Tribofilm Formation and Run-In Behavior in Ultra-Low-Wearing Polytetrafluoroethylene (PTFE) and Alumina Nanocomposites

Abstract

Polytetrafluoroethylene (PTFE) and alumina nanocomposites have been of great interest to the tribological community due to the significant reduction in wear imparted by filling PTFE with low weight percentages of alumina nanofillers. The mechanisms of these three order, four order, and sometimes five order of magnitude reductions in wear have been commonly explored through evaluation of a transfer film. The present article evaluates the tribofilm formed on the wear surface of the polymer. PTFE composites with 2, 5, and 8 wt% alumina fillers were tested to evaluate wear of the polymer and how it is affected by the tribofilms formed on the wear surface of the polymer. Furthermore, the link between a transient "runin" wear period and the formation of that tribofilm is observed and discussed. Nanomechanical measurements of the tribofilm reveal significant alterations in the mechanical properties of the surface films formed during sliding.

A Review of Transfer Films and Their Role in Ultra-Low-Wear Sliding of Polymers

“…They found that running film hardness and modulus increase with increased sliding distance and decreased wear rates [24]. They suggested that changes in mechanical properties reflect filler accumulation and tribochemical degradation of PTFE, which is consistent with their more recent papers on the evolution of interface chemistry [27,29] and particle enrichment [30].…”

“…The transfer films had the same mechanical properties as the bulk composite during the run-in, but hardness and modulus both increased significantly following the transition to ultra-low-wear sliding, as shown in Figure 7a,b. Both figures share the same trend as the mechanical evolution of running films reported by Krick et al [24]. Furthermore, there were strong correlations between transfer film mechanical properties and the system wear rater (Figure 7c,d), although the causal relationship remains uncertain; improved transfer film cohesive strength may simply be the consequence of the friction-induced polymer degradation and the accumulation of filler at the sliding interface [27,29,42].…”

“…In addition to producing thinner and more uniform transfer films, low-wear alumina-PTFE transfer films are obviously discolored [13,18,20,[23][24][25][26][27]29,31,32,[37][38][39], which suggests chemical reactions had taken place and possibly enhanced adhesion of the transfer film. In 2013, Ye and Burris used optical microscopy to determine the degree to which transfer films persist during sliding [20].…”

“…Krick et al [24] used nanoindentation to study the mechanical properties of ultra-low-wear alumina-PTFE running films (on the composite's running surface). They found that running film hardness and modulus increase with increased sliding distance and decreased wear rates [24].…”

Interrelated Effects of Temperature and Environment on Wear and Tribochemistry of an Ultralow Wear PTFE Composite

“…There were early indications that chemistry played a role based on the observation that these ultra-low wear surfaces always exhibited a brown discoloration after transitioning to low wear [16][17] . Based on this observation alone, it is impossible to determine whether chemistry was the cause or consequence of low wear.…”