Tribological Behaviors of Self-Assembled Dual-Layer Films in Atmosphere and in Vacuum

Three kinds of self-assembled dual-layer films with various tail groups and chain length were prepared on APS film substrated by silicon wafer. Using an atomic force microscopy, the tribological behaviors of these films were detected both in atmosphere and in vacuum. Compared to those in atmosphere, the films revealed smaller adhesion and friction forces in vacuum. The more hydrophobic film was found to exhibit the less difference between the friction forces in vacuum and in atmosphere. The reason may be partly attributed to the adsorbed water layer on the samples, since which will show relatively weak effect on the friction force on the hydrophobic surface. No obvious damage was observed on the self-assembled films after the friction tests in vacuum at an applied load of 140 nN by a Si3N4 tip. As the initial stage of nanowear process by a diamond tip, a series of hillocks were observed on silicon surface along the scratching line. All the films can effectively enhance the antiwear ability of silicon surface and the self-assembled dual-layer film terminated by long chains (STA/APS) or -C6H5 groups (PAA/APS) performed much better than that terminated by short chains. Finally, the microwear abilities of the films were also examined on a universal micro-tribometer at the normal load from 50 to 200 mN. The wear life varied for different films and good antiwear performance was assigned to STA/APS and PAA/APS. This work can be indicative in the application of self-assembled films in MEMS/NEMS.