Development of W-coating with functionally graded W/EUROFER-layers for protection of First-Wall materials

To protect First-Wall components, made of reduced activation ferritic martensitic steel, against the plasma of future fusion reactors, tungsten coatings are a feasible option. The difference in coefficient of thermal expansion between the coating and the steel substrate can be compensated using functionally graded material layers. Such layers were successfully produced by vacuum plasma spraying. This technique reduces, however, the hardness of the substrate surface near zone. Modified spraying parameters moderate the hardness loss. The parameters may, though, affect also the layer bonding toughness which is evaluated in this work by four point bending tests. Furthermore, the layers behavior on First-Wall Mock‐ups and under different thermal loads is investigated by finite element simulations. The measurement of the layer adhesion indicates that the layer adhesion decreases only for modified spraying parameters that do not reduce the substrate hardness. It follows also from the toughness calculation that without layer residual stresses the toughness values depend on coating thickness. In regard to the Mock‐up behavior the simulations show that intermediate steps are necessary during heating and cooling to prevent artificial stresses and inelastic deformation. It is, however, not possible to avoid stresses and inelastic deformation completely as they originate from the residual stresses.