22D13
Development of High-Grade VPS-Tungsten Coated on Plasma-Facing
Component with a High Heat Load Characteristic
高い耐熱負荷特性を有するVPS-タングステン被覆プラズマ対向機器の開発
Tomonori TOKUNAGA1, Hideo WATANABE2, Naoaki YOSHIDA2,Takuya NAGASAKA3, Ryuta KASADA4, et al.
徳永知倫1, 渡辺英雄2, 吉田直亮2, 長坂琢也3, 笠田竜太4,李泳柱4, 木村晃彦4, 時谷政行3, 光原昌寿1, 檜木達也4, 中島英治1,増崎貴3, 高畠剛5, 黒木信義5, 江里幸一郎6, 鈴木哲6, 秋場真人6
1

Interdisciplinary Graduate School of Engineering Sciences, Kyushu University6-1, Kasuga-koen, Kasuga-city, Fukuoka 816-8580, JAPAN2Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan
3National Institute for Fusion Science, Gifu, Japan
4
Institute of Advanced Energy, Kyoto University, Kyoto, Japan
5Tocalo Co. Ltd, Hyogo, Japan
6
Japan Atomic Energy Agency, Ibaraki, Japan.
1
九州大学総合理工学府(〒816-8580 福岡県春日市春日公園6-1)、2九州大学応用力学研究所、
3
核融合科学研究所、4京都大学エネルギー理工学研究所、5トーカロ株式会社、6日本原子力研究開発機構
To endure very high loads of heat, neutrons, and also plasma particles, it is necessary to develop the high-grade W
coating method by vacuum plasma-spraying-technique (VPS). The forming crack in the boundary of W grain stuck atthe time of plasma spraying. In order to control inner layers deviation, it is effective to form the columnar grain
structure which grows long over W particles epitaxially. In this paper, influence of substrate temperature and size of W
powder on the various properties, such as thermal conductivity, density, microscopic structure, of the coated W were
examined to develop high-grade W coating on F82H.

1. Introduction
Plasma facing surfaces of nuclear fusion
reactors must endure very high loads of heat,neutrons, and also plasma particles [1]. Theadvantage of W is the high melting point, lowtritium inventory and low erosion rate underplasma-loading. Coating with W is a promising
method to protect the surfaces against these loads.
Vacuum plasma-spraying (VPS) technique would
be the suitable method for coating of the plasma
facing surfaces because it is rather easy to coatthickly on large area. It has been confirmed that
the VPS coated W layer (VPS-W) has a good
physical properties compared with the atmosphereic plasma spraying (APS) W layer (APS-W)[2].
If we would want to create the VPS-W layer on
the first wall materials such as a reduced activation ferritic/martensitic steel (F82H), surface
temperature of the F82H should be kept below
1040 K during the coating process because its
microstructure is changed above this temperature.
It was reported that columnar-like grain
structure of the W should be created to avoid theintergranular fracture of the W layer. In this paper,
influence of the columnar-like grain structure forthe thermal conductivity was studied, and then,the requirement for getting the highly controlledcolumnar-like W grain was discussed.
2. Experiment Process
2.1 VPS Coating productive method
The F82H substrate was installed in thedecompressed chamber, and then, molten W powders (18～37 m) were sprayed up to the thicknessof about 1 mm by using the scanning plasma spraying gun which can move back and forth repeatedly.
During the coating phase, the surface of the F82H
substrate was heated up to around 873 K due to thethermal conduction from the molten W powders.
2.2 Coatings Assessment Procedure
In order to know the details of the microstructure,
cross-sectional specimens were observed bySEM/EBSD, and very thin cross-sectional TEM specimens were picked up by using FIB. Re-solidified
grains and columnar grains were defined more than
20 m2 and the aspect ratio > 2.5, respectively.
Then, the area ratio of columnar grains and re-solidified grains was analyzed by Orientation Imaging
Microscopy (OIM), and each coating structure wascompared.
2.3 Quality of Coating Improvement
The effects of various conditions, such as grain
size of W powder, spraying distance, and substratetemperature ware examined. In order to acquire theoptimal coating condition, the coating layers wareformed on various coating conditions, and thequality of coating improving effect was examinedfrom rate of the columnar grain.
3. Results and Discussions
3.4 Substrate Temperature and W Powder Grain
Size
Influence of substrate temperature and size of W
powder were examined with several substratetemperature and two type of W powder (18 m, 37m), respectively. As substrate temperature became
high, microcrack decreased and the rate of the columnar grain became higher. Moreover, comparedwith the coating which made of 18 m, the ratio ofcolumnar grain of W layer which made of 37mincreased to two times, because liquid state of large
particle could be kept long due to large heat
capacity. Fig.1 shows that correlation between
columnar grain rate, the density and the thermaldiffusivity. It is observed that the density and thethermal diffusivity are highly sensitive to changes
in ratio of columnar grain. When the ratio of acolumnar grain was 40% or less, density andthermal diffusivity increased in proportion to therate of a columnar grain increasing. It is consideredthat the fraction of a columnar grain have a major
effect on inhibition of microcrack formation. On theother hand, when the ratio of columnar grain was
upward of 40%, density and thermal diffusivity are
not sensitive to changes in ratio of columnar grain.
It is considered that the microcrack decreased
enough according to the increase effect in acolumnar grain, but a constant amount of poresremained. These results indicate that if the fractionof the columnar grains is more than 40%,
micro-cracking can be suppressed well.
3.6 Spray Coating Improvement with 37mparticle
W layers were formed on some conditions which
kept substrate temperature about 873K to compare
influence of columnar grain. The cross-sectional
micrograph of Sample1 and Sample2 is shown inFig.2. Condition of Sample1 was basal condition.
And, condition of Sample2 which was coated withW powder that cut off under 25 m from anaverage size of 37 m, can get the highest ratio ofcolumnar grain. Ratio of columnar grain ofSample2 was increase from 33.6% to 45.5%. On theother hand, the ratio of re-solidified grain wasdecrease from 18.25 % to 13.5 %. Coating
conditions are about 873 K during VPS coating, andcontribute to increasing of ratio of columnar grain.
Especially, when particle-size-distribution of W
powder is narrowed, W powder poured into theplasma jet can be kept molten state more uniformly.
4. Summary
Micro-cracks in the direction parallel to the W
layer causes heat conductivity decrease. When theratio of columnar grain up to 40%, increase ofcolumnar grain have a major effect on inhibition ofmicrocrack formation. This indicates that one
should reduce un-melted/re-solidified particles toimprove adhesion. Rather large grain size (37m inaverage) is also effective for increasing of large
columnar grains due to their high heat capacity.
Moreover, on the coating conditions of Sample2,
the ratio of the columnar grain was able to obtain
more than 40% also with the substrate temperature
873K.

Fig.1: Correlation between ratio of columnar grain,
the density and the thermal diffusivity

Fig2: Cross-sectional SEM/EBSD micrograph of VPS-W(Sample1) basal coating condition with 37 m(Sample2) W powder which cut off under 25 m
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