Development of new coatings to prevent environmental embrittlement of titanium aluminides

Abstract: For temperatures above 750°C, TiAl alloys still show insufficient oxidation resistance and suffer from environmental embrittlement. This work focuses on the surface modification of alloys and development of coatings against environmental embrittlement, as well as on testing of mechanical properties after high temperature oxidation. Aluminum enriched coatings (between 50 and 60 at.%) containing alloying elements, i.e. Cr, Nb, Si, Y, to improve the oxidation behavior and the corrosion resistance have been produced by MO-CVD, CVD, PVD and thermal spraying techniques (HVOF, APS), and have subsequently been chemically modified with halogen elements, notably fluorine. The mechanical properties have been studied by means of 4-point bending and tensile tests on coated samples after 100h oxidation at 900 °C in laboratory air. The CVD process combined with fluorine treatment using plasma immersion implantation (PI³) of F offers the best combination to remedy environmental embrittlement. It has been shown in particular that 90% of the initial fracture strain and fracture stress can be maintained.