Mechanisms of Ultra-Low Wear Polymeric Solid Lubricants*

Polymeric solid lubricants can address the many limitations of more traditional lubrication systems including complexity, cost, maintenance requirements, vapor pressure, and sensitivity to environmental contamination, temperature, and composition. At present, commercially available materials have been designed based on the conventional wisdom that wear reductions require increased strength and toughness, and friction reductions require increased availability of soft/lamellar phases. Unfortunately, friction and wear reductions compete in this framework, which fundamentally limits overall tribological performance. The fundamental problem with the conventional wisdom is that it neglects to account for the transfer film, a layer of adhered debris that ultimately protects the solid lubricant from the counterface and vice versa. It is now understood that the friction and wear properties of these systems depend as much (and likely more) on the formation, properties, and stability of the transfer film as they do on the composition, structure, and properties of the solid lubricant itself. Unfortunately, the relevant transfer film properties have proven difficult to measure and the extent to which they contribute to friction and wear reduction remains unclear. In this talk, I will review some of the ultra-low wear polymeric solid lubricants we have developed and studied over the last decade, our efforts to understand the attributes of and specific roles played by the transfer film, and how we have applied these insights to a more useful materials design framework that prioritizes the properties and stability of the transfer film.