Evaluation of wear mechanisms of PVD and CVD coatings deposited on cemented carbide substrates applied to hard turning

A good understanding of the coating wear mechanism is essential in relation to tailoring the coating properties to the application, with a focus on increasing the tool performance. When a coated cemented carbide tool is applied in processes with high thermal and mechanical loads (e.g., hard turning process), the cutting tool can suddenly collapse with the deterioration of the coating. The focus of this research was to track the sequence of events that leads to the deterioration of coating materials (PVD TiAlN and MT CVD TiCN/Al2O3/TiN), to reveal the real wear mechanisms associated with coated cemented carbide cutting tools applied to hard turning. The tool wear was evaluated by focus variation microscopy (FVM) and scanning electron microscopy (SEM), before and after cutting edge collapse, during the hard turning of quenched and tempered AISI 4340 steel. The mechanisms associated with the progression of wear on MT CVD coatings involve abrasion, crack nucleation, propagation, the formation of crack networks, delamination, detachment between coating layers, and spalling. The deterioration of PVD coatings is related to the abrasion wear mechanism and the high deformation values at the cutting edge, which leads to the nucleation of cracks in the coating, reducing the bonding strength between the coating and the tool substrate, leading to spalling of the coating. The tool life of the PVD coating was three times longer than that of the CVD coating. The wear mechanisms acting on the coating is the main factor that influences the end of tool life.