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

Abstract

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 Si 3 N 4 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.

The Role of Configuration Entropy in Boundary Lubricants Based on the n-Perfluoropropylene Oxide Main Chain Structure

“…This is attributed to the decreased segmental mobility of the stiffer main chain, as expected on the basis of the theoretical studies discussed above. Recent investigations of nanoscale kinetic friction dissipation mechanisms on comparatively thicker *4 to *200 nm polymer films on surfaces indicate higher friction (more wear) for stiffer main chains [20][21][22]. The propensity for the formation of lubricant moguls [1,2] on the disk surface can also be used to probe the effect of PFPE main chain stiffness to lubricant disturbance caused by low-flying sliders.…”

Tribological Properties of Ultra-Thin Functionalized Polyethylene Film Chemisorbed on Si with an Intermediate Benzophenone Layer

“…Our earlier study has shown that polymer with benzene side groups such as polystyrene would give high coefficient of friction despite high water contact angle [39]. Further reduction in friction was observed as PFPE was applied as the top layer due to its low surface energy and superior lubricating properties.…”

“…Hence, the presence of fPE bonded to Si with the help of benzophenone has helped increase wear performance. This would not have been possible with a polymer layer such as polystyrene because PE has linear molecular chains which provide excellent wear resistance in coated form [39]. Benzophenone acts as very effective bonding agent for the substrate Si and the fPE layer.…”

“…A clear wear track is also observed on the image of Si/Ph 2 CO/fPE after 1,000 cycles of sliding. Satyanarayana et al [39] have studied the wear resistance of Si/APTMS/fPE film and observed a wear life under similar contact condition of 4,000 cycles. The results show that though Si/Ph 2 CO and Si/Ph 2 CO/fPE can increase the wear resistance of Si, neither Ph 2 CO nor Ph 2 CO/fPE alone is adequate to protect Si substrate for extended use.…”

Tribological Performance of UHMWPE and PFPE Coated Films on Aluminium Surface

“…The mobility and flexibility of the UHMWPE backbone (polyethylene-based) makes it less resistant to the sliding motion. Also, PEs do not contain any bulky side group making this family of olefins most suitable for lowfriction applications [8]. The film is able to withstand several hundreds of cycles of dry sliding demonstrating excellent adhesion with the substrate.…”