The optimum composition of Me–C:H coatings on drills for cutting performance

This study investigates the drilling of AISI 1045 steel workpieces using Ti-based (Ti–C:H, Ti–C:H/TiC/TiCN/TiN and TiC/TiCN/TiN) and Cr-based (Cr–C:H/CrC/CrCN/CrN, Cr–C:H and CrC/CrCN/CrN) Me–C:H coated high-speed steel drills. The thrust forces, torque, flank wear and hole surface roughness are measured in order to evaluate the cutting performance of each coated drill. The coating comprising a Ti–C:H top layer deposited over an intermediate layer of TiC/TiCN/TiN results in a low and stable cutting force (thrust force and torque) throughout the drilling tests. This coated drill also demonstrates the highest drill wear resistance and produces a superior machined surface finish. The excellent cutting performance of this particular drill is due to the fact that the multi-layer composition generates an excellent solid lubricant effect, which improves the tribological interaction between the drill and the workpiece, hence resulting in a significant reduction in material transfer at the drill edge. Drills coated with a single thick Ti–C:H topcoat or with a single thick TiC/TiCN/TiN intermediate layer demonstrate poor cutting characteristics, including high flank wear, high thrust force, high torque and poor drilled hole quality. In general, the Cr-based coated drills exhibit a relatively poorer cutting performance and an inferior drilled hole quality than the Ti-based coated drills since the Cr element in the coating possesses a high chemical affinity for Fe.