Tailoring the structure of RF-ICP tungsten coatings

Abstract Nuclear fusion is considered to be one of the future sources of sustainable energy. Prior to its widespread application, several critical issues need to be solved though. One of them is the choice of materials for the fusion reactors, in particular for plasma facing components (PFCs). At the moment, one group of candidate materials for this task are tungsten-based materials. Despite having many advantages (such as the highest melting point of all metals, high density), tungsten also possesses several drawbacks (e.g., poor machinability and weldability), making production of large-scale components particularly difficult. One of the considered production routes that could potentially overcome these problems is plasma spraying. Unfortunately, the conventional atmospheric plasma torches have critical limitations toward this task, resulting from the susceptibility of tungsten to oxidation at elevated temperatures. In this study, a radio-frequency inductively-coupled plasma (RF-ICP) torch was used. This technology enables spraying under controlled atmosphere, which helps to prevent detrimental oxidation of the metal. To examine the process window of tungsten spraying, twelve coatings were deposited on graphite substrates under two torch powers of 12 kW and 15 kW and using two feedrates. Furthermore, three different gases were used for feeding of the W powder: pure argon, a mixture of Ar/7.5%H2, and pure helium. All combinations led to production of thick coatings. It was shown that spraying using pure argon carrier gas was sensitive to the used power and feedrate, resulting in dense coatings only for a combination of the higher power and the lower feedrate. Using the Ar/H2 mixture led to an improved melting of the individual W particles where the densest microstructures were obtained for the higher torch power level only. Helium carrier gas resulted in the best coating quality, where the powder particles were properly melted in all power/feedrate combinations.