Behavior of Polar Compounds in Lubricating-Oil Films

The effect of oil films was investigated under comparatively mild conditions of boundary lubrication (up to 10 MPa) using electrical methods. For a dynamic investigation, a pin-on-disk-type friction apparatus was uses. The capability of oil films for preventing metal-metal contact was estimated by measuring the contact resistance of the mating surfaces. For a static investigation, the characteristics of oil films while being squeezed from a very narrow slit between mercury and steel surfaces was observed by measuring the capacitance and conductance of oil films. Squalane was used as a base oil. Saturated straight chain carboxylic acids (C6-C18), and methylesters and alcohols which had the corresponding chains to the acids were used as additives. The following were found for the capability of oil films for preventing metal contact: 1) Prevention depended on the concentration of the additives (5—20 mol·m-3), and increased with the increase of the concentration. 2) Prevention did not necessarily correspond to the adsorptivity of the polar groups of the additive molecule. 3) A chain-matching effect was observed with the compounds having a C10 Chain-length which was not the same as that of the base oil molecule. From the above results, it is concluded that the oil film must have a structural property, and its load-bearing capability for preventing metal-metal contact depends not only on the absorbed film but also on its structural property.