(279c) On the Investigation of Frictional Properties in Pure and Mixed Fluorocarbon/Hydrocarbon Monolayers

Fluorinated materials are known to exhibit low critical surface tension and high thermal and mechanical stability; however while fluorinated monolayer coatings are more stable against elevated temperature and humidity than hydrocarbon silane monolayers, they are known to be less effective as lubricants than their hydrocarbon counterparts. The likely reason for the weaker performance is that the perfluorinated monolayer is not as well-ordered as the hydrocarbon SAM due to their van der Waals diameter being larger than the distance between the active sites on the SiO2 surface. The concept of mixed monolayers composed of hydrocarbon and fluorocarbon chains could therefore enable the formation of well-ordered perfluorinated SAMs on silica. To investigate the frictional behavior of pure and mixed hydrocarbon/ fluorocarbon monolayers molecular dynamics simulations have been performed of fluorinated chains and mixed fluorinated/hydrogenated chains on silica. The adhesion and friction between nanoconfined monolayers as a function of normal load, shear velocity, and chemical composition of the SAM coating has been investigated and mixed fluorocarbon/hydrocarbon monolayers were found to out-perform both pure fluorocarbon and pure hydrocarbon monolayers. The simulations suggest that a liquid-like layer is formed by the longer hydrocarbon chains that protrude above the shorter fluorocarbon chains, which aids in friction reduction. Surface coverage is also found to have a significant effect on the performance of all systems tested, with higher surface coverage resulting in a lower coefficient of friction.