Perfluoropolyalkylether lubricants under boundary conditions: Iron catalysis of lubricant degradation

Abstract

Perfluoropolyalkylether (PFPAE) oils and oil-based greases exhibit signifcant degradation under boundary lubrication conditions. In the absence of significant concentrations of oxygen, as in the case of spacecraft orbital environments, the degradation mechanism consists of two steps: (1) the initial reaction of the PFPAE molecules with freshly exposed iron to form the Lewis acid, FeF3, followed by (2) the reaction of the FeF3 with unreacted PFPAE molecules, which results in the cleavage of the etherate linkages. The result is an autocatalytic degradation that takes place at temperatures below the onset of thermal decomposition. This mechanistic hypothesis is supported by the following experimental results: (1) The reaction of PFPAE with FeF3 has been shown to give rise to fluorinated-ketone and acid fluoride ether-cleavage products that are more reactive than the parent PFPAE molecules and have lower molecular weights and thus poorer lubricating ability. (2) FeFx compounds are formed in the wear tracks of wear-test components under boundary lubrication conditions. On the basis of these experimental results, poor performance can be predicted for PFPAE oils under conditions in which high temperatures (approximately 350 °C) and freshly exposed metal surfaces are present (i.e., the conditions of boundary lubrication). This prediction is corroborated through wear tests in which the performance of PFPAE is compared to the performance of commercial petroleum-based and synthetic- hydrocarbon lubricants to which lead naphthenate or antimony dialkyldithiocarbamate have been added. The test results confirm the well-known fact that the antiwear additives are very important to prolonged wear life under boundary lubrication conditions. In addition, the results show that, compared to the other lubricants tested, the PFPAE lubricants do not demonstrate adequate lubrication pevormance. Both the PFPAE degradation mechanism and the fact that soluble antiwear additives are not currently available imply that PFPAE lubricants are not currently suitable for applications in which boundary lubrication conditions exist, especially when high loads are involved.