J. Plasma Fusion Res. SERIES, Vol.3 (2000) 260-264

Thermal Response and Material Degradation of TungstenCoated Carbon Divertor Mock-ups by High Heat Flux
TOKUNAGA Kazutoshi*, YOSHIDA Naoaki, KUBOTA Yusuker, NODA Nobuaki',
IMAMURA Yoshio2, KURUMADA AKira2, OKU Tatsuo2, SoGABE Toshiaki3,
SUZUKI Tatsushia, KATO Toshiyukis and pI-6CUl Laurenz6
Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan

lNational Institutefor Fusion Science, Toki, Gifu 509-5292, Japan
2Faculty of engineering, Ibaraki
University, Hitachi, Ibaraki 316-8511, Japan
3Toyo Tanso Co., LTD., Mitoyo-gun, Kagawa 769-1612,
Japan
aKawasaki Heavy Industries, Ltd.,Kohtoh-ku, Totcyo 136-8588, JapansNippon Plansee K.K., Chiyoda-ku, Tokyo102-0083, Japan
6Plansee Aktiengesellschaft, A-6600

Reutte, Austria(Received: 18 January 2000 / Accepted: 12 May 2000)

Abstract
CX-2O02U and IG-430U coated by VPS-W were developed as a lighrweight high-Z plasma-facing
material. After brazing them on OFHC blocks using a titanium foil and silver based materials, their
thermal response and thermal fatigue properties were examined. The targets were actively cooled understeady state high heat flux. Heat load resistance of the VPS-W coated CX-2002U/OFHC was much betrer
than that of the VPS-W coated IG-430U/OFHC due to the high thermal conductivity of CX-2002U (350WmK). Neither cracks nor exfoliation were observed on the W surface and the braze interface even after
160 cycles of heat load for 20 s at l0 MWm2 in the case of Ti brazing. This result indicates that the Tibrazing is a promising alternative to Ag-brazing forjoining carbon to Cu and it is a potential candidate

for a high heat resistance iumor material on plasma facing components.

Keywords:
W-coated carbon, high heat flux, active cooling

1. Introduction

properties.

Although the utilization of low Z materials like

Disadvantages of tungsten as a plasma facing
material are related to its heavy weight and poor

carbon for plasma facing components has enabled an
improvement in plasma confinement, high erosion rates

workablity. One way to circumvent these disadvantages
is to deposit tungsten on carbon materials that haveshown good heat shock resistance in the present plasma
confinement devices. Tungsten coatings on graphite, bymeans of plasma spray (PS) or physical vapor

at elevated temperatures create serious problem.
Degradation of thermal conductivity by neutron damage
and high tritium retention will be serious issues in next
generation of D-T fusion machines [l]. Tungsten seems

to be a promising candidate material for plasma facing

deposition (PVD), have been produced and theirperformance under high heat flux loading has been

components in steady state plasma discharge devices
because of its low sputtering yield and good thermal*Corresponding author's e-4ail: tokunaga@ riam.kyushu-u.ac jp260@2000 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchTokunaga K. et al., Thermal Response and Material Degradation of Tungsten-Coated Carbon Divertor Mock-ups by High Heat Flux

examined [2,3]. From the viewpoints of thermal
conductivity and mechanical strength, it seems thatcarbon/carbon fiber composites (CFC) are preferable asa substrate material for high heat flux loading. Thicktungsten coatings on CFC and isotropic fine grainedgraphite have successfully been produced by vacuum
plasma splay (VPS) technique and their good thermal
and adhesion properties has been confirmed by highheat flux tests [4,5].

In the present work, CFC tile and isotropic finegrain graphite tiles coated with VPS-W were jointed ona OFHC block by using either silver-free titanium andsilver based brazings. In most cases, silver brazing

technique has been used for carbon-Cu system.
Development of Ag-free joints is needed because oftransmutation of silver into cadmium during neutronc)G20mulGrt3oU

irradiation. This work was focused on testing of thermal
response and thermal fatigue lifetime tests under steadystate high heat flux to evaluate the silver-free brazings.

3mmtc
2. Experimental

lD:7mm,OD:1Omm

Tiles (20 mm x 20 mm x l0 mm) of carbon/carbon
composite CX-20O2U and isotropic fine grain graphite
IG-430U [6] made by Toyo Tanso Co. were coated withno hnist tspe

tungsten by VPS technique. They are denoted as WCX2O02U and WIG-430U, respectively. The carbon or thegraphite substrates had received PVD multilayer

Fig. 1 Photograph and cross-sectional view of VPS-W
coated CX-2002U/OFHC mock-up.

diffusion barrier layers of rhenium and tungsten prior tothe coating in order to inhibit the formation of brittletungsten carbide [5]. A thickness of the diffusion barrier
layer was about 28 pm. Heat treatments were performed
to stabilize the microstructure of the sample. Thethickness of the VPS tungsten layer was 0.5 mm and 1.0mm and its density was 92.57o of the theoretical value.
Mock-ups were made by brazing the tiles on
OFHC block with a cooling tube. Two methods wereapplied: a) Ti brazing by inserting a Ti foil of 0.05 mmthick in between at 1000"C in Ar atmosphere. b) silverbase brazing at 750/850'C. The mock-ups are denoted
here as W|CX2O02U/OFHC and W/IG-430U/OFHC.
Fig. I shows a mock-up and a cross-section of the W
CX-ZOOZU|OFHC. Mock-ups of CX-2002U/OFHC andIG-430U/OFHC without W coating were also prepared
to compare with the W coated specimens. Heat load
experiments were performed using the Active Cooling
Teststand (ACT) of National Institute for Fusion
Science (NIFS) [7]. Uniform electron beam of 30 keV
was used to irradiate the tungsten surface. Ramp-up,
plateau and ramp-down phases of the inadiation were
20 s,2O s and I s, respectively. Heat flux was from 2 to

10 MWm2 and it was evaluated using a net electric
current, which was measured by applying a bias voltageof +90V to the test sample in order to suppress
secondary electrons induced by the irradiation. Ejectionof thermal electrons from the heated surface was also
suppressed by the bias voltage. Thermal response tests
were performed by stepwise increase of the heat flux.

Thermal fatigue tests were also carried out up to 160

cycles at a heat flux of l0 MW/m2. The surface
temperature of the central part about 5 mm in diameter
was measured with an optical pyrometer (300-3000'C).
Temperatures of the upper side (Tl) and the lower side(T2) of the brazing interface were measured by inserting
thermocouples in deep holes at those positions. The
thermocouples used was K and sheath type. The
diameter of the thermocouples and the sheath was 0.18mm and I mm. The tip of the sheath of thethermocouples was mechanically attached. The flow
rate, pressure and inlet temperature of the cooling water
were 14.4 m/s, 0.5 MPa and 25oC, respectively. Afterthe heat load experiments, surface morphology was261

Tokunaga K. et al., Thermal Response and Material Degradation of Tungsten-Coated Carbon Divertor Mock-ups by High Heat Flux

because brazing temperature of the silver was lower
than that of Ti brazing.
Figure 2 shows typical time evolutions of theobserved by scanning electron microscopy.

3. Results and Discussion

electric current through WICX-2OO2U/OFHC mockup(Ti brazing) (a) and temperatures measured at theIn the case of WIG-430U/OFHC by Ti brazing,
micro cracks were formed at corners of the IG-430U
part during the brazing process, but not in IG-430U/
OFHC. This means that the cracks were formed bysurface, upper (Tl) and lower (T2) parts of the brazing
interface (b) under heat loading of 10.1 MWm2. For
this 0.5 mm coating, the temperature profiles closely
followed the change of the beam current changes.
Figure 3 shows the heat flux dependence ofplateau
temperatures measured at the surface, Tl and T2 for Wl

residual stress introduced into the VPS-coating layer
during and after the brazing. No damage was observedin the W/CX-2002U/OFHC and CX-2002U/OFHC
mock-ups. The CTEs of CX-2002U are (1.6 - 3.2) x
610-6 /K for X and Y directions and (3.6 - 6.6) x l0 /Kfor Z direction. The CTE of IG-430U is 5.2 x 10-6 /K for
X, Y and Z direction. The CTE of Cu is 16.7 x 10-6 /K.
These show that the difference of the CTEs of CX2O02U and Cu is larger than that of IG-430U and Cu.
Therefore, the excellent mechanical strength of CX2002U may suppress crack formation. In the case ofsilver brazing, no damage was observed in all the mockups under investigation. It is expected that this is150
140130IG-430U/OFHC and IG-430U/OFHC with Ti brazing
and the W coating thickness of 1.0 mm. It can be seenthat the temperature increases monotonically with theincreasing heat flux. Surface temperature of the WIG-430U/OFHC was always higher than that of the noncoated IG-430U/OFHC. For instance, the difference wasabout 250'C at the heat flux of l0 MW/m2.

Figure 4 shows plateau temperatures at the surface,

T1 and T2 for WICX-?OO2U/OFHC and CX-ZOOZU|OFHC with Ti brazing. It is remarkable that the surface
temperature at 10 MWm2 for WCX-2002FVOFHC wasabout 700oC lower than that of WIG-430U/OFHC at

KAC.258(a) nnthe same heat load. Table I shows thermal conductivityof CX-2002U and IG-430U. This large difference may
be caused by the difference in thermal conductivity asshown in Table 1. However, high temperature at lower
heat flux may be due to hot spots caused by surfacel-cunentl

110100{90-980
trav

Poobo,?

roughness.

Figure 5 shows plateau temperatures at the surface,

3020Tl and T2 for WICX-ZDO?U/OFHC and CX-ZOO2U|

l0100

150

Time (s)IG-430U based case

KAC-258ou

1300
1200
1 100
1000
e00

H800
zoo
600

e3oo

2000
1800
1600

raoo
Qeg l2oo

F 5oo
EEi 4oo!o rooo
I1000800o ^^^FOW

200

400050ro1. , luo 200 250

200t lme (s/0Fig.2 (al Time evolution of the electric current throughO2,1681012
Heat Flux(MVm')

WICX-20O2U|OFHC mock-up and (b) temperatures

Fig. 3 Heat flux dependence of plateau temperatures
measured at the surface, Tl and T2 for WIG-II3OU/

at the surface, upper (T1) and lower (T2) parts ofthe brazing interface under heat loading of 10.1

OFHC and lG-43OU/OFHC (Ti brazing). The
thickness of W-coating was 1.0 mm.

MWm'z. The thickness of the VPS-W layer was 0.5mm.
262

Tokunaga K. et al., Thermal Response and Material Degradation of Tungsten-Coated Carbon Divertor Mock-ups by High Heat Flux

CX-2002U based case

Table 1. Thermal conductivity ot CX-2OO2!J and lG-ut3OU.

2200
2000
1800

r600

(J 1400

With W-coatingII|

ifl rzoo
tooots!

a8. aoo400200....................:....

cx-2002u,i''!.

i r : s;[i t:tr,i,:;T;
: t4

|

1t:,

il-r,

Yz

350

422

140

IG-430U

t46.......... ....1.................1iii...1 . .1..o -n!::---r-:',..-'::.'

Fi.o? ..A.F.-.
i| l8r

KACR-1

VPS-WCX-2002U(#2O)/OFHC

10 MVm'z.20s;

ttftttt

0t2

Heat Flux(MW/mz)

t\ 1200-a4i rooo!o3u*ofu

Fig. 4 Plateau temperatures at the surface, Ti and T2 for
WICX-2O02U/OFHC and CX-2002U/OFHC (TiBrazing). The thickness of W-coating was 0.5 mm.

400

Irg

200

CX-2002 based case(Ag baso brazing)

tt:tt'r : Surface
r:T1^:T2eooL'

2200

X:

WithoutW_coatingt."..
E:Surfacet , ;600.oF

Thermal conductivity (WmK)

ii:: Surface

O:Tl
L:r2

aofarror ..lla..
$ ^A^ AA.A A02000*"ru"ioor,io"o.'#:;i1800

160 r8o 2oo1600

Fig. 6 Thermal fatigue test up to 160 cycles (10 MWm,,20 s) for WICX-2OO2UIOFHC (Ti brazing). Changeof the temperatures at the surface, the upper (T1)and lower ff2) sides of the brazed interface.

(J l4oof rzoo

f= tooo
8. eoo.o 600-400

change much. The surface morphology observed by
SEM also did not change. These results indicate that no2000

failure occurred at the braze interface or in the W

0216810t2

coating during cyclic heat load.

Heat Flux(Mw/mz)

In the case of WIG-430U/OFHC with Ti brazing,
micro cracks at the corners of the IG-430U part, which
had been formed during the brazing process, expanded
during testing. This was probably caused by theaccumulation of stress on the edge of the cracks byFig. 5 Plateau temperatures at the surface, Ti and T2 for
W ICX-2OO2UIOFHC and CX-2OO2U|OFHC (silverBrazing). The thickness of W-coating was 0.5 mm.

OFHC with silver brazing. Surface temperature of theWICX-20O2U|OFHC was higher 35'C than that of thenon-coated CX-2O0?U|OFHC. Surface temperaturesdifference between them is smaller than that of the Ti
brazed mock-ups. The reason for this is probably related
to the defects formed in the inside of the WCX-?OO?U|OFHC and WIG-430U during Ti brazing process.
Detailed observation of cross section of the mock-ups
insides structure will be required to confirm this.
Figure 6 shows results of thermal fatigue test up tol60cycles (10MWm2, 20 s ON/65 s OFF) for WCXZOO2UIOF}IC with Ti brazing. Though surface
temperature gradually increased from 1250'C to 15fi)"C
during the cycle test, temperatures at Tl and T2 did notthermal expansion during heating.

4. Summary
New high-Z coated plasma facing components weredeveloped by VPS coating of CX-2002U and IG-430U,and brazing them on the OFHC with a cooling tube.
Thermal response and thermal fatigue tests on the mock-ups were carried out under active cooling conditions
using an electron beam. The most important conclusions
are following.

(l) In the case of WIG-430U/OFHC, micro-cracks
were formed at the corners of the IG-430U part
during Ti brazing, but not for WCX2002U/OFHC.
263

Tokunaga K. et aL, Thermal Response and Material Degradation of Tungsten-Coated Carbon Divertor Mock-ups by High Heat Flux

facing component with high thermal conductivity
and high strength.

(2) No cracks and no exfoliation were formed in Wcoating and at braze interface by heat loads of up to10 MWm2.

References(3) In case ofTi brazing, surface temperature increased
250-300'C due to a VPS-W coating of I mm

tllthickness. On the other hand. in case of silver
brazing, that increased by 35'C due to a VPS-W
coating of 0.5 mm.
(4) Surface temperature increase of the VPS-W coatedCX-2O0ZU|OFHC was much lower than that of theVPS-W coated IG-430U/OFHC due to excellent
thermal conductivity of CX-2002U.
(5) It was showed that the Ti-brazed mock-ups
successfully withstood at least 160 cycles of heatloads at 10 MWm2 for 20 s. This demonstrates that
the Ti-brazing is good alternative to silver brazing
for the carbon-copper system.

W.O. Hofer and J. Roth et al.,Physical Processesof the Interaction of Fusion Plasmas with Solids(Academic Press, 1996) 341.

t2l R. Neu et al., J. Nucl. Mater.24l-243 (1997) 678.
t31 C. Garcia-Rosales et al., Fusion Technol.32 (1997)
263.

t4l K. Tokunaga et aL, J. Nucl. Mater. 258-263 (1998)
998.

t5l K.Tokunaga et al.,J. Nucl. Mater. 266-269 (1999)
1224.

[6] T. Matsuda et al., Proc. Japan-US Workshop-P243
on High Heat flux Components and Plasma Surface

Interaction for Next Fusion Devices (1995\ 366.
t7l K. Kubota et al.,Fusion Eng. Des. 39&40 (1998)(6) These results indicate that the WICX-ZOO2U/OFHC
shows the most promising properties as a plasma

247.

264