Nano-structure and stability of ultra-thin lubricant films.
详细信息
文摘
This work focuses on developing a model to study and design the HDI and to analyze the nano-structure and the stability of molecularly-thin films. Contact angle measurements are performed to calculate the surface free energy of the carbon overcoats and the lubricant films to analyze the interaction between them. It was observed that the interaction between the endgroups of perfluoropolyether (PFPE) Zdol and Ztetraol and the surface of the hydrogenated carbon overcoat was characterized by hydrogen bonding.;A theoretical model is then developed utilizing the calculated surface free energy to analyze the thermodynamic stability of the lubricant film. The model predicted that the presence of the head slider over the film can destabilize the lubricant film.;A bead-spring, off-lattice model is developed for PFPEs demonstrating their flexible ether bonds and rigid fluorocarbon backbone units, and the model was employed in Monte Carlo simulations to investigate the static nano-structure of the thin films. The films demonstrated layered structures with each layer thickness characterized by the anisotropic radius of gyration of the molecules. The functional endgroups couple with each other leading to a characteristic surface tension behavior as a function of thickness and spreading profiles. The nonfunctional and "lightly" functional endgroups were segregated toward the film surface and the interface between the film and the wall. However, strongly functional endgroups were not segregated to the film surface. It was also shown that this same mechanism caused lower molecular weight fractions to segregate to the interfaces.;Molecular dynamics simulations were also performed using the bead-spring model. The adhesive properties were dependent on the molecular weights, the deformation rates, and the chain-end functionalities. The surface tension was obtained from simulations of the molecularly-thin films with chain-end functionality.;The shape of the capillary bridge appearing between the two facing films was described by a theoretical form based on the augmented Young-Laplace equation. Disjoining pressure of a film on solid substrate was calculated for the first time by fitting the simulated shape to the theoretical one, and film stability is discussed in terms of the disjoining pressure of the molecularly-thin films.