Evaluation of tensile and elastic properties of W/Cu cold-spray coatings for application to the FGM DEMO divertor concept

The ITER based conceptual design of EU DEMO divertor envisages tungsten monoblocks as plasma-facing material joined to a CuCrZr cooling tube with a thick Cu-OHFC interlayer. Since divertor configuration will define the power exhaust capabilities of the reactor, additional concepts have been developed within the EUROfusion WPDIV project. In this regard, FGM (Functionally Graded Material) concept is one of the proposed alternatives. The primary motivation of this concept is to replace the thick copper interlayer with a W/Cu compositionally graded thin film for armour-to-pipe joining with the aim of minimizing the stresses at the interfaces while assuring the structural integrity of the component. This FGM interlayer will be composed of stacked elementary layers manufactured by Cold-Spray with the following four target compositions: 100 vol.%Cu, 25 vol.%W + 75 vol.%Cu, 50 vol.%W + 50 vol.%Cu, 75 vol.%W + 25 vol.%Cu. The elastic modulus of these coatings is an especially important materials parameter since it has a strong influence on both FGM fracture and stress concentration at the interface. Therefore, it is of fundamental importance to determine the elastic modulus of the asdeposited coatings. In this work, the mechanical and microstructural characterisation of each elementary layer is presented, and different methods for the evaluation of their elastic modulus have been evaluated, and their agreement has been assessed. Tensile tests were conducted on the free-standing coatings to understand the strengthening and deformation mechanisms of each composition. The ultimate tensile strength increased proportionally with the W content, showing 50% higher tensile strength on the addition of less than 40 vol.%W since crack arresting tungsten particles were observed as strengthening mechanism.