A study of the tribological behaviour of perfluoro-polyethers

Abstract

The tribological behaviour of perfluoropolyethers (PFPEs) was investigated by performing four-ball extreme-pressure (EP), Falex, four-ball wear, Reichert, SRV (both point-plane and plane-plane contacts), Amsler and TE 77 determinations on four commercially available Fomblin Y fluids, which cover the entire viscosity range. For comparison a petroleum-based hydraulic oil was subjected to the same tests. In load-carrying capacity measurements (four-ball EP and Falex tests) the PFPEs also surpassed the hydraulic oil as an EP gear lubricant. The performances of the PFPEs in determinations of their wearpreventing characteristics were variable. They were better than the hydraulic oil in Reichert tests but behaved in a roughly identical way in four-ball wear determinations and were inferior to it in the SRV (point-plane contact) method. The irregular behaviour is due to the different severity of operating conditions. The harsh strains exerted in four-ball EP, Falex and Reichert tests led to elevated contact temperatures, thus causing the decomposition of fluorinated macromolecules. The subsequent reactions of decomposition products originated the fluoride films to which the relevant performances of PFPEs are ultimately due. As the stresses to be endured in the SRV (point-plane contacts) tests had not been so extreme, the temperature at which the thermal decomposition of PFPEs would have begun was not attained. The more fluid the grade, the better was its performance in four-ball wear tests, since a lower viscosity led to a more abundant supply of lubricant at the rubbing spots and thus to a smaller wear rate. Lubrication was hydrodynamic in SRV (plane-plane contact), Amsler and TE 77 tests, and hence no damage took place in them. Friction coefficients, which depended on both testing machine and test conditions, were slightly higher for PFPEs than for the hydraulic oil.