Tribological behavior of graphene-filled polytetrafluoroethylene (PTFE) composites

Abstract

For many decades now, polytetrafluoroethylene (PTFE) has been used as a solid lubricant in various tribological applications. PTFE offers low friction coefficients during dry sliding against typically encountered countersurfaces, and this ability has been one of the primary reasons for its tribological uses. However, PTFE is known to wear at high rates, which can approach 10⁻³ mm³/Nm, under commonly encountered conditions. Such high wear rates limit its widespread application.

Thermal analysis of graphene-filled PTFE composites was performed using differential scanning calorimetry (DSC). Clear correlations could not be found between the measured wear rates and properties such as crystallinity, peak melting temperatures, and enthalpies of phase transitions happening near ambient temperatures. However, the investigation of wear-resistant graphene-filled PTFE composites, using wide-angle x-ray scattering (WAXS), revealed that the crystalline regions within the PTFE matrix had a greater resemblance to those found in the tougher phase-I of PTFE. Additional testing performed on wear-resistant PTFE composites with nanoparticles of α-phase alumina and activated carbon also showed that the increasing wear resistance of these composites was accompanied with the increasing structural resemblance to the tougher phase-I of PTFE.

The effectiveness of graphene platelets, in providing wear resistance, was investigated for a few other polymers having some similarities with the molecular structure of PTFE. When added to high-density polyethylene (HDPE), graphene platelets failed to improve the wear resistance, possibly because unfilled HDPE itself already had significantly lower wear rate (~3.6×10⁻⁷) mm³/Nm). The addition of ~1.25 nm and 8 nm thick graphene platelets to fluorinated ethylene propylene (FEP), at 0.32 vol% and 1.1 vol%, respectively, also failed to lower the wear rate.

Unworn and worn surfaces of unfilled PTFE and various graphene-filled composites were also studied using attenuated total reflectance (ATR) spectroscopy. The ATR-IR spectra obtained from worn surfaces of only highly wear-resistant (typically, ~10⁻³ mm³/Nm and lower) graphene-filled composites contained bands which originate due to vibrations in metal chelates. These bands were largely absent in the spectra obtained from unfilled PTFE and the unworn surfaces of its composites. The data further showed that the wear rates decreased monotonically with increasing intensities of these bands. This indicated that the presence of graphene platelets within the PTFE matrix could possibly initiate the formation of a chemically bonded and well-adhered transfer film over metallic countersurfaces, which could protect the composite during sliding wear and effectively lower its wear rate. Similar trends were also observed for other highly wear-resistant composites of PTFE with activated carbon nano-particles and micro-particles of silicon nitride and ??-phase alumina.

This work is mainly focused on understanding the role of graphene platelets— a micro/nano mixed-scale filler— in imparting wear resistance to PTFE. Composites of PTFE were prepared at different filler loadings, using graphene platelets with typical thicknesses ranging between ~1.25 nm and 60 nm. Wear tests showed that the addition of graphene platelets could lower the steady-state wear rates by more than three orders of magnitude, in comparison with unfilled PTFE. Moreover, at a given loading, the thinner graphene platelets typically provided greater wear resistance to PTFE than their thicker counterparts. Additionally, the wear behavior of the various graphene-filled PTFE composites collapsed around a “master-curve", when the wear rates were plotted against graphene surface area present in a unit mass of the composite. The friction coefficients were also largely reduced with the addition of graphene platelets to PTFE.

Composites of PTFE, with micron- and nanometer-sized particles of copper, silicon nitride and ??-phase alumina, as representative example filler materials, were prepared using the mix/press/sinter technique. Pin-on-plate type wear tests conducted on these composites demonstrated that micron-sized fillers typically outperform their nanometer-sized counterparts in imparting wear resistance to PTFE. The size of the generated wear debris indicated that larger micron-sized filler particles impart wear resistance by a common mechanism of arresting subsurface delamination crack propagation and limiting the size of the wear debris.