On the Mechanism of Boundary Lubrication. I. The Action of Long-Chain Polar Compounds

The effect of long-chain polar compounds on the coefficient of kinetic friction under boundary conditions has been studied using the Boerlage four-ball friction apparatus in various modifications. With steel balls of the highest grade, coefficients of friction for a great number of lubricants were measured as a function of the relative velocity of the rubbing surfaces. The structure of thin films of these lubricants rubbed on polished mild steel surfaces was investigated by electron diffraction. It was found that lubricants showing little or no surface orientation had a constant coefficient of friction of about 0.1 over the available velocity range from 0 to 1 cm./sec. With oils which showed high surface orientation imparted by addition of long-chain polar compounds, a sudden decrease of the coefficient of the friction was observed at various velocities of the sliding surfaces, depending upon the com pound used. Investigation of a great number of compounds gave a direct correlation of this effect with molecular orientation: those compounds causing the effect to occur at the lowest velocities were found to be most highly oriented with their carbon chains most nearly perpendicular to the surface. Since such a change of the coefficient of friction can only be explained by the wedging of oil under the surface (oil drag), the effect was termed the ‘wedging effect’ leading to a type of lubrication which may be called ‘quasi-hydrodynamic’. By measuring the electrical resistance between the sliding surfaces it was found that the regions of sudden decrease of the coefficient of friction correspond to a change from metallic contact to extremely high resistance. The investigation shows that long-chain polar compounds act primarily by inducing the ‘wedging effect’ and not by giving a direct protection to the surface.