Development of functionally graded tungsten/EUROFER coating systems
Reduced activation ferritic/martensitic (RAFM) steels, e.g. EUROFER are primary structural material candidates for the First Wall of DEMO, a demonstration reactor towards future fusion power plants. For protecting first wall material from physical and chemical sputtering of plasma, tungsten coating is considered as the candidate material for first wall armor in the current helium cooled divertor concept, primarily due to its high melting point, low sputtering yield, high thermal conductivity and low activation. However, residual stresses, generated from the large mismatch of thermo-physical properties between tungsten coating and steel substrate, may lead to the failure of the coating system. The application of functionally graded material (FGM) is considered to be a good solution for the thermal mismatch problem. In the present study, erosion protective tungsten coatings with W/EUROFER FG-layer (FG tungsten/EUROFER coating system) on EUROFER substrates are investigated performing finite element (FE) simulations, which aiming at the determination of the thickness for the FG-layer and assessment of creep damage as well as allowable cyclic lifetime during the operating phase.
A field variable f ranging from 0 to 1 is used to indicate gradation level and the linear gradation is investigated here. The model is loaded by a homogeneous temperature field varying over time. In the simulations of the cooling down phase, all the materials are considered to behave as isotropic, linear elastic and perfectly plastic. Norton power law of creep is taken into account for the simulations of the operation phase, in which the temperature alternates between 20°C and 600°C with a dwell time 24h at 600°C. The von Mises stress is limited by the yield stress of tungsten, and the equivalent plastic strain in tungsten is decreasing with increasing FG-layer thickness during the cooling down phase. In contrast, the equivalent plastic strain is not observed in EUROFER. The maximum equivalent creep strain of all investigated thicknesses increases over thermal cycles, but the thicker the FGM, the smaller the creep strain. Particularly for FG-layers thicker than 0.7mm a shakedown is observed. In addition it becomes almost constant over the number of cycles for a 1.0mm thick FG-layer and it is even negligible for a 1.2mm and 1.5mm thick FG-layer.
Based on these results and with view to vacuum plasma spraying (VPS) selected for fabrication of the FG layer, 0.7mm is proposed as maximum thickness for the FG-layer in the first planned fabrication experiments since with respect to the adhesion between the vacuum plasma sprayed layers and between them and the substrate the thinner the better. Three and five layers with linear variation of the compositional ratio are introduced as representatives of stepwise linear gradation of FG-layer. The as-received surface morphology and cross-sectional structures of both types are shown in Fig.2. The surfaces have a smooth structure which indicates that particles melted well during the spraying process and re-solidified forming a pancake-like structure. In addition, few spherical particles of about 10µm exist on the surface, which are un-melted tungsten particles. The cross-sectional SEM micrographs show that stepwise linear gradation is formed successfully between the tungsten coating and the EUROFER substrate and that there are no obvious interfaces among layers of the FG-layer, especially for coating with five layers which has a finer gradation. No delamination is observed for both coatings. Nominal and true thicknesses of FG-layer are shown in Fig.3, the comparable thickness are obtained successfully as designed. Produced FG tungsten/EUROFER coating systems have proper nominal microstructure with the pancake-like features, columnar grains as well as low porosity and sound seamless interfaces with good toughness. Nano and micro hardness measurements also reflect successful functional gradation and global homogeneity. The assessment of the coating quality as FW application are performed, in particular, thermal load resistance including ELMs-like thermal shock tests and thermal fatigue tests, which show good thermal load resistance comparing to bulk tungsten. FG W/EUROFER coating system with total thickness over 2mm will be fabricated in the further based on the experiments up to now.
D.D. Qu, W.W. Basuki, J. Aktaa, Numerical Assessment of Functionally Graded Tungsten/Steel Coating System for First Wall Applications, Fusion Engineering and Design 98–99 (2015) 1389–1393
D.D. Qu, W.W. Basuki, J. Gibmeier, R. Vaßen, J. Aktaa, Development of functionally graded Tungsten/EUROFER coating system for first wall application, Fusion Science and Technology, 68 (2015): 578-581