Forming an oxidation protective coating on titanium and titanium-base alloys

We report on the fabrication of a coating for protecting α-Ti and Ti-base alloy surfaces from environmental oxidation and embrittlement at elevated temperatures. The fabrication process involves the deposition of Ti and Al by dual magnetron sputtering followed by vacuum annealing (700°C, 8 h) to form a layer of γ-TiAl, which is finally treated by plasma-based ion implantation of fluorine to provide the necessary conditions for activating the so-called halogen effect. The resulting coating forms a protective alumina-containing scale upon subsequent high-temperature oxidation in air, thereby rendering the coated substrate material resistant to oxygen diffusion and embrittlement. Two types of F-containing precursor gases, namely a mixture of difluoromethane and argon (CH2F2+25% Ar), and a mixture of silicon tetrafluoride and argon (SiF4+25% Ar) have been employed for implanting fluorine. It has been shown that the SiF4/Ar plasma-based process is much more efficient in terms of implantation time (twice as short compared with that of the CH2F2/Ar-based treatment). Moreover, the co-implantation of Si is helpful as it results in an additional enhancement of the halogen effect. A variety of analytical techniques such as XRD, ERD, RBS and XTEM in conjunction with EELS and EDXS have been used for characterization. We present analysis data detailing the microstructure and the phase formation in the coating material. The structure and quality of the coating appear to be independent of the substrate material. Moreover, the ability to form, in a reproducible manner, thin films of γ-TiAl on various substrates has also relevance to high-temperature microelectronics applications (diffusion barriers, conduction lines etc). Thus, the results of this study may serve to broaden the range of TiAl uses.