Coating of W fibres for use in W/W hybrids for fusion power plants and analysis of constitution and mechanical properties

C. Ziebert, S. Ulrich

Forschungszentrum Karlsruhe, Institut für Materialforschung I, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany

In the most promising concepts of future fusion power plants, plasma with a very high energy density is confined by magnetic fields which direct the plasma for exhausting of reaction products and further impurities to the thermo-mechanically most loaded part, called a divertor. The requirements, to be met by the materials of the plasma facing divertor components, suggest the selection of tungsten as refractory and structural material. Due to the brittleness of tungsten, its applicability as a structural material is limited to high temperatures (> 650°C). Solving this challenge, hybrid material systems have to be developed, examined, optimised and realised. One possible approach is a W-fibre /
W-powder composite which is expected to have enhanced toughness in comparison to pure tungsten and thus to be applicable at lower temperatures. The development and optimisation of the hybrid should be based on modelling, fabrication and characterisation at different levels. Of special importance is a surface engineering of the tungsten fibres by depositing a multifunctional PVD coating, which combines three functions: i) a diffusion barrier, ii) a protection that prevents for the oxidation of the fibre surface and iii) the maintenance of the separation of the two W phases in the W-fibre and the W-powder during the hot isostatic pressing in order to produce a W/W composite material. Through this approach the properties of the fibre-matrix interface, such as debonding fracture energy and frictional stress, shall be optimised so that controlled cracking and friction at this interface increase the energy dissipation over an extensive inelastic strain range and thus enhance the composite toughness. Therefore constitution, microstructure and mechanical properties of the PVD coated tungsten fibres have to be analysed by electron microscopy, diffraction and scattering methods, micro- and nanoindentation and hot-hardness testing. These activities should be accompanied and guided by multi-scale modelling of the fibre fabrication as well as damage modelling and simulation of the composite behaviour.