Effect of TiN- and TiC- tool coatings on wear, surface roughness, and chip morphology in machining Ti-6Al-4V titanium alloy

Titanium alloy Ti-6Al-4V is widely used in aerospace, biomedical, and automotive industries due to its high strength and corrosion resistance, but it is a challenge to machine a workpiece made of this alloy. This study presents the first systematic comparison of TiN and TiC coated carbide inserts under controlled wet machining conditions using two industrially relevant lubricants soluble oil (water-based emulsion) and HJ 20 industrial oil. Experiments used TiN- and TiC-coated carbide inserts under dry and lubricated conditions, with soluble oil and HJ 20 industrial oil as lubricants. A 3 × 3 matrix of coated and uncoated tools was tested on a CNC turning center. Surface roughness and tool wear were analyzed using measurements and SEM imaging. Results reveal that coating-lubricant interactions significantly outperform either factor alone. Among all combinations, the TiN-coated tool with soluble oil delivered the best performance, cutting surface roughness by ≈ 65.6% (Ra = 1.412 μm) compared to uncoated dry machining (Ra = 4.116 μm) and limiting tool wear to 0.100 mm V_bmax. It also produced more stable, ductile chip shapes demonstrating TiN coatings, contrary to assumptions about chemical affinity, provide improved diffusion barrier properties when paired with appropriate coolants.