Detection of Asperity Contact for Precise Gap Determination in Thin-Film Nanorheometry

Understanding the mechanical properties of a nanometer-thick liquid lubricant film coated on a magnetic disk surface is essential for effective lubrication design of head-disk interface in hard disk drives. We have developed a highly sensitive shear force measurement method, named the fiber wobbling method (FWM), that enables us to measure the viscoelastic properties of such films. For these measurements, the shearing gap needs to be determined precisely since the thickness of the film is around 1–2 nm, and the accuracy of the gap determination depends on the sensitivity of the solid contact point detection, which is considered to correspond to a gap width of zero. In this study, we present a sensitive method for solid contact detection in the FWM that relies on the monitoring of the resonant oscillation of a shearing probe excited by asperity contact. The feasibility of the method is verified by both numerical calculations and experiments. The applicability to a magnetic disk with a lubricant film is also examined and we confirmed the method had an enough sensitivity to determine the nanometer-sized gap width.