Polymers Tribology Exposed: Eliminating Transfer Film Effects to Clarify Ultralow Wear of PTFE

Abstract

PTFE composite wear rates are known to vary by 1000 × depending on the size and strength of their nanofiller aggregates. While these effects have been attributed to variations in subsurface reinforcement, debris regulation, transfer films, and filler abrasivity, the chain of causation has proven difficult to test. This study aimed to clarify these causal relationships by eliminating confounding transfer film effects on wear reduction. We conducted indexed reciprocation experiments and tracked the interfacial development for PTFE filled with 5 wt% nano-alumina aggregates of varying strength (weak, strong, or a fully dense control). Weak aggregates were broken down most by processing, created the fewest abrasions, and produced the largest wear debris (~ 10 μm). Strong aggregates were largely retained following processing, produced the densest abrasions, and radically reduced debris size (< 100 nm). Despite these key interfacial differences, the composites produced comparable wear rates (2–7 × 10–5 mm3/Nm). The results provide the first direct evidence of the following: (1) even weak nanoparticle aggregates can be extremely abrasive; (2) ultralow wear rates (10–7 mm3/Nm) require transfer film stability; (3) the wear-reducing effects of unstable transfer films and loose debris are negligible; and (4) fillers directly reduce debris size even without a protective transfer film. The results suggest that successful fillers reduce debris size directly, that small debris nucleates a stable transfer, that stable transfer films reduce transfer wear rates, and that interfacial stability provides the time for needed for tribochemical reinforcement.