Protection of titanium aluminides by FeCrAlY Coatings

The effectiveness of Fe+15%Cr+5%Al+0.3%Y coatings, produced by sputter ion plating, in improving the oxidation behaviour of alpha-2 Ti3Al at 800 degrees C and gamma-TiAl at 900 degrees and 1000 degrees C, during cyclic exposures in air of up to 1000 hours duration, has been studied. The 45-135 mu m coatings were tested in both the as-coated condition and following densification by peening. The kinetics of attack and of spallation were followed gravimetrically, while the reaction product compositions were examined by a range of surface analytical techniques. Oxidation protection was afforded by the formation on the coating surface of a alpha-Al2O3 oxide scale but chemical reactions at the coating-substrate interface with accompanying voidage development, eventually caused the coating to fail mechanically. To surmount this problem the potential of several ceramic interlayers produced by the same coating route, and interposed as diffusion barriers between the FeCrAlY overlayers and the Ti3Al substrate was examined in 1000 hour oxidation tests at 800 degrees C.