Independently tuning surface and subsurface reinforcement to optimize PTFE wear

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.