RAMAN MEASUREMENTS OF LOCALIZED PRESSURE VARIATIONS IN LUBRICANTS ABOVE THE GLASS-TRANSITION PRESSURE

Spatial pressure variations in synthetic lubricants contained in a sta tic high-pressure diamond anvil cell (DAC), as well as in a loaded mod el bearing contact device, have been measured using the frequency shif t of the lubricant's Raman vibrational modes. Long-lived pressure fluc tuations of +/-0.5 GPa, with a relaxation time of several days, are ob served in the static high pressure systems at an average pressure of 2 .5 GPa. Evidence for rapidly varying pressure fluctuations in a concen trated contact is inferred from the increase in lubricant Raman linewi dths. These results mise questions about key assumptions made in model ing EHD contacts. It is suggested that the present results are closely linked to recent observations of shear localization made by Winer and Bair.