Liquid Nanodroplets on Thin Film Magnetic Recording Disks©

Abstract

As magnetic recording slider size and flying height evolve to smaller dimensions, previously insignificant levels of contamination begin to play a role in slider media tribology. This article describes a new type of contamination. Liquid nanodroplets on disks originate with electrostatic deposition of hygroscopic ultrafine particles, also referred to as cloud condensation nuclei (CCN). On lubricated disks, the CCN equilibrate with atmospheric moisture and become partially overcoated with disk lubricant, which acts as a fluorocarbon surfactant. Dark-field microscopy measured deposition rates of 0.001 to 0.006 #/mm 2 /sec on initially clean disks exposed to ambient air in nonconductive cassettes. Tapping mode atomic force microscopy determined that the sites were deformable nanodroplets 70 to 300 nm in diameter and up to 150 nm high. From AFM profiles, the contact angle of the spherical capped nanodroplets with the disk was between 40 and 90 degrees. Nanodroplet contamination is characterized, and its effect on friction, acoustic emission, and slider smears is demonstrated. A surface chemical thermodynamic model is developed and employed to estimate that the average initial dry nucleus diameter is 110 nm. The estimated size range and composition of the initial nuclei are consistent with those well known in the atmospheric sciences. Nanodroplets were absent from disks that were stored in a CCN-free environment, and the deposition rate was reduced 10× by air ionizer or conductive cassette.