Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Plasma immersion ion implantation (PI3) of fluorine has been used to enhance the oxidation resistance of several commercial TiAl alloys (γ-MET, TNB and TNM) with the aim of expanding their high-temperature (750°-1050°C) structural potential for advanced aerospace, power generation and automotive applications. The mechanism that operates in the F-implanted TiAl alloys, enabling one to achieve oxidation protection up to 1050°C, is the so-called halogen effect.
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 F. A variety of analytical techniques such as X-ray diffraction (XRD), elastic recoil detection (ERD), Rutherford backscattering spectrometry (RBS) and cross-sectional electron microscopy (XTEM) in conjunction with electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDXS) have been used for material characterization. The degree of oxidation protection has been evaluated by testing F-implanted samples under conditions of both isothermal and thermocyclic oxidation in air employing thermal gravimetric analysis (TGA). The formation of a protective scale has been studied by metallography, EDXS, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA).
For the CH2F2/Ar-based PI3 process, ERD analyses have revealed in the F-implanted γ-MET and TNB alloy samples anomalously broad, high-concentration (up to 70 at. %) F profiles of either Gaussian or plateau-like shape extending to much larger depths than those predicted by theory. It has been found that the broad F implant profiles are not associated with F diffusion, but rather result from a complex amorphization/phase transition process, which occurs via the implant zone/substrate interface progressing toward the bulk. As distinct from the implanted fluorine, the co-implanted carbon forms a shallow surface peak of a concentration of about 15 at. %, much in accordance with the theoretical predictions. Optimized PI3 of F into γ-MET and TNB alloys in this case leads to a dramatic improvement in their environmental durability due to the formation of a stable, adherent and highly protective α-Al2O3 scale on the alloy surface upon subsequent high- temperature oxidation. On the basis of the results obtained, components of complex geometry (turbine blades and turbochargers) made of TiAl alloys have been successfully implanted with F for times as short as 22 min, and a commercially viable PI3 technique has been developed.
Furthermore, we have demonstrated for the first time the possibility of fabricating yttria- stabilized-zirconia thermal barrier coatings (YSZ-TBCs) on PI3-treated γ-TiAl to improve turbine efficiency, thereby enabling turbines made of γ-TiAl to be used for longer times at higher service temperatures. TBCs have been deposited by electron-beam physical vapor deposition (EB-PVD) at 900° and 1000°C on γ-TiAl that had been pre-implanted with F and then subjected to oxidation. Excellent adhesion of the TBCs to the Al2O3 scale present on the alloy surface has been observed, with the coatings retaining adherence and the halogen effect still lasting under cyclic oxidation at 900°C in air. Another aspect of our research has been to investigate the halogen effect resulting from the PI3 of F in a new class of TiAl materials containing a β-phase, namely TNM-B1. The preliminary results have been positive, strongly indicating that further work is warranted.
The SiF4/Ar-based PI3 process is still being optimized. Under certain implantation conditions, adequate oxidation protection has been achieved in both γ-MET and TNB alloys for an implant time twice as short (11 min) compared with that used in the CH2F2/Ar process. This can be explained by the combined effect of F and Si. The latter element is believed to reduce the rate of oxygen diffusion into the incipient protective scale. A non-linear time dependence of the retained F dose has been observed, presumably due to enhanced ion etching and material removal for longer implant times. These phenomena are currently under study.