Harnessing Ti2AlN MAX Phase Based PVD Coatings on Titanium Aluminide Alloys for High Temperature Applications

Abstract

Nanolaminate coatings based on MAX phases (where M= Ti, A=Al, and X is nitrogen) exhibit a distinctive combination of ceramic and metallic material properties, offering considerable potential for utilization in high-temperature environments. It is unfortunate that the application of MAX phases as coating material on various Ti- or Ni-based alloys results in degradation due to interdiffusion processes between the coating and the alloy, which is accompanied by an Al-depletion. A promising strategy involves combining a γ-TiAl substrate with a MAX phase coating, as the substrate can serve as an Al reservoir, replenishing the coating through outward diffusion of Al. This approach could also enhance the mechanical properties of such coated components compared to other protective but brittle intermetallic coatings on TiAl alloys. In this study, a Ti2AlN-MAX phase-based coating was deposited using reactive magnetron sputtering using pure elemental targets of Ti, Al and nitrogen as a reactive gas. The deposition process was studied using a variety of substrates, including inert Al₂O₃ and MgO substrates, as well as an already been used γ-TiAl alloy. This alloy TiAl48-2-2 (48Ti–48Al–2Nb–2Cr in at.%), supplied by GfE–Gesellschaft für Elektrometallurgie in Nuremberg, Germany, was utilized for all oxidation and interdiffusion experiments. The two-fold rotation ensures homogeneous deposition, with a thickness of 10 µm and near to the requisite stoichiometric composition of the Ti2AlN MAX phase. Due to the low substrate temperature during deposition, the resulting layer was X-ray amorphous. A post-annealing treatment was performed at 800°C in a high vacuum furnace for one hour for crystallization. Additionally, high-temperature X-ray diffraction (HT-XRD) in a vacuum atmosphere was conducted from room temperature to 1000°C to observe in-situ the phase formation in the Ti2AlN coating. Following the production of Ti2AlN MAX Phase, a series of oxidation tests are conducted to assess the coating's performance. These include isothermal oxidation for 10 hours at 850°C in laboratory air. Hereby, the Ti2AlN MAX phase based coating develop a thermally grown layer of predominantly alumina, which is suitable as protection in high temperature environments. Below the alumina layer the Ti2AlN MAX phase as well as the intermetallic Al-rich phase. Analysis techniques included GD-OES for chemical composition, XRD for phase analysis, and SEM/EDS and TEM for structural evaluation are used.