Lubricant evolution and depletion under laser heating: a molecular dynamics study

Abstract

Understanding the performance of polymeric perfluoro-lubricants under femtosecond laser irradiation is of great fundamental importance in enhancing the stability and durability of micro- and nano-devices. In this paper, molecular dynamics simulations of perfluoropolyether are carried out to investigate the evolution and depletion of molecularly thin lubricants when subjected to laser heating. Ultrathin perfluoropolyether lubricant films are modeled by the coarse-grained bead-spring model and are coated on an inert substrate. Periodical surface morphology and layered film structure are formed in the equilibrium lubricant system due to the polar interaction of functional beads. It is found that the lubricant undergoes severe depletion with an increase in laser heating duration, resulting in aggravated lubricant evaporation and raised ridges. A temperature gradient is formed in the radial direction due to the heat transfer between the heated beads and the surrounding lubricants. During the cooling process, the strong functional interaction between end-beads and the substrate layer hinders the recovery and redistribution of depleted lubricant beads, resulting in an undersaturated film. The mechanism of lubricant depletion under laser heating is further demonstrated by analyzing the temperature dependence of surface tension. The detailed analyses of lubricant depletion provided in the present work are expected to be guidelines to design novel perfluoropolyether lubricants.