PTFE Tribology and the Role of Mechanochemistry in the Development of Protective Surface Films

Abstract

The wear and friction behavior of ultralow wear polytetrafluoroethylene (PTFE)/α-alumina composites first described by Burris and Sawyer in 2006 has been heavily studied, but the mechanisms responsible for the 4 orders of magnitude improvement in wear over unfilled PTFE are still not fully understood. It has been shown that the formation of a polymeric transfer film is crucial to achieving ultralow wear on a metal countersurface. However, the detailed chemical mechanism of transfer film formation and its role in the exceptional wear performance has yet to be described. There has been much debate about the role of chemical interactions between the PTFE, the filler, and the metal countersurface, and some researchers have even concluded that chemical changes are not an important part of the ultralow wear mechanism in these materials. Here, a "stripe" test allowed detailed spectroscopic studies of PTFE/α-alumina transfer films in various stages of development, which led to a proposed mechanism which accounts for the creation of chemically distinct films formed on both surfaces of the wear couple. PTFE chains are broken during sliding and undergo a series of reactions to produce carboxylate chain ends, which have been shown to chelate to both the metal surface and to the surface of the alumina filler particles. These tribochemical reactions form a robust polymer-on-polymer system that protects the steel countersurface and is able to withstand hundreds of thousands of cycles of sliding with almost no wear of the polymer composite after the initial run-in period. The mechanical scission of carbon−carbon bonds in the backbone of PTFE under conditions of sliding contact is supported mathematically using the Hamaker model for van der Waals interactions between polymer fibrils and the countersurface. The necessity for ambient moisture and oxygen is explained, and model experiments using small molecules confirm the reactions in the proposed mechanism.

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]. Ye [42] conducted similar measurements on transfer films from the same system.…”

“…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].…”

“…In 2015, Pitenis et al [29] and Harris et al [27] conducted spectroscopic studies on transfer film chemistry of the same low wear alumina PTFE nanocomposite. After one cycle, they found peaks typically associated with PTFE (Figure 6a).…”

“…They proposed that these metal chelate salts represent the bond between the polymer (perfluorinated carboxylic acid) and the counterface (metal). Harris et al [27] found evidence showing that PTFE chain ends not only chelate to the steel surface under the transfer film, but also to the surface of the alumina filler particles. Thus, tribochemistry in this system appears to stabilize the transfer film and the sliding surface of the solid lubricant, thus compartmentalizing damage and reducing wear [39].…”

“…Thus, tribochemistry in this system appears to stabilize the transfer film and the sliding surface of the solid lubricant, thus compartmentalizing damage and reducing wear [39]. As a group with unprecedented expertise in tribology and fluoropolymer chemistry, Harris et al [27] proposed a likely tribochemistry-based wear reduction mechanism in PTFE-related systems with the following essential steps: (1) PTFE chains break during sliding and form perfluoroalkyl radicals at new chain ends; (2) the perfluoroalkyl radicals react with atmospheric oxygen to form acyl fluoride end groups; (3) the acyl fluoride end groups hydrolyze in ambient humidity to form carboxylic acids; and (4) the perfluorinated carboxylic acids chelate to metals (Fe in the steel counterface and Al in the alumina fillers), thus stabilizing the transfer film and near surface of the pin. This hypothesis explains the observed environmental dependence of the PTFE nanocomposite's wear performance and represents a breakthrough in our understanding of ultra-low-wear PTFE.…”