Effect of various fillers on the friction and wear of polytetrafluoroethylene-based composites
Abstract
The friction and wear behaviour of polytetrafluoroethylene (PTFE)-based composites incorporating various fillers was determined under a constant load and at various sliding speeds when composite pins were rubbed against a steel disk. Factors influencing the wear-reducing mechanism of the fillers were studied. The friction of PTFE-based composites was generally independent of the type of filler. Fibre and particle fillers of suitable size were more effective than lamellar solid lubricants and very small hard particles. The load-supporting action and the prevention of large-scale destruction of the banded structure of the PTFE matrix at frictional surfaces contribute to the wear-reducing action of the fillers. The effects of the material and the shape and size of the fillers on the wear of composites are discussed. The load-supporting action of the fillers is discussed theoretically.
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Clarifying the importance of the running film to the ultra-low wear of the polymer composite by eliminating its individual effect
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Recent studies have shown that PTFE achieves ultra-low wear rates through a combination of surface and subsurface reinforcement. However, these two wear-reducing effects depend on filler material, size, and loading in ways that have proven difficult to isolate and study to date. In a recent study, we used 5 wt% of a secondary subsurface reinforcing phase (PEEK) to independently isolate and optimize surface reinforcement by trace nanofillers. To our knowledge, these hybrid composites are the first to demonstrate optimal wear reducing effects of nanofillers at trace loading (« 1 wt%). The present study independently varied PEEK and nanofiller loading to optimize both subsurface and surface reinforcement, respectively. In the absence of nanofillers, a minimum wear rate of 8 × 10−8 mm3/Nm was achieved at 20 wt% PEEK, which is 4-fold the loading used in our prior study. The addition of 0.1 wt% nano-alumina reduced the wear rate by 5-fold to 1.6 × 10−8 mm3/Nm. For all PEEK loadings, the addition of 0.1 wt% nanofiller reduced the corresponding PEEK-PTFE wear rate by between 4 and 40-fold. Wear performance and tribofilm quality degraded monotonically with increased nanofiller loadings beyond 0.1 wt% at all PEEK loadings. This study shows that surface and subsurface reinforcement are separable and independently tunable using fillers whose sizes and loadings differ by orders of magnitude. Conversely, it suggests that prior optima from single-filler-in-PTFE studies reflect a compromise between surface and subsurface reinforcement rather than an optimum of either.
Wear resistance effects of alumina and carbon nanoscale fillers in PFA, FEP, and HDPE polymers
2022, WearFollowing numerous studies demonstrating the ability of nanostructured alpha-alumina filler to reduce the prohibitively high wear rate of polytetrafluoroethylene (PTFE) down to extremely small values (∼10−7 mm3/Nm) well below those of conventional PTFE micro-composites, alpha-alumina filler was also shown capable, in a couple recent studies, of imparting similar performance to another fluoropolymer, perfluoroalkoxy (PFA) copolymer, which otherwise was similarly lacking wear resistance in the unfilled state. In addition to duplicating such alpha-alumina performance in PFA, in this study such an extreme wear-reducing capability has also been demonstrated using nanocarbon powder, as an example representative of several other forms of nanoscale carbon filler that like alpha-alumina had been shown capable of providing extreme resistance to PTFE. Fluorinated ethylene propylene (FEP) copolymer, whose nanocomposites have not been tribologically explored previously, was thus more fully investigated here with not only alpha-alumina and nanocarbon, but also other nanotube (CNT) and mesoporous forms of nanoscale carbon fillers. In preliminary testing, only the alpha-alumina filler indicated an ability to impart its wear-reducing capability to FEP; a wear rate of ∼0.8 × 10−6 mm3/Nm was observed at 2 wt% alpha-alumina concentration. While this is an impressive reduction in wear rate, it is not quite as extreme a reduction as that observed in PTFE or PFA. The transport of the extreme wear resistance of nanocarbon powder in PTFE and PFA was even more partial and incomplete in FEP, with the ∼0.3 × 10−3 mm3/Nm unfilled FEP wear rate only reduced to ∼10−5 mm3/Nm, while the CNT and mesoporous carbon fillers were even less effective. Correspondingly, ATR-FTIR spectra from FEP wear surfaces displayed sizable peaks evident of the chelation of chemical interactions known to be associated with wear resistance for PTFE and PFA matrices only in the most wear-resistant alpha-alumina case and to a lesser extent for the nanocarbon. Finally, it is demonstrated that such fillers demonstrate such strongly beneficial effects only in polymers that otherwise lack wear resistance, and may actually be deleterious for polymers such as high density polyethylene (HDPE) already having some inherent wear resistance in their unfilled state.
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Present address: Nippon Denso Co. Ltd., Kariya, Aichi 448, Japan.