3694 IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 9, SEPTEMBER 2007 Lubricant Dynamics in the Sub-Nanometer Clearance Regime Bruno Marchon, Qing Dai, Bernhard Knigge, and Remmelt Pit Hitachi GST, San Jose Research Center, San Jose, CA 95120 USA This paper reviews the present state of understanding of lubricant-slider interactions in a rigid disk drive. As slider flying heights are rapidly approaching the intrinsic limit of a disk topography (1–3 nm), it has become apparent that disk-slider clearance of less than a nanometer can be achieved, at least on a laboratory setup. We will discuss the implication of such low spacing on the lubricant film behavior, and provide general physical arguments that attempt to highlight lubricant attributes that are relevant under those clearance conditions. Index Terms—Disk drive, lubrication, tribology. I. INTRODUCTION T HE advent of thermal flying height control (TFC) now allows the dynamic adjustment of the head/disk clearance to the sub-nanometer range, with a precision better than an Angstrom, and a dynamic response in the 1–10 kHz range [1]. This technological breakthrough might well allow us to reach the Terabit per square inch mark without the need to develop a reliable contact recording interface [2]–[4]. However, a flying, sub-nanometer slider-to-disk clearance, is not without technological challenges. In particular, the resulting shear stress in the megapascal range exacerbates slider effects on the molecularly-thin lubricant film of the disk [5], [6]. As a result, the lubricant film is prone to locally redistribute according to the disk topography [6]–[8], or slider natural frequencies [5], [6], [9], [10]. In this paper, we will review the present state of under- standing of the physics of slider/lubricant interactions. In the first section, we will describe perfluoropolyether (PFPE) lubri- cant characteristics (structure, viscosity, disjoining pressure), and how they can be modeled. In the second part, some approx- imations will be described, allowing some simple fundamental equations dictating lubricant stability to be applied. II. LUBRICANT FILM CHARACTERIZATION A. Structure It is generally accepted that the polar lubricants used on rigid disks (e.g., Zdol, Ztetraol) adopt a structure where the polar end- groups attach themselves to the disk overcoat surface through hydrogen bonding. As the thickness of the lubricant is increased, crowding of the molecular arrangement occurs, and the lubri- cant chains coils up, until dewetting occurs [11]–[13]. Whereas early papers describe the dewetting thickness limits in the random coil configuration, where the critical thickness scales at the square root of molecular weight (MW), more recent studies performed on Zdol suggest that the dewetting thickness scales linearly with MW [12]. The latter result is actually more realistic, as strong end-group to surface interactions is likely to result in a constant molecular footprint area, regardless of MW. It is interesting to point out that assuming Digital Object Identifier 10.1109/TMAG.2007.902972 Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. where : dewetting thickness, one can estimate the molecular footprint of the Zdol on carbon surface as (1) being the Avogadro number, and the density of the lubri- cant. Using Kg [12] and g/cm , one obtains nm . This result therefore gives us a convenient order of magnitude of the smallest possible surface area covered by the lubricant molecule, before dewetting occurs. B. Thermodynamics As far as the energetics of the lubricant film is concerned, several levels of complexity have been proposed to account for both polar and dispersive components [14], [15]. A convenient expression for the film energy as a function of thickness, that captures most of the behavior in the sub-monolayer range, is as follows [16]: (2) is the Hamaker constant J , the distance of closest approach [15], and the amplitude of the oscillatory polar component ( 2.5 mN/m) [12]. C. Dissipative Properties Viscous dissipation in these lubricant films is certainly a field of growing interest, albeit at present very poorly understood. Al- though it is well documented that nonfunctional Fomblin oils (Z series) behave like bulk liquid under shear or disjoining pressure gradient, even in the sub-nanometer range [15], [17], [18], little is understood of the dissipation behavior of functional PFPEs. Historically, lubricant engineers have assumed a binary view of lubricant viscous behavior, wherein a chemically “bonded” layer of infinite viscosity is covered with a “mobile” lubricant film with a viscosity matching the bulk value. More recently, it was found that lubricant (Zdol) viscosity was behaving in a more complex fashion, exhibiting either an exponential rise at low thickness [19], or a two-layer characteristics, with a mobile layer 2 to 10 times the bulk viscosity on top of a “restricted” layer of 20–100 times that value [20]. In a more qualitative view, it is generally well accepted that increasing the number of polar (hydroxyl) end-groups in the PFPE chain from two (Zdol) to four (Ztetraol) to eight (ZTMD [16]) tends to magnify sub- strate-lubricant interactions, increasing the effective surface vis- cosity. 0018-9464/$25.00 © 2007 IEEE