The effect of stress and TiC coated balls on lifetime of a perfluoropolyalkylether using a vacuum rolling contact tribometer

A vacuum spiral orbit tribometer (SOT) was used to determine the relative l ifetimes of a branched perfluoropolyalkylether (PFPAE) on 440C stainless st eel. The effect of varying the mean Hertzian stress (0.75, 1.0, 1.5 and 2.0 GPa) and the use of TiC coated balls on lubricant lifetime was studied. Ot her conditions included: similar to 100 rpm, similar to 50 mu g of lubrican t, an initial vacuum level of < 1.3 x 10(-6) Pa (< 1.0 x 10(-8) Torr), and room temperature (similar to 23 degrees C). Increasing the mean Hertzian st ress from 0.75 to 2.0 GPa results in an exponential decrease in lubricant l ifetime for both material combinations. However, substituting a TiC ball fo r the 440C ball quadrupled lifetime at low stress levels (0.75 and 1.0 GPa) and doubled life at higher stresses (1.5 and 2.0 GPa). The reduced reactiv ity of the TiC surface with the PFPAE lubricant is considered to be the rea son for this enhancement. Decreasing lifetime with increasing stress levels correlated well with energy dissipation calculations.