The effect of chain density on the frictional behavior of surfaces modified with alkylsiloxanes and immersed in n-alcohols

Abstract

Concepts of viscoelastic behavior of bulk polymer systems were used to describe maxima in the friction behavior of self-assembled monolayers ͑SAMs͒ measured with lateral force microscopy ͑LFM͒ as a function of sliding velocity and applied normal load in the presence of solvents ͑plasticizers͒. The objective of this study was to investigate whether decreases in the chain density of the SAMs caused maxima in the friction force to shift to higher sliding velocities; such a shift is indicative of shorter chain relaxation times in less dense, less ordered SAMs. Complete SAMs were formed from octadecyltrichlorosilane ͑OTS͒ and partial SAMs were formed from n-octadecylmethyldichlorosilane ͑2Cl͒ and n-octadecyldimethyldichlorosilane ͑1Cl͒, all on silicon/silicon dioxide substrates. With decreasing chain density, solvent partitioning into the monolayer should increase as OTSϽ2ClϽ1Cl. In butanol and pentanol, the maxima in the friction force shifted to higher sliding velocities with decreasing chain density, and the relaxation times calculated for the partial 2Cl films were an order of magnitude shorter than those for SAMs of OTS. For both SAMs of OTS and 2Cl films, maxima shifted to lower sliding velocities with increases in the applied normal load and with increases in the chain length n of the solvent. The higher compressibility of 2Cl films caused greater shifts in the maxima for similar increases in the applied normal load. The increase with n was consistent with both a mechanism of solvent partitioning controlled by the free volume distribution in the SAM and a mechanism of insertion. The relaxation times of the alkyl chains were related to a molecular model of energy dissipation involving the adsorption and desorption of the chain ends to and from the surface of the probe tip.

Microtribological properties of silicon and silicon coated with self-assembled monolayers: effect of applied load and sliding velocity

“…On the other hand, the increase with velocity for SAMs is usually discussed in terms of viscoelasticity [19,20]. The molecules under the counter sample are compressed and adsorbed on the ball before the motion starts.…”

“…An increase of the friction force with increasing velocity was found and attributed to a stable tilting of the molecules due to the reciprocating motion of the ball. Another study [19] investigated the frictional behavior of SAMs in n-alcohols and interpreted the increase with sliding velocity and the plateau at a certain velocity in terms of viscoelasticity. The viscoelasticity was found to be different in different alcohols due to a different solubility of the alcohols in the monolayer.…”

“…While various tribological aspects of SAMs have been extensively examined in various reports [12][13][14][15][16][17][18][19][20][21], there exist only few reports involving SAMs which provide microtribological information based on systematic studies on the effect of the size of the contact area on the increase of the friction force with sliding velocity. A lower packing density and lower film order was correlated to higher friction due to an increased area of contact and increased number of paths to dissipate energy in [12][13][14][15].…”

Nanotribological characterization of perfluoroalkylphosphonate self-assembled monolayers deposited on aluminum-coated silicon substrates

“…The efficiency, power output and steady state operation of MEMS/NEMS devices can be critically influenced by adhesion, friction, and wear (Bhushan 1998(Bhushan , 2003(Bhushan , 2004(Bhushan , 2005Liu and Bhushan 2004a, b;Bhushan and Liu 2004a). Further, adhesion and friction are known to show dependence on the operating parameters, such as relative sliding velocity and environmental conditions, e.g., relative humidity (RH) and temperature (Liu et al 1996;Lio et al 1997;Schoenherr and Vancso 1999;Tian et al 1999;Clear and Nealey 2001;Bhushan 2002, 2003;Bhushan and Liu 2004b;Bhushan et al 2004;Kasai et al 2005;Bhushan 2004, 2005a, b, c, d). This has necessitated the application of low friction and low adhesion, ultrathin lubricant films for the protection of contacting surfaces in MEMS/NEMS.…”

Entropic solvation force between surfaces modified by grafted chains: a density functional approach

“…[17][18][19][20][21]. In the last decade, much research has been focused on synthetic and chemically modified substrates for specific applications [22][23][24][25][26].…”