluids with a core–shell structure were successively synthesized via functionalizing silica nanoparticles with a charged corona and ionically tethering with oligomeric chains as a canopy. It was demonstrated that the as-synthesized nanofluids were endowed with outstanding dispersion stability and fluidity. Importantly, when used as a lubricant and additive, the nanofluids showed excellent friction-reducing and antiwear properties. That is, blending even a small fraction of nanofluids into polyethylene glycol base oil dramatically reduced the friction and wear of a steel–steel contact. Depending on the specific shell structures, the lubricity of the nanofluids exhibited a responsive characteristic to electrical potential. The friction coefficients can be adjusted by loading or unloading the external power and changing the direction of the electrical potential. Formation of a unique double electric layer consisting of both organic and inorganic materials during the friction was proposed, which can be essential for yielding robust lubrication of the adsorption film. Moreover, a nanostructured tribofilm comprising a significant fraction of silica and tribo-chemical products of the organic layers was identified, and it was believed to be significant in improving the boundary lubrication performance.