XPS ANALYSIS OF THE EFFECT OF FILLERS ON PTFE TRANSFER FILM DEVELOPMENT IN SLIDING CONTACTS

The development of transfer films atop steel counterfaces in contact w ith unfilled and bronze-filled PTFE has been studied using X-ray photo electron spectroscopy. The sliding apparatus was contained within the vacuum of the analytical system, so the effects of the native oxide, h ydrocarbon, and adsorbed gaseous surface layers of the steel upon the PTFE transfer behavior could be studied in-situ. For both the filled a nd the unfilled PTFE, cleaner surfaces promoted greater amounts of tra nsfer. Metal fluorides, which formed at the transfer film/counterface interface, were found solely in cases where the native oxide had been removed to expose the metallic surface prior to sliding. These fluorid es also were found at clean metal/PTFE interfaces formed in the absenc e of frictional contact. A fraction of these fluorides resulted from i rradiation damage inherent in XPS analysis. PTFE transfer films were f ound to build up with repeated sliding passes, by a process in which s trands of transfer filled in the remaining counterface area. Under the se reported test conditions, the transfer process is not expected to c ontinue atop previously deposited transfer films. The bronze filled co mposite generated greater amounts of transfer than the unfilled PTFE. The results are discussed relative to the observed increase in wear re sistance imparted to PTFE by a broad range of inorganic fillers.