Friction and durability characteristics of ultrathin perfluoropolyether lubricant film composed of bonded and mobile molecular layers on diamond-like carbon surfaces

Molecularly thin perfluoropolyether (PFPE) lubricant film composed of bonded and mobile molecular layers was dip-coated on diamond-like carbon (DLC) surfaces, and experiments using a ball-on-disk tribotester were carried out for the friction and durability characteristics of the thin lubricant film. The average thicknesses of the bonded and mobile layers were varied independently up to 1.5 and 4 nm, respectively, by changing the concentration of the PFPE solution and the curing time after the dip-coating. The experimental results show that the friction is reduced with an increase in the average thickness of either layer but is more dependent upon the thickness of the bonded layer. When the total PFPE thickness is kept constant, the friction reduction is enhanced by an increase in the bonded ratio, which is defined as the average bonded layer thickness divided by the average total PFPE thickness. Based on the durability test, it was found that the optimum film thicknesses of the bonded and mobile layers existed for the longest lifetime of the PFPE lubricant film, and that the excess PFPE molecules on them only reduced the lifetime of the PFPE lubricant film.