The Effect of Humidity on the Wear Behavior of Bearing Steels with RfO(n-C3F6O)xRfPerfluoropolyalkylether Fluids and Formulations©

A reciprocating tribometer and four-ball tribometer were used under controlled environmental conditions to determine the wear behavior of bearing steels with a linear perfluoropolyalkylether (PFPAE) lubricant which does not contain difluoroacetal groups. Formulated lubricants (base oil plus additive) containing potential antiwear additives were also tested and results compared to those for the unformulated base oil. Using the reciprocating tribometer, wear of M-50 steel in air was studied under sliding boundary lubrication conditions at 50°and 150°C with relative humidity ranging from 1 to 95%. Wear was observed to decrease sharply as humidity increased from 1 to 5%, then did not change significantly as humidity increased to 95%. These results indicate that the high wear previously observed in a PFPAE fluid at low humidities was not due to the presence of difluoroacetal groups, but may be characteristic of PFPAE fluids. Reduced wear observed at low humidities in screening tests with several formulated lubricants demonstrates several types of soluble PFPAE additives exhibiting antiwear properties. Using the four-ball tribometer, wear of 52100 steel was determined with the unformulated base oil under sliding boundary lubrication conditions in a nitrogen atmosphere at 75°C with 0% relative humidity. These results were then compared to those obtained with formulated lubricants under similar conditions. These screening tests again revealed several types of PFPAE soluble additives that exhibit antiwear properties. In spite of the major differences in the reciprocating tribometer and four-ball test conditions, a triphenylphosphine, a polar nonionic endgroup additive, a diphenylether, and an alcohol additive exhibited significant wear reduction under both sets of conditions. While the triphenylether considerably reduced wear in the reciprocating tribometer tests, it increased wear in the four-ball wear tests. Preliminary results of grazing angle microscope Fourier transform infrared spectra of thin films formed on wear surfaces have provided insight into the chemical reaction mechanism for wear of metals with these lubricants and additives.