Nanometer-Thick Fluorinated Ionic Liquid Films as Lubricants in Data-Storage Devices

Abstract

Media lubricants are critical to the reliability of hard disc drives (HDDs). Ideally, the lubricant should have high thermal stability and low monolayer (ML) thickness. Unfortunately, the state-of-the-art lubricant perfluoropolyether (PFPE) only has limited thermal stability, and its ML is relatively thick due to the polymeric chain structure. In this study, we showed that an ionic liquid (IL) with imidazolium cations and fluorinated anions, [Bmim]­[FAP], has higher thermal stability than PFPE Z-tetraol based on thermogravimetric analysis (TGA). Moreover, both AFM and friction test results demonstrated that the ML thickness of [Bmim]­[FAP] is only ∼50% of that of PFPE Z-tetraol, which can be attributed to the smaller molecular size of [Bmim]­[FAP]. These findings suggest that ILs are promising candidates for the next-generation media lubricants.

Effect of Solid Substrates on the Molecular Structure of Ionic Liquid Nanofilms

“…Unlike on negatively charged mica and silica, neither aggregation nor double-layering quantized growth has been observed during the film growth processes on amorphous carbon, indicating a fairly smooth increase in the local film thicknesses. At the interface between IL and amorphous carbon, π–π + stacking interaction occurs between the delocalized π + electrons from the cation imidazolium ring and the localized π electrons from the sp 2 carbon on the amorphous carbon surface. , The random distribution of the sp 2 carbon in the amorphous carbon provides high conformational flexibility for the respective IL cations to arrange themselves and fill in vacant solid surface areas. The IL anions subsequently pack on the cation layer as a result of the electrostatic interaction between the IL ions to maintain electroneutrality .…”

“…The IL studied was 1-butyl-3-methylimidazolium tris­(pentafluoroethyl)­trifluorophosphate ([Bmim]­[FAP], ≥ 99.0%), which contains a cation imidazolium ring and was acquired from Millipore Sigma. [Bmim]­[FAP] has exceptional hydrophobicity, and it can be readily dissolved in 2,3-dihydrodecafluoropentane (trademark as Vertrel XF by Chemours, purchased from Fisher Scientific, and used as received) to make dilute solutions . The vapor pressure of Vertrel XF is 30.1 kPa (0.297 atm) at 25 °C.…”

“…During the thin film fabrication processes, solid substrates were vertically dipped into and withdrawn from the dilute IL solutions at the speed of 1 mm/second. , As a common solvent in the hard disk drive industry, Vertrel XF evaporates instantly and completely after dipcoating. As the nanofilm thickness trends with the concentration of the dipcoating solution, various stages of the nanofilm growth were realized by fabricating the IL nanofilms independently using solutions with a sequential increase in concentration. ,, The thicknesses of IL nanofilms on silica and amorphous carbon were measured by ellipsometry using a J.A. Woollam alpha SE spectroscopic ellipsometer at the incident angle of 70°.…”

“…They are usually made of organic cations and organic or inorganic anions with bulky and asymmetric conformations. As a result, the electrostatic force and the lattice formation are dampened, leading to the low melting point. − Since the successful synthesis of air- and water-stable ILs, ILs have attracted a great deal of academic and industrial research interest due to their exceptional physiochemical properties, and ILs with a variety of chemical structures have been designed, synthesized, and studied. − Among all the booming applications of ILs, the IL/solid interface is particularly important to the revolution of many cutting-edge applications, e.g., media lubricants in the data storage industry, − additives for engine lubrication, − gas separation and carbon capture media, − green electrocatalysis, − and battery electrolytes. − Fundamental understanding of the interfacial molecular structure of ILs is critical to the performance of solid-confined ILs.…”

Ionic Liquid Lubricant for Ultra-Low Head-Media Spacing in Hard Disk Drives

“…Wang et al used atomic force microscopy (AFM) to show that the monolayer thickness of the IL is only half of that of Z-Tetraol (MW: approximately 2000). They concluded that smaller monolayer thickness of the IL is attributed to the smaller molecular size of the material [15]. The IL in that study was general one which has an typical imidazolium cation and a fluorinated anion.…”