Chain matching effects of base oils and additives in nanotribology of boundary films

An apparatus has been built in order to estimate the nanotribological properties of boundary films confined between two metal surfaces with practical roughness. A spherical specimen (SUJ2, diameter: 5 mm) is held by a double-cantilever spring (stiffness: 7400 N/m) in a sample liquid, and a planar specimen (SUJ2) is moved toward it with a constant speed (1 μm/s). After a running-in treatment, the displacements of the two specimens and the voltage between them are measured simultaneously, and the changes of film thickness, normal force and electrical resistance are obtained in a series of measurement. Experimental results with systematic combinations of base oils (normal saturated hydrocarbons) and additives (normal saturated fatty acids) show that chain matching effect is not clearly observed in the effective viscosity of mobile layers, whereas the thickest immobile films are formed when the chain length of base oil is coincident with that of additive, which indicates the importance of the interaction between base oils and additives in forming boundary films. 1