Thickness and density of adsorbed additive layer on metal surface in lubricant by neutron reflectometry

The thickness and density of the adsorbed additive layer on a metal surface in lubricant were directly measured by neutron reflectometry. First, two kinds of metal surfaces, iron and copper, on ultra-flat silicon blocks were prepared by physical vapor deposition. After that, each target surface was analyzed by neutron reflectometry in air, in base oil and in base oil with an additive. Poly-alpha-olefin was used as the base oil, while deuterated acetic acid was used as an additive. Fitting operation based on Parratt's theory showed that the thicknesses of the adsorbed layers on the iron and copper surfaces were quite thin, only 2.0 nm. The friction coefficients of the metal surfaces measured by a ball-on-disk tribometer decreased considerably when the acetic acid was added to the base oil. It was concluded that the additive adsorbed layers on the metal surfaces considerably affected friction properties despite being only several nanometers thick.

► The thickness and density of the adsorbed additive layer was measured by neutron reflectometry.
► The adsorbed layers of acetic acid on the iron and copper surfaces were quite thin, only 2 nm.
► Friction coefficients of the metal surfaces decreased considerably with acetic acid in base oil.
► The additive layers affected friction properties despite being only several nanometers thick.