CHARACTERISATION OF PLASMA-SPRAYED BORON CARBIDE AND TUNGSTEN LAYERS FOR FUSION APPLICATIONS

In nuclear fusion devices plasma-facing materials (PFM) have to sustain extreme environments. The first wall of fusion experiments or future power plants is loaded with thermal radiation and high-energy particles, eroding the wall material. In current experiments low-Z-materials such as graphite and carbon composites are preferred PFM, but basic disadvantages are the high erosion rate and accumulation of hydrogen. For the new WENDELSTEIN 7-X fusion experiment (W7-X), now under construction at Greifswald (Germany), 70 m2 of water-cooled wall protection will be coated with B4C layers by vacuum plasma spraying (VPS). VPS provides the required high purity of coating materials and is an effective industrial method. The thermal expansion mismatch of the ceramic B4C coating and stainless-steel (SS) substrate and, the high processing temperature require an interlayer to reduce residual stress. An interlayer made of SS mixed with B4C has been successfully developed. An alternative high-Z PFM with very low erosion is tungsten, which shows good plasma-physical and thermo-mechanical behaviour in fusion experiments. The VPS-B4C technology developed is transferable to coatings of W on SS. Tungsten will possibly be employed as a protective material for the first wall in the next fusion experiment, ITER.