Atomistic and molecular effects on the surface morphology and film conformation of perfluoropolyether lubricant layer

Summary form only given. As the head-media spacing of head-disk interface (HDI) is reduced to 7 nm, interfacial configurations of molecularly thin perfluoropolyether (PFPE) lubricant film such as surface morphology and film conformation become critical for the stability of head-disk operation. Therefore, for the stable protection layers in HDI with ultra thin conformation as well as enhanced tribological performance, novel lubricant materials modified by the molecular structure and functionality have been developed. For example, ztetraol multidentate was invented to improve the film stability and conformation by adding hydroxyl functional group in the middle of the chain. Branched PFPEs were developed to enhance the functionality as well as thermal stability by modifying backbone structure. Here, we selected the molecular structure and functionality as the key parameters, which provide the direction of researches on novel lubricant marching forward. In this paper, we investigated the surface morphology and film conformation of PFPE films as interfacial properties by modifying molecular structures with backbone structures and endgroup functionality. Molecular dynamics simulation using a coarse-grained, bead-spring method was performed to generate the PFPE films from submonolayer to a few layer. We modified PFPE backbone structure by tuning the flexibility of bead-spring chain and the functional groups by tuning functional group-functional group and functional group-carbon surface interactions independently. The force field parameters were calculated based on the ab-initio calculation. To characterize the surface morphology, we introduced the wavelet transform entropy as well as correlation length and the roughness exponent. Different morphological states were found for various oligomers backbones, resulting from strong tethering of functional groups on the solid surface, and as a function of surface coverage. In addition, the correlations between the surface morphol- gies, transport properties, and the wavelet entropy were investigated. Our simulation results of the atomistic and molecular structures on the interfacial properties of film will provide the lubricant molecular design criteria, which will be combined to the researches on the HDI including air-bearing design.