Thermo-mechanical decoupling at the tool-workpiece interface: A comparative study of coating architectures in orthogonal cutting of Nickel 718 alloy

This study investigates the decoupling of thermal and mechanical effects in the orthogonal machining of Nickel 718 alloy, evaluating four coating architectures (DLC, DLC-Si, DLC-W and AlTiSiN) under variations in edge radius (9-27 µm) and cutting speed (5-20 m/min). The comprehensive analysis combines force measurements, temperature measurements using IR thermography, and surface hardness measurements to elucidate the interaction mechanisms at the tool-workpiece interface. The results reveal that DLC-Si emerges as the optimal choice for low-speed operations (5 m/min), generating the hardest surfaces (453 HV-5) with the best thermo-mechanical balance under these conditions. This coating demonstrated a unique combination of low friction (µapp = 0.434) and effective thermal management, outperforming the other coatings in terms of surface quality. For medium-high speed applications (20 m/min), AlTiSiN maintained its traditional advantage with the most consistent friction coefficient (0.453) and lower tool temperatures. Contrasting behaviours were identified: DLC-W showed thermal degradation with massive heat transfer to the workpiece, while conventional DLC presented limitations in thermal management. Correlation analysis showed that surface hardness is compromised by thermal softening (Tpiece > 50°C), while high ΔT_shear (>80°C) correlates with greater work hardening. The study proposes DLC-Si as a novel and promising alternative for low-speed precision operations where surface integrity is critical. It also proposes a thermo-mechanical quality index that identifies the optimal coating option, combining high tribological efficiency, balanced thermal management, and maximum operational consistency. These findings provide selection criteria based on fundamental mechanisms for precision machining in critical low-speed cutting process applications.