Systematic Study of the Effect of Disorder on Nanotribology of Self-Assembled Monolayers

Abstract

The adhesion and friction between pairs of ordered and disordered self-assembled monolayers on SiO2 are studied using molecular dynamics. The disorder is introduced by randomly removing chains from a well ordered crystalline substrate and by attaching chains to an amorphous substrate. The adhesion force between monolayers at a given separation increases monotonically with chain length at full coverage and with coverage for fixed chain length. Friction simulations are performed at shear velocities between 0.02-2 m/s at constant applied pressures between 200 and 600 MPa. Stick-slip motion is observed at full coverage but disappears with disorder. With random defects, the friction becomes insensitive to chain length, defect density, and substrate.

Effects of Interfacial Bonding on Friction and Wear at Silica/Silica Interfaces

“…Friction and adhesion of silica are also of fundamental interest for geophysics and earthquake mechanics, since quartz is a common component of rocks and shallow tectonic earthquakes are known to result from frictional instabilities in crustal faults [4,5]. Consequently, it is not surprising that friction of silica has been studied extensively in different contexts [6][7][8][9][10]. One aspect of the silica studies, which is particularly relevant to this paper, is related to the effects of surface chemistry on friction [11][12][13].…”

Friction between solids

“…All the periodic quantities associated with sliding exhibited a start-up sliding effect with the final shape of the period oscillation achieved after two or three unit cells. This sort of start-up effect is also apparent in the simulations of two, well-ordered hydrocarbon monolayers on SiO 2 in sliding contact (Chandross et al 2004). In the analysis of kinetic friction, data from these first few cycles should be ignored.…”

“…Therefore, while the examination of idealized geometries using computer simulations yields important information, there is a need to examine systems which fall outside this classification. Some of the variables that have been examined are packing density (or defect density) (Mikulski & Harrison 2001a;Chandross et al 2004;Harrison et al 2004), mixed chain lengths (Mikulski et al 2005a), the odd-even effect (Mikulski et al 2005b) and surface termination (Park et al 2003). Mikulski & Harrison (2001a) examined the effects of packing density, or defect density, in SAMs by randomly removing 20 chains (approx.…”

“…As a result, the friction force as a function of sliding distance loses the periodicity associated with the (2!2) unit cell and the average friction is higher (Mikulski & Harrison 2001a). The effect of defect density on the friction between two SAMs composed of alkane chains attached to SiO 2 was examined by Chandross et al (2004) using MD and the COMPASS force field. Friction between commensurate, defect-free SAMs and SAMs with 10, 30 and 50% of the chains randomly removed was examined.…”

Temperature control in molecular dynamic simulations of non-equilibrium processes

“…These substrates are subsequently annealed until the substrate has a silanol density that is consistent with that observed experimentally (≈ 4.6 OH/nm 2 ) [77]. A full description of the algorithm can be found in [78,79,80]. The alkoxylsilane molecules were added to the substrate to achieve a surface density corresponding to an area of 0.25 nm 2 per chain by removing randomly chosen silanol groups from the silica surface and placing the alkoxylsilane chain in its place.…”