Characterization of surface layers on M-50 steel exposed to perfluoropolyalkyethers at elevated temperatures

Complex surface layers have been identified on M-50 steel samples exposed to a linear perfluoropolyalkyether (PFPAE) in the presence of air at 260°C. An understanding of the mechanisms that drive the formation of these layers is believed to be crucial for efforts to develop PFPAEs as effective lubricants for advanced high temperature turbine engines. Work presented here shows that the extreme surface region contains physisorbed PFPAE and Fe fluoride under which an Fe3O4 layer is observed followed by an FeF2 layer which is adjacent to the substrate. Similar layered structures have been observed for Fe-containing materials exposed to HF but have not been reported for PFPAE systems. X-ray photoelectron spectroscopy (XPS) and conversion electron Mössbauer spectroscopy (CEMS) were used to unambiguously characterize these layers.