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 ...

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

http://research.me.udel.edu/~dlburris/papers/JA32.pdf

Transfer film evolution and its role in promoting ultra-low wear of a PTFE nanocomposite

Over the last decade, researchers have explored an intriguing polymer composite composed of granular polytetrafluoroethylene (PTFE) 7C and alumina particles. This material is extraordinary because a very small amount of alumina additive (<5 wt%) decreased the wear rate of the PTFE composite by over four orders of magnitude. Previous studies have shown that this wear resistance was initiated and maintained by the formation of a stable, robust, and uniform polymeric transfer film on the surface of the countersample. Although its importance to this tribological system is clear, the transfer film itself has not been well understood. Careful spectroscopic analysis throughout the stages of transfer film development revealed that tribochemistry plays a major role in the significant wear rate reductions achieved in PTFE and alumina composites. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy reveal that PTFE chains break due to the mechanical stresses at the wear surface and, in the presence of oxygen and water in the ambient environment, produce carboxylic acid end groups. These carboxylic acid end groups can chelate to the exposed metal on the steel surface and nucleate the formation of the transfer film. The resulting thin and robust fluoropolymer transfer film protects the surface of the steel and changes the sliding interface from polymer on steel to polymer on polymer transfer film. These effects keep friction coefficients and wear rates low and stable. Ultimately, the real mechanisms responsible for the exceptional wear performance of these materials are all about the tribochemistry.

Ultralow Wear PTFE and Alumina Composites: It is All About Tribochemistry

From the context of a contemporary understanding of the phenomenological origins of friction and wear of materials, we review insightful contributions from recent experimental investigations of three classes of materials that exhibit uniquely contrasting tribological behaviors: metals, polymers, and ionic solids. We focus on the past decade of research by the community to better understand the correlations between environment parameters, materials properties, and tribological behavior in systems of increasingly greater complexity utilizing novel synthesis and in situ experimental techniques. In addition to such review, and a half-century after seminal publications on the subject, we present recently acquired evidence linking anisotropy in friction response with anisotropy in wear behavior of crystalline ionic solids as a function of crystallographic orientation. Although the tribological behaviors of metals, polymers, and ionic solids differ widely, it is increasingly more evident that the mechanistic ori...

Mechanistic Studies in Friction and Wear of Bulk Materials

Polytetrafluoroethylene (PTFE) is widely regarded as an excellent candidate for solid lubrication in vacuum. However, it is often precluded from many prac- tical applications due to its intrinsically high wear rate. Over the past decade, it has been discovered that small loading fractions of alumina nanofillers can increase the wear resistance of PTFE by three to four orders of mag- nitude. This dramatic increase in wear resistance has in turn prompted numerous tribological studies to examine the robustness of this performance. In this study, the wear and friction behavior of unfilled PTFE and PTFE and alumina nanocomposites were evaluated under a broad range of vacuum environments from 760 to 4 9 10 -6 Torr. The nanocomposites of PTFE/alumina showed a dramatic increase in wear of over two orders of magnitude at the highest vacuum conditions. There appears to be an optimal vacuum environment around 1-10 Torr, in which these samples achieved the lowest wear rates of approximately 2.5 9 10 -7 mm 3 /(Nm).

/pdf/in-vacuo-tribological-behavior-of-polytetrafluoroethylene-4ypd4s575o.pdf

In Vacuo Tribological Behavior of Polytetrafluoroethylene (PTFE) and Alumina Nanocomposites: The Importance of Water for Ultralow Wear

Environmental dependence of ultra-low wear behavior of polytetrafluoroethylene (PTFE) and alumina composites suggests tribochemical mechanisms

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 “run-in” 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.

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

Embodiments of the present disclosure provide for mixtures, methods for making mixtures, articles, methods for making articles, and methods of using articles.

Jeffrey J. Ewin

Papers

Environmental dependence of ultra-low wear behavior of polytetrafluoroethylene (PTFE) and alumina composites suggests tribochemical mechanisms

In Vacuo Tribological Behavior of Polytetrafluoroethylene (PTFE) and Alumina Nanocomposites: The Importance of Water for Ultralow Wear

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

Mixtures, articles having low coefficients of friction, methods of making these, and methods of using these