Abstract
Optimization of the composition of a new generation of bi-nano-multilayered TiAlCrSiN/TiAlCrN-based coatings is outlined in this study for the machining of direct aged (DA) Inconel 718 alloy. Three types of TiAlCrSiN/TiAlCrN-based bi-nano-multi-layer coatings with varying chemical compositions were investigated: (1) a previous state-of-the-art Ti(0.2)Al(0.55)Cr(0.2)Si(0.03)Y(0.02)N/Ti(0.25)Al(0.65)Cr(0.1)N (coating A); (2) Ti(0.2)Al(0.52)Cr(0.2)Si(0.08)N/Ti(0.25)Al(0.65)Cr(0.1)N with increased amount of Si (up to 8 at.%; coating B); (3) a new Ti(0.18)Al(0.55)Cr(0.17)Si(0.05)Y(0.05)N/Ti(0.25)Al(0.65)Cr(0.1)N coating (coating C) with an increased amount of both Si and Y (up to 5 at.% each). The structure of each coating was evaluated by XRD analysis. Micro-mechanical characteristics were investigated using a MicroMaterials NanoTest system and an Anton Paar-RST3 tester. The wear performance of nano-multilayered TiAlCrSiN/TiAlCrN-based coatings was evaluated during the finish turning of direct aged (DA) Inconel 718 alloy. The wear patterns were assessed using optical microscopy imaging. The tribological performance was evaluated through (a) a detailed chip characteristic study and (b) XPS studies of the worn surface of the coated cutting tool. The difference in tribological performance was found to correspond with the type and amount of tribo-films formed on the friction surface under operation. Simultaneous formation of various thermal barrier tribo-films, such as sapphire, mullite, and garnet, was observed. The overall amount of beneficial tribo-films was found to be greater in the new Ti(0.18)Al(0.55)Cr(0.17)Si (0.05)Y(0.05)N/Ti(0.25)Al(0.65)Cr(0.1)N nano-bi-multilayer coating (coating C) than in the previous state-of-the-art coatings (A and B). This resulted in over two-fold improvement of this coating's tool life compared with those of the commercial benchmark AlTiN coating and coating B, as well as a 40% improvement of the tool life of the previous state-of-the-art coating A. Multi-scale self-organization processes were observed: nano-scale tribo-film formation on the cutting tool surface combined with micro-scale generation of strain-induced martensite zones as a result of intensive metal flow during chip formation. Both of these processes are strongly enhanced in the newly developed coating C.