Ultralow Wear PTFE and Alumina Composites: The Role of Tribochemistry and Nanomechanics

Mark A. Sidebottom1, Kasey Campbell2, Cooper Atkinson2, Christopher P. Junk3, Gregory S. Blackman4 Brandon A. Krick2*
1Mechanical and Manufacturing Engineering, Miami University; 2Mechanical Engineering and Mechanics and 3Materials Science, Lehigh University and 4DuPont

INTRODUCTION The wear rate of polytetrafluoroethylene (PTFE) can be reduced by 10,000 times (K ~ 10-7 to 10-8 mm3/Nm) by addition of low concentration (0.25 to 5 vol. %) α alumina1–8. The true mechanisms responsible for this extraordinary wear reduction have remained elusive, largely due to their multiple, size, length, and force scale origins.

A mechanistic clue is not all varieties of alumina fillers can achieve this dramatic reduction in wear; in fact, there is a three order of magnitude variation in wear depending on the type of alumina filler used. Here, we study how the nanomechanics of nanostructured alumina fillers couple with tribochemical reactions that contribute to the formation of stable tribofilms that are structurally, morphologically, chemically and mechanically different from the bulk. While investigations are still underway to discern the exact role of the low volume percent alumina fillers, the true mechanisms for wear reduction are likely coupled with complex multi-scale chemical, physical, mechanical and material phenomena that result from the tribologically-induced temporal and spatial variations of the interface composition. The prevailing discoveries are 1) a tribochemically formed transfer film promotes adhesion to a countersample and 2) multi-scale mechanics of the filler dominate the variation in wear rate with filler


RESULTS AND DISCUSSION Mechanistically, the bulk material (a low volume % random dispersion of alumina in PTFE) transforms in situ during the initial sliding cycles, resulting in a hierarchically-structured, tribochemically altered composite  system. The wear reducing mechanisms rely on the transient formation of tribofilms on the surface of the worn composite (“running film”) steel countersample (“transfer film”), preventing  steel-bulk composite contact. The mechanistic hypotheses are:

  1. Mechanochemically formed tribofilms (Figure 1 ii, iii) chemically bond to the surface of the metal countersample through carboxyl-metal salts (Figure 1 vi).
  2. Nanostructured microscale alumina fillers are not abrasive to the transfer film because they break up into nano-scale fragments at the interface (Figure 1 i).
  3. Nanoscale alumina accumulates and improves the robustness of the tribofilms through mechanical (reinforcement) and chemical (crosslinking) mechanisms (Figure 1 v).

Each hypothesis is supported through multi-scale experiments and characterization techniques, including x-ray photoelectron spectroscopy, infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, nanoindentation and atomic force microscopy.


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