Effects of various kinds of fillers on the tribological behavior of polytetrafluoroethylene composites under dry and oil-lubricated conditions

Polytetrafluoroethylene (PTFE)-based composites filled with various inorganic fillers in a volume fraction of 30% were prepared. The tribological behavior of the PTFE composites sliding against AISI52100 steel under dry and liquid paraffin-lubricated conditions was investigated on an MHK-500 model ring-on-block test rig. The morphologies of worn surfaces and wear debris were observed with a scanning electron microscope (SEM) and an optical microscope. As the results, different fillers show different effects on the tribological behavior of the PTFE composites, while the composite shows much different tribological behavior under lubricated conditions as compared with dry sliding. The tribological behavior of the PTFE composites under dry sliding is greatly related to the uniformity and thickness of the transfer films. Only the PTFE composites with a transfer film of good uniformity and proper thickness may have excellent tribological behavior. The PTFE composites show much better tribological behavior under lubrication of liquid paraffin than under dry sliding, namely, the friction coefficients are decreased by i order of magnitude and the wear rate by 1-3 orders of magnitude. Observation of the worn composite surfaces with SEM indicates that fatigue cracks were generated under lubrication of liquid paraffin, owing to the absorption and osmosis of liquid paraffin into the microdefects of the PTFE composites. The creation and development of the fatigue cracks led to fatigue wear of the PTFE composites. This would reduce the mechanical strength and load-supporting capacity of the PTFE composites. Therefore, the tribological behavior of the PTFE composites under lubrication of liquid paraffin is greatly dependent on the compatibility between the PTFE matrix and the inorganic fillers. In other words, the better is the compatibility between PTFE and fillers the better is the tribological behavior of the composites.