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

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 <i>n</i>-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 <i>n</i>-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 <i>n</i>-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 <i>n</i>-pentanol vapor. The failure of <i>n</i>-pentanol to lubricate polymeric materials may be due to vapor ingress into the polymer and the absence of an adsorbed surface layer