Development of a Ball-Suspension Assembly for Measuring Speed-Dependent Friction Characteristics of Thin Lubricant Films Coated on Magnetic Disks

To measure the speed-dependent friction properties of molecularly thin lubricant films coated on magnetic disks, we developed a novel ball-suspension assembly (BSA) that allows us to slide a ball on a disk with steady contact at high sliding speed up to 210 mm/s. With increasing sliding speed, the frictional force exerted on the contacting point of the ball increases, and the frictional moment that serves to rotate the ball increases. The frictional moment is more likely to excite the out-of-plane vibration of the suspension than the frictional force itself. In the newly designed BSA, only a small portion of the ball protrudes from the suspension surface so that the frictional moment is minimized. We confirmed that, as compared with a conventional BSA, the new BSA satisfactorily suppresses the out-of-plane vibration of the suspension and the horizontal vibration of the friction transducer to which the BSA is connected. Using the new BSA, we measured speed-dependent friction properties of monolayer perfluoropolyether Zdol4000 lubricant film in the sliding speed range of 2.1–210 mm/s. The measured frictional force was found to be nearly proportional to the 1/3 power of the sliding speed.