Nanomechanical Filler Functionality Enables Ultralow Wear Polytetrafluoroethylene Composites

Surprisingly, certain alpha-phase alumina filler particles at one to five weight percent can reduce the wear rate of polytetrafluoroethylene (PTFE) by 10,000 times, while other, seemingly comparable alpha-phase alumina particles provide only modest-by PTFE composite standards-100 times improvements. Detailed studies reveal that size, porosity, and composition of the particles play important roles, but a quantitative metric to support this mechanism is yet to be developed. We discovered the mechanistic importance of friability of the particles, for example, the ability of the particles to fragment at the sliding interface. This work establishes the importance of functionally friable metal-oxide filler particles in creating ultralow wear PTFE-metal-oxide composite systems. We used in situ nanoindentation/electron microscopy experiments to characterize the fracturability of candidate filler particles. A mechanistic framework relating apparent particle fracture toughness and wear is established, where porous low-apparent fracture toughness particles were observed to promote ultralow wear by breaking up during sliding and forming a thin, robust tribofilm, while dense, high-apparent fracture toughness particles abrade the countersurface, limiting the formation of ultralow wear promoting tribofilms. This framework enables use of a new metric to select filler particles for multifunctional, ultralow wear PTFE composites without relying solely on empirical tribological tests of polymer composite materials.