Friction and wear life evaluation of silane based self assembled monolayers on silicon surface

Abstract

The tribological properties of monomolecular layers of Octadecyltrichlorosilane (CH3 (CH2)17SiCl 3, OTS) and 3-aminopropyltrimethoxysilane (NH2 (CH 2)3Si(OCH3)3, APTMS) were investigated. The monolayers were obtained by the self-assembly process. These ultra thin films were characterized using AFM, Contact angle measurements and Ellipsometry. The morphology has shown the formation of ordered and uniform layers of OTS and non uniform layers of APTMS. Water contact angles have shown the hydrophilic nature of bare Si, hydrophobic nature of OTS and intermediate nature of APTMS. Ellipsometry has confirmed the formation of the two films. The friction and wear properties of these two layers were investigated by ball-on-plate and ball-on-disk tribometers, respectively. Friction and wear life were evaluated by stainless steel and ceramic silicon nitride balls, respectively. OTS decreased the coefficient of friction (COF) of bare Si (0.2-0.3) to 0.07 and APTMS showed higher COF of about 0.3-0.4 at a normal load of 50gm and 0.1 mm/sec sliding velocity. OTS has shown higher wear life at 5 gm in comparison to APTMS and bare Si. The coefficient of friction and wear life performances of these self-assembled monolayers are explained in terms of hydrophobic/hydrophilic nature, uniformity in the film and reactive terminal groups. The contribution of the ordered film to the higher wear resistance is explained. This paper also shows that composite layers of self-assembled monolayers can be more effective in engineering higher wear resistance surface if appropriate layers are selected. © 2005 Elsevier B.V. All rights reserved.