Decomposition of Perfluoroalkylpolyethers (PFPE) in Ultra-High Vacuum Under Sliding Conditions

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Abstract
Reactions of three different types of perfluoroalkylpolyether (PFPE) liquids were studied during sliding contact with stainless steel (440C) specimens under ultra-high vacuum conditions. All three liquids reacted with the steel specimens during sliding. One PFPE liquid, which has acetal linkages, decomposed under the sliding conditions and generated gaseous products, namely, COF2 and fluorinated carbons, which were detected by a quadrupole mass spectrometer. Gaseous products were not detected for the other two PFPE liquids (these two liquids lacked the acetal linkages). The amount of gaseous products from the acetal containing PFPE liquid increased with increasing sliding speed. At the end of the sliding experiments, the specimens were removed from the vacuum chamber and the wear scar and the deposits on the specimens were examined by small-spot-size X-ray photoemission spectroscopy (XPS). The oxide layer on the specimen surface was removed during sliding, and metal fluorides were formed on the worn surface. The surfaces of the wear scar and the deposits were covered with adsorbed PFPE. On the basis of these results, it was concluded that the decomposition reaction of the acetal containing PFPE liquid began when the liquid contacted the fresh metal surface formed during sliding.