Behaviour of polymer-thickened oils in lubricated concentrated contacts

Abstract

Measurements of elastohydrodynamic (EHD) oil film thicknesses have been carried out to assess the lubricating performances of polymer-thickened oils using an optical interferometric technique. Five commercially available polymethacrylates (PMAs) with weight average molecular weights of 40 000–900 000 were used as polymer additives. The film thicknesses of all PMA-containing solutions became less than those of corresponding straight oils. Furthermore, provided that the PMA solutions had the same viscosity at a given experimental temperature, the EHD film thicknesses decreased to that of the base oil with increasing average molecular weight of the PMA. In order to elucidate the results and to clarify the influences of permanent viscosity loss, further experiments have been conducted for PMA-thickened oils previously degraded by means of a sonic shear tester of frequency 10 kHz. It is very interesting that an increase in the EHD film thickness with a decrease in the ambient viscosity resulted from a molecular weight decrease. In order to evaluate the above results with respect to antiwear properties, four-ball wear experiments were also conducted with PMA-thickened oils having the highest and the lowest average molecular weights. These results showed that the wear scar diameters for oils containing a PMA of higher molecular weight were larger than those for lower molecular weight oils. It is suggested that it is difficult for the polymer molecules of larger size to pass through the contact region; therefore these molecules may accumulate at the inlet region and/or sweep around the contact. The behaviour of PMA-thickened oils has been investigated by taking photomicrographs in the presence of graphite particles under pure sliding conditions.