Fretting Wear Behavior and Damage Mode Modification of Cu-Ni-In Coated Ti6Al4V during Gross-Slip to Partial-Slip Transition

Abstract Fretting wear in Ti6Al4V dovetail joints threatens aerospace structural integrity and fatigue life. This work investigates the tribological behavior and damage mechanisms of air plasma sprayed Cu–Ni–In coatings on Ti6Al4V under 20–100 N loads. Friction evolution, fretting loops, and 3D topography analyses show the steady friction coefficient decreases with load, driven by a gross-to-partial slip transition. In the gross-slip regime, loose abrasive debris induces severe micro-cutting, whereas the partial-slip regime promotes the formation of a compacted, coherent tribolayer that effectively shields the surface. Crucially, the fretting behavior differs notably between polished and unpolished Cu-Ni-In coatings, with surface roughness affecting the slip regime transition and wear severity. A limited comparison further reveals a fundamental divergence in damage modes: while uncoated Ti6Al4V substrates suffer from severe plastic deformation and material pile-up, the coating fails through gradual material removal without significant plastic accumulation. The findings highlight Cu–Ni–In coatings modify damage modes to mitigate stress concentrations, providing insights for aerospace fatigue-critical coating design.