A comparative study of the tribological behaviors of CH₃- and CF₃-terminated bilayer films

Two types of bilayer films were constructed on silicon substrates by a two-step self-assembly strategy. 3-[2-(2-aminoethylamino)ethylamino]propyl-trimethoxysilane (TA) was chosen to create underlayer with amino-terminated surface, then n-octanoic acid (nOA) and perfluorooctanoic acid (PFOA) were chemically grafted to the amino-derivatized substrates, respectively, to construct CH3- or CF3-terminated bilayer film, denoted as TA–nOA and TA–PFOA, respectively. The formation and surface properties of the films were evaluated by ellipsometry, contact angle goniometers, X-ray photoelectron spectroscopy, and atomic force microscope (AFM). The nano- and micro-tribological properties of TA–nOA and TA–PFOA bilayers were comparatively investigated via AFM and ball-on-plate tribometer, respectively. Results show that these two types of bilayer films exhibit good anti-adhesive and friction-reducing properties, and TA underlayer enhances the stability of these films through interchain hydrogen bonding. At nanoscale, TA–PFOA film exhibits lower adhesion and friction force because of –CF3 terminals with lower work of adhesion. However, the micro-tribological property of TA–PFOA is inferior to that of TA–nOA film. This could be attributed to the fact that –CF3 terminal groups with larger size tend to form less-ordered structure. Moreover, larger terminal groups lead to more energy dissipation during sliding.