Assessing quantitative metrics of transfer film quality as indicators of polymer wear performance

• Tested the relationships between polymer wear rates and transfer film morphology using representative polymers.
• Demonstrated that transfer film free-space length provides the best correlation to and prediction of polymer wear rates.
• The free-space length appears to be the best measure of transfer film quality in the context of polymer wear.

Reduced wear rates of filled polymeric tribomaterials consistently accompany improvements in the appearance of the transfer film, a protective layer of debris that adheres to the counterface. As a result, wear reductions are often attributed to the favorable effects of the filler on transfer film quality. However, the cause-effect relationship between fillers, transfer film quality, and polymer wear performance remains uncertain due, in part, to a lack of standard metrics for assessing transfer film quality. Methods for quantifying transfer film thickness and area fraction have been proposed previously; although some studies show strong correlations between these parameters and wear rate, others have demonstrated a lack of general applicability. In a more recent study, it was proposed that the characteristic size of the gaps in the transfer film (free-space length) may better reflect visual differences in transfer film quality and more directly relate to debris size and wear rate. In this paper, a representative collection of common tribological polymers and composites were subjected to wear testing and transfer film topology characterization to better generalize the link between polymer wear performance and transfer film topology (thickness, coverage, and domain size-scales). The free-space length provided the best correlation with steady state wear rates of the tribopolymers in this study and prediction of wear rates from previous studies. The results suggest that, among the metrics considered, the free-space length provides the best independent measure of transfer film quality in the context of polymer wear.