Hydrogen diffusive transport parameters in W coating for fusion applications

Combined Magnetron Sputtering and Ion Implantation (CMSII) technology was used for W coating of carbon based materials (Carbon Fibre Composite – CFC and fine grain graphite) for the first wall in fusion devices. The coating thickness was 10–15 μm or 20–25 μm depending on the position of the tile at the wall. While such coatings successfully passed the demanding thermo-mechanical tests, not much is known about its hydrogen interaction. The latter is particularly important for the assessment of tritium retention. Due to the low hydrogen diffusivity and very small volume of W in the coated layer, the gaseous hydrogen permeation measurement at 400 °C was selected for the experimental technique, where increasing & decreasing transient and steady state permeation flux was monitored. Problems that could arise with the CFC membrane sealing were overcome by deposition of the identical W layer on the 0.5 mm Eurofer substrate. Two such membranes were investigated. Obtained hydrogen permeability in tungsten layer (∼10−13 mol H2/m s Pa0.5) is comparable to the upper range of published data. Measured diffusivity (∼10−14 m2/s) is several orders of magnitude lower compared to the average of published data for tungsten, while the measured solubility (∼1 mol H2/m3 Pa0.5) is several orders of magnitude higher. The explanation is given in terms of hydrogen trapping that has significant impact on hydrogen migration.