Viscoelastic Properties of Monolayer Lubricant Films during Touch-Down and Take-Off Behavior Measured by Fiber Wobbling Method

Abstract

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.