An understanding of the viscoelastic properties of molecularly thin lubricant film is essential to clarify tribological issues of head-disk interface (HDI) in high-density recording hard disk drives. Characteristic conditions for the HDI occur when lubricant molecules are extremely confined in the gap between the head and the disk surfaces, and the surfaces slide at high speeds. The lower the flying height, the more this confinement affects the flying characteristics. However, a few attempts have been made at clarifying the dynamic viscoelastic properties of confined lubricant molecules. This is because a method of measuring the dynamic shear force has not yet been established. Fiber wobbling method enables us to measure the shear force with a detection limit of less than 1 nN. Additionally, frequency of shear can be set at several kHz. Further, the gap which confines the lubricant is controlled with a resolution of 0.1 nm. Using the FWM, we investigated the effect that confinement had on the dynamic viscoelastic properties of perfluoropolyether lubricants on a magnetic disk. We found that the viscosity started to increase at a gap width that was less than a few hundred nanometers, which is hundreds of times larger than the molecular size. On the other hand, elasticity suddenly appeared at a gap width that was less than a few nanometers, which is equivalent to a few molecular sizes. Both the viscosity and elasticity increased monotonically as the gap decreased.
It is essential to clarify the interaction between the magnetic head and the monolayer lubricant film coated on the magnetic disk in the design of next-generation hard disk drives with ultra-low flying height. However, with previous measuring methods such as AFM, it was difficult to measure the mechanical properties of molecularly thin lubricant films with a precisely controlled nanometer-sized gaps. In this study, we applied the fiber wobbling method to clarify the viscoelastic properties of monolayer lubricant films during touch-down and take-off behavior. The fiber wobbling method is the highly-sensitive measurement of shear force we developed and can control the shearing gap with sub-nanometer resolution. The gap dependence of viscoelastic properties was evaluated both in the approaching and the separating processes and their differences and dependencies on the types of lubricant films were discussed. As sample lubricants, we used non-polar and polar perfluoropolyether lubricants: Fomblin Z03 and Fomblin Zdol4000. Three different types of monolayer lubricant films were prepared. The first one consisted of mobile molecules of Z03 only. The second one contained both mobile and bonded molecules of Zdol4000. The third one was only made of bonded molecules of Zdol4000. Our experimental results indicated that the mobile molecules caused the formation of a liquid bridge between solid surfaces and it leads to the hysteresis of viscoelastic properties between approaching and separating processes. On the other hand, the sample only consisted of bonded molecules did not show such a hysteresis.
In the head disk interface (HDI) of a magnetic recording system, lubrication caused by a monolayer thick lubricant film is necessary to achieve stable relative motion between a magnetic disk and a magnetic head. Viscoelastic properties of lubricant films should be clarified for improvement of lubrication performance, however, measurement methods have not been established yet. In this study, we present a new method precisely detecting the starting point of asperity contact of sliding solid surfaces in order to measure viscoelastic properties of the molecularly thin lubricant film on the magnetic disk.
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