Critical design factors for multi-layer coating systems that influence heat partition in the secondary shear deformation zone and machining performance

High thermal loads during the cutting process can have a major influence on tool performance. Multilayer coated tools can have a positive impact in improving machining performance, especially at high cutting velocities. In this study TiCN/Al2O3/TiN, TiCN/Al2O3 and TiCN/Al2O3-TiN multilayer chemical vapour deposition (CVD)-coated carbide cutting tool inserts, were investigated in dry turning of AISI/SAE 4140 over a wide range of cutting speeds of between 200 m/min and 879 m/min. While the TiCN/Al2O3/TiN and TiCN/Al2O3 coating layers were deposited uniformly around the insert, TiCN/Al2O3-TiN has TiCN/Al2O3 on the rake face and TiCN/Al2O3/TiN on the flank/clearance faces. For clarity, in this article it is addressed as (TiCN/Al2O3/TiN) flank (TiCN/Al2O3) rake and termed the functionally graded system. The investigations were based on experimental determination of cutting forces, chip compression ratio, tool-chip contact phenomena, and flank wear, as well as finite element modelling of heat partition into the cutting tool. Results show that two layered TiCN/Al2O3 deposited in series gives a better performance against the three layered TiCN/Al2O3/TiN and the functionally graded TiCN/Al2O3-TiN tools. Results from the cutting tests and finite element analysis suggest that selection of an appropriate top coating and optimised coating thickness are two important factors for achieving effective cutting performance.