Deposition and qualification of tungsten coatings produced by plasma deposition in WF6 precursor gas

A strong R and D programme is at present ongoing to qualify tungsten as the plasma-facing material in ITER and possible future devices. Since the use of solid W tiles poses a number of problems, mainly with respect to increased forces, weight and costs, W coatings on graphite wall tiles are often used instead. These coatings are deposited in a special deposition apparatus, while in situ plasma-assisted techniques are widely used for low-Z wall coatings such as, for example, in carbonization, boronization or siliconization. To develop a similar procedure for the in situ coating of plasma-facing materials with tungsten, a deposition technique based on a mixture of 95% H2 and 5% WF6 was analysed. Tungsten coatings up to 0.5 μm have been deposited in laboratory experiments and the coatings were characterized by a number of methods. The W coatings deposited on fine-grain graphite have a dense structure with a rough surface and are free of fluorine. The coatings show excellent high-heat-flux resistance with no failures in transient heat flux tests up to 600-700 MW m-2 for 1 ms.