To ensure the reliability and durability of hard disk drives, molecularly thin polymeric liquid films of perfluoropolyether (PFPE) coated on the disk surfaces possess a mixed structure consisting of molecules bonded to the disk surfaces and non-bonded mobile molecules. Aiming to clarify the influence of film composition (mobile/bonded) on tribological performance, we measured friction properties of two types of 2-nm-thick PFPE films (functional Zdo12000 and nonfunctional Z03) under lightly loaded (loading force : 0-1 mN) and quasi-static (low rotational speed : 2.1 mm/s) conditions as a function of elapsed time. The friction force of Z03 remained unchanged with time and increased linearly with loading force as described by Amontons' law. In contrast, induced by the development of the molecules' bonding in time, the friction force of Zdo12000 increased and exhibited a nonlinear increase with loading force as time proceeded. The nonlinear friction-load relationship of Zdo12000 in the equilibrium state was well characterized by the Johnson-Kendall-Roberts model.
