2504 IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006 Fomblin Multidentate Lubricants for Ultra-Low Magnetic Spacing B. Marchon , X.-C. Guo , T. Karis ,H. Deng , Q. Dai , J. Burns , and Robert Waltman Hitachi GST, San Jose Research Center, San Jose, CA 95120 USA Hitachi Global Storage Technologies, San Jose, CA 95139 USA This paper discusses a new class of lubricants for disk application, designed for ultra-low magnetic spacing. By tying down the middle of the long perfluoropolyether (PFPE) Fomblin chain to the overcoat surface with additional functional groups, one can achieve the very low clearance needed for the future, sub-5 nm flying height head-disk interface. Bulk synthesis and characterization are described, and tribological performance is investigated. Benefits of this novel lubricant family can be attributed not only to its shorter free chain length, but also to disjoining pressure attributes. Index Terms—Head-disk interface, lubricant, tribology. I. INTRODUCTION F OR areal densities in the Tb/in range, it will be necessary for the head-media spacing (HMS) to be within 5–7 nm [1]. At this level, the disk lubricant thickness is no longer a negligible part of this HMS budget, and it will have an increas- ingly stronger impact on read/write performance [2]. Recently, dynamic effects of lubricant displacement from shear effect of low-flying sliders have been observed [3] and successfully mod- eled [4]. These dynamic effects are believed responsible for loss of appropriate slider/disk clearance. In this paper, we present an extension to functionalized end-groups. By introducing ad- ditional functional hydroxyl groups in the middle of the perfluo- ropolyether (PFPE) chain [5], we will demonstrate that one can achieve reduced clearance, while still achieving overall drive reliability. In a recent publication [6], we have described the synthesis of a Ztetraol multidentate, or ZTMD, whose structure is represented in Fig. 1(a). This molecule is composed of two oligomeric Ztetraol (1000 Daltons) units, coupled by a diepoxy agent, leading to a structure similar to Ztetraol (2000 Daltons), but with four additional hydroxyl groups in the middle of the chain. In this paper, additional performance data is discussed. In particular, water adsorption, altitude clearance, and behavior after on-track flying will be shown. II. EXPERIMENT Using both Zdol and Ztetraol (1000 Daltons), two mul- tidentate lubricants (ZDMD and ZTMD respectively) were synthesized by Exfluor Research Corporation (Round Rock, TX), using a diepoxy coupling agent, according to the reaction (shown for Zdol only) in Fig. 1(b). Throughout this paper, only the results for ZTMD [Fig. 1(a)] will be shown. Narrow molecular weight fractions of the syn- thesized lubricant were further separated by supercritical fluid fractionation (Phasex Corp., Lawrence, MA), and characterized by nuclear magnetic resonance (NMR) and thermogravimetric analysis (TGA) [6]. Digital Object Identifier 10.1109/TMAG.2006.878628 III. RESULTS A. Water Uptake at High Humidity Because of this higher number of -OH groups, the water affinity in its bulk state is higher than other lubricants, as illustrated in Fig. 2. In that figure, mass uptake of different lu- bricants when exposed to a water-saturated nitrogen atmosphere at room temperature is shown. Water absorption clearly scales, at least qualitatively, with the number of hydroxyl groups in the lubricant chain going from ZDPA [7] (no hydroxyl groups) to A20H (one), Zdol (two), Ztetraol (four), and ZTMD (eight -OH groups). This behavior contrasts dramatically with the data obtained at thin film level (0.8–1 nm) on carbon overcoat. For these mea- surements, lubricated disks were first heated to 70C for a few seconds to desorb any residual adsorbed water, and then allowed to cool under dry air for 20 mn in a temperature and humidity- controlled chamber. Water adsorption was measured by in-situ ellipsometry, incrementing relative humidity by 5% steps, and allowing the surface to equilibrate for one minute. Water adsorp- tion for ZTMD is equivalent to Ztetraol and lower than with a UV-treated, 50-50 mixture of A20H and Zdol (Fig. 3). It is be- lieved that an increase in the number of hydroxyl end-groups help tie up hydrophilic sites on the carbon surface, in effect making them less available to water adsorption. Water contact angle (WCA) data (Table I) is consistent with this picture, with Ztetraol and ZTMD having the highest values, showing a higher degree of hydrophobicity and lower surface energy. B. Thermal Stability Hydroxyl groups have been shown to passivate catalytic sites for PFPE degradation [8]. As a result, another significant attribute of ZTMD, derived from the multiplicity of hydroxyl groups, is an improved thermal catalytic stability. Isothermal thermogravimetric analysis tests verified a significantly longer induction time for ZTMD on Al O catalyst. C. Clearance Properties Clearance property was probed by varying pressure to simu- late flyability under high-altitude conditions [9], using a slider 0018-9464/$20.00 © 2006 IEEE