The behavior of colloidal solid particles in elastohydrodynamic contacts

In recent years there has been considerable interest in employing dispersed, solid colloidal particles in the 1-500 nm diameter range to enhance the friction and wear characteristics of liquid lubricants. One important question concerning the use of these additives is the extent to which the tiny solid particles pass through rolling and sliding contacts. This paper describes a study of the lubricating behavior of two simple colloidal sol systems, one based on silver particles dispersed in an alkane and the other dispersed carbon black in mineral oil. Elastohydrodynamic film thickness measurements, in-contact visualization, friction and wear measurements are combined to investigate the extent to which colloidal solid particles enter a lubricated contact. It is shown that bath of the colloidal sols studied contribute to film formation at very slow speeds, when the lubricant film thickness is less than the particle diameter. However they make no contribution to film thickness at higher speeds. The silver colloid reduces friction and wear in the thin film regime but the carbon black colloid shows a negligible beneficial effect.