Stability of beryllium-tungsten coatings under annealing up to 1273 K

Abstract

Tungsten and beryllium were foreseen to be the plasma facing materials (PFMs) for the first wall and divertor in ITER. Particles eroded from PFMs surfaces will be transported by the plasma and co-deposited in different locations of the vessel walls. Elemental mixing in the exposed surfaces enables the formation of distinct phases that may influence the stability of PFMs in a wide temperature range. In the present work, Be and W coatings were deposited on W and Be plates, respectively, and annealed in vacuum in the 673–1073 K temperature range. Additionally, Be75W25 coatings deposited on Be plates were annealed in the 873–1273 K range. The Be-W chemical reactivity, phase formation and coatings stability were followed by ion beam analysis, X-ray diffraction and electron microscopy. The reactivity is too weak up to 973 K. It becomes fast at 1073 K with a competitive formation of Be2W and Be12W. At 1173 K, a strong Be22W formation occurs, and Be22W is the only stable phase at 1273 K. The growth of BeO occurs at the coatings surface and increases with temperature. Despite the presence of distinct phase structures, the experiment evidences that the growth of tungsten beryllides enhances the mechanical stability of the coatings from 973 K to 1073 K. In opposition, Be22W formation at 1173 K and 1273 K induced fracture and split off from the Be plates.