Adhesion and friction of hydrogenated DLC films with adsorbed water studied by in-situ AFM/FFM

Influence of relative humidity on tribology of diamond-like carbon (DLC) has been studied using an in-situ microtribometer. In order to simulate the relative humidity in tribological experiments, water vapor is often back-filled in a vacuum chamber at an appropriate pressure. However, the relation between vapor pressure and relative humidity has not yet been fully understood. Simultaneous use of a hygrometer, a high-pressure vacuum gage and a quartz crystal microbalance (QCM) solved this problem in this study. For DLC films deposited on QCM, an abrupt increase in adhesion and friction forces occurred at the relative humidity of 70-80%, resulting from the meniscus force by the condensation of water vapor. For atomically smooth DLC films on the Si(001), the highest adhesion and friction forces were found at the relative humidity of 40-60%.