The effect of the polarity of the lubricant upon capacitance measurements of EHD contacts

Abstract

Electrical methods, such as capacitance or resistance, allow the study of film formation in elastohydrodynamic (EHD) metal/metal contacts. This makes them more applicable to real machine elements, such as rolling element bearings, gears or cams. Measurements of electrical resistance give an indication of the amount of direct metal/metal contact, while electrical capacitance provides the separation between the surfaces. From the theory of capacitance it is known that electrical capacitance increases with decreasing film thickness, making it very suitable for studies in the thin film region. Measurements of film thickness by electrical capacitance were successfully performed by the authors on both glass/steel and steel/steel contacts with a nonpolar lubricant for films thinner than 20nm. All commercial lubricants contain polar additives, or are polar themselves; hence it is of practical interest to examine the influence of a lubricant's polarity on capacitance measurements. For this reason, three different types of fluids were studied: a nonpolar polyalphaolefin (PAO), glycerol—a strongly polar fluid, and polyethylene glycol (PEG) with polarity between that of PAO and glycerol. The results indicate that PAO shows a very good agreement between the film thickness measured with optical interferometry and evaluated from capacitance measurements. In case of glycerol and polyethylene glycol, however, it was found that the film thickness extracted from capacitance deviates from the optically measured values in the thinner film region. This suggests that EHD conditions may have an effect on the dielectric behaviour of polar lubricants, and that this effect becomes stronger as the polarity increases.