Gas-driven hydrogen permeation through tungsten-coated graphite

Hydrogen gas-driven permeation through graphite coated with different types of tungsten coatings with thicknesses up to 200 μm has been investigated. The substrate material was fine-grain graphite R5710 and R6710, which properties with respect to hydrogen transport are well known . Magnetron-sputtered W coatings of thicknesses 1 and 3 μm and two coatings of ASDEX Upgrade were investigated: A 3 μm thick layer of tungsten deposited by physical vapor deposition (PVD-W) and 200 μm thick layers of tungsten deposited by plasma-spaying in vacuum (VPS-W). The gas-driven permeation was investigated at a pressure gradient of 10-2-150 Pa. The gas-driven permeation occurs through the carbon base-materials by hydrogen molecular gas flow through the internal porosity network rather than hydrogen atom diffusion through the graphite lattice. It was found that W coatings with thicknesses up to 3 μm are transparent for hydrogen gas penetration and do not influence the permeability of coated fine-grain graphite, because the open porosity system of graphite remains open. Even a 200 μm thick layer of VPS-W has an open system of connected pores, which connects the front and rear surfaces of the deposited layer.