Processing and tribological evaluation of irradiated/unirradiated fluoropolymer composites

Abstract

Electron irradiation in air improved the steady-state wear resistance of PTFE and FEP in contact with polished counterfaces over three orders of magnitude at 30 Mrad. The wearreduction mechanisms proposed are related to the control of the shape and size of debris produced. Friction of PIFE initially increases with increasing dose, reaching a maximum value at 5 Mrad, then decreases with subsequent increase in dose, attaining a value similar to that of unirradiated PTFE at 30 Mrad.

These irradiated fluoropolymers were reheated at 360° to 380°C to observe whether the wear resistance can be retained through subsequent thermal processing necessary for the sintering of unirradiated PTFE matrix powder. The most wear-resistant of the reheated samples was chosen as a particulate filler to mix with unirradiated PTFE powder, pressed, and sintered at 360° to 380°C, yielding the composite material. PTFE either irradiated in air or irradiated at 300°C in nitrogen and FEP irradiated in air cannot be accepted as fillers. Only FEP irradiated at 300°C in nitrogen maintained the improved wear resistance through subsequent thermal processing and therefore can be accepted as a filler for wear-resistant PTFE matrix composites. These bulk composites showed a combination of low wear resistance, low friction, and non-abrasivity to the mating counterfaces. Moreover, the friction of these bulk composites was less than or equal to that of unirradiated PTFE alone.

The tribological performance of these irradiated fluoropolymers and composites was characterized using pin-on-disk friction and wear test equipment, with pins sliding against smooth stainless steel counterfaces. Wear and friction measurements were recorded throughout the duration of sliding tests by computer-assisted data acquisition. FI -IR spectroscopy and Differential Scanning Calorimetry (DSC) were used to investigate the changes of chemical structure (scission, cross-linking, and branching), molecular weight, and the crystallinity of the electron-irradiation PIFE and FEP.

Although PTFE (polytetrafluoroethylene) polymer is widely used as a dry bearing material because of its low friction coefficient when rubbed against smooth, clean mating surfaces, it suffers from poor wear resistance and this limits its applications. This unacceptable high wear rate can be improved by the incorporation of hard particle fillers or irradiation processing. However, hard particles can increase the friction and abrade the mating counterface. Irradiated PTFE is wear-resistant and non-abrasive, but irradiation of PTFE causes higher friction. Therefore, the objectives of this study are production and tribological evaluation ofPTFE matrix composites containing irradiated fluoropolymers (PTFE and FEP) as fillers. These composites will combine the advantage of unirradiated PTFE and irradiated fluoropolymers to make wear-resistant, self-lubricating, and non-abrasive PIFE matrix composites.

In this research, fluoropolymer (PTFE and FEP) particles and bulk disks were irradiated by electrons (10 MeV) at room temperature and 300°C in air and nitrogen environments with several doses (0 to 100 Mrad) to see whether wear resistance of fluoropolymers can be radiation-induced.

In addition to bulk composites, irradiated/unirradiated composite surfaces were produced through patterning PTFE surfaces with masks and lower energy 225 keV electrons. The composite unirradiated/irradiated PTFE surfaces produced no abrasive damage to the mating stainless steel counterfaces, provided wear resistance for sliding distances of several kilometers, and exhibited a friction even lower than that experienced by unirradiated PTFE. As with the bulk composites, the wear resistance of the surface composites appears to result from the limitation of debris size imposed by the irradiated fraction, while the retained low friction is provided by the unirradiated PTFE.