Development of Functionally Graded Tungsten/EUROFER Coating System for First Wall Application

Reduced activation Ferritic/Martensitic (RAFM) steels, e.g. EUROFER are to be used as structural material for the First Wall (FW) of future fusion power plants. The interaction between plasma and FW, especially physical sputtering will limit the FW lifetime under normal operation. Therefore tungsten coating is selected to protect the FW due to its very low sputtering yield and low activation. However, the mismatch in thermo-physical properties between tungsten and EUROFER can lead to large residual thermal stresses and even failure. To overcome the issue of erosion a protective tungsten coating with a functionally graded (FG) tungsten/EUROFER layer (FG tungsten/EUROFER coating system) on EUROFER substrate will be developed and optimized.

Non-linear finite element simulations are performed to predict optimal parameters of the coating system. Thereby the potential of the FG-layer in reducing inelastic strains and improving lifetime is demonstrated, and the investigated thickness of FG-layer is suggested. Based on the simulation results samples are fabricated by vacuum plasma spraying (VPS) with three different thicknesses of FG-layer. The microstructural observations revealed that the coating system has fine gradation and variable thickness as designed, low porosity, as well as a sound interface. Berkovich and Vickers hardness identify basic properties of those layers.