On the Effect of the Chemical Composition of a Steel Counterbody on the Wear Rate of a Low-Filled PTFE/SiO2 Composite

The wear of PTFE/SiO2 composites on steel 40X with low and steel 40X13 with high chromium content is studied. Tribochemical processes occurring during the wear of the composite are characterized by changes in the chemical structure and composition of the running film of the composite using attenuated total reflection technique of FTIR spectroscopy and DTA results of thermal oxidative degradation of model blends of PTFE with iron, chromium and silicon oxides. It is found that higher wear rate of the composite on the stainless steel is accompanied by a lower extent of oxidation/chelation of the polymer and a greater content of accumulated silica in the running film, while at friction on the low chromium steel a lower wear rate is matched with higher content of oxidation/chelation products, lower accumulation of silica in the running film and appearance of double bonds, indicative of the polymer defluorination. The principal differences in wear rates and running film composition are attributed to the catalysis of PTFE defluorination and oxidation by iron oxides of the low chromium steel counterface, based on IR-spectroscopic evidence of similar chemical changes in PTFE on sintering in presence of Fe3O4, as well as DTA results showing catalysis of PTFE degradation in air and nitrogen in presence of nano-Fe3O4 and drastic improvement in wear resistance of PTFE/silica composite modified with small addition of nano-Fe3O4. It is hypothesized that the sharp increase in the wear resistance of PTFE nano- and microcomposites, containing highly abrasive particles, at friction on a stainless steel, observed by several research groups, is due to the destruction of a mechanically strong passivation surface layer of chromium oxide of the counterbody and the entry of freshly formed iron oxides into the friction zone, which catalyze the tribochemical oxidation of PTFE and improve the transfer film adhesion and life.