Comparison of TiN-coated tools using CVD and PVD processes during continuous cutting of Ni-based superalloys

• PVD- and CVD-deposited TiN coatings are tested in continuous cutting of Ni-based superalloy.
• Damage modes of TiN coatings during cutting were fractures at coating defects and plastic deformation.
• CVD-deposited TiN coatings showed thermal stability and softening resistance at high temperatures.
• High correlation was found between coating defects and initial wear during machining.
• High correlation was found between the high-temperature softening of TiN coatings and steady wear rate during machining.

High-strength, low-conducting Ni-based superalloys require higher cutting force and cutting temperature than other materials during the machining process. To understand how the coating characteristics such as defects formation and physical and mechanical properties affect cutting performance, TiN coatings were deposited by three physical vapor deposition (PVD) methods (arc ion plating, sputtering, and hollow cathode) and chemical vapor deposition (CVD). Sputtering and CVD yielded TiN coatings with fewer defects than other methods. Subsequently, the damage to the TiN coatings during machining of alloy 718 was investigated under continuous turning. Tools coated by sputtering and CVD exhibited micro-abrasion wear without heavy fracture or delimitation of the coating. Furthermore, the CVD coating showed the best cutting performance and lower plastic deformation than the PVD coatings. TiN coatings were characterized by X-ray diffraction, nanoindentation, electron back-scatter diffraction, and transmission electron microscopy. As the hardness of the coatings was measured before and after heat treatment, the CVD coating was more resistant to softening due to heating than the PVD coatings. The reduced hardness of the PVD coatings was attributed to strain release, atomic rearrangement, and grain growth in the PVD coatings. Such high-strength coatings with good high-temperature stability and low-defect density offer strong protection to cutting tools during machining of Ni-based superalloys. Based on the results of this study, fine structure CVD-deposited coatings may be good candidate coating for cutting of the Ni-based superalloys.