Electric Response of CuS Nanoparticle Lubricant Additives: The Effect of Crystalline and Amorphous Octadecylamine Surfactant Capping Layers.

Octadecylamine-coated CuS nanoparticles were designed and confirmed to play an important role in their electric response and boundary lubrication in the ester lubricant. For the case of CuS nanoparticles coated with crystalline surfactant, the surface potential is 18.47 ± 0.99 mV higher than with amorphous surfactant, owing to the random chain conformations of the octadecylamine molecules. When used as a lubricant additive, CuS nanoparticles (in the form of nanoplates or nanoarrays) with a crystalline surfactant were positively charged due to the presence of the amino headgroup in octadecylamine. The observed friction coefficient decreased from 0.18 to 0.09 and 0.05, respectively, when negative potential (for the copper lower pair) was applied across untreated CuS nanoparticles. However, thermally treated CuS nanoparticles showed good lubricating effect, but almost no effect of potential control since the amino groups were obscured by the disordered carbon chains, hindering electron transfer and weakening the response to externally applied electric field.