Polymeric solid lubricant transfer films: Relating quality to wear performance

Abstract

Polymers and polymer composites are often described as solid lubricants because they provide relatively low friction coefficients and wear rates in unlubricated and other extreme tribological conditions. However, few, if any, are inherently lubricious or wear resistant. These materials achieve useful tribological properties by depositing a layer of debris onto the mating counterface; this sacrificial layer is called a transfer film and it shields the polymer from damage by the harder counterface. The friction and wear performance of these systems depend as much on the formation, evolution, and stability of the transfer film as they do on the structure and composition of the solid lubricant itself. Although it is understood that transfer films are essential for high tribological performance of polymers and polymer composites, the causal relationship between transfer film qualities and wear resistance remains uncertain due largely to the difficulty in quantitatively measuring their properties. There have been increased efforts, particularly in the last 10 years, to develop quantitative methods to assess the topographical, adhesive, mechanical, and chemical properties of polymer transfer films. This chapter reviews the latest efforts to measure transfer film qualities and quantitatively relate them to the tribological performance of solid lubricant polymers.