Friction and Adhesion Hysteresis of Fluorocarbon Surfactant Monolayer-Coated Surfaces Measured with the Surface Forces Apparatus

Abstract

Friction and adhesion hysteresis experiments were carried out on fluorocarbon surfactant monolayer-coated surfaces using the surface forces apparatus. Measurements were made as a function of temperature, load, sliding velocity, and relaxation time, and the resulting properties are contrasted with those of hydrocarbon monolayers and also with bulk fluorocarbon surfaces (e.g., Teflon, PTFE). The dynamic adhesion measurements show that the adhesion hysteresis and friction of fluorocarbon monolayer-coated surfaces are related: large friction forces being associated with large adhesion hysteresis. The results also show that the overall tribological properties of fluorocarbon surfactants follow the same generic “friction phase diagram” behavior as do hydrocarbon surfactants. However, the friction phase diagram for fluorocarbon surfactant has at least two peaksone well above and the other well below room temperatureindicating that two different molecular relaxation processes are involved in friction and adhesion energy dissipation. Apparently, chain interdigitation, which is the most important molecular relaxation mechanism in the friction and adhesion hysteresis of hydrocarbon materials, does not play a major role with fluorocarbon surfaces. Instead, the surface topography and its change at the molecular and submolecular levels during shear is the most important factor determining the friction of these surfaces, but only so long as the monolayers remain molecularly smooth or “undamaged”. Reasons for the beneficial tribological properties of fluorocarbon surfaces are discussed.

Nanotribology of Symmetric and Asymmetric Liquid Lubricants

“…In order to investigate the static friction behaviors of confined lubricants, SFA stop-start experiments were performed by a number of authors [14,15,[18][19][20]30]. Figure 5 shows the example of a SFA stop-start experiment for the molecularly confined film of an asymmetric liquid lubricant, 1,3-dimethylbutyl octyl ether (DBOE) in the hard-wall thickness (D = 1.7 nm) [18,20].…”

“…Hence, the nucleation time τ n is the time required for the conformational (orientational) rearrangements of molecules from shear-ordered configuration to disordered liquid-like configuration and then to a static solid-like structure. The τ n is of course dependent on molecular mobility, and increasing applied load or decreasing temperature generally increases τ n because both effects decrease the molecular mobility in the film [14,18,20,30] (see Figure 6a). The brief mechanism discussed here is rather suitable to describe the behavior of asymmetric liquid molecules; symmetric liquids include some different features (discussed below).…”

Fundamental mechanisms of micromachine reliability

“…That is, these interferograms look identical for increasing and decreasing voltages. Yet, hydrocarbon and fluorocarbon-tailed monolayer films deposited by the Langmuir-Blodgett technique exhibit adhesion hysteresis in surface force apparatus experiments [29,30], as well as in similar experiments between polydimethyl siloxane and fluoroalkylsiloxane coated mica [31]. The proposed mechanisms for the hysteresis are chain interdigitation when surfactant films are in an amorphous state [29], and molecular as well as submolecular changes in a fluorocarbon surfactant due to the bulky fluorocarbon groups [30].…”

“…These results gives us information on the average amount of water on the surface, which we interpret in terms of a classical BET adsorption isotherm [30]. For the hydrophilic Si02 surface below 50% RH, a sub-monolayer film of water exists on the surface.…”

“…We shall refer to these as the hydrophobic samples. ODTS has been investigated as a promising molecule for minimization of adhesion in MEMS [8,22,29,30]. The head group of the organosilane is hydrolyzable, which facilitates the formation of covalent siloxane bonding to the substrate hydroxyl groups as well as between neighboring silane molecules to form a well attached organosilane layer [31].…”