Lubricant evolution and depletion under laser heating: a molecular dynamics study Yong Li, a Chee How Wong, * a Bei Li, a Shengkai Yu, b Wei Hua b and Weidong Zhou b Received 8th December 2011, Accepted 15th March 2012 DOI: 10.1039/c2sm07326a 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. 1. Introduction With the rapid development of exquisite nanofabrication tech- niques and novel nano-materials, the emerging micro- and nano- devices have been proven to be one of the most promising technologies in the automotive industry, biological technology and medical care, customer electronics, etc. 1 However, profound influence of friction and wear on the performance and reliability of such devices has been demonstrated during the miniaturiza- tion of components. 2 One of the challenges in commercializing sophisticated micro- and nano-devices is reducing the tribolog- ical effects, such as the adhesive and frictional forces, which are more important and critical in such small devices than macro- scopic moving parts due to large surface area-to-volume ratios. 3 In the continuous search for improvements in the tribological properties of the micro- and nano-devices, lubrication has arisen as a very promising technique because it plays a determining role in reducing galling, friction, and wearing of moving parts. 4 During the last few decades, different lubrication techniques were thoroughly explored to enhance the reliability and dura- bility of the micro- and nano-devices, such as surface coating of self-assembled monolayers (SAMs), bound-plus-mobile phase lubricants, and ionic liquids and perfluoro-lubricants. 5 Among the techniques used to reduce friction, perfluoropolyether (PFPE) lubricants have been proven to be potential candidate lubricants for use in the micro- and nano-devices. The main features of PFPE lubricants are their unreactive, nontoxic, and non-flammable fluid properties. 6,7 Recently, polar PFPE with hydroxyl terminal groups, such as Fomblin Z-dol, 6 is frequently used to improve the bonding of lubricant to the solid surface together with the excellent lateral mobility to diffuse and recover the worn surface regions. 5 Recent studies show that the laser technique has revolutionary advances in the field of micro- and nano-machining, surface texturing and modification, printing technique, optical projec- tion, hard disk storage, etc., due to its high peak power, instant high energy density values and localized heating. 8–11 However, the numerous advantages of a femtosecond pulsed laser may yield counterproductive effects on the performance and reli- ability of the devices. To date, incomplete understanding of the mechanism of lubricant failure due to laser heating remains one of the critical issues that retards the rate of introducing these devices into practical applications. 12 Tao and Bhushan studied the degradation and environmental effects on Zdol lubricants and found that decomposition by triboelectrical reaction and mechanical scission are the principal mechanisms of degrada- tion. 13,14 Heller et al. investigated the effect of heating a PFPE a School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore. E-mail: chwong@ntu.edu.sg; Fax: +65 6792 4062; Tel: +65 6790 5913 b Data Storage Institute, DSI Building, 5 Engineering Drive 1, Singapore 117608, Singapore This journal is ª The Royal Society of Chemistry 2012 Soft Matter , 2012, 8, 5649–5657 | 5649 Dynamic Article Links C < Soft Matter Cite this: Soft Matter , 2012, 8, 5649 www.rsc.org/softmatter PAPER Downloaded by Nanyang Technological University on 15 May 2012 Published on 16 April 2012 on http://pubs.rsc.org | doi:10.1039/C2SM07326A View Online / Journal Homepage / Table of Contents for this issue