Length scale effects on the fretting wear of DLC coating system

Fretting of diamond-like carbon (DLC) coatings has been investigated previously using piezo-driven micro-tribometers, AFM-based nanofretting, and micro-scale reciprocating systems. These studies revealed dependencies on contact size, displacement amplitude and probe sharpness, but lacked the ability to track true sliding amplitude and fretting-regime evolution in real time. The present work introduces a nano-fretting methodology integrated into an ultra-low-drift nanomechanical testing platform, enabling continuous measurement of actual track length and on-load displacement throughout accelerated fretting experiments. Tests were conducted on multilayer DLC coatings using sphero-conical diamond probes of two radii (15µm and 163µm) across partial-slip, gross-slip and full-sliding regimes. Mechanical properties were characterised via partial-load nanoindentation, followed by reciprocating nano-fretting under controlled loads, track lengths and cycle numbers. Real-time depth sensing allowed decomposition of total penetration into elastic, plastic and wear components. Local dissipated energy density was calculated from friction–displacement loops to evaluate energy-based wear relationships. Complementary nano-scratch tests provided contact radii, mean pressure estimates and independent corroboration of deformation modes. The study shows that wear behaviour depends strongly on the contact length scale. The blunt probe exhibits a consistent energy-controlled mild-wear response, whereas the sharp probe displays pressure-activated transitions leading to accelerated coating failure. Dissipated-energy wear coefficients differ by nearly two orders of magnitude between probe geometries. This work demonstrates that contact size fundamentally governs the mechanisms of nano-scale fretting wear in DLC systems and provides the first direct comparison of energy-based wear coefficients across length scales using a real-track-length nano-fretting method.