Deposition of Ti2AlC MAX-phase based coating on γ-TiAl to improve the oxidation resistance

Abstract

Currently, the application of intermetallic γ-TiAl alloys is limited by their deterioration in strength and creep resistance at elevated temperatures as well as by reduced oxidation resistance above 800 °C. The deposition of protective coatings is a promising opportunity to enhance the oxidation resistance by several orders of magnitude. Moreover, additive manufacturing processes have enabled the production of components with more complex geometries. Thus, a design of turbine blades with internal cooling made of γ-TiAl is feasible in the near future. In this context, the deposition of thermal barrier coatings and, therefore, protective coatings become important. MAX-phases are of increasing interest as coating material for high temperature applications due to their unique combination of metallic and ceramic properties. Especially the alumina forming MAX phases of Cr2AlC, Ti2AlC or Ti2AlN are promising as oxidation resistant coatings. Unfortunately, degradation of MAX phases is observed when applied on various Ti- or Ni-based alloys by interdiffusion processes between coating and alloy and the associated Al-depletion. This degradation is not present when MAX-phases are applied on the Al-rich γ-TiAl based alloys, which leads to an inward diffusion of Al from the substrate alloy into the coating and finally to a stabilization of the thermally grown alumina layer. Moreover, MAX-phases are known as a ductile material and could therefore prevent the deterioration of the mechanical properties especially the fatigue behavior of such coated components in contrast to the common intermetallic, protective but brittle coatings on γ-TiAl alloys. In the present work a Ti2AlC MAX-phase based coating were deposited by DC magnetron sputtering. Using three, pure elemental target materials of Ti, Al and C and a two-fold rotation a homogenous all-around coating was applied on the γ-TiAl alloy TiAl48-2-2 with a coating thickness of 10 µm. After the deposition process the stochiometric Ti2AlC coating was x-ray amorphous, therefore a post-heat treatment at 800°C for 1 h was performed to achieve the desired hexagonal MAX-Phase. Finally, the MAX-phase coated TiAl48-2-2 alloy was subjected to the cyclic oxidation test at 850°C in laboratory air up to 100 1h-cycles. The Ti2AlC MAX-phase coated TiAl48-2-2 alloy exhibits an excellent oxidation behavior at 850°C, due to the formation of a thermally grown alumina top layer. The phase formations during the heat treatment were analysed by HT-XRD measurements showing a homogenous coating microstructure of the hexagonal Ti2AlC phase below α-Al2O3 oxide layer. The interface reaction zones between the coating and the TGO, as well as between the coating and the TiAl48-2-2 substrate alloy were analyzed by SEM with EDS and especially by high-resolution STEM.