Molecular dynamics simulations of adsorption behavior of organic friction modifiers on hydrophilic silica surfaces under the effects of surface coverage and contact pressure

Abstract

The effects of surface coverage and contact pressure on the adsorption of organic friction modifiers (OFMs) on the hydroxylated SiO 2 surfaces were studied by molecular dynamics. Two typical OFMs with different polarities (R–OH and R–NH 2 ) were discussed. R–OH molecules formed a dense monolayer which was strengthened by increasing coverage and pressure. R–NH 2 molecules formed loose adsorption layers, which were further dispersed by increasing coverage but transformed to a compact monolayer by increasing pressure. The adsorption amount of R–NH 2 decreased clearly with the increase of coverage, but increased from ~70% to 100% with the increase of pressure. As for R–OH, the adsorption amount was always kept above 90%. Obviously, the high pressure benefits the OFM adsorption at static conditions. Also, the high surface coverage enables more adsorbed molecules on SiO 2 surfaces, which enhances the anti-pressure ability of adsorption layers and thereby reduces the friction and wear. • The adsorption of OFMs on SiO 2 surfaces with effects of surface coverage and pressure is studied. • High contact pressure benefits the OFM adsorption at static conditions. • High surface coverage enhances the anti-pressure ability of OFM monolayers.