Effect of Molecular Structure on Friction and Wear of Polymer Thin Films Deposited on Si Surface

In this study, the influence of the molecular structure (linear or with bulky side groups) of polymer films covalently attached to Si surface on tribological properties is investigated. Two polymers, PE (polyethylene) and PS (polystyrene), are selected where PE has simple linear molecular structure whereas PS has linear molecular structure but contains bulky benzene groups located at the sides of the linear chain. PE and PS molecules, both with reactive maleic anhydride groups, are chemisorbed onto Si via an intermediate APTMS SAM (3-aminopropyltrimethoxysilane self-assembled monolayer). Water contact angle measurements, AFM (atomic force microscopy), ellipsometry, and XPS (X-ray photoelectron spectroscopy) are used to identify and characterize the polymer films. Tribological properties are studied using a microtribometer where a 4 mm diameter Si₃N₄ ball is used as the counterface. Among the two polymer films investigated, Si/APTMS/PE has shown very low coefficient of friction (0.08) and high wear life (∼4,400 cycles) than those of Si/APTMS/PS. Surprisingly, Si/APTMS/PS did not show any improvement in tribological properties when compared to that of bare Si. The present study proves that the polymer with linear molecular structure without the bulky side groups show good tribological properties even when it is coated as a thin film and hence such polymers can be used as thin-films for reducing friction and wear of substrates such as Si or other materials.