Study on the Transfer Film Layer in Sliding Contact Between Polymer Composites and Steel Disks Using Nanoindentation

In the present work, nanoindentation experiments were carried out to characterize the localized transfer film layer (TFL) on a steel disk, which resulted from a sliding contact of the latter against a polymer composite pin. It was found that the hybrid nanocomposites filled with both nanoparticles and traditional tribo-fillers were more effective to form durable TFLs on the steel counterpart, associated with desirable tribological properties of the sliding system, i.e., a low friction coefficient and a low wear rate. By studying the load-displacement behavior of polymeric TFLs on metallic substrates, the thickness of TFLs could be estimated, thus, allowing the comparison of TFLs formed under different sliding conditions in a quantitative way. Based on the experimental data, the effects of TFLs on the tribological performance of polymer composites were further discussed in terms of a “transfer film efficiency factor” λ, which was calculated by the ratio of the average thickness of the TFL to the surface roughness of the steel counterpart. The factor mainly considered the relative contributions of the TFL and the metallic counterface to the wear process of the polymer-on-metal system. Accordingly, the wear rate and the friction coefficient of the sliding system could be analyzed as a function of the transfer film efficiency factor, resulting in a Stribeck type diagram. The analyses provided new insight into the role of TFLs in polymer tribology.