Characterisation and thermal loading of low-Z coatings for the first wall of W7-X

Low-Z coatings with a thickness up to 500 μm are being developed as plasma facing material on stainless steel first wall panels for the W7-X stellarator under construction at Greifswald, Germany. The materials under investigation are boron carbide (B4C) and a silicon–boron–carbide (SIBOR, manufactured from Plansee A.G., Austria), both applied by vacuum plasma spraying. Thermal loading was performed in the First Wall Test Facility (FIWATKA) at the Research Centre Karlsruhe. In particular, stepwise increasing heat loads from 50 to 500 kW/m2 and cyclic heat loads up to 1000 cycles of 3 min duration were applied to characterize the thermo-mechanical behaviour of the different coatings. Additionally, 2D and 3D finite element modelling is used to support the experiments and to predict the failure threshold of the coatings, which is also verified experimentally.