Tribomechanical Performance of Functionally Graded Ti–TiC–TiNbC and Ti–TiC–TiNbC–TiNbCN DLC Coatings on M2 Tool Steel

ABSTRACT This study investigates the structural, mechanical, and tribological performance of two functionally graded diamond‐like carbon (DLC) coatings—Ti–TiC–TiNbC–a:C and Ti–TiC–TiNbC–TiNbCN–a:C—deposited on AISI M2 high‐speed steel via closed‐field unbalanced magnetron sputtering. The multilayer design was developed to mitigate residual stress and adhesion limitations commonly observed in conventional DLC films. Cross‐sectional SEM and XRD analyses confirmed the formation of dense, nanocrystalline TiC, NbC, and Ti(C,N)/TiN phases embedded in an amorphous carbon matrix, with significant niobium enrichment verified by EDS. Mechanical testing revealed that the Ti–TiC–TiNbC–a:C coating achieved a hardness of 1250 HK 0 . 01 , while nitrogen incorporation in the Ti–TiC–TiNbC–TiNbCN–a:C structure increased hardness further to 1500 HK 0 . 01 . Both coatings maintained ultralow and stable friction coefficients (0.10–0.11), yet the nitrogen‐enriched film exhibited a higher wear rate (8.0 × 10 − 5 vs. 3.2 × 10 − 5 mm 3 /N·m). Scratch adhesion tests showed superior interfacial strength for the thinner Ti–TiC–TiNbC–a:C coating (37 N) compared to the thicker Ti–TiC–TiNbC–TiNbCN–a:C variant (28 N), suggesting that excessive layer thickness and nitrogen incorporation exacerbate residual stresses and reduce adhesion. Overall, the findings demonstrate that while Nb‐ and N‐rich graded DLC architectures improve hardness, they compromise wear resistance and adhesion, emphasizing the importance of carefully optimizing layer composition, thickness, and stress management for practical industrial applications.