Tribological properties of confined water and graphite-based surfaces probed by atomic force microscopy

Abstract

To understand the lubrication mechanism on ice, a deep understanding of the atomic-scale friction characteristics of the water layer is required. Two-dimensional materials such as graphene have definable atomic flatness, providing an excellent model for examining the tribological properties of water. Atomic force microscopy (AFM) is a versatile technique that gathers various tribological information via the inter-atomic interaction at the sharp tip/sample interface. Many studies such as finding out the isotopic effect of adsorbed water on the frictional force have been conducted with it. In this dissertation, Chapter 1 explains the relations between surface chemistry and friction on the aspect of energy transfer at the interface. In Chapter 2, the operating principles of atomic force microscopy in terms of configuration and experimental examples is discussed. In Chapter 3, the correlation between the friction force and each confined water region intercalated between graphene and hydrophilic mica was investigated by AFM. Phonon density of state (DOS) calculation suggests that the friction dependence of graphene and water layers was highly correlated with the overlapping of phonon vibration modes for each model environment. If water was mixed with heavy water, the slower vibration rate of heavy water caused a decrease in friction force. This friction behavior was shown similar when graphene was replaced by MoS2, indicating that the friction on confined water is a general phenomenon. In Chapter 4, the nanoscale friction behavior of confined water layer under various relative humidity (RH) conditions was investigated by using AFM. In ambient conditions, SiO2/Si had higher friction than that on single-layer graphene (SLG). At high RH (>98 %), superlubrication behavior was observed on SiO2/Si compared to SLG. These results suggest that the slippery surface consisting of adsorbed water molecules at the contacting interface induces gliding motion in high RH environment. Overall, these findings show that friction is mediated by phonons, which is heavily governed by the surface characteristics.