W-Si-Cr Compounds as Plasma-Facing Material for Fusion Reactors

F. Koch (Sp), H. Bolt, Max-Planck-Institut für Plasmaphysik, Garching (Germany) 
For a future nuclear fusion power reactor tungsten is the main candidate material for the plasma-facing protection due to the low sputter erosion under bombardment by energetic D, T and He ions.
A potential problem may be the formation of volatile WO3 compounds under accidental conditions. The total loss of coolant in an He-cooled reactor could cause a temperature rise to 1100 °C after 10 to 30 days due to the afterheat of the nuclear reactions induced by the high energetic neutron radiation. In such a situation an additional ingress of air into the reactor vessel would lead to the strong exothermic formation and subsequent evaporation of radioactivated WO3-clusters.

A substoichiometric W-Si compound was produced by sputter deposition on inert substrates. Oxidation experiments with a thermobalance under synthetic air showed above 600°C the formation of a protective SiO2 film at the surface which seals the reactive W from further air contact and oxidation. Below 600°C the formation of the protective film is inhibited and the creation of WO3 takes place.

To protect the system against oxidation below 600°C the W-Si compound was doped also with low temperature oxide formers like chromium. Films produced by sputter deposition and bulk materials were investigated. It was found that the oxidation of W-Si is additionally reduced by the addition of Cr in the temperature range from 400 °C to 1000 °C. Compared to pure W the oxidation rate of the W-Si-Cr compound at 1000 °C was decreased by a factor of 104.

The high oxidation resistance of this compound potentially opens applications outside the fusion energy area where tungsten or tungsten silicides are not used because of their oxidation sensitivity.