Oxidation behavior of Ti2AlC MAX phase-based coating on a γ-TiAl alloy TiAl48-2-2 produced by DC magnetron sputtering

Abstract

The application of intermetallic γ-TiAl-based alloys is limited by the deterioration in strength and creep resistance as well as reduction in oxidation resistance above 750 °C. The deposition of protective coatings is a promising opportunity to enhance the oxidation resistance or the lifetime with regard to the sustainability by several orders of magnitude. Moreover, additive manufacturing processes could enable the production of components with more complex geometries in the future. Thus, designs of γ-TiAl turbine blades with internal cooling systems could be feasible in the near future. In this context, the deposition of thermal barrier coatings and, therefore, oxidation protective bond coatings became important. MAX phases are of increasing interest as coating materials for high temperature applications due to their unique combination of metallic and ceramic properties. Especially the alumina forming MAX phases such as Cr2AlC, Ti2AlC or Ti2AlN are promising coating materials.In the present work a Ti2AlC MAX-phase based coating was deposited by DC magnetron sputtering. Using three pure elemental target materials - 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 phase. Finally, the MAX-phase coated TiAl48-2-2 alloy was subjected to a cyclic oxidation test at 850 °C in laboratory air. The Ti2AlC MAX-phase coated TiAl48-2-2 alloy exhibits an excellent oxidation behavior, due to the formation of a thermally grown alumina top layer with the desired hexagonal Ti2AlC MAX-phase below. Extensive TEM analyses also showed the dependence of Al2O3 formation on post-heat treatment. θ-Al2O3 with a lower protective effect and the concomitant formation of TiN under the TGO was observed with post-heat treatment in Ar, while post-heat treatment in air lead to the formation of a protective α-Al2O3 layer.