Tribology of Ultra-Thin Polymer Films Covalently Bonded to Silicon Surface: Effect of Molecular Structure

Abstract

The primary objective of the present study is to investigate the influence of the molecular structure (linear or with bulky side groups) of polymer films (of few nanometers thickness) covalently attached to Si surface on tribological properties. PE (polyethylene) and PS (Polystyrene) were selected to test the objective where PE has simple linear molecular structure while PS also has linear molecular structure but contains bulky benzene groups located at the side of the linear chain. PE and PS contains maleic anhydride groups and are chemisorbed onto the APTMS (3-aminopropyltrimethoxysilane) modified Si surface. AFM (atomic force microscopy), XPS (X-ray photoelectron microscopy) and water contact angle measurements are used to identify and characterize the polymer films. A micro-tribometer is used to identify the tribological properties in a sliding test using 4 mm diameter Si3N4 ball as the counterface. PE film has shown very low coefficient of friction and high wear durability when compared to PS film and moreover PS film did not show any improvement in the tribological properties when compared to bare Si surface. This study proves that even in film form polymer with linear molecular structure without the bulky side groups show good tribological properties and hence such polymers can be used as thin-films for reducing friction and wear of substrates, Si or other materials. The chemisorbed PE film may be able to find applications in micro-components made from Si surface as anti-stiction and anti-wear coating.