On the influence of varying the crystallographic texture of alumina CVD coatings on cutting performance in steel turning

Understanding the mechanisms at the tool/chip interface during metal cutting is crucial in the production of almost every metallic component used in engineering applications. It is critical to have rapid, durable, and reliable machining processes. This work contributes to the understanding of mechanisms occurring on the tool in the secondary shear zone, and it is focusing on the tool side of the contact. Crystallographic textured Chemical Vapor Deposited (CVD) alpha-Al2O3 coated cutting tools are dominating the steel turning area, as they show an increased performance compared to coatings with randomly oriented grains. In this study, we investigate the effect of three different CVD alpha-Al2O3 textures on the initial rake crater behavior. This was done using a turning test designed to generate crater wear only in the alumina layer, which was deposited onto an inner Ti(C,N) layer, which in turn was deposited on a cemented carbide insert. With this approach, the influence of the underlying coating layer and substrate was reduced. Pre- and post-machining characterization of the different contact areas on the surfaces of the three textured CVD alpha-Al2O3 coatings, (0001)(0001), (01 (1) over bar2) and (11 (2) over bar0), was performed using scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and energy dispersive Xray spectroscopy (XEDS). Plastic deformation, micro-rupture, abrasion and chemical reactions with the workpiece material are all identified as mechanisms involved in crater formation during turning. For the (0001)-textured coating, the observed low wear-rate is attributed to homogeneous basal-slip dominating plastic deformation, while for the (01 (1) over bar2) and (11 (2) over bar0) textures the main deformation mechanisms are attributed to heterogeneous plastic deformation, causing micro-rupture and abrasion, leading to higher wear-rates. The effect of a larger coating grain size is mainly seen as the formation of wider ridges and valleys, while the effect on wear rate was limited.