Development of novel castellated functionally graded W/EUROFER coatings for fusion application

• Reduction of residual stress by castellation of W/EUROFER coatings shown by FEM. • Optimization of depth of castellation. • Castellation experiments using optimized notch depth show a reduction in bending. • In thermal fatigue tests of castellated coating no crack observed for at least 5000 thermal cycles. W/EUROFER functionally graded material (FGM) coatings have application in the First Wall of fusion plants as a shielding layer. In this work, a novel castellation of W/EUROFER coatings has been developed. Castellation is defined as cuts or notches created in material to reduce the thermal stress. Finite element simulations of castellated coatings were performed to evaluate the advantage of the castellation. Simulations suggested a reduction in the bending of coated components as well as a reduction of residual stress. Afterwards, simulations were performed to find out the optimized depth of castellation. Simulations of castellated and non-castellated coated sections under a heat flux cycle showed a reduction in bending of the coated section with castellation as compared to the non-castellated coating section. Castellation experiments were executed on coated samples using the optimized depth. After castellation, recovery in the original coating-induced deflection of the samples indicated the feasibility of the castellation approach. Additionally, thermal fatigue tests were performed on castellated coated samples to determine the thermal load stability of the notch. Neither cracks at the notch tip nor the interface are observed for at least 5000 cycles of thermal load typical for the envisaged fusion application.