Tribology of PFPE Overcoated Self-Assembled Monolayers Deposited on Silicon Surface: Effect of Thermal Treatment

Effect of thermal treatment on the friction and wear of monomolecular layers of SAMs (Self-assembled monolayers) overcoated with PFPE was studied using ball-on-disk tribometer. Two SAMs, Octadecyltrichlorosilane (OTS) and 3-aminopropyltrimethoxysilane (APTMS) were formed on the Si substrate by self-assembly followed by dip-coating with perfluoropolyether (PFPE). Modified Si samples are thermally treated at 150 °C under vacuum for 2h. The physical and chemical properties of modified samples, before and after thermal treatment, are analyzed using contact angle measurements, X-ray photoelectron spectroscopy (XPS). Tribological properties are evaluated using Universal Micro Tribometer (UMT). A Si₃N₄ ball of 4mm diameter is used as the counterface. Thermal treatment of the PFPE coated SAMs and Si surface reduced the coefficient of friction to slightly lower values in all cases. Thermal treatment has shown slight increase in wear life in the case of PFPE coated APTMS SAM surfaces whereas it decreased the wear life in the case of PFPE coated OTS SAM and Si surfaces. The exact reasons for this behavior are not clear at present, but it is being speculated that thermal treatment effects the extent of bonding between the surface groups of PFPE and SAMs and mobile characteristics of the PFPE, which finally influence the wear durability characteristics. Therefore, the differences in the wear durability characteristics are analyzed using changes in the surface energies and the amount of strongly adsorbed PFPE, as a result of thermal treatment.