Effects of Ultraviolet Irradiation on Molecularly Thin Lubricant Films Coated on Magnetic Disk Surfaces (Influence of End Group Functionality)

Abstract

To investigate the effects of ultraviolet (UV) irradiation on perfluoropolyether (PFPE) lubricants with identical main chains but different end groups, three types of PFPEs (UV sensitive AM3001, less UV sensitive but functional Zdo12000, and less UV sensitive and nonfunctional Z03) coated on magnetic disk surfaces at nanometer thickness were irradiated by 172 nm UV rays, and bonded lubricant thickness, surface energy, and lubricant spreading measurements were subsequently conducted. Over the irradiation time range investigated, Zdo12000 showed a linear increase in bonded thickness, whereas AM3001 and Z03 showed a saturating increase and the saturating bonded thickness matched the gyration diameter (2.8 nm) of bulk AM3001 and the cross-sectional diameter (0.7 nm) of Z03 molecules, respectively. For surface energy, except for Z03, the polar component decreased significantly with UV irradiation compared to the dispersive component, suggesting the preferential effects of UV irradiation on the functional end groups rather than the main chains. Spreading of AM3001 to bare disk surface was drastically decelerated by UV irradiation; for Zdo12000 and Z03, however, it was worth noting that the thickness profiles after UV irradiation changed to a layered structure featuring a forward-spreading shoulder. A peak/valley structure caused by molecular flow from the non-irradiated toward the irradiated areas was observed for functional AM3001 and Zdo12000, but was not observed for nonfunctional Z03.