Abstract
In dry sliding conditions, polytetrafluoroethylene (PTFE) composites can form thin, uniform, and protective transfer films on hard, metallic counterfaces that may play a significant role in friction and wear control. Qualitative characterizations of transfer film morphology, composition, and adhesion to the counterface suggest they are all good predictors of friction and, particularly, wear performance. However, a lack of quantitative transfer film characterization methods and uncertainty regarding specific mechanisms of friction and wear control make definitive conclusions about causal relationships between transfer film and tribological properties difficult. This paper reviews the state of the art in the solid lubricant transfer film literature and highlights recent advances in quantitative characterization thereof.
Highlights
Polytetrafluoroethylene (PTFE) and its composites are attractive for use in tribological applications due to low friction coefficients [1,2], high melt temperature, and chemical inertness of the parent polymer
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), stabilizing the transfer film and near surface of the pin
Apart from transfer film morphology, improved mechanical properties of the transfer film have been suggested as an important contributor of wear reduction in PTFE and other polymer systems
Summary
Polytetrafluoroethylene (PTFE) and its composites are attractive for use in tribological applications due to low friction coefficients [1,2], high melt temperature, and chemical inertness of the parent polymer. Tanaka and Kawakami were the first to study sub-micron filler particles in PTFE; they performed poorly relative to the microparticles in the field and the authors concluded that nanoscale fillers were too small to disrupt large-scale destruction of the banded structure [14]. It was not appreciated at the time, that the combination of large loadings (20 wt %) and small particles (300 nm) are likely to lead to agglomeration, ineffective sintering, and a substantially weakened polymer composite. The reduction in wear rate was accompanied by decreased debris size and improved transfer film attributes, like thickness and uniformity
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