Friction and wear behavior of PTFE composites filled with rare earth compounds under oil-lubricated conditions

Polytetrafluoroethylene (PTFE)-based composites, filled with CeO₂, CeF₃, and La₂O₃ in volume contents of 5, 10, 15, 20, and 30%, were prepared. Then, the friction and wear behavior of these PTFE composites sliding against GCr15 bearing steel under both dry and liquid paraffin-lubricated conditions was evaluated using an MHK-500 ring-on-block wear tester. Finally, the worn surfaces and the transfer films of these PTFE composites were investigated using a scanning electron microscope (SEM) and an optical microscope. Experimental results showed that filling CeO₂, CeF₃, and La₂O₃ into PTFE can reduce the wear of the PTFE composites by 1-2 orders of magnitude. When the content of CeO₂ in PTFE is 15%, the friction and wear properties of the CeO₂-filled PTFE composite are the best. Meanwhile, when the content of La₂O₃ in PTFE is between 15 and 20%, the PTFE composite filled with La₂O₃ exhibits excellent friction and wear-reducing properties. However, the friction coefficient of the CeF₃-filled PTFE composite increases but its wear decreases with increase in the content of CeF₃ from 5 to 30%. The friction and wear-reducing properties of CeO₂-, CeF₃-, and La₂O₃-filled PTFE composites can be greatly improved by lubrication with liquid paraffin, but the limit loads of the PTFE composites decrease with increase in the content of CeO₂, CeF₃, and La₂O₃ in PTFE (from 5 to 30%) under the same conditions. Investigations of worn surfaces show that the interaction between liquid paraffin and the CeO₂-, CeF₃-, and La₂O₃-filled PTFE composites, especially the absorption of liquid paraffin into the microdefects of the PTFE composites, creates some cracks on the worn surfaces of the PTFE composites and that the creation and development of the cracks reduces the mechanical strength and the load-supporting capacity of the PTFE composites. However, with increase of the content of CeO₂, CeF₃, and La₂O₃ in the PTFE, the microdefects in the PTFE composites also increase, which would lead to increase in the number of the cracks on the worn surfaces of the PTFE composites under load and, so, in turn, lead to the reduction of the limit loads of the CeO₂-, CeF₃-, and La₂O₃-filled PTFE composites under lubrication with liquid paraffin.