THIN NITRIDE LAYERS AS PERMEATION BARRIERS

Permeation barriers represent one of the crucial fields in materials development for thermonuclear fusion. Primary objective of the barriers is to suppress the permeation of hydrogen isotopes (mainly tritium) from future thermonuclear fusion facilities. Secondary objective is to reduce hydrogen retention in structural materials, potentially also improving their corrosion resistance. Expected reactor conditions put high demands on the material, as well as on the final barrier quality. Key properties are tritium permeation reduction, absence of defects (especially cracks), high-temperature stability and corrosion resistance, and compatibility with structural materials (mostly ferritic-martensitic steels). Regarding industrial scale production, ability of the deposition method to coat large complex surfaces is desirable. Thin nitride layers, identified as promising permeation barriers, were prepared by diffusion-based nitridation and physical vapour deposition (PVD) and characterized

Thick functionally-graded W-316L composite coatings for nuclear fusion applications

Nuclear fusion is a potential pathway to finding a sustainable, carbon-free energy source. Some critical components of the fusion reactors are planned to be coated by tungsten. For the task, thermal spraying in vacuum or protective atmosphere can be employed, offering several advantages such as easy preparation of advanced feedstock for deposition of functionally graded composites. Such coatings could be a viable approach to avoid the thermal expansion coefficient mismatch between the W coating and the steel components. In this study, radio-frequency inductively-coupled plasma spray method was used to deposit W-steel composite coatings of three different W ratios, as well as a functionally graded coating consisting of the three composites and a pure W top coat. The coatings exhibited a high-quality microstructure, without intermetallic or oxide phases formation. Thermal diffusivity and conductivity of the coatings was measured at 100 °C and 600 °C, with the values falling into range between the bulk steel and plasma sprayed W. In conclusion, we have shown that the RF-ICP technology is suitable for preparation of tungsten-steel graded deposits and the outputs are now prepared for other testing and a following upscaling to the industry-relevant size

High Heat Flux Testing of Graded W-Steel Joining Concepts for the First Wall

The realization of the first wall (FW), which is composed of a protective tungsten (W) armor covering the structural steel material, is a critical challenge in the development of future fusion reactors. Due to the different coefficients of thermal expansion (CTE) of W and steel, the direct joining of them results in cyclic thermal stress at their bonding seam during the operation of the fusion reactor. To address this issue, this study benchmarks two joining concepts. The first concept uses an atmospheric plasma sprayed graded interlayer composed of W/steel composites with a varying content of W and steel to gradually change the CTE. The second concept uses a spark plasma sintered graded interlayer. Furthermore, in order to benchmark these concepts, a directly bonded W-steel reference joint as well as a W-steel joint featuring a vanadium interlayer were also tested. These joints were tested under steady-state high heat flux cyclic loading, starting from a heat flux of 1 MW/m2 up to 4.5 MW/m2, with stepwise increments of 0.5 MW/m2. At each heat flux level, 200 thermal cycles were performed. The joints featuring a sintered graded interlayer survived only until 1.5 MW/m2 of loading, while the joint featuring plasma sprayed graded interlayer and V interlayer survived until 3 MW/m2

Process maps for plasma spray. Part II: Deposition and properties

This is the second paper of a two part series based on an integrated study carried out at the State University of New York at Stony Brook and Sandia National Laboratories. The goal of the study is the fundamental understanding of the plasma-particle interaction, droplet/substrate interaction, deposit formation dynamics and microstructure development as well as the deposit property. The outcome is science-based relationships, which can be used to link processing to performance. Molybdenum splats and coatings produced at 3 plasma conditions and three substrate temperatures were characterized. It was found that there is a strong mechanical/thermal interaction between droplet and substrate, which builds up the coatings/substrate adhesion. Hardness, thermal conductivity, and modulus increase, while oxygen content and porosity decrease with increasing particle velocity. Increasing deposition temperature resulted in dramatic improvement in coating thermal conductivity and hardness as well as increase in coating oxygen content. Indentation reveals improved fracture resistance for the coatings prepared at higher deposition temperature. Residual stress was significantly affected by deposition temperature, although not significant by particle energy within the investigated parameter range. Coatings prepared at high deposition temperature with high-energy particles suffered considerably less damage in wear tests. Possible mechanisms behind these changes are discussed within the context of relational maps which are under development

High heat flux testing results of various W-FGM-steel joints

For a future commercial fusion reactor the First Wall of the breeding blanket will require a joint between the structural steel and the plasma facing material tungsten. However, the difference in the coefficient of thermal expansion (CTE) between them results in thermal stresses at their interface during operation and this could result in premature failure of the joint. A functionally graded material (FGM), as an interlayer between tungsten and steel could reduce these stresses. In this study two processes, atmospheric plasma spraying (APS) and spark plasma sintering (SPS), are utilized to manufacture four different kinds of W-FGM-steel stacks: the first two include FGMs prepared by APS; W-V-75W-50W-25W-steel and W-50W-25W-steel, the other two include FGMs made by SPS; W-75W-50W-25W-steel and W-50W-25W-steel. This investigates: i) the influence of an additional V-interlayer; ii) the comparison of APS- and SPS-FGMs; and iii) the influence of FGM composition and thickness. In all these W-FGM-steel stacks, with a surface area of 12 mm x 12 mm, the thickness of each FGM sublayer (75W, 50W, 25W) is about 0.5 mm, whereas the bulk-W and bulk-steel are 3 mm thick. A sample of direct diffusion bonded W-steel joint is used as a reference. A high heat flux benchmark test was performed to investigate and compare the potential of the different joining technologies. For this, the stacks were soldered on a copper cooling module and exposed to high stationary loads at the JUDITH-2 facility. At each power level of 1 MW/m2, 2 MW/m2, 3 MW/m2 and 4 MW/m2 the samples were subjected to a component screening cycle to determine the component quality followed by up to 200 on/off cycles (30/30 s). By monitoring the surface temperature using an IR camera, the cooling capabilities of each sample and any local overheating as indication of bond failure can be determined

Data publication: Radiation damage evolution in pure W and W-Cr-Hf alloy caused by 5MeV Au ions in a broad range of dpa

Positron annihilation lifetime spectroscopy data taking for ELBE proposal POS19101496 by Sandrina Fernandes, Rez, CZ. Role of open volume defects in irradiated structural materials for fusion applications. Measurements performed 16.3.2019 at the MePS facility