The Effects of Substrate Bias and Edge Proximity on the Mechanical Properties and Machining Performance of TiN-Coated Carbide Inserts

The effects of substrate bias on magnetron sputter-deposited TiN-coated carbide inserts were examined with a focus on the property changes that occur near the cutting tool edge. Sandvik SPG-422 polished carbide inserts were coated with TiN using substrate bias levels ranging from −25 to −150 V. The hardness was measured using the nano-indentation method and x-ray diffraction was used to measure residual stress. The bias strongly affected the appearance of the region 1-2 mm from the tool edge with defect formation, cracking, and delamination observed at the higher substrate bias levels. For samples deposited at bias levels of up to −50 V, the coating composition was not strongly affected by edge proximity and the hardness exhibited only a small increase with bias of about 3 GPa. The residual stresses were measured using the sin2ψ method, and it was found that both the increasing bias level and proximity to the edge increased the degree of compressive stress, reaching −3.2 GPa at edge location of the −50 V bias sample. The machining performance was assessed by turning hardened 4340 steel, and only the samples deposited at −25 and −35 V exhibited substantial improved tool wear in comparison to an uncoated edge. These results correlated well with the appearance of the cutting tool edge and the measured stress levels.