Tribology Letters Vol. 11, No. 3–4, 2001 151 Lubricant spin-off from magnetic recording disks T.E. Karis, B. Marchon, V. Flores and M. Scarpulla IBM Research Division, Almaden Research Center, San Jose, CA 95120, USA Received 3 February 2001; accepted 28 July 2001 As the rotation rate of magnetic recording disks increases over the next few years, lubricant spin-off from the disk surface may be significant. Lubricant thickness was measured as a function of spin time at 10 000 rpm on typical carbon overcoated magnetic recording disks initially lubricated with 10–135 Å of perfluoropolyether Zdol. The viscosity of the lubricant film increased as the film thickness decreased with spin time. Lubricant spin-off in response to air shear stress on the free surface was approximately described by viscous flow. The rate of lubricant removal by evaporation was compared to the spin-off removal rate in films between 10 and 50 Å thick. Dispersion interaction and chemisorption are expected to retain a molecularly thin film of lubricant on the disk surface. KEY WORDS: magnetic recording; rotating disk; spin-off; perfluoropolyether; Zdol; lubricant; viscosity; tribology 1. Introduction Perfluoropolyether lubricant is removed from the thin film magnetic recording disk surface in several ways. Low molecular weight lubricant evaporates at elevated tempera- tures [1]. Perfluoropolyethers are subject to tribochemical degradation [2–4] in asperity contacts between the magnetic recording head and the disk. Lubricant is displaced by air shear and pressure gradients which originate from the action of the flying slider [5,6]. However, among the many driving forces, only two of them lead to a net radial movement of lubricant, or spin-off. These are the centrifugal force and the shear stress from the air that spirals outward across the lubricant film [7–9]. The goal of this study was to find out how much of a typical perfluoropolyether lubricant Zdol is expected to re- main on thin film magnetic recording disks made with cur- rent technology after spinning at 10 000 rpm for a period of years at typical disk drive operating temperature. Spin-off was measured with lubricant films between 10 and 135 Å thick. The film thickness was measured as a function of spin time at 10 000 rpm and 55 ◦ C to obtain the spin-off re- moval rate. Two types of viscous flow are considered to fit the spin-off data, and to estimate the film thickness expected to remain on the disks when starting with thinner films and spinning for longer periods of time. Dispersion interaction between lubricant molecules and solid surfaces becomes significant as lubricant films ap- proach molecular dimensions. Dispersion interaction in- creases the activation energy for evaporation, and this shows up as a decrease in the evaporation rate from the disk sur- face [1]. The viscosity of thin films also increases as the evaporation rate decreases, because the flow-activation en- ergy is proportional to the activation energy for vaporization. In this study, the dispersion-enhanced viscosity of thin lubri- cant films was theoretically derived from the spin-off meas- urements on thick films. The dispersion-enhanced viscosity was then employed to calculate the minimum lubricant film thickness on magnetic recording disks as a function of time up to five years. 2. Theory 2.1. Spin-off Spin-off of liquid lubricant is caused by air shear stress on the free surface [8,9]. An exact solution of the air flow problem is available for the case in which the column of air above the rotating disk is unbounded [10]. The expression for the radial component of the air shear stress is τ = 0.51rω √ η a ρ a ω ≈ rω √ η a ρ a ω 2 , (1) where γ a and θ a are the air viscosity and density, respec- tively, r is the radius, and ω is the disk angular velocity. For a Newtonian fluid in the r and z dimensions of cylin- drical coordinates, the shear rate ˙ γ z,r = dv/dz = τ z,r /η, where v is the fluid velocity in the radial direction in the plane parallel to the surface, τ z,r is the shear stress in the ra- dial direction in the plane parallel to the surface, and z is the distance along the axis perpendicular to the surface. 2.2. Effective viscosity Integrating the Newtonian fluid equation with respect to z, with the viscosity and shear stress independent of z, and a no-slip boundary condition at the surface (v = 0 at z = 0), gives the linear velocity profile v = (τ z,r /η)z. The volumetric flow rate in the radial direction per unit length of circumference, q , is obtained by integrating the velocity pro- file over distance normal to the surface. Equating the shear stress in the film to the air shear stress, τ z,r = τ , gives 1023-8883/01/1100-0151$19.50/0  2001 Plenum Publishing Corporation