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iTSSe TSS A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 1 9 4 7 iTSSe TSS tritium. Combining these would result in the formation of heli- um nuclei carrying 3.5 MeV of energy (used to further heat the fuel) and free neutrons with energies of 14.1 MeV (used for en- ergy production and breeding of tritium) [2] . The plasma confined in the tokamaks could reach tem- peratures up to ∼ 10 8 K. This presents one of the major chal- lenges in the surface engineering of materials used to build the tokamak vessel. Despite the plasma being confined by strong magnetic fields preventing direct contact with the surface, the selection of materials for the so-called plasma-facing com- ponents (PFCs) is difficult. Currently, the most suitable ma- terials considered for the task are those based on tungsten. Tungsten and its alloys have many favorable properties, such as a high melting point (3695 K for pure W), high thermal stress resistance, high thermal conductivity, low erosion (i.e., a high energy threshold to suffer from sputtering), and low tri- tium retention. WORKING WITH TUNGSTEN Unfortunately, some of tungsten’s other properties such as high ductile-to-brittle transition temperature, poor weld- ability and machinability, and thermal expansion coefficient mismatch with materials used in tokamaks complicate the fabrication of PFCs from these materials. Tungsten is also sus- ceptible to oxidation above ∼ 770 K, promoting fast cracking of the material. At normal vessel operation, this should not present a threat as the entire system is vacuum tight. Howev- er, with safety in mind, rapid oxidation would present a major threat in case of an accident. The solution is self-passivation, i.e., inducing formation of a protective oxide scale on the surface of W or W-based alloys upon exposure to oxygen. This could be triggered by adding various oxide-forming elements. Currently, the most promising combination appears to be a solid solution of Cr in the W lattice [3] . That said, fabrication of pure W and W-Cr bulk alloys via traditional metallurgy routes is complicated. Aside from the economic aspects (e.g., high melting temperatures), the reac- tivity of bothmetals presents amajor problem. Given that, the research focus is shifting toward the potential of surface engi- neering methods and manufacturing techniques. This field is largely unexplored andwill require significant effort to achieve perfect armor coatings for the W-based components. Initial research results are now emerging: These include cold spray attempts and the first WandW-Cr coatings deposited using ra- dio-frequency inductively coupled plasma (RF-ICP) spraying at the Institute of Plasma Physics in Prague. RADIO-FREQUENCY INDUCTIVELY COUPLED PLASMA RFICP is a thermal spray coating deposition method that operates in a controlled atmosphere (inert gas, reactive gas, or vacuum), thereby effectively eliminating oxidation of the molten metal. The temperatures involved in the process are high enough to melt tungsten and ensure its proper deposi- tion [4] . Importantly, the technology is commercially available both at the laboratory and industrial scale. This enables re- searchusing lower powermachines and subsequent upscaling for target applications. The device uses the principle of electro- magnetic induction to ionize gas atoms as they pass through the center of an electromagnetic coil. Analogous to other thermal spray technologies, the gas is heated due to the ion- ization, enabling the introduction of solid powder particles, which are melted and sprayed onto a substrate (Fig. 2). This presents yet another potential advantage as the armor coat- ings could be produced directly onto PFC surfaces without the need for a second step such as joining or welding [5] . EXPERIMENTAL METHOD The experimental setup of W and W-Cr coating fabri- cation involved several routes of feedstock preparation. Up to six tungsten powders from various manufacturers were used during this stage, differing by average particle sizes (4 to 100 µm), purities (always >98.5% and mostly >99.95%), as well as particle morphologies and mechanical properties associated with different production routes [6] . A 30-µm Cr powder of 99.5% purity was purchased from US Research Nanomaterials Inc., Houston. Experiments initially involved spraying different pure W powders, partially serving as bench- marks. W-Cr powders were either mechanically mixed (blend- ed), co-milled to obtain an intermixed layered structure of the two constituents, or mechanically alloyed to obtain W-Cr solid solution. Initially, two different compositions were test- ed, W-10Cr and W-12Cr (wt%). Examples of the W and W-Cr (milled) powders are shown in Fig. 3. Deposition was carried out using TekSpray 15, a 15-kW laboratory device from Tekna, Canada. Samples were pro- duced under different spray conditions (9-15 kW torch pow- er, 60-100 kPa chamber pressure, and 46-70 mm stand-off distance) with a wide range of coating thicknesses. Argon and hydrogen were used as the main plasma forming gases. Preheated graphite discs were used as substrates. However, a Fig. 2 — Deposition of W-based powders on 60-mm-diameter graphite substrates via radio-frequency inductively coupled plasma (RF-ICP). FEATURE 7