Lubrication by Physisorbed Molecules in Equilibrium with Vapor at Ambient Condition: Effects of Molecular Structure and Substrate Chemistry

Abstract

The effects of physisorbed organic vapor molecules on friction and wear were studied for various materials with different surface chemistries (metals, ceramics, glasses, carbons, polymers) and adsorbed species with distinct functional groups (short linear-chain, branched, and fluorinated alcohols with alkyl chain lengths up to five carbons as well as acetone and n-decane). Friction test results of stainless steel under equilibrium vapor adsorption conditions indicated that the longer chain length of the adsorbed alcohols results in lower friction and that n-pentanol gives the lowest friction and wear among the molecules investigated. The adsorption isotherm measurements revealed that the functional groups of the adsorbed molecules appear to play important roles in lubrication. Friction coefficients that ranged from 0.02 to 0.9 for the various materials in dry and humid environments converged to ∼0.15 for the inorganic solid materials tested in n-pentanol. These findings indicate that the molecular lubrication by the physisorbed species dominates the tribological behaviors of the inorganic solid materials, regardless of bulk mechanical properties. Tribotests using polymeric materials did not show the same lubricating effects for n-pentanol vapor. The failure of n-pentanol to lubricate polymeric materials may be due to vapor ingress into the polymer and the absence of an adsorbed surface layer.

Vapors in the ambient—A complication in tribological studies or an engineering solution of tribological problems?

“…This study also showed that the friction coefficient of stainless steel pairs in vapor phase lubrication condition vary slightly depending on the molecular structure of the adsorbing molecule [211]. For the normal alcohol series, the friction coefficient decreases as the alkyl chain length increases.…”

“…This was proven by testing various simple organic molecules on a number of solid materials with different modulus, hardness, and surface roughness (Fig. 13) [211]. Figure 13(a) shows how different pairs of materials gave varying friction coefficients in dry environment.…”

“…It should be noted that adsorbed pentanol vapor lubricates even on surfaces where the mean roughness is orders of magnitude larger than the adsorbed molecule. The absence of wear debris suggested that plastic deformation occurred rather than breaking of asperities [211]. This also indicates that the presence of the adsorbed molecule shifted the shear plane from the solid−solid interface to the adsorbate/solid interface, protecting surfaces from wearing.…”

“…Lubrication by adsorbed vapor layer(s) falls in the boundary lubrication regime. However, the thickness of the adsorbed vapor molecules on solid surfaces (on the order of angstroms or a nanometer) is much smaller than the roughness of most macro-scale surfaces used in engineering applications [211]. Therefore, it was often thought that adsorbed vapors would not be sufficient to lubricate asperities of rough surfaces; but this is an incorrect misconception originated from picturing the vapor phase lubrication similar to the mixed lubrication in liquids.…”

Origin of Ultra-Low Friction of Boric Acid: Role of Vapor Adsorption

“…These studies argued that intercalated molecules weaken binding forces between the basal planes, allowing easy shear between the planes. However, a recent study on lubrication by physisorbed organic molecules showed that the friction coefficient of one monolayer of organic molecules at sliding interfaces is typically within 0.15-0.2 for most inorganic materials covering a wide range of modulus, hardness, and surface roughness [19,20]. This value is much larger than the friction coefficients of graphite, MoS 2 , and H 3 BO 3 under super-lubricious conditions, suggesting that the interfacial shear of physisorbed molecules does not appear to give super-lubricity.…”