Preparation and tribological characteristics of sulfonated self-assembled monolayer of 3-mercaptopropyl trimethoxysilane TiO2 films

Silane coupling reagent (3-mercaptopropyl trimethoxysilane (MPTS)) was prepared on silicon substrate to form two-dimensional self-assembled monolayer (SAM) and the terminal–SH group in the film was in situ oxidised to–SO₃H group to endow the film with good chemisorption ability. Thus, TiO₂ thin films were deposited on the oxidised MPTS-SAM to form composite thin films, making use of the chemisorption ability of the–SO₃H group. Atomic force microscope (AFM), and contact angle measurements were used to characterise TiO₂ films. Adhesive force and friction force of TiO₂ thin films and silicon substrate were measured under various applied normal loads and scanning speed of AFM tip. Results showed that the friction force increased with applied normal loads and scanning speed of AFM tip. In order to study the effect of capillary force, tests were performed in various relative humidity (RH). Results showed that the adhesive force of silicon substrate increases with RH and the adhesive force of TiO₂ thin films only increases slightly with RH. Research showed that surfaces with more hydrophobic property revealed the lower adhesive and friction forces.