Thermo-mechanical performance of a TiAlN-coated micro-drill: effect of cutting speed and point angle on tool wear, cutting forces, and surface integrity

Abstract

Cobalt Chromium Molybdenum (CoCrMo) alloys are essential for biomedical applications. However, their high hardness and low thermal conductivity severely hinder machinability. In micro-drilling, a geometric trade-off typically exists, where smaller point angles improve penetration but increase thermal loading, while larger angles reduce thermal stress at the expense of higher cutting resistance. Although recent studies highlight the importance of chip evacuation and cutting mechanics, the combined thermo-mechanical effects of advanced coatings and dedicated drill geometries in CoCrMo microdrilling remain insufficiently explored. This study evaluates the performance of a TiAlN-coated micro-drill incorporating an effective chip evacuation architecture. The effects of cutting speed (50, 65, and 80 m/min) and point angle (118°, 130°, and 140°) on axial thrust force, surface roughness, and steady-state tool wear were analyzed using Two-Way ANOVA. The results revealed that a cutting speed of 50 m/min with 118° point angle is the optimal machining condition, yielding the lowest axial thrust force (95.42 N), minimal flank wear (85.72 µm), and the finest surface finish (0.51 µm). Mechanistically, this condition preserves the TiAlN coating while enabling efficient chip segmentation and evacuation, thereby reducing thermal accumulation at the tool-chip interface. Conversely, higher cutting speeds promote rapid tool degradation and a shift towards chip-dominated material removal. Overall, the findings show that combining coating properties with chip evacuation geometry can significantly improve the performance of CoCrMo micro-drills, offering more stable operating window and wear resistance.