Formation and compaction of sub-10 nm transfer films in metal–ceramic fretting

Nanoscale transfer in metal–ceramic fretting originates at microcontacts but has remained experimentally unresolved. Here, we use an atomic force microscopy-based topographical difference method to visualize the formation of sub-10 nm transfer films at a sapphire–chromium interface subjected to fretting. Transfer proceeds via adhesive wear of chromium at micrometer-scale contacts, followed by plastic compaction of the chromium debris into a conformal ≈5 nm transfer film. Excess debris is expelled from the circular contact and accumulates in a curvature-defined peripheral ring around the contact. These results connect microcontact fracture, debris evolution, and nanoscale transfer-film formation in fretting contacts.