MOLECULAR-ORIENTATION OF POLYMER LUBRICANT FILMS - ITS TRIBOLOGICAL CONSEQUENCE

The objective of this paper is to demonstrate, from experiments and mo deling, how and why molecular orientation of functional end groups of perfluoro-polyether (PFPE) lubricants play an important role in the tr ibological performance of thin film magnetic disks. These disks typica lly have an amorphous carbon overcoat upon which a thin lubricant laye r is deposited using dip-coating technique. Glancing-angle FTIR (Fouri er Transform Infra-Red Spectrometry) is used for measuring molecular o rientation of planer functional end groups. A molecular orientation in dex (MOI) was defined as 1 for randomly oriented functional end groups . The MOI is mathematically derived as 3 (maximum) for lubricant molec ules oriented with their functional end groups perpendicular to the su rface, and as 0 (minimum) if lubricant molecules oriented with their f unctional end groups parallel to the surface. The MOI is shown to depe nd on processing conditions and lubricant film thickness. The tribolog ical performance of the lubricant films was evaluated using drag-mode contact start-stop testing. It was found that wear durability of the l ubricant film (similar to 2 nm) with MOI similar to 1.5 is a few times better than those with MOI similar to 0.5 to 1.0. No significant diff erence in the amount of bonded lubricant film was detected over the ra nge of MOI studied. Nor was there a detectable relationship with hydro phobicity. It was inferred from decreased MOI values due to thermal ef fects and storage time that a smaller MOI value corresponds to a lower free energy state of the lubricant film. Interestingly, MOI values fo r bonded lubricant films for Process A are found to be close to 3.0, s uggesting that almost all functional end groups in the bonded films ar e oriented perpendicular to the carbon surface, close to 2.0 for proce ss B, and close to 0 for process C, meaning that almost all functional end groups in the bonded films from process C are oriented parallel t o the carbon surface. Relationship between physical/chemical bonding c onfigurations and MOI values are graphically presented in detail. Base d on this relation, a simple model on lubricant film structures for th e three processes studied is presented. The model MOI values agree ver y well with measured MOI values as a function of lubricant thickness f or all three processes, and the model also appears to account for the observed tribology performance for the MOI values studied (0.5 similar to 1.5).