Chemical characterization of antiwear films generated by Tris-[ p-(perfluoroalkylether)phenyl] phosphine using X-ray absorption spectroscopy

Perfluoropolyalkylethers (PFPAEs) are primary candidates as high temperature oils for the next generation of turbine engines due to their chemical and thermal stability. However, the usefulness of the PFPAE base fluids are hindered by corrosive wear in dry environments. This problem can be minimized and overall wear properties improved by the addition of soluble additives, such as Tris-[ p-(perfluoroalkylether)phenyl] phosphine (PH3). Currently, little work has been reported on the mechanism by which this additive actually improves overall wear performance. This paper provides critical insight regarding the interactions of the PFPAE additive PH3 with Fe-based alloys in a pin-on-flat tribological environment. It is found that the PH3 decomposes on the surface, within the wear track, forming a tribofilm composed of a polyphosphate glassy material. At low relative humidity (∼0%), the polyphosphate antiwear film substantial improves the wear performance of the fluid which is reflected by a decrease of ∼325% in width of the measured wear scar. Contrasting this result, at high relative humidity (∼50%), little improvement is found in the wear properties of the fluid. This is due to the formation of carboxylate multilayers produced by PFPAEs in a moist environment, which serve as their own antiwear film. The formation and protective properties of these films are controlled by three important environmental factors. First, oxygen must be present in order to form the polyphosphate. Second, tribomechanical scission and hydrolysis of the additive is required to drive the reaction to completion. At low humidity, a large amount of unreacted and intermediate material was found within the wear track. Third, the test temperature combined with the relative humidity was shown to control the overall useful lifetime of the additive. In order to gain some understanding on how this additive works, a series of tribological experiments were performed at different temperatures, relative humidities and rubbing times. The worn specimens were examined by X-ray absorption near edge structure spectroscopy (XANES) and imaging photoelectron spectromicroscopy (MEPHISTO).