Investigation of several PVD coatings for blind hole tapping in austenitic stainless steel

Abstract

During the last 10 years, the usage of stainless steel materials increased continuously in various industrial applications. However the machineability of these materials is difficult, especially austenitic stainless steels. Blind hole tapping in such materials is problematic due to difficult chip formation, chip transport, the tendency of cold-welding and build-up edges on the tool. HSS-tools are commonly used for tapping. The surface is either untreated, annealed in steam or coated with PVD. In most cases TiN, TiCN or TiAlN coatings are used. In this study, blind hole tests were accomplished with HSS M8-taps in 1.4571 austenitic stainless steel. The tools were coated with hard coatings such as CrN, CrC, TiN, TiAlN, CrAlN; lubricant coatings such as WC/C and DLC, or double layers such as TiCN + WC/C. To investigate the behavior of the different coatings in this cutting process, the cutting torque maxima, the torque slope and different quality criteria for the finished thread were analyzed. The used tools were analyzed by SEM regarding tool wear and cold welding. In addition mechanical properties such as hardness, abrasive wear resistance and friction coefficient were measured. To conclude which coating is suited for this application the combined analysis of the machining data, the SEM-investigations and the mechanical properties were conducted. Coatings with low friction coefficient have the lowest fluctuation of the cutting torque and are the only one having the ability to reduce the reverse torque by a half. Overcoating e.g. TiCN with DLC and WC/C this property will be preserved. In this case the excellent abrasive and adhesive wear resistance of TiCN was combined with excellent friction behavior and abrasive wear resistance of DLC. Out of the accomplished tests no precise conclusion about the lifetime can be made, but to obtain a stable blind hole threading process in austenitic stainless steel a coating system which combines excellent adhesive and abrasive wear resistance as well as excellent friction properties is required.