Enabling Cr2AlC MAX Phase as Sustainable Bond Coat for Ni-Superalloys

Abstract

Al2O3-forming MAX phases, such as Cr2AlC, are a promising alternative to state-of-the-art MCrAlY bond coats (BC) for Ni-superalloys. Not only do they offer great corrosion resistance, desirable mechanical properties and a crack self-healing effect, but they are also more affordable and sustainable by avoiding elements such as Ni and Co. However, depletion of Al from the MAX phases due interdiffusion with the substrate is a major issue. Diffusion barriers (DB) are therefore required for enabling the full potential of MAX phases as BCs for Ni-superalloys . In this work, CrN is investigated as a potential DB material. Specimens with a CrN DB (ca. 1 µm thick) between IN718 substrates and Cr-Al-C coatings were compared to specimens without a DB. A surplus amount of Al was deliberately added, resulting in a hybrid Cr2AlC-Cr5Al8 coating (ca. 10 µm thick), where Cr5Al8 serves as Al reservoir phase. The CrN coatings were deposited using reactive magnetron sputtering, whereas Cr2AlC was synthesised using conventional DC magnetron sputtering. Both coating systems were tested in isothermal and cyclic oxidation experiments at 800°C for up 100 h and 100 cycles (60 min heating, 10 min cooling) in air, respectively. Subsequently, the oxidation and interdiffusion behaviour of the specimens was characterised by means of XRD, SEM, EDX, WDX, and TGA. The CrN DB successfully suppresses interdiffusion under the investigated circumstances. Not only was the loss of Al completely prevented (Fig. 1), but no elements from the substrate alloy were detected in the BC either, indicating optimal conditions for growing a protective Al2O3 top layer. While the microstructure of the coating systems with DB was almost unaffected, a pronounced interdiffusion zone (IDZ), with no remaining Cr2AlC phase, formed in specimens without DB for oxidation times equal to or longer than 25 h or 25 cycles, respectively. Furthermore, porous Cr-carbides and other mechanically disadvantageous phases, such as brittle beta-NiAl, formed in the IDZ due to Al diffusion into the IN718 substrates in specimens without DB. In conclusion, it is evident that CrN is an effective DB under the investigated conditions, increasing the life time substantially and thus enabling Cr2AlC as a sustainable and affordable BC material.