Failure mechanisms of physically vapour deposited coated hardmetal cutting inserts in turning

Thin hard physical vapour deposition (PVD) coatings attribute excellent cutting performance to cemented carbide inserts, allowing a beneficial exploitation of modern CNC machine tools. Based on the continuous chip formation, coatings experience distinct failure mechanisms in turning. To clarify the influencing parameters on coating wear resistance release, turning experiments with TiAlN derived coatings, deposited on cemented carbide inserts have been carried out. At a variety of cutting speeds, the experimental results are explained through analytical ones obtained by means of finite elements method (FEM) calculations indicating a coating static decohesion at low cutting speeds, owing to its overstressing. On the other hand, at higher cutting speeds tribomechanical abrasive phenomena are dominant. Nevertheless, the cutting accomplishment of coated tools is impressive, in as much as even after the local initiation of coating failures the tool still exhibits adequate cutting reserves, continuing its severe service up to the end of its operational life due to abrasive flank wear. As the overall cutting length increases theoretical and experimental results show that a progressive local coating decomposition occurs while the cutting forces remain practically stable. Herewith chip formation alterations such as a gradual increase of the chip compression ratio and a consequent reduction of the tool contact stresses are induced.