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Carbon (i.e. CFC) is considered as plasma-facing material in future fusion devices like ITER. The reactivity of carbon against hydrogen species (chemical erosion) is the main drawback, causing life-time problems of the components and formation of hydrocarbon layers in hidden parts of the reactor vessel. This is of safety concern if radioactive tritium — the fuel for fusion — is used. Doping of carbon with transition metals strongly reduces chemical erosion, which is the motivation for the developement of new doped graphite and CFC materials. To investigate the underlying mechanism of the reduced chemical erosion well-defined deuterium ion beam experiments were performed with metal-doped amorphous carbon thin films (a-C:Me). In order to evaluate and understand the influence of their micro- and nanostructure on the erosion process, it is important to characterize these coatings detailed on different length scales.
Hydrogen-free a-C:Me films (Me = Ti, V, W and Zr) with metal contents up to 19 % are synthesized by dual-source magnetron sputter deposition. Post-annealing of the films (at 700 K — 1300 K) induces structural ordering, like carbide grain formation and growth and changes of the carbon matrix.
For W-doped films carbide grain formation is evident even after deposition, and their growth can be studied by XRD. For low metal concentrations of Ti, V, and Zr, carbide crystals are only detectable after annealing. Structural information of the X-ray amorphous samples is gained by applying X-ray absorption fine structure spectroscopy (EXAFS) which probes the local atomic environment of the metal.
The amorphous carbon matrix is studied by C 1s NEXAFS and Raman spectroscopy. The results show that metal doping affects the structural properties of the carbon matrix: The addition of even small amounts of metal and annealing to low temperatures (700-900 K) is reflected in the Raman spectra (D/G ratio, G peak position). For higher annealing temperatures (1300 K) the D/G ratio of all doped films is comparable to an undoped film.
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