Quartz crystal microbalance measurements of sliding friction of inert gas films on lead, copper, nickel, and graphene surfaces

The Quartz Crystal Microbalance (QCM) has been used to record adsorption and sliding friction data for molecularly thin inert gas films, at 77K, on metal surfaces prepared under Ultra High Vacuum (UHV). Adsorption of xenon was studied on nickel, carbonized nickel, copper and lead. Adsorption of krypton was studied on carbonized and clean nickel. Even sub-monolayer quantities of inert gas produce changes in QCM mechanical properties that may be used to estimate coefficients of sliding friction. At 77K, these inert gases are known to adsorb on various surfaces, forming two-dimensional phases analogous to the solid and gas phases exhibited by bulk substances. While previous QCM studies have emphasized the role of two-dimensional solid and liquid phases in damping QCM motion, this work examines the possibility that the two-dimensional gas phase may be the dominant cause of damping at low coverage. QCM data suggest that the sliding motion of the two-dimensional gas phase of inert gases tends to decay within a characteristic time approaching 10 nanoseconds. The 2D solid phase routinely cited as a low friction phase has an order of magnitude higher friction than the 2D gas phase.