Tribological performance of magnetic thin-film glass disks: Its relation to surface roughness and lubricant structure and its thickness

Chemically strengthened glass substrates have recently been used in the construction of magnetic thin-Aim rigid disks for their rigidity, shock resistance, and smoothness over commonly used Ni-P coated aluminum substrates. The effects of disk roughness, lubricant film thickness and viscosity on coefficients of static and kinetic friction and on disk durability were investigated for disks constructed with glass substrates with three roughnesses, as well as for aluminum-substrate disks with one roughness for comparison purposes. Three nonpolar and one polar perfluoropolyether lubricants with wide range of viscosities were chosen in the experiment. Disks were lubricated using a dip-coating technique with 1 to 10 nm thickness. Coefficients of static and kinetic friction and durability of each of the disks were measured. It was found that above a critical lubricant-film thickness, coefficient of static (stiction) and kinetic friction increased rapidly with increasing lubricant-film thickness and decreasing lubricant viscosity. The critical film thickness was approximately proportional to the lubricant viscosity and disk roughness. The friction force increase was more rapid for a smoother disk. The disk durability increased with decreasing disk roughness and lubricant viscosity, and with increasing lubricant thickness. For glass-substrate disks, the polar lubricant provided best disk durability among the four lubricants tested. Failure mechanisms for glass-substrate and aluminium-substrate disks appear to be similar, however, aluminum-substrate disk exhibited better disk durability, as compared to glass-substrate disk. These experimental observations are attributed to the interplay of meniscus formation around contacting and near-contacting asperities, lubricant depletion and/or migration at contacting asperities, as well as ploughing of contacting asperities. For the first time, a kinetic meniscus model to account for time and viscosity dependence of meniscus formation is presented, which explains the experimental trends.