Frictional properties of bilayered mixed lubricant films on an amorphous carbon surface: effect of alkyl chain length and SAM/PFPE portion

Abstract

Twenty-Å-thick mixed lubricants with bilayered structures, which consist of alkylsilane monolayers (C6, C8, and C10) and mobile perfluoropolyether (PFPE) lubricants, were deposited on a hydrogenated amorphous carbon surface. The friction and durability properties of the mixed lubricants on the carbon surface were measured using an atomic force microscope and a micro-scratch tester, which were compared with 20 Å-thick perfluoropolyether lubricant coated carbon surfaces. The friction and durability properties of the mixed lubricant on a carbon surface were mainly dependent on the silane monolayers, that is, the friction force became low and the critical load was enhanced by increasing the alkylsilane chain length although the amount of the mobile lubricant decreased. On the other hand, the mixed lubricants on the carbon surfaces showed a higher friction force compared to that of the Fomblin Zdol lubricant that has a satisfactory molecular flexibility, whereas it showed a higher durability due to a higher surface coverage of the chemisorbed molecules and the rigid nature of the silane monolayers. The frictional properties of the perfluoropolyether lubricants on a carbon surface are related to the radius of gyration, R g, of the molecule. The excess mobile molecules contribute to the high friction due to the capillary effect, that is, an optimum thickness of the mobile lubricant exists for a lower friction, which is comparable to R g.