Evaluating the Elastic Moduli of Ultra-Thin Liquid Perfluoropolyether Lubricant Films on Diamond-Like Carbon Films Using Ar-Gas Cluster Ion Beams

The mechanical properties of ultra-thin films with monolayer thicknesses adsorbed onto a solid surface are of great interest when understanding tribological phenomena. In this study, we measured the Young's moduli of ultra-thin liquid perfluoropolyether (PFPE) lubricant films on diamond-like carbon (DLC) films using Ar-gas cluster ion beams (Ar-GCIBs). Two types of PFPE lubricants with different mainchain structures (D-4OH and Z-tetraol) were used in this study. The dissociated ion yields of Ar-2(+)/(Ar-2(+) + Ar-3(+)) were measured by time-of-flight secondary ion mass spectroscopy using Ar-GCIB as the primary ions. The dissociated ion yield was calibrated based on the relationship between Young's modulus and the dissociated ion yield of seven different samples. Using this calibration, the Young's moduli of the two PFPE lubricant films on the DLC surfaces were estimated. They showed relatively good agreement when compared with those derived from the intermolecular pressure calculated via molecular dynamics simulations. It can be concluded that the Young's modulus measurement method using Ar-GCIB is an effective tool for evaluating ultra-thin adsorbed films. In the future, it will be possible to measure the Young's moduli of tribofilms and boundary lubrication films using this technique.