Abstract
The addition of 0.2–5% nanoscale (40–80 nm) α-phase or microscale (40 μm) γ-phase Al2O3 particles in PTFE effectively reduce the matrix wear rate by 99.99%, whereas microscale (> 0.5 μm) α-Al2O3 or nanoscale (40–80 nm) γ-Al2O3 only reduce PTFE wear by ~ 90% under identical loading, dispersion and testing conditions. This paradoxical material system best illustrates the complexity of tribology and the importance of filler–matrix interactions at small scales. We studied the independent effect of the Al2O3/PTFE interface area and alumina structure by systematically varying the particle size over two orders of magnitude for both α- and γ-Al2O3/PTFE composites. Detailed characterizations of filler size, surface area and tribofilm’s chemical composition were conducted. The results found: (1) DLS median particle sizes conformed reasonably to vendor reported values and percentages of microscale filler aggregates correlated weakly with wear rates, (2) electron microscopy of the as-worn composite surface suggested a strong relation between the characteristic size of ‘unreinforced’ polymer domain and composite wear rate, (3) third bodies (i.e., transfer films, debris) played an important role in counterface abrasion, (4) wear rate correlated strongly with filler’s specific surface area and ultralow wear was only maintained ~ 0.3–10 m2/g nominal specific filler–matrix area values, (5) ultralow wear coincided with perfluorinated carboxylic salt rich tribofilms which supported a previously proposed wear reduction mechanism that mechanochemically degraded PTFE chelate with alumina and cause crosslinked and wear-resistant tribofilms, (6) tribofilm Al-F bond signal increased with filler surface area and high wear coincided with excessive tribofilm Al-F signal for γ-Al2O3/PTFE systems. Based on these results and literature hypothesis, we proposed that (1) the 1 μm α-Al2O3 provided the least filler–matrix interface and largest unreinforced polymer domain in PTFE, which lead to the least crosslinked and compartmentalized tribofilms; (2) in γ-alumina filled composites, Al-F bond forms as a product of mechanochemically degraded PTFE but also blocks chelation between the degraded PTFE and alumina fillers, (3) the 20 nm γ-Al2O3 provided the most filler–matrix interface which leads to excessive aluminum fluoride that blocked the filler–matrix chelation, prevented the tribofilm crosslinking and lead to high wear rates. This hypothesis was additionally supported by small molecule experiments in this study. However, this study provides no direct insight into how sensitive the filler–matrix tribochemical interaction is to filler phase or aggregate strength (strong, weak or fully dense).
Published Version
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