Degradation Mechanisms and Environmental Effects on Perfluoropolyether, Self-Assembled Monolayers, and Diamondlike Carbon Films

Abstract

The degradation mechanisms and durability of selected lubricants and environmental effects on the lubricants which could be used for microelectromechanical/nanoelectromechanical systems (MEMS/NEMS) applications were studied in this paper. The degradation of perfluoropolyether (Z-DOL), four self-assembled monolayers (SAMs)-hexadecane thiol, perfluoroalkylsilane, and alkylsilane (C8 and C18)-and diamondlike carbon (DLC) films was investigated in high vacuum. Gaseous products and friction force were detected using a quadrupole mass spectrometer and strain gauges. It is believed that triboelectrical reaction and mechanical scission cause the degradation of Z-DOL. SAMs are believed to degrade by cleavage at an interfacial bond accompanied with triboelectrical reactions. DLC is believed to degrade by mechanical shear and thermal oxidation. Environmental effects on lubricant films were studied in high vacuum, argon, and air at various humidity levels. It was found that the environment has a significant influence on the lubricant performance. The lubricant films exhibit high friction and low durability in high vacuum. Oxygen in the air can cause the thermal oxidation of SAMs and DLC films. Water molecules can act as a lubricant for Z-DOL films at a moderate humidity level, while they can penetrate the Z-DOL films at a high humidity level. Water molecules can detach the SAM molecules from the substrate, whereas, for DLC films, water molecules can act as a lubricant.