Investigation on protective tungsten-based coatings produced by HiPIMS for liquid metal divertor concepts for EU-DEMO

The more demanding Plasma Wall Interactions expected in EU-DEMO compared to ITER require examining heat exhaust alternatives to the baseline W divertor. The use of liquid metals as Plasma Facing Materials has been considered due to their low sensitivity to neutron damage and improved heat removal, especially during transients [1]. The design proposed by ENEA consists of a porous structure to support liquid tin (Sn) [2]. However, the corrosive action of this metal requires protective coatings on structural components, especially those made of CuCrZr alloy. In this respect, W-based films show promising thermo-chemical properties. Concerning the coating fabrication method, Physical Vapor Deposition (PVD) techniques offer superior control over nanoscale properties [3]. Specifically, High Power Impulse Magnetron Sputtering (HiPIMS) applies high amplitude voltage pulses at a low duty cycle to achieve sputtering. The increased peak power improves the plasma density above the cathode and the degree of ionization of sputtered species [4]. Thus, the energy of such particles can be tuned by applying a bias voltage to the substrate. It is therefore possible to obtain smooth and dense coatings and control their properties. In addition, HiPIMS provides conformal coverage of complex substrates, an important requirement for the deposition on structural components. Here we report on the HiPIMS deposition of compact W-based coatings on fusion-relevant CuCrZr substrates as protective barriers against liquid Sn corrosion. HiPIMS pulses (100 µs-long, duty cycle of 1.75%) were applied to the tungsten cathode at a fixed gas pressure of 0.5 Pa. We employed argon as working gas to produce W coatings, while a reactive deposition in a mixture of argon and nitrogen allowed to obtain WNx films of varying stoichiometry. We explored different deposition conditions to tune the coatings properties. Growth, morphology, and nanostructure of the films were characterised. Lastly, we present preliminary results about liquid Sn exposure of coated copper substrates in divertor-like conditions.