| To understand the surface mobility of lubricants, the spreading of thin perfluoropolyalkylether (PFPE) films with thicknesses less than 20 nm on amorphous carbon surfaces has been studied using scanning micro-ellipsometry, as a function of film thickness, end group functionality, molecular weight, hydrogen or nitrogen content in carbon films, temperature, and humidity. The thickness-dependent diffusion coefficient was calculated from the thickness profiles by applying the Matano interface method. A complex layered structure in thickness profiles was observed for PFPE with functional end groups, such as Zdol, with the first layer being diffusive and subsequent layers exhibiting sharp steps of unequal heights. The thickness of the layers is on the order of the radius of gyration of PFPE. No layering was observed in the non-functional analogs, Z. Zdol and Z display very distinct diffusive behaviors. The spreading of Zdol is much slower (nearly one order of magnitude) than that of Z. With increasing molecular weight, the thicknesses of the layers increase, and the diffusion coefficient decreases. A conformation of molecules on solid surfaces is proposed. Monte Carlo simulations have been conducted to explain the major experimental observations. Results show that for molecules without reactive end groups, a layered structure in the spreading profiles can be obtained for stronger inter-molecular interactions. With decreasing the interaction strength, the layering is smoothed out, which corresponds to Z. For molecules with reactive end groups, the end groups were represented by the spin assigned to each molecule. By assuming a head-to-head interaction between the spins, a layered structure similar to that in Zdol spreading profiles was obtained. The replenishment of lubricants used in this work has been characterized by solving the diffusion equation using the thickness-dependent diffusion coefficients obtained from experimental results. This work will be useful in optimizing the lubricant-carbon overcoat system to improve the reliability and durability of recording media. |
