Influence of ta‑C, a‑C, and a‑C:H properties and low-viscosity lubricants on the mild wear behaviour of DLC

Diamond-like carbon (DLC) coatings are widely employed to extend the service life of components exposed to severe tribological conditions, particularly where uncoated surfaces would fail under boundary and mixed lubrication. For reliable application, a mechanistic understanding of mild wear processes in such regimes is essential. In this study, in-situ wear rate measurements were performed using a reciprocating tribometer to evaluate DLC-coated specimens against steel counter bodies under low-viscosity lubricants with distinct chemical compositions, namely water and diesel fuels (fresh and aged). A series of hydrogenated and hydrogen-free DLC coatings with varying sp ³ /sp ² hybridisation ratios and hardness were investigated. The results indicate that under mild wear conditions, lubricant properties — most notably dynamic viscosity (~ 1–3 mPa·s) and oxidation state — dominate the wear response, while the DLC coating’s sp ³ /sp ² ratio and mechanical properties exert a secondary influence. Hydrogenated, sp ² -rich DLCs exhibited superior wear resistance due to their enhanced ability to form passivating hydroxyl layers, which suppress graphitisation-induced degradation. In contrast, water-based lubrication, while promoting hydroxylation, led to increased wear due to steel counter body corrosion and third-body abrasion. Aged diesel lubricants resulted in lower DLC wear rates, attributable to increased oxidation promoting surface passivation. These findings highlight the necessity of jointly considering lubricant chemistry and DLC structural characteristics when designing tribological systems for low-wear applications.