Effects of Tail Group and Chain Length on the Tribological Behaviors of Self-Assembled Dual-Layer Films in Atmosphere and in Vacuum

In the present study, three kinds of self-assembled dual-layer films with various tail groups and chain length were prepared by adsorption of different carboxylic acids (stearic acid, STA; propionic acid, PPA; and phenylacetic acid, PAA) to the top of 3-aminopropyltriethoxysilane (APS) film on silicon surface. Using an atomic force microscopy, the films were found to reveal smaller adhesion and friction forces in vacuum than in atmosphere. Due to the effect of the adsorbed water layer on the samples, the more hydrophilic film exhibited the larger difference between the friction forces in vacuum and in atmosphere. For the dual-layer films either in atmosphere or in vacuum, the densely packed long chains can lead to lower friction than the poor-packed short chains, and the tail phenyl groups may induce higher friction than the methyl groups. In the initial stage of nanowear process by a diamond tip, a series of hillocks were observed on silicon surface along the scratching line. It was found that 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 examined on a universal micro-tribometer. With the increase in normal load from 50 to 200 mN, the wear life varied for different films and good antiwear performances were also assigned to STA/APS and PAA/APS. This work can be indicative in the application of self-assembled films in the micro/nanoelectromechanical systems.