Promoting direct formation of a single α-Al2O3 scale on a silicon-aluminizing coating on TiAl alloys by a novel fluorination method

Metastable θ-Al 2 O 3 rather than stable α-Al 2 O 3 scale is preferentially formed during oxidation between 850 °C and 1000 °C, which decrease the long-term service life of the coating. In this study, a novel method for promoting the formation of single α-Al 2 O 3 through the fluorination modification of a silicon-aluminizing coating on TiAl alloys was proposed. The results indicated that fluorination resulted in notable differences in the oxidation behavior of the coatings. The unfluorinated silicon-aluminizing coating predominantly formed needle-like θ-Al 2 O 3 during the initial oxidation stage while the fluorinated silicon-aluminizing coating facilitated the direct formation of granular α-Al 2 O 3 at the initial oxidation stage. The α-Al 2 O 3 scale with high thermal stability and good adhesion significantly improved the long-term service life of the coatings. This provides a novel strategy for improving the life of this high- temperature protective coating with alumina as protective scale.