Boundary lubrication effect of organic residue left on surface after evaporation of organic cleaning solvent

Abstract

Although samples are visibly clean, organic deposits could have tremendous impacts on tribological measurements and interpretation of friction and wear behaviors. This paper discusses the boundary lubrication effects of invisible residues from organic solvents that have been widely used in tribological studies in ambient conditions. Stainless steel, soda lime glass, and copper substrates were cleaned using UV/ozone treatment or organic solvents such as alcohols where the solvent was allowed to evaporate from the surface. In ball-on-flat tribo-tests, all UV/ozone cleaned samples showed high friction and catastrophic wear immediately upon sliding due to the absence of lubricants at the sliding interface, while samples cleaned by organic solvent experienced low friction and minimal wear for hundreds of reciprocating cycles. Analyses of the surface by atomic force microscopy and vibrational spectroscopy indicated that the solvent cleaning deposits hydrocarbon residue on metal and oxide substrates. This residue is due to trace impurities with low vapor pressures inevitably present in the bulk liquid, regardless of solvent purity, which become concentrated upon vaporization of the high vapor pressure solvent. These results demonstrated that drying after cleaning with organic solvents is equivalent to a dip-coating of low vapor pressure organics onto the sample surface. This dip-coating effect can be avoided if surfaces are cleaned using UV/ozone or if the surface is immediately rinsed with DI water and blow-dried after cleaning with organic solvent. These results indicate that the friction coefficients of the samples with organic residues should not be interpreted with the shear strength model of friction.