Interfacial Isolation-Layer Stability Controlling Friction and Wear of a Pinion–Rack Steel Pair: Greases versus a DLC-Like Carbon Coating

Pinion–rack contacts in rack railway systems run under high stress in open environments, where wear limits reliability. Reciprocating ball-on-flat tests on a 20CrMnMoA/42CrMo steel pair benchmarked four interfacial strategies: unlubricated (dry) sliding, an OEM grease (L-WGR000-17), an open-gear grease (CEPLATTYN 300), and magnetron-sputtered diamond-like carbon (DLC) coating. The coefficient of friction (COF) and infrared contact-zone temperature were recorded in situ, and wear/tribochemistry were examined by 3D profilometry, SEM/EDS and XPS. Steady-state COF ranked as unlubricated (0.478) ≈ CEPLATTYN 300 (0.453) > DLC (0.163) > L-WGR000-17 (0.120). The unlubricated condition produced severe adhesive tearing and third-body ploughing with strong tribo-oxidation. CEPLATTYN 300 reduced wear but formed undulated scars with oxygen-rich, patchy deposits, indicating an unstable boundary/deposit layer. L-WGR000-17 delivered ultra-low wear but occasional localized micropitting, suggesting transient film breakdown. DLC achieved grease-free low wear with smoother scars and carbon-rich interfacial coverage. Overall, friction, temperature rise and wear are governed by the continuity and stability of the interfacial isolation layer, providing a benchmark for protecting exposed, highly loaded steel contacts.