An in-depth investigation of the machining performance of Ti1-Xalxn PVD coatings during high-speed machining of 316 stainless steel

Abstract

PVD coatings are being intensively developed for cutting tools to increase the productivity of machining processes and enhance the quality of machined parts. The temperatures and pressures at the tool/chip interface are extremely high during high-speed machining of hard-to-cut materials. Such conditions drive the need for effective surface protection, which can be accomplished by the application of Ti-Al-N-based coatings. This study investigates the influence of Al content on the cutting performance of the Ti-Al-N coating during high-speed machining of 316 stainless steels. Detailed investigation of tribo-film formation was performed using surface sensitive methods (X-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES)). Wear volume measurements were carried out with an Alicona infinite microscope. A study of chip characteristics was performed by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) methods. Finally, the surface integrity of the machined parts was studied using nano-indentation, SEM and surface profilometry. Although the thicknesses of the alumina tribo-oxide layers are nearly equal in Ti0.40Al0.60N and Ti0.27Al0.73N coatings during the initial machining stage, the ability of the Ti0.40Al0.60N coating to constantly rebuild an alumina-based tribo-layer in response to external stimuli, results in an 8× longer tool life compared with the Ti0.27Al0.73N coating. This difference can be attributed to self-organization. The inferior micro-mechanical properties of the Ti0.27Al0.73N coating had also promoted cratering during machining, and in addition to built-up edge (BUE) formation, made it the predominant wear mechanism. Chip characteristics were favorable due to the formation of beneficial tribo-films on the surface of the Ti0.40Al0.60N coating and less sticking/tearing occurred during machining. This had also altered the frictional condition at the tool/chip interface. Thus, the chips underwent a lesser amount of heat transfer and plastic deformation, resulting in a lower fraction of recrystallization. Since the surface integrity was improved, minimal work-hardening was observed in the Ti0.40Al0.60N coating due to the favorable frictional condition. The surface of the machined parts was found to be relatively smooth. It can be therefore concluded that the Ti0.40Al0.60N coating is best suited for intense tribological conditions.